DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS

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Process Safety Guide:
GBHE-PSG-019
DESIGN OF VENT GAS
COLLECTION AND
DESTRUCTION SYSTEMS
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DESIGN OF VENT GAS COLLECTION AND
DESTRUCTION SYSTEMS
CONTENTS
1 INTRODUCTION
1.1 Purpose
1.2 Scope of this Guide
1.3 Use of the Guide
2 ENVIRONMENTAL ISSUES
2.1 Principal Concerns
2.2 Mechanisms for Ozone Formation
2.3 Photochemical Ozone Creation Potential
2.4 Health and Environmental Effects
2.5 Air Quality Standards for Ground Level Concentrations of Ozone, Targets
for Reduction of VOC Discharges and Statutory Discharge Limits
3 VENTS REDUCTION PHILOSOPHY
3.1 Reduction at Source
3.2 End-of-pipe Treatment
4 METHODOLOGY FOR COLLECTION & ASSESSMENT OF PROCESS
FLOW DATA
4.1 General
4.2 Identification of Vent Sources
4.3 Characterization of Vents
4.4 Quantification of Process Vent Flows
4.5 Component Flammability Data Collection
4.6 Identification of Operating Scenarios
4.7 Quantification of Flammability Characteristics for Combined Vents
4.8 Identification, Quantification and Assessment of Possibility of Air Ingress
Routes
4.9 Tabulation of Data
4.10 Hazard Study and Risk Assessment

4.11 Note on Aqueous / Organic Wastes
4.12 Complexity of Systems
4.13 Summary
5 SAFE DESIGN OF VENT COLLECTION HEADER SYSTEMS
5.1 General
5.2 Process Design of Vent Headers
5.3 Liquid in Vent Headers
5.4 Materials of Construction
5.5 Static Electricity Hazard
5.6 Diversion Systems
5.7 Snuffing Systems
6 SAFE DESIGN OF THERMAL OXIDISERS
6.1 Introduction
6.2 Design Basis
6.3 Types of High Temperature Thermal Oxidizer
6.4 Refractories
6.5 Flue Gas Treatment
6.6 Control and Safety Systems
6.7 Project Program
6.8 Commissioning
6.9 Operational and Maintenance Management
APPENDICES
A GLOSSARY
B FLAMMABILITY
C EXAMPLE PROFORMA
D REFERENCES
DOCUMENTS REFERRED TO IN THIS PROCESS GUIDE

TABLE
1 PHOTOCHEMICAL OZONE CREATION POTENTIAL REFERENCED
TO ETHYLENE AS UNITY
FIGURES
1 SCHEMATIC OF TYPICAL VENT COLLECTION AND THERMAL
OXIDIZER SYSTEM
2 TYPICAL KNOCK-OUT POT WITH LUTED DRAIN
3 SCHEMATIC OF DIVERSION SYSTEM
4 CONVENTIONAL VERTICAL THERMAL OXIDIZER
5 CONVENTIONAL OXIDIZER WITH INTEGRAL WATER SPARGER
6 THERMAL OXIDIZER WITH STAGED AIR INJECTION
7 DOWN-FIRED UNIT WITH WATER BATH QUENCH
8 FLAMELESS THERMAL OXIDATION UNIT
9 THERMAL OXIDIZER WITH REGENERATIVE HEAT RECOVERY
10 TYPICAL PROJECT PROGRAM
11 TYPICAL FLAMMABILITY DIAGRAM
12 EFFECT OF DILUTION WITH AIR
13 EFFECT OF DILUTION WITH AIR ON 100 Rm³ OF FLAMMABLE GAS

1 INTRODUCTION
1.1 PURPOSE
The purpose of this guide is to provide guidance on the safe design of
vent gas collection and destruction systems including, in particular,
thermal oxidizers and their associated equipment for destroying volatile
organic compounds (VOCs). It is based on experience gained from
operating units and capital projects and on the application of sound
engineering practice and good safety principles.
The standards which are applied to any particular project or plant will differ
based on the geographic location and local legal requirements as well as
site and business preferences. Any relevant company, local, national or
international codes or standards should therefore be applied to the design
of the system.
Most operating problems that are experienced with thermal oxidizers
derive from process deviations upstream of the unit. Therefore, in any
project or installation it is essential to consider the vent collection headers
and the destruction unit as a complete system and not as an assembly of
separate entities.
1.2 Scope of this Guide
This guide does not replace, or provide a substitute for, national or
international standards but should be considered in conjunction with them.
When consulting this document it should be remembered that it is
intended as a guide and not a set of hard and fast rules. Good engineering
judgment should be applied to the design at all times in order to produce a
safe and efficient collection and destruction system.
This guide is applicable to the safe design of:
o Vent collection headers whether connected to destruction units,
flare stacks or vent stacks;
o Ancillary equipment including knock-out pots, fans, pumps etc.;
o Thermal oxidizer units;
o Process and vent gas burner control systems.

It also covers:
o Flammability and explosion hazards in vent headers;
o Environmental aspects of vents treatment and destruction systems;
o Heat recovery systems;
o Flue gas scrubbing;
o Specification and purchase of destruction units.
This guide does not deal with:
o Detailed mechanical or engineering design of the thermal oxidation
unit itself, except where applicable to safety issues;
o Choice of materials of construction for oxidizer refractory linings;
o Choice of specific type of oxidation unit, except for general
considerations around environmental and safety performance.
Guidance on different types of VOC abatement technology can be found
in Process Safety Guide: GBHE-PSG-017
PRACTICAL GUIDE ON THE SELECTION OF PROCESS
TECHNOLOGY FOR THE TREATMENT OF AQUEOUS
ORGANIC EFFLUENT STREAMS
.
Guidance on the detailed design and operation of flare stacks can
be found in Process Safety Guide: GBHE-PSG-008
PRESSURE RELIEF

1.3 Use of the Guide
This guide is split into six main Sections:
1 Introduction.
2 Environmental Issues.
3 Vents Reduction Philosophy.
4 Methodology for Collection & Assessment of Process Flow Data.
5 Safe Design of Vent Collection Header Systems.
6 Safe Design of Thermal Oxidizers.
Section 2: discusses environmental issues, mechanisms for ozone
depletion and air quality standards.
Section 3: provides guidance on reduction at source in compliance with
the principles of inherent SHE.
Section 4: outlines a methodology for collecting and assessing the data
required to design a vent header system. This is based on
previous experience on a number of previous projects in
GBHE.
Section 5: contains guidance on the design of vent header systems.
This is equally applicable to all header systems whether
venting to atmosphere, flare stack or thermal oxidation unit.
Section 6: deals with the design of thermal oxidizers. These are the
most common form of destruction system used for VOCs.
Specific guidance on the design of flare stacks can be found
in GBHE-PSG-008 PRESSURE RELIEF

2 ENVIRONMENTAL ISSUES
2.1 Principal Concerns
Some VOCs are toxic and some are implicated in damage to the
stratospheric ozone layer. However, the principal concerns with most
VOCs are:
(a) Their involvement, together with oxides of nitrogen and in the
presence of sunlight, in the production of photochemical oxidants in
the lower atmosphere (see Section 2.2).
(b) Odors which may be offensive at concentrations well below the
Occupational Exposure Limit (OEL).
VOCs can be classified according to their Photochemical Ozone Creation
Potential (POCP) referenced to a standard of unity for ethylene (see
Section 2.3). Ozone is the photochemical oxidant that has been studied
most widely but there are others including peroxyacetyl nitrate (PAN) and
hydrogen peroxide. Ozone can pose a health risk and cause
environmental damage (see Section 2.4).
Some VOCs also present an odor nuisance, even at very low
concentrations. For example, ethyl acrylate has an odor threshold of about
0.02 ppb. This can create major difficulties for design and operation as the
emission to atmosphere of only a few mg/sec can cause odor problems. It
is therefore vital that odorous materials are contained within process
equipment. Where this cannot be achieved, then destruction or capture
techniques should be very efficient and stacks discharging directly to
atmosphere should usually be very tall.
2.2 Mechanisms for Ozone Formation
The atmospheric chemistry of ozone formation is very complex and
involves a multitude of interacting chemical reactions [Refs. 2 & 3]. The
principal reactions are shown below which illustrate the involvement of
VOCs in a simplified form.

Nitrogen dioxide absorbs natural radiation and breaks down into nitric
oxide and oxygen radicals:
The oxygen radicals combine with oxygen to form ozone:
However, ozone oxidizes nitric oxide to nitrogen dioxide:
Hence there is a natural balance of ozone concentrations at ground level
involving oxides of nitrogen. However, peroxy radicals (RO2) produced by
the attack of hydroxyl radicals (OH) on VOCs act as a sink for nitric oxide
and thereby disturb the above equilibrium towards higher concentrations
of ozone:
It is believed that hydroxyl radicals are formed in the atmosphere by
photochemical dissociation of ozone and subsequent reaction with water.
It should be noted that the above reactions require the simultaneous
presence of precursors in the appropriate meteorological conditions.
Furthermore, not only are some of these reactions slow, but ozone, once
formed, can persist for several days and so may be transported long
distances. Therefore, elevated ozone concentrations often appear over
widespread areas up to several hundred kilometers from the sources of
the precursors.
2.3 Photochemical Ozone Creation Potential
As stated above, VOCs and other substances can be classified according
to their POCP referenced to a standard of unity for ethylene [Ref. 5] as
shown in Table 1.

2.4 Health and Environmental Effects
A high concentration of ozone can affect human soft tissues such as the
eyes and nose. It may also affect respiratory functions including changes
to the airways and an increase in the sensitivity to some inhaled allergens
such as pollen. Although there is no evidence that it can cause asthma, it
has been claimed that it might trigger allergic reactions and it is widely
reported to be involved in the significant rise in reported cases of asthma.
It is recognized that ozone at commonly found concentrations can damage
a wide variety of crops and other vegetation including grapevine, beans,
beet, spinach, clover, peanut, cotton and turnip. It has been reported that
soybean yield is reduced by up to 15% by concentrations of ozone at
about 50 ppb.
Ozone and other photochemical oxidants cause material damage to
rubber, plastics, painted surfaces, dyed fabrics and synthetic elastomers
which is estimated to cost billions of US dollars annually.

It is well known that smog in warm still air, such as regularly experienced
in the Los Angeles area, can be caused to some degree by photochemical
oxidants.
It is worthy of note that ozone is the only atmospheric pollutant that is
commonly present in concentrations that can be significant fractions of the
occupational exposure limit (OEL). Further information on the health and
environmental effects of ozone can be found in Refs. 4 and 5.
2.5 Air Quality Standards for Ground Level Concentrations of Ozone,
Targets for Reduction of VOC Discharges and Statutory Discharge
Limits
The World Health Organization guideline for ground level ozone
concentrations on an 8-hour average basis is 50-60 ppb. The National
Ambient Air Quality Standard for ozone in the USA is 120 ppb hourly
average, not to be exceeded on more than one day per year. The UK
Expert Panel on Air Quality Standards has proposed an Air Quality
Standard of 50 ppb as a running 8-hour average [Ref. 4]. The 8-hour time
weighted average (TWA) occupational exposure limit (OEL) for ozone is
100 ppb; the 3-minute TWA limit is 300 ppb.
A 1991 Protocol to the 1979 United Nations Economic Commission for
Europe (UNECE) Convention on Long Range Transboundary Air
Pollution, calls for voluntary reductions in VOC emissions across Europe
and North America by at least 30% by 1999 relative to 1988 levels.
There is increasing pressure from both legislative authorities and public
opinion to completely eliminate all vents containing VOCs.
In general, discharge limits for VOCs are set at national level and are
usually in the form of emission concentration limits. Some of these are
defined by statute as in TA Luft [Ref. 6] in Germany whereas others
appear as strict guidance limits as in IPR Guidance Notes [Ref. 7] in
the UK. Although the principles of POCP are becoming generally
accepted, it is likely to be some time before they are adopted formally by
the statutory control authorities.

3 VENTS REDUCTION PHILOSOPHY
3.1 Reduction at Source
It is most important that, wherever possible, vents should be eliminated at
source according to the principles of inherent SHE. This is not only
environmentally responsible, but also good business practice as vented
material is wasted material and, furthermore, end-of-pipe treatment is
invariably expensive. If vents cannot be eliminated at source, they should
be reduced as far as possible or mitigated. Large volumes of
vented material will require proportionately larger and more expensive
collection and treatment systems and have higher operating and
maintenance costs. Vents minimization can therefore have a large positive
benefit on the overall project cost.
Technical options for control at source include the:
o Increased vessel design pressure may eliminate the need for
pressure relief systems at minimal extra cost for the stronger
vessel. Consideration should also be given to the possibility of
uprating the design pressure of existing vessels, tanks and pipe
work. Stock tanks should be fitted with PV valves instead of open
vents;
o Instrumented, high integrity protective systems may be fitted
utilizing reaction quench technology or dump tanks. It should be
noted that in North America and some countries subscribing to
ASME codes, containment or instrumented protective systems may
not be allowed;
o If water-based solvents or solvents with lower volatility can be used,
VOC discharges can generally be reduced significantly;
o Subject to considerations of safety, cross-contamination and plant
layout, a number of stock tanks can sometimes be connected to a
common venting system to reduce the overall volumetric flow rate.
This is particularly effective when transfers are made between the
tanks in question;
o Similarly, the vent on a road tanker or other transportable container
that is being loaded or unloaded to a stock tank should, wherever
possible, be connected (i.e. back-balanced) to the stock tank vent
system;

o Where it is necessary to exclude oxygen or moisture by using
nitrogen, this should be achieved by means of a pressure-
controlled nitrogen supply and a pressure-controlled vent rather
than a continuous nitrogen sweep in order to minimize the volume
of gas vented. Furthermore, the nitrogen inlet and the vent outlet
should be located close to each other in order to minimize the
concentration of VOCs in the vent. Disturbance of the vapor space
should be minimized by connecting the nitrogen flow via a large
nozzle thus reducing the gas velocity;
o It is claimed that floating-roofs can reduce evaporative losses from
stock tanks by up to 90% compared to conventional fixed roof
tanks. Multiple and secondary seals also reduce evaporative
losses;
o The liquid inlets to stock tanks should, wherever possible, be below
the liquid level in order to minimize the disturbance of the vapor
space. This reduces evaporative losses;
o Hydraulic and pumped liquid transfers, rather than pneumatic
transfers, can significantly reduce VOC losses as vapor and mist in
the vent at the end of the transfer;
o The charging of material through an open lid or charge port into a
vessel containing VOCs usually results in VOC losses to
atmosphere;
o If the vessel is at or above atmospheric pressure, the losses occur
locally. If the vessel is under some vacuum, there will be an ingress
of air which could result in a VOC discharge to atmosphere
remote from the charge point. Furthermore, air sucked in could
result in fuel-rich mixtures becoming flammable in the vessel or in
downstream vent collection pipe work;

o Ιf the material to be charged is a liquid or can be dissolved in a
liquid, a closed charging system should be used. Where this is not
possible, a charge hopper should be considered with a narrow
entry point and a rotary, ball or slide valve into the vessel;
o As a general rule, the flow rate of inerts that come into contact with
VOCs should be minimized. Unnecessary purging and draughting
should be avoided. Attention should be paid to poorly designed or
faulty pneumocators, valves on nitrogen blowing or blanketing
systems that are passing or left open, etc. Correct location of
nitrogen blanketing on the vent line to the thermal oxidizer can
reduce vapor losses, but in some cases it may be necessary to
sweep the vapor space (e.g. if corrosive gases are evolved from
the liquid);
o High quality maintenance can reduce fugitive losses from poorly
seated relief valves, pin holed bursting discs, flanged connections,
control valve stems, pump glands, etc.. Fitting bursting discs to
relief valve inlets may eliminate fugitive emissions but their effect
on the relief stream capacity should be checked;
o Alternative process equipment may reduce fugitive losses e.g.
glandless or canned pumps, soft seat relief valves, bellows sealed
valve stems and improved gasket materials.

3.2 End-of-pipe Treatment
Some possible types of end of pipe treatment are:
 Condensation;
 Adsorption;
 Absorption;
 Thermal oxidation;
 Catalytic oxidation;
 Biological filtration;
 Membrane separation.
End-of-pipe solutions should always be regarded as a last option in view
of their capital and operating cost. Destruction systems can also have
inherent problems of statutory authorization and social pressures which
invariably take a significant amount of time, effort and money to overcome.
The additional cost to the business of these factors should not be
underestimated. The overall energy and environmental impact balance
should be considered carefully before selecting the appropriate, if any,
vents destruction system. The impacts of such things as additional support
fuel usage, discharges to atmosphere of thermal oxidizer flue gas,
discharges to water of scrubbing liquor blowdown or waste solids disposal
of spent adsorbent should be addressed opposite the environmental
improvement of treating the vent gas in question. This exercise is required
by statute under Best Practicable Environmental Option (BPEO)
assessments in the UK and under Best Available Control Technology
(BACT) assessments in the USA.
The above principal end-of-pipe treatment options are described in more
detail in GBHE-PEG-015 which also provides guidance on the selection of
the appropriate option together with names and addresses of suppliers. It
may be advantageous to use a combination of techniques such as
refrigerated condensation, adsorption or membrane separation in order to
concentrate or reduce the amount of VOCs prior to destruction by thermal
oxidation. This will result in a smaller and thus cheaper destruction unit.

The safety and environmental aspects of thermal oxidation are discussed
further in Section 6 of this guide.
4 METHODOLOGY FOR COLLECTION & ASSESSMENT OF PROCESS
FLOW DATA
4.1 General
Vent gas collection and destruction systems are complex plants in their
own right. Hence, in order to ensure a safe design, a methodical approach
to the design basis and basis of safety is essential. This Section provides
a framework methodology which can be adapted to specific project
requirements.
Considerable effort is required to collect the information on flows,
compositions, component data, flammability data and scenarios which is
needed to produce the basis of safety for the system and the Hazard
Study. The use of a spreadsheet will assist in this process. This process is
especially difficult for batch plants where flows are intermittent and highly
variable.
For existing plants and processes it is essential to obtain the full co-operation of
the plant personnel in the information gathering process since they will have
experience of many of the possible deviations from normal operation which can
occur. It should be noted that some possible occurrences may never have been
experienced in the life of the plant due to their extremely low potential frequency.
The range of possible scenarios should be established by consultation with the
plant operations team and by examination of the Hazard Study records for the
project. If necessary, further Hazard Studies may be required to establish a
range of worst cases. Full transmittal of this information from the plant to the
project (or between members of the project team for new plants) is essential. For
new plants, all possible operating scenarios should be identified at the design
stage, again using information from the Hazard Study process. Other useful
techniques for hazard assessment and reduction are fault tree analysis, process
hazard review, failure mode and effect analysis and consequence analysis [Ref.
17].

The use of a standard proforma may be helpful in allowing clear and concise
collation of the data. It is essential to ensure that the person responsible for
completing the proforma is aware of the importance of the data being supplied.
One-to-one discussions are invaluable to avoid confusion. The proforma should
be comprehensive in the information requested. An example proforma is shown
in Appendix D. If the information supplied on the proforma is incomplete or
incorrect, it will have serious consequences for the design of the system,
possibly even making it unsafe. If errors are discovered in the information on
vents flow and compositions the rework required will almost certainly be costly in
terms of both man hours and new equipment. There are examples where VCDS
have been grossly undersized or there have been fluctuations of the composition
into the hazardous region due to a failure to identify the maximum short term
flows.
If possible, the vent collection system should be installed at least a year before
final design of the destruction system in order to provide time for comprehensive
monitoring of the flows and compositions in the header system under operational
conditions. This has benefits to the project in that the data collected during this
period enables a more efficient destruction unit to be designed with consequent
savings in design and operational costs. Regulatory authorities, however,
generally require the collection and destruction systems to be installed
simultaneously.
The proposed methodology for safe design consists of the following steps:
 Identification of vent sources;
 Characterization of vents;
 Quantification of process vent flows;
 Component flammability data collection;
 Identification of operating scenarios;
 Quantification of flammability characteristics for combined vents;
 Identification and quantification of possibility of air ingress;
 Tabulation of data;
 Hazard Study assessment.

The processes involved at each step are described further in Sections 4.2
to 4.9 (inclusive):
4.2 Identification of Vent Sources
It is essential that all vent sources should be identified before starting the
design of the header system. These may include:
 Tank breathing vents;
 Relief and breather valves;
 Tanker loading points;
 Reactor vents;
 Vacuum pump exhausts;
 Lute pots and siphon breakers.
It is important that all sources are identified, as the number and location
will have an impact on the size and complexity of the collection system. It
may be possible to identify a number of vents which could be eliminated,
recycled economically or minimized by other means at this stage. Any
existing vent or flare header systems should also be identified (e.g.
common purging of tank farms), and a strategy for dealing with these
included.
During this part of the project, the plant engineering line diagrams (ELDs)
should be updated for existing plants and vent sources for new plants
clearly marked. This information should also be carried over onto site plot
plans and general arrangement drawings and will aid both estimation of
project costs and mechanical design of the header system.
4.3 Characterization of Vents
The results from this part of the design process will have major
implications on the number, type and size of headers, the conditions in the
system and ancillary equipment needed. Vents may be characterized in
several different ways. Typical characterization groupings are:

 Fuel-rich, fuel-lean or flammable;
 Continuous or intermittent;
 Condensable or non-condensable;
 Corrosive;
 Toxic;
 Wet/dry;
 Mixed or variable properties.
This activity will indicate which of the vent headers is the most appropriate
to use for each vent stream and any treatment which is needed to make
the header safe if it would otherwise operate within the flammable region.
During the characterization process, the effect of any interactions between
vent compositions should be evaluated to ensure that the flows and
compositions in the system do not operate in the flammable region and
that there are no undesirable chemical reactions between the different
materials. This is particularly important where there may be polymeric
material which can clog the system. Any base load of inerts, support fuel
or dilution air should be included. An interaction matrix should be used to
ensure that all possible combinations are identified and assessed.
Interactions should also be examined between the VOCs and the
materials of construction of the header system. An example of this is
shown in Ref. 17.
Certain conditions such as fire relief and other types of emergency vent
may be exempt from treatment on the basis that they are likely to occur
extremely infrequently and have such large flow rates that they would
need the construction of a much larger destruction unit. Such matters
should be assessed during the quantification of process vent flows and, if
appropriate, discussed with the local regulatory authorities.
Vents often have varying compositions depending on the particular
operating scenario at the time; hence the "mixed or variable properties"
heading. These may need special consideration if they can transit from
fuel-rich to fuel-lean or vice versa. Similarly, consideration may be
required if the composition in the vent can change drastically or if a
material with extreme combustion properties such as hydrogen or a
material with an unusual flammability diagram such as ethylene oxide can

be present. The effect of such changes will have an impact on the design of the
headers in dictating the type of ancillary equipment or systems needed (e.g.
flame arresters or inert gas provision). This is of particular importance in batch
process where several reaction steps or unit operations may be carried out in a
single vessel.
4.4 Quantification of Process Vent Flows
Vents collection and destruction systems can only be designed safely with
full knowledge of the range of flows and compositions which may be
encountered not only during normal operation but also in abnormal
conditions (e.g. relief valve operation, process deviations etc.). For most
processes, whether batch or continuous, both the vent flows and
compositions are likely to be highly variable. Typically, the following
operations should be considered:
• Flowsheet (normal operation);
• Batch operating cycle;
• Tank breathing as a result of thermal expansion and contraction,
pumping etc.;
• Process deviations;
• Relief situations;
• Maintenance purging of some or all plant items;
• Start-up, shut-down and stand-by modes;
• Other abnormal operations.
Where possible, monitoring of flows and compositions should be carried
out over an extended period of time where applied to existing plants to
ensure that all normal situations are covered. Where this is not possible,
soundly based estimates should be made. It is unlikely that worst case
conditions will be seen during the monitoring period since the frequency of
combined events occurring may be very low. A judgment should therefore
be made as to the worst credible case, taking into account equipment
failures, process deviations, operator error, etc. Some of this information

may be carried over from pressure relief documentation, especially relief
philosophy and bursting disc/relief valve data sheets and. If possible, the
vent collection system should be installed prior to the destruction system
in order for performance monitoring to be carried out. This will yield
valuable design information for the destruction unit.
In-depth plant knowledge will be needed to fully identify all the possible
deviations and resulting vent compositions and flow rates. Once again,
any base load of inerts, fuel or dilution air should be included. As stated
above, a proforma may be useful for the transferral of information from
plant and operations personnel to the project team, although this is no
substitute for face-to-face discussions with plant personnel and should not
be used in isolation from other information sources. The data can be
classified into a number of flow rate/composition scenarios such as:
 Zero;
 Normal / flowsheet;
 Minimum flowsheet;
 Maximum flowsheet;
 Maintenance condition;
 Maximum plus over-design allowance.
It may be impracticable to install a vent gas collection and destruction
system that can cope with the simultaneous occurrence of the "worst
case" flows from all vent sources. The likely frequency and duration of
deviations from flowsheet should, therefore, be estimated in order
to determine which combination of vent flows will be accommodated and
which will be dealt with by other means. Common cause events should be
identified as these often lead to comparatively large vent flows e.g. power
failure. When calculating the flows due to relief valve operation, the relief
stream capacity should be used rather than the required relief rate.
A spreadsheet may be helpful to correlate the data in order to identify
those scenarios which would cause operational difficulties or process
hazard.

The quantification of vent flows may be particularly difficult for batch
processes which, by their nature, have intermittent flows and
compositions. In this case it is sensible to consider the maximum possible
flows from the process and the full range of flows from zero to the
maximum. For batch processes, consideration of the possibility of process
deviation and cross contamination is especially important. The reduction
of emissions from batch processes is discussed in Ref. 1.
It may be advisable to carry out a Hazard Study on the upstream process
plant at this stage to consider the feasible deviations which could occur
resulting in different emissions to the vent collection system. When
applying the Hazard Study guide words, consideration should be given to
the special cases which may be generated (e.g. more fuel, more air, less
fuel etc.).
Typical deviations which should be considered for all process plant, but
especially for batch processes, are:
 Charging wrong reactants (other materials stored in area or wrong
materials delivered);
 High or low process temperatures;
 High or low pressures;
 Overfilling of tanks, reactors or distillation columns;
 Purging, venting or pressure letdown;
 Agitator failure;
 Heating failure;
 Cooling water failure;
 Instrument air failure;
 Power failure.
Overfilling can be a major problem as it may result in liquid entering the
vent gas collection header system. This should be avoided as it can cause
a number of hazards as described in Section 5.3. Frothing of reactor or
tank contents may also result in liquid entering the header system.

Suitable precautions should be taken to prevent this situation occurring,
including the provision of liquid interceptors (knock-out pots) or liquid level
alarms if appropriate. Temperature, pressure, bubble and dew points for
each component and composition may be needed if there is a possibility
of flashing liquid entering the header and also to evaluate any possibility of
volume shrinkage of the gas on cooling or condensation after entering the
header. (Shrinkage may cause air to be drawn into the header giving rise
to a flammable mixture). This will also give an indication of whether
lagging or heat tracing of lines is needed and whether there are any
potential solidification or icing problems.
Incomplete quantification of data is likely to result in incorrect specification
of equipment including the vent collection pipe work, safety equipment
such as flame arresters, KO pots and downstream plant such as a thermal
oxidizer. It is therefore vital that the quantification process is carried out in
full. This can only be achieved by appropriate allocation of resources and
time in the overall project program (see Section 6.7). Particular regard
should be paid to the presence of more hazardous components such as
hydrogen, acetylene, ethylene oxide etc..
Chemical interactions should also be quantified at this stage using the
interaction matrix developed in Section 4.3. Undesirable reactions may
occur when mixing vent streams causing, for example, polymerization,
condensation or exothermic reaction. Such situations should be avoided.
4.5 Component Flammability Data Collection
Flammability data, particularly LFL, UFL and MOC, is required for each of
the components in the vent system in order to construct the flammability
diagrams for the different compositions and scenarios which may occur
(see Section 5.2). If possible, experimentally determined flammability
diagrams should be used. If flammability diagrams are not available then
they may be constructed for each of the worst case compositions for each
of the vents. For further explanation of flammability diagrams see
Appendix C.
In some systems there is synergy between the more reactive and less
reactive components of the gas mixture, hence relatively small amounts
of, for example, hydrogen have a disproportionate effect on the
flammability characteristics. If there are multiple components or significant
quantities of reactive gases present then experimental determination of
flammability characteristics should be considered.

These diagrams can also be used to assess the possible consequences of
air ingress into fuel-rich systems. The flammability characteristics for
mixtures should be estimated using Le Chatelier's rule (see Appendix C).
A spreadsheet may be useful for this. Critical flammability estimates
should be backed up with experimental data.
4.6 Identification of Operating Scenarios
The range of operating scenarios which are appropriate to the individual
process sources should be identified. The scenarios to be considered may
include:
• Start-up from cold;
• Re-start after trip;
• Shut-down;
• Stand-by;
• Normal operation;
• Low rate operation;
• VOC/fuel excursion;
• Oxidant excursion;
• Inert excursion;
• Commissioning standby equipment or after maintenance;
• Depressurizing or venting down;
• Vacuuming down;
• Purging.
Any other possible scenarios should be identified as part of the individual
project.

There may be differences in the way that the system performs or in the
conditions of each vent under the different operating scenarios which lead
to differences in the flow rate or composition of the vent. The range of
operating scenarios identified depends on the operation of the plant, for
instance the conditions for a cold start up may differ from those which
occur after a plant trip. The identification process is intended to detail the
full envelope of operating conditions which can be generated by the plant.
The list produced should include those scenarios which would be
generated by failures of trips or controls. Obviously a full working
knowledge of the plant and its associated control systems and safety trips
is required to identify all the possible scenarios.
4.7 Quantification of Flammability Characteristics for Combined Vents
A brief description of flammability diagrams and associated terminology is
given in Appendix C. The flammability characteristics for the possible
combinations of vent sources under each of the possible operating
scenarios should be calculated. The compositions calculated can be
placed into one of the following categories:
• Fuel-lean;
• Fuel-rich (oxidant lean);
• Inerted;
• Flammable.
Fuel-lean vents are those which have fuel concentrations below the LFL
and which are therefore safe under all air ingress conditions. Fuel-rich
vents have compositions above the UFL which could, in theory, enter the
flammable region in the case of air ingress whether they are oxidant lean
or inerted. Flammable vents are those operating inside the flammable
region. As stated previously in Section 4.6, excursions should be
considered which could change the composition of the vent.
Flammable vent compositions should be avoided if at all possible or
treated to take them out of the flammable region (e.g. by inerting). If they
cannot be avoided, a full risk assessment of the likelihood and
consequences of incidents should be carried out.

Design of the system to cope with overpressure due to deflagration or
detonation may be necessary in exceptional circumstances (see Section
5.2.6).
It should be noted that air is not the only source of oxidant. In particular
chlorine and oxides of nitrogen may act as oxidizing agents. These may
originate in the upstream process.
4.8 Identification, Quantification and Assessment of Possibility of Air
Ingress Routes
Obviously, if the vent collection header is operating under positive
pressure at all times then air cannot be sucked into the system from the
atmosphere. Hence, it may be possible to eliminate all or the majority of
possible air ingress routes in the header system by operating at positive
pressure. However, this may not be possible for all vents or for upstream
process equipment operating under vacuum. Therefore it is essential that
all possible upstream air ingress routes are considered as well as those
relating directly to the header system. There may also be a number of
cases where failures mean that a nominally positive pressure system may
become negative pressure.
There may be a number of possible routes for air ingress into fuel-rich
vent headers. For each source, all possible openings or paths for air
leakage into the system should be identified and the potential ingress
rates estimated. It is important to include all flanged joints, instrument
connections and also possible failures of the header pipe work. Some
typical situations and operations which may lead to possible air ingress
routes are:
• Maintenance operations involving removal of equipment such as
isolation, control and relief valves, instruments, blank ends, flanges,
slip plates, etc;
• Failure of seal liquid supply to, or failure to top up, lute pots or
leakage of seal liquid. This may lead to the seal running dry thereby
opening up a route direct from atmosphere;
• Accidental damage to pipe work (e.g. vehicle damage to exposed
lengths of header adjacent to roadways);

• Corrosion of pipe work and fittings including cracks and weld
defects;
• Manual operations involving breaking/making connections such as
road/rail tanker purges to vent collection headers, opening of
inspection and charging ports, sampling operations, physical
changeover of batch unit operations, etc.;
• Process equipment operating under vacuum;
• Failure to adequately purge equipment prior to start-up, causing air
to be displaced into the header;
• Failure to completely purge headers and laterals from process
vents through to vents treatment unit;
• Inadequate isolation e.g. leaving sample points open, failure to
blank off, passing valves (including thermal oxidizer bypass valves);
• Oxygen generation by process.
It may be possible to eliminate a number of air ingress routes by
minimizing the number of flanges, equipment connections etc.. Similarly,
purge and sample points may be equipped with “dead man’s handle” type
valves to prevent them being left open inadvertently. Other operations
may also be modified to reduce the possibility of air ingress.
Where stacks are involved, many nominally positive pressure systems
may in fact be under a slight vacuum due to the chimney effect. This is
particularly apparent where the vent gas is above ambient temperature or
the molecular weight is significantly less than that of air. This
effect is more pronounced at low flow rates and can result in air ingress
causing a flammable mixture. The stack may be fitted with a liquid seal at
the base to prevent the header system operating under negative pressure.
Even systems that are nominally under positive pressure may in fact be
under negative pressure due to the chimney effect where no seal pot is
installed.

Once again, this identification process requires a thorough knowledge of
the plant operation, layout and maintenance procedures if all possible
hazards are to be identified. Some knowledge of the proposed operating
pressure in the header system will also be needed to calculate the air
ingress rates from the various routes. This may mean taking some early
design decisions in order to get an early estimate of possible
consequential hazards.
The frequency and consequence of each possible air ingress combination
should be estimated. Air leakage into the system will alter the composition
in the header, possibly taking it into the flammable region. Additionally, the
flows from other sources and the pressure profiles in the header may be
affected by the leakage. The full possible range of operating scenarios for
other vent sources discharging into the same header should be
considered, including the effects of the air leakage. Similarly, the
interactions caused by air leakage should be identified. An interaction
matrix should be used for this evaluation process.
4.9 Tabulation of Data
A "control chart" should be created that lists the activities and events
(normal and abnormal) which would result in deviations from flowsheet
conditions whether resulting from process variations or by air ingress.
By inspection, those scenarios which would not result in a flammable
mixture occurring in the vent header system should be eliminated. It is
extremely important to identify the likelihood of any transient incursions
into or through the flammable region, as well as new steady state
flammable conditions.
The remaining scenarios are therefore the ones which would result in a
potentially hazardous situation.
The tabulation of data is an important aid to understanding the
complexities of the numerous operating scenarios and simplifies the
identification of potentially hazardous situations.

4.10 Hazard Study and Risk Assessment
An assessment of the potential for flammable mixtures and the likely
consequences should be carried out in the Hazard Studies. Whether or
not required in order to satisfy the Hazard Study criteria, the opportunity
should be taken to consider the possibility of making some of the
hazardous vents inherently safe or more safe or of reducing the duration
or frequency of potentially unsafe situations. It is most important that a
trained, accredited practitioner carries out, or at least coordinates, the
Hazard Studies and Risk.
4.11 Note on Aqueous / Organic Wastes
One way of dealing with aqueous effluent contaminated with organic
waste is to air strip it in a packed column. The air can then be used as
combustion air in a thermal oxidizer. There are, however, a number of
potentially hazardous situations that could arise including the following:
• Enclosing organic contaminated water in a tank may lead to a flammable
mixture arising in the vapor space of the tank. The Henry's law coefficients
of the contaminants should be examined to check for the possibility of a
flammable mixture above the liquid;
• If a large quantity of organic liquid gets into the aqueous waste stream
there is a risk of free phase organics getting into the stripping column. If
this occurs, then the air stream coming from the stripper may again be
flammable;
• The column may also be prone to clogging due to dissolved or suspended
solids. Reaction of scrubbing liquor with atmospheric gases or
constituents of the vent gas may also cause clogging (e.g. alkalis with
CO2);

• During start-up, shut-down or process interruption the vapor space in the
stripper column head may become flammable. On restart the flammable
mixture may be carried forward into the header system with consequent
risk of explosion hazard;
• Low air flow, however caused, can cause a flammable mixture to develop
in the air stripper (e.g. fan failure, damper failure, partial blockage).
4.12 Complexity of Systems
There may be considerable pressure both from environmental and
business sectors to increase the number of vents being treated by a single
thermal oxidizer. As the number of vents increases, the number of cases
to be considered may increase exponentially. This is reflected in the
increased amount of work needed during the design methodology
described previously.
Cost and business pressures often dictate that a single large thermal
oxidizer is installed rather than a number of smaller dedicated units. The
provision of two or more smaller VCDS may make the system inherently
safer due to the reduced complexity and lower number of possible failure
modes. When all factors including maintenance, availability and the cost of
down time are taken into account, the economics of a number of smaller,
independent systems may in fact be better than for a single large system.
4.13 Summary
The methodology described above is intended as guidance which can be
adapted to the particular requirements of any project. It does, however,
contain the basic steps which should be considered for the assessment of
potential incidents in the formal Hazard Study of a vent header system. It
is important that this work results in an auditable design trail.
It is essential that, wherever possible, vents should be eliminated at
source according to the principles of inherent SHE. This is not only
environmentally responsible but also good business practice as
vented material is wasted production which cannot be recovered and,
furthermore, end-of-pipe treatment is invariably expensive to design, build
and operate. If vents cannot be eliminated at source, they should be
reduced as far as possible or mitigated.

5 SAFE DESIGN OF VENT COLLECTION HEADER SYSTEMS
5.1 General
A typical vent collection and destruction system (VCDS), including a
thermal oxidizer, is shown schematically in Figure 1.
FIGURE 1 SCHEMATIC OF TYPICAL VENT COLLECTION AND THERMAL
OXIDIZER SYSTEM
All pipelines carrying potentially flammable liquids or gases have some risks
attached. These risks may stem from external factors such as corrosion or
impact damage or internal factors such as process composition changes or the
failure of a fan or pump. The risks can be minimized by good engineering design
as described in the following guidelines.
The quality of the design, maintenance and operation of the vent header system
is critical to the safety of the thermal oxidation unit, since many safety problems
with VCDS originate in the vent headers or upstream process plants.

5.2 Process Design of Vent Headers
5.2.1 Basis Of Safety
The principal requirement of the basis of safety for vent headers should be
control of the vent gas composition such that the header does not operate
in the flammable region during normal operation or abnormal situations.
This can be done properly only after systematically identifying,
characterizing and quantifying the vent streams using a rigorous
methodology such as that described in Section 4. This approach is based
on inherent safety and is important since there may be possible sources of
ignition in the vent header system itself (fans, pumps etc.) and there is, of
course, a permanent source of ignition in the thermal oxidation unit. Even
in the absence of obvious ignition sources in the header system there is
still a possibility of static electricity discharge, especially in non-conductive
or mixed conductive and non-conductive pipe work systems. The
probability of an ignition occurring may be low but cannot be assumed to
be zero. Operation in the flammable region could therefore result in an
ignition in the header leading to deflagration or detonation.
It can be difficult to design vent headers to have a sufficiently low
frequency of deflagration or detonation, particularly if the consequences of
such an event would be the rupture of a long vent header. Where a
header passes through a number of different plant areas, the domino
effects from the rupture of a header are potentially serious.
Notwithstanding the precautions taken to prevent vent headers operating
in the flammable region, process deviations, equipment failures or other
unforeseen circumstances may arise which result in the formation of a
flammable mixture within the system. Typical of such events are leakage
of air into the system from maintenance activities, process deviations on
start-up or shut-down or failure of instruments. These failures or
deviations, however unlikely, will have a finite potential frequency. Since
there is also some finite probability of ignition sources being present, it is
prudent to consider installing a second form of protection to further reduce
the possibility of a flame front propagating into the header pipe work or
other pieces of equipment with consequent hazards.
Secondary protective systems are not designed to provide continuous
protection against the permanent or extended presence of a flammable
mixture in the header but do provide protection for a limited period
enabling the system to be shut down safely or the flammable condition to
be removed.

Typical secondary protection systems are flame arresters or flame
suppression systems and shutdown or diversion systems actuated by
oxygen analyzers.
Flame arresters are designed to prevent the propagation of a flame along
a header from an ignition source. To be effective they should be placed as
close as possible to the source and should be considered for fitting in the
vent headers both upstream and downstream of any potential ignition
sources in the header itself and also immediately upstream of the thermal
oxidation unit. Positions of flame arresters in a typical vent header system
are shown in Figure 1. All flame arresters, including those on diversion
stacks or vents, should be fitted with high temperature trips or alarms to
warn of an ignition occurring in the system.
Some additional protection from flashback from the thermal oxidizer may
be provided by the velocity of the gas through the burner nozzle if it is
greater than the turbulent burning velocity. However, it is extremely
difficult to estimate turbulent burning velocities. There is also some doubt
as to the possibility of flame creep back along the walls of the burner
nozzle and back into the header. Thus this approach cannot be used as
the basis of safety against flashback.
It may be tolerable to operate very short sections of the header system in
the flammable region, if this condition is unavoidable, depending on the
hazard consequences and the probability of an ignition occurring. In this
situation, the length of line operating in the flammable region and the
probability of an ignition occurring should be minimized. An example of
this is the section of line at the exit from a scrubbing system or reactor
where dilution air or inert gas can only be injected after leaving the
scrubber or reactor thus creating a small flammable region prior to dilution.
In some cases it may be possible to design the plant to withstand
deflagration or detonation where this condition is known to exist.
The consequences of a detonation occurring in a line should be
considered very carefully with particular emphasis paid to possible injury
or plant damage from missiles. Domino effects by missile impact into other
pieces of plant and equipment, e.g. tanks holding toxic or flammable
materials, should also be taken into account.

Requirements of the basis of safety in addition to the avoidance of
flammable mixtures, include protection against internal and external
corrosion, mechanical impact damage, liquor logging and back pressures
that could adversely affect upstream plant. It may be necessary
to segregate vent streams in parallel headers in order to manage these
issues or even to install a number of independent, smaller VCDS.
5.2.2 Process Design Basis for Vent Collection Header Systems
Vent header systems should be designed to avoid the possibility of
flammable mixtures as described above in Section 5.2.1. For the initial
design, three main types of header should be considered: fuel-rich, fuel-
lean and inerted. Several branches may connect into each header at the
process plant end. It is, therefore, important to ensure that a flammable
mixture cannot result from the mixing of a fuel-rich vent from one branch
mixed with a fuel lean vent from another branch.
The design basis should also take into account the quantity of material
vented and the design pressure of the upstream process equipment. For
example, low pressure storage tanks may not have a high enough design
pressure to provide the necessary driving force for the required flow of
material down the vent header and hence a suction fan may be needed.
This may, however, introduce new hazards from pulling air into the system
or sucking the tank in.
Complications may also be introduced into the design by the presence of
high and low pressure vents and high and low temperatures, especially if
venting into the same header. For high pressure vents the possibility of
back flow and over pressurization or contamination of low pressure
sources (e.g. stock tanks) should be considered. High temperature may
cause damage to headers or take the mixture above its auto-ignition
temperature. High temperature and high pressure may also affect the UFL
and LFL. For further information on the change of flammability limits with
temperature and pressure see list of Best Contacts in Appendix B.
Separate header systems or additional processing equipment may be
required to avoid these issues. Low temperature (e.g. from vaporization of
liquefied gases) may cause condensation in the line and liquid logging or
even freezing. In cold climates it is often necessary to lag and heat trace
headers to prevent condensation or icing.

If VOCs do condense in the header during cold periods and then vaporize
on warming, the destruction unit can be overloaded. There are recorded
instances where this has occurred. In carbon steel headers embrittlement
and damage may result from sub-zero temperatures. (see Section 5.3.3).
5.2.2.1 Fuel-Rich Headers
Wherever practicable, fuel-rich headers should be operated under positive
pressure rather than under suction since a leak of gas to the atmosphere
will usually be less hazardous than an ingress of air which could possibly
result in the mixture becoming flammable. The exception to this is where
the material in the line is not only flammable but also toxic or highly
damaging to the environment; in which case the consequences of a
release should be considered carefully against the consequences of air
ingress.
The pressure of a sub-atmospheric header will probably have to be raised
above atmospheric at some stage upstream of a thermal oxidizer.
Therefore, it is generally better to provide the boost in pressure as far
upstream as possible in order to minimize the length of header subject to
possible air ingress.
A major consequence of a leak of non-toxic gas from a fuel-rich header
operating under positive pressure is likely to be a torching fire which may
impinge on other adjacent equipment. The possibility of consequential
ignition or damage to other equipment in this event needs to be
considered. With vent systems it is unlikely that sufficient gas will be
released to cause a significant fireball or flash fire; however, if the release
occurs in a confined space there is a risk of a confined explosion.
Significant overpressure is only generated when the flammable cloud has
a degree of confinement. Most vent headers run in unconfined areas so
the risk of a confined explosion is generally small. For assistance with
explosion and consequence modeling see Best Contacts in Appendix B.
5.2.2.2 Fuel-Lean Headers
Fuel-lean headers can be operated above or below atmospheric pressure
without increasing the risk of generating a flammable mixture through air
ingress. If the vent gas is toxic or particularly damaging to the
environment, then consideration should be given to operating at
sub-atmospheric pressure.

5.2.2.3 Inerted Headers
Inspection of the operating position on the flammability diagram is
necessary to determine the effect of air ingress into an inerted header.
Where it is possible for air ingress to result in a flammable mixture, the
header should, wherever practicable, operate above atmospheric
pressure.
5.2.3 Modifying Composition of Vent Headers
Wherever practicable, flammable mixtures should not be sent to the vent
collection system. If the vents arising from a plant or process are in, or
very close to, the flammable region then they should be made safe prior
to, or immediately after, entering the vent header system. Similarly, if it is
possible for a flammable mixture to be generated within a vent header by,
say, the mixing of fuel-rich and a fuel-lean vent streams or condensation
of VOCs in a fuel-rich stream, the possible consequences should be
evaluated and, where appropriate, corrective action taken. This can be
done in a number of ways (see 5.2.3.1 to 5.2.3.4):
5.2.3.1 Enriching
The vent gas may be enriched by adding fuel gas to take the composition
above the UFL. Some of the thermal oxidizer support fuel can be added in
this way. The amount required to make the vent "safe" should be
calculated based on the variability of the composition and flow and
possible air ingress rates. For reactive gases, such as those containing
significant quantities of acetylene, ethylene oxide, hydrogen etc., it is
difficult to specify an upper "safe" limit because of the size of the
flammable region. Hence, the flammability characteristics of
the gas mixture should be taken into account when specifying the
appropriate amount of enrichment.
5.2.3.2 Diluting
The vent gas can be diluted with air to below the LFL. From NFPA 69 a
value considered "safe" for this would be LFL/4 without composition
monitoring or up to 60% of the LFL with monitoring, but other values may
be appropriate on consideration of the factors described above. Operation
above LFL/4 with or without monitoring, should be considered very
carefully. This value is chosen because of the variability of process flows
and the difficulty of estimating compositions accurately for upset
conditions.

Reasons for the choice of factor should be described in the basis of
safety. The amount of confidence in the accuracy of flow and composition
data should be considered when making the decision.
It should be noted that, whereas application of the NFPA standards is
mandatory in the US and Canada and may also be mandatory in some
other countries and is strongly recommended for use throughout the
Americas, it may not be accepted in others. It is necessary to check with
local regulatory authorities before making a final decision.
5.2.3.3 Inerting
Inert gas can be injected into the vent in order to reduce the concentration
of oxygen in the header to below the minimum oxygen concentration
(MOC) to sustain combustion. NFPA 69 suggests a limit of 60% of the
MOC with monitoring or 40% of the MOC if the MOC is below 5%. If not
continuously monitored, the oxygen concentration should be checked on a
regular basis (see NFPA 69). Again, the variability of the vent flow and
composition should be considered along with the measurement accuracy.
There may be circumstances where it is appropriate to use a larger safety
factor such as 25% of the MOC depending on the variability of vent flows,
process deviations and confidence in the data. The reasons for the
choice of dilution factor should be detailed in the basis of safety. As
above, it should be noted that application of the NFPA guides may be
mandatory in some countries.
5.2.3.4 Combination of Vent Headers
Combining vent streams should be considered very carefully. Although
mixing vent streams to ensure operation outside the flammable region is
possible, the various combinations of flow and composition should be
quantified in detail as deviations in one or other of the streams may result
in the header becoming flammable [Ref. 17]. This method of ensuring
operation outside the flammable region is not generally recommended
unless there is a high degree of certainty about vent flows, compositions
and equipment reliability.

The interactions of the components in each of the vent streams, along with
the possible range of compositions, should also be examined to ensure
that there are no undesirable reactions or other consequences of the
combination.
Chemical reactions may occur in the header causing, for example,
corrosion, condensation or polymerization of material in the line. Using the
combination of vent headers as the basis of safety will result in a
significant amount of additional work in order to provide sufficient
justification and hazard quantification. An interaction matrix should be
used to check for undesirable interactions between the streams being
mixed.
Complex vent collection systems connecting several plants or units to a
common destruction unit may cause the propagation of an incident from
one plant to the others. It may, therefore, be preferable to have several
smaller systems instead of one large system. A consequence analysis
should be performed to consider the options. The benefits of scale for a
single, large vent header and destruction system may not be significant
when considered against the additional burden of design engineering
needed to ensure the safety of the system.
5.2.4 Summary
The decision on which of the above methods to apply depends on the
starting composition of the vent. In order to decide the best route for
altering the vent composition, the flammability diagram for the vent
composition should be considered. Flammability diagrams are described
further in Appendix C.
5.2.5 Flame Arresters
5.2.5.1 General
Flame arresters are designed to prevent the propagation of a flame front.
They are classified as a form of secondary protection and are effective for
a limited period before burn through or overheating occurs. Each arrester
is designed for a specific duty based on the composition, flow rate and
operating conditions in the line.
The presence of a flame arrester can provide time for the plant to be shut
down or the fault condition to be rectified before an incident occurs.

The design of flame arresters is a complex subject. Only a short synopsis
of the main features is provided here. For further advice on design and
specification of flame arresters see the list of Best Contacts in Appendix B.
5.2.5.2 Types
The most common type of arrester in use is the conventional crimped
metal type (such as supplied by IMI Amal in the UK and Enardo in the
USA) but other types are available including flat or perforated plate and
liquid seal. Arresters are designed for a specific range of duties. An
arrester designed to cope with potential detonation will be designed to a
more stringent standard than one designed for deflagrations. There are a
number of standards applicable to testing of flame arresters including ISO,
BS, Canadian, Underwriters Laboratories [Ref. 14] and US Coast Guard
[Ref. 15]. The USCG tests are reckoned to be the most stringent. It should
be noted that suitable approval will be needed for flame arresters before
installation in the USA (Factory Mutual or Underwriters Laboratories) and
some other countries. Again, local authorities should be consulted.
Plate type arresters are less common in use than crimped metal types and
are limited to the less reactive gases and therefore are not suitable for
mixtures containing hydrogen or acetylene. This type is made by several
manufacturers, particularly in the USA, including Protectoseal and
Westech.
Liquid seal arresters are less common but are useful when dealing with
gases containing particulates or mists. There are no known published
design methods for this type; however, empirical design procedures have
been used in GBHE. Under conditions of high gas flow the seal may break
down and a gas path exist through the arrester. This type of arrester
should not be specified without reference to GBHE.
Pebble bed arresters are another example of a type which was used
extensively in the past but is little used today. Again there are no known
design criteria for this type of unit.

5.2.5.3 Specification
Most arrester manufacturers have their own specification sheets which
should be filled in as far as possible for the enquiry. Information is
required on gas composition, flow rate, materials of construction and
acceptable pressure drop. The key design parameter is the minimum
experimental safe gap (MESG). This may have to be determined
experimentally for gas mixtures although it is known for many single
component gases.
The location of the arrester in relation to the ignition source is also
important as it affects the flame velocity and whether there is likely to be a
deflagration or detonation. Based on this specification and the
manufacturer’s knowledge of the performance of their own designs, a
flame arrester will be proposed. Much of the design knowledge for the
arresters is based on performance of actual units in operation and is not
available in the public domain.
Flame arresters are designed to stop deflagrations or detonations. The
latter are significantly larger, stronger and more expensive than the
former. The two types are not interchangeable. Deflagration arresters are
intended to stop relatively low velocity flames whereas detonation
arresters are designed for supersonic flame fronts and shock waves.
5.2.5.4 Pressure Drop
There is always some pressure drop across the arrester which varies with
the type and duty. Crimped metal and plate type arresters are designed
with larger cross sectional areas than the pipe in which they are installed,
partly to minimize the pressure drop and partly to reduce the flame speed.
Where more reactive gases are present and the gas flow channels in the
arrester smaller, the pressure drop will be higher. It should be noted that
flow through the arrester is likely to be laminar due to the large diameter.
For liquid seal arresters the pressure drop is dictated by the head of liquid
needed to make the seal. Typically the liquid level is in the region 300-400
mm. Some liquid may be lost via the overflow due to level swell and liquid
can be lost by vaporization to the vent gas. An adequate source of make-
up liquid is therefore required.
For some low pressure vent collection systems, the pressure drop may be
critical and should be discussed with the arrester manufacturer.

Pressure drop can be increased by deposits forming on the arrester
element, some of which may not be easily visible on inspection, or by
liquid logging if the arrester is installed in the horizontal plane. Dust and
polymeric materials are a particular problem in crimped metal or
plate type arresters. Liquids which may polymerize on the element may
also cause difficulty.
When installed on the vent of "atmospheric" or low pressure tank vents, a
separate emergency relief should also be fitted. High and low pressure
alarms should also be provided to warn of the operation of emergency
vents. NFPA 30 gives guidance on the design and use of flame arresters
on storage tanks.
5.2.5.5 Instrumentation
Instrumentation of flame arresters is an important part of the safety for the
header system. In the event of an ignition occurring, a high temperature
can be detected in the flame arrester. Since flame arresters are designed
only to give protection for a limited time, it is essential that corrective
action be taken to eliminate the source of flammable gas and extinguish
the flame before burn through of the arrester occurs.
For this reason, all flame arresters should be fitted with a thermocouple
and high temperature alarm in order to detect the presence of a flame.
The thermocouple may be installed on one or both sides of the arrester
element. This enables the fault to be detected and rectified or the plant to
be shut down safely. Where the arrester is critical to the safety of the
system (i.e. the majority of cases) more than one independent
temperature probe should be fitted. Thin wall thermocouple pockets
should be used in this application for a rapid response time. Large
arresters may need more than one thermocouple to guard against
localized burning.
For systems where pressure drop is critical, or those where clogging of the
arrester may be expected, the pressure drop across the arrester should
also be monitored. Typical systems which may cause fouling of the
arrester are those containing solid particles and those containing
monomers which are prone to polymerizing on the element of crimped
metal types in particular.

Instrumentation for liquid seal arresters should include not only high
temperature detection, but also liquid level detection and high and low
level alarms. A low level in the arrester may indicate failure of the makeup
liquid supply or excessive carry-over of liquid in the vent gas due to high
flow. High level in the arrester may mean liquid flowing into the arrester
from the vent or blockage of the overflow line.
Level alarms should be of appropriate types to deal with the temperature,
pressure and composition of any liquid which may enter the arrester. Float
or capacitance probe instruments are prone to fouling or differences in
composition which can give false readings. Ultrasonic or other non-
intrusive types are therefore preferred for this application.
In the event of a deflagration or detonation occurring in the arrester the
instrumentation should be inspected for damage and replaced or re-
calibrated as necessary.
5.2.5.6 Installation and Maintenance
Installation depends on the type of arrester. Crimped metal arresters
should be installed in a vertical plane so that the element is self draining.
Offset designs are available which can be installed in the horizontal plane
but are not recommended for use as they are more prone to blocking and
corrosion.
Arresters may be fitted with liquid drains if installed in the horizontal plane.
If not luted then arrangements should be made for regular draining. Drains
may be prone to blocking or freezing and precautions need to be taken to
prevent blockage occurring (e.g. heat tracing). Luted drains may be
installed and may further be protected by a "dead man’s handle" spring
loaded valve on the drain line to prevent air ingress.
Maintenance of crimped metal arresters is limited to inspection and
cleaning of the elements. This should be done carefully, as the element is
constructed from relatively thin metal strip and is prone to mechanical
damage. If the element is damaged then it may result in enlargement of
one or more of the passages through the element which may allow
transmission of a flame. Endoscope ports may be fitted to the arrester
body to check for blockages. These are particularly recommended if the
arrester is to be used on a duty where fouling is expected.

Cleaning of the element should be done by non-contact means (e.g.
solvent bath or low pressure steam). Regular cleaning should be
considered carefully as it increases the chance of mechanical damage to
the arrester element thus compromising its integrity.
For systems where clogging is expected or experienced, it may be
possible to fit two arresters in parallel, one on line and the other off line
being cleaned or on standby. Isolation and purging systems will be
needed and for larger sizes special lifting gear may also be necessary.
Due to the physical size of flame arresters relative to the pipe and the
necessity of designing for high pressure, lifting or support gear should be
considered for pipe diameters above about 100 mm.
Metal plate arresters are more robust but may still be prone to mechanical
damage.
Liquid seal arresters may require regular cleaning and flushing. The
design of overflows and drains is important to minimize the possibility of
blockage due to solid deposition.
5.2.6 Design Pressure of Header Systems
If the probability of deflagration or detonation is unacceptably high after all
reasonable precautionary measures have been taken, then it may be
necessary to consider designing the line for containment. For
hydrocarbons, deflagrations typically generate 8 times the initial pressure
and it may be acceptable to design for containment. Detonations can
generate up to 20 times the initial pressure in a straight line with impulses
of up 40 times the initial pressure at bends and junctions and localized
pressure spikes of up to 100 times initial pressure. Designing for these
pressures will significantly increase the cost and complexity of both the
header and supports which will have to be designed to cope with the
pressure wave impulse.
The temperatures and pressures generated by deflagrations and
detonations can be predicted with a degree of accuracy, although whether
a particular mixture will run up to detonation is not accurately predictable.
Similarly, there are as yet no reliable design methods for predicting the
effect of shocks on pipes, pipe supports and support structures. The
amount of effort needed to ensure the safety and integrity of the design
should not be underestimated. For calculation of deflagration and
detonation pressures see list of Best Contacts in Appendix B.

The cost of designing the line for containment should be balanced against
the cost of making the system safe by preventing the occurrence of a
flammable mixture. As well as the line, the cost of designing vessels to
cope with very high pressures may be prohibitive. Fans, in particular, are
normally designed only for low pressures and, although they may be
designed for containment of moderate pressures, damage to the casing
should be expected in the event of an incident. In the limit, secondary
containment may be necessary for equipment with low design pressures
(e.g. locating in blast bays or explosion proof enclosures).
Although the thermal oxidizer itself may also have a very low design
pressure, it should not suffer significant damage from a deflagration or
detonation in the vent headers since:
· There should be no significant quantity of unburned fuel in the oxidizer,
hence the explosion should be snuffed out;
· The pressure surge passing through a flame arrester should be
dissipated in the large volume of the combustion chamber.
It should be noted that it is not generally possible to design the oxidizer
combustion chamber for full pressure relief since the low design pressure
of the unit would require a very large relief area. Additionally, the high
operating temperature would make the construction and sealing of vent
apertures on the combustion chamber impracticable.
It is possible to design an instrumented protective system (IPS) to provide
a similar level of integrity as a pressure relief device.
5.3 Liquid in Vent Headers
Liquid (non-flashing as well as flashing) should be avoided in vent headers
if at all possible. The philosophy should be to eliminate or minimize the
carry-over of liquids into the header, especially those which may solidify or
cause fouling through, say, polymerization. The possibility of condensation
should also be minimized.

5.3.1 Sources
Liquid may enter the vent header system in a number of ways, for
example:
 Liquid carry-over from over-filled upstream plant;
 Liquid from relief valves (especially thermal reliefs);
 Condensation can occur due to contact of warm gas with cold pipe
work or by the mixing of hot and cold vent streams;
 Storage tanks vents are normally saturated with vapor, hence diurnal
temperature changes may cause condensation;
 Liquid can be collected in a knock-out pot and then re-enter the header
if the pot becomes over-filled;
 Water from, say, maintenance activities (although this should not
remain in a properly designed header after re-commissioning).
5.3.2 Potential Consequences
5.3.2.1 Liquid Logging
Liquid collecting at low points in headers can cause partial or total
obstruction. This may cause excessively high pressure drops or back
pressure problems bearing in mind that many vent headers operate with
only a few mill bars of pressure differential.
If a vent header intended for gas is filled, even partially, with liquid then
damage can be caused by the weight of liquid either by bending the pipe
itself or by damage to the pipe supports. At best this can lead to low spots
being formed, thus worsening the liquid logging, and at worst in a shear
failure of the header.

5.3.2.2 Thermal Oxidizer Upsets
Liquid, even as a spray, fed in a line to a thermal oxidizer designed to burn
gas is likely to cause major upsets including the possibility of flame-out or
even explosion. If the liquid is flammable, then it may cause a flame out
followed by ignition of the flammable vapor from the hot refractory which
could result in an explosion.
5.3.2.3 Flammable Hazard Due to Evaporation of Liquid
The evaporation of a very small amount of liquid VOCs in a fuel-lean vent
header can push the composition into the flammable region extremely
quickly.
5.3.3 Design
5.3.3.1 Elimination of Liquid Carry-over
Carry-over of liquid into vent collection headers should be eliminated at
source wherever practicable. Possible methods include condensation of
vapor, mist elimination, inclusion of knock-out pots and careful
management of liquid levels in vessels connected to vent headers.
Dilution of vaporized liquid is not generally favored as it is likely to cost
more in downstream treatment than it would save in avoiding
condensation by other methods. For small quantities of liquid carry-over,
trace heating of the header may be sufficient. For guidance on design of
pipe work systems see Best Contacts in Appendix B.
5.3.3.2 Sloping of Lines
All headers should be constructed with an appropriate slope for drainage
of any condensation forming in the system (water vapor or high boiling
point organics), regardless of whether any liquid is expected in the
system. A suitable gradient is likely to be of the order of about 1:100
depending on the type of pipe work and distance between supports. For
small bore plastic lines which are prone to sagging, a steeper slope may
be appropriate whilst for large bore metal lines a more gentle slope (not
less than 1:250) may be used.

Gas superficial velocities should be low enough and line sizes should be
large enough to prevent entrainment of liquid as this could cause
excessive pressure drops, surging and liquid hammer as well as
significant hazards in a thermal oxidizer. If possible, the slope should be
in the downstream direction (i.e. with the gas flow) otherwise a steeper
gradient may be needed to ensure liquid flow. Low points should be fitted
with drain points.
5.3.3.3 Pipe Supports
Appropriate pipe supports should be specified for the weight of the line
filled with liquid if there is any possibility of significant quantities of liquid
entering the system. If hydraulic testing of the line is required, then the
design should take this into account with the provision of appropriate
maintenance drain points and procedures. For design of pipe supports,
the appropriate best contact should be consulted from the list in Appendix
B.
5.3.3.4 Design Temperature
Consideration should be given to the possibility of liquefied gases flash
vaporizing down to very low temperatures. This can cause localized
thermal stresses at low points and in KO pots. In particular, welded joints
can be prone to thermal stress cracking. There are numerous examples of
incidents occurring on vent header systems due to low temperatures
caused in this way. The design temperature of the system should be low
enough to cope with the flash vaporization temperature of any liquid that
may be discharged into the header or the lowest credible ambient
temperature, whichever is the lower. In certain parts of the world, winter
temperatures can fall below the boiling points of ammonia, propane and
chlorine. Steel becomes embrittled at low temperature, hence a suitable
grade should be specified.
On systems where steam cleaning may be employed, the upper design
temperature should be appropriate. High temperature can cause damage
to lines due to thermal expansion or melting of plastics.

5.3.3.5 Knock-Out Pots
In long headers it may be necessary to install a number of KO pots or
condensate drain points to remove liquid from the line at intervals. The
complexity of equipment needed and relative cost makes the inclusion of
KO pots generally unattractive. Ideally, KO pots should only be used to
provide security against an abnormal occurrence of liquid carry-over into
the header or of condensation in the header. Whereas KO pots can be
used safely on fuel-rich or inerted systems, their inclusion in fuel-lean
headers can cause problems if liquid vaporizes at such a rate that it sends
the composition into the flammable region. KO pots should not therefore
be used in fuel-lean headers unless the liquid being removed has a low
vapor pressure or is inert (e.g. water).
KO pots may be designed to either remove liquid flowing down the line or
to disentrain liquid droplets from the vapor stream. A typical configuration
is shown in Figure 2. If at all possible, the use of KO pots should be
avoided as they are an additional cost and can also be difficult to operate
and maintain.
The design basis for a liquid collection KO pot depends on the draining
arrangements for the system. If the pot is equipped with a continuous
drain (luted or continuously operating drain pump) then the pot will have to
hold sufficient liquid such that the drain system can cope with the
maximum expected rate of ingress over any particular time period. The
lute needs careful design to be able to cope with the maximum pressure
and any possible fluctuations.

FIGURE 2 TYPICAL KNOCK-OUT POT WITH LUTED DRAIN
If the pot is equipped with a manual drain, then it should have sufficient
capacity to hold the maximum amount of liquid which could collect
between drain periods under normal and abnormal conditions including
emergency relief. It is essential that a proper management system is in
place to ensure that manual drains are properly maintained and operated.
Calculations of the maximum inflow, drain rates and any allowances for
vaporization should be detailed in the design basis for the KO pot and
associated drain equipment and should form part of the basis of safety for
the header.
Lagging and heat tracing systems should be designed to the minimum
anticipated ambient temperatures for the area. If designed for the 1% or
2½% lowest probability of recorded temperature then there is a finite
likelihood that the system will freeze at some point during the life of the
plant. The consequences of freezing are far more costly than the marginal
extra cost of the increased specification.
All KO pots should be equipped with suitable high liquid level alarm
systems in order to prevent flooding of the vent header due to unforeseen
circumstances or process deviations.

Alarms should give sufficient time for process operators to respond before
flooding of the header occurs. Temperature monitors may also be
necessary to check that heat tracing is working and to guard against
freezing.
Some liquid VOCs collected in a KO pot may be removed by vaporization.
Heat can be supplied from a heating coil, trace heating or jacket and
should be sufficient to vaporize any liquid collected during normal
operation. The vapor flow from the KO pot should not overload the
destruction system. During times of abnormal liquid flow, the level in the
KO pot could rise. A level gauge and high level alarms are therefore
necessary. There should also be a separate, independent high/high level
alarm. Monitoring of the heat source may also be considered necessary
as it is critical to the safety and operation of the system. Reliance on
the integrity of a steam trap for the safe operation of the header system is
not generally acceptable. Vaporization of VOCs from a KO pot may cause
other problems with the composition of gases in the header.
Selection of an appropriate level sensor is important as liquid density may
vary. Pressure cell and float types may give a false reading under these
circumstances. Capacitance probes or ultrasonic type sensors should be
considered. For selection of appropriate instrumentation see list of Best
Contacts in Appendix B.
A drain system should be provided to remove liquid collecting in the pot
which does not vaporize or which is being collected to be recycled. The
pot may be some distance from the plant and the liquid collected may
need to be pumped a considerable distance to be recycled to the process
or treated prior to disposal. The equipment associated with this operation
may be costly, and is likely to include a pump with power supply and
associated pipe work. The operation of the pump should be monitored as
it may be critical to the safe operation of the system.
Drain systems which rely on pumps which are normally idle are prone to
failure due to non-starting. Regular maintenance and checking is required
to ensure that these systems remain operable.
Automatic drain systems which operate by lutes can be reliable if lagged
and heat traced, although solids deposition may still be a problem (rust,
scale etc.) and the drain pipe work should be designed in such a way that
it can be unblocked easily if problems do occur. However, lutes are prone
to failure due to pressure surges and may not re-seal under these

conditions; they should not therefore be used on systems which are prone
to extreme pressure fluctuations. Luted drains are not suitable for volatile
flashing liquids as the liquid seal may fail on vaporization.
If liquid in KO pots and drain lines can freeze, especially during cold
weather, heat tracing and lagging should be fitted. The heat input and
lagging thickness specified should be appropriate for the worst weather
conditions likely to be experienced in the geographic area. In the UK a
minimum temperature of at least -20°C should be used, but in parts of
Canada or the USA a much lower design temperature may be necessary.
The above problems with KO pots are likely to be more severe if the
installation is located in a remote area of the plant or in an area not
frequently visited.
5.4 Materials of Construction
Materials of construction should be suitable for the range of process
materials and operating conditions expected in the system. The possibility
of getting water vapor into the system should also be considered, as it
may cause corrosion in combination with other substances such as HCl
(even in trace quantities). In this case, the use of corrosion resistant
materials should be considered. Typical of these are Hastelloy, lined
carbon steel or plastic/GRP.
The suitability of materials depends on several factors including:
• Temperature range;
• Corrosion resistance;
• Operating pressure range;
• Electrical conductivity;
• Whether design for explosion containment is required;
• Cost.

5.5 Static Electricity Hazard
Static electricity can provide a source of ignition. The flow of any gas or
liquid through a pipe work system may be sufficient to build up a charge
large enough to cause a spark although, in general, liquid droplets and
mists are more likely to cause problems. It should be noted that solid
particles can also create a charge when carried in a gas stream.
The amount of energy required to ignite a gas mixture is known as the
Minimum Ignition Energy (MIE). Some gases, such as hydrogen or
acetylene, have very low MIEs. Even in systems of conducting pipe work,
sufficient electrostatic charge may be generated to ignite flammable
mixtures containing gases with very low MIEs.
Non-conductive pipe work should be avoided particularly for gas mixtures
which have a low MIE. It is possible to make plastic pipe work conductive
by carbon filling. Pipelines and equipment should be checked for
conductivity across flanges and joints prior to commissioning the plant and
on a regular basis thereafter (and especially after maintenance). Any joints
where the resistance is above 1 Ohm should be fitted with electrical
bonding strips across the flanges. Electrostatic build up can be hazardous,
particularly across changes in material specification (e.g. changes in pipe
specification from metallic to non-metallic and across non-conductive
gasket material). All fittings, valves and instruments should be assessed
for static electric generation potential.
Valves and fittings in the header should be designed and specified to an
appropriate antistatic standard. Problems may occur in systems containing
particulates or materials which can polymerize if a non-conductive coating
is formed on the inside of conductive equipment.
Static electricity is a complex topic.

5.6 Diversion Systems
5.6.1 General
Diversion systems are used to redirect vent gases away from the normal
feed inlet to a thermal oxidizer prior to oxidizer start up or in the event of
an abnormal situation such as:
• Thermal oxidizer trip;
• High oxygen in a fuel-rich vent header;
• High fuel in a fuel-lean vent header;
• High pressure in vent header;
• High temperature in flame arrester;
• High liquid level in final KO pot;
• Failure of inert gas system.
The diversion system enables corrective action, such as re-starting the
thermal oxidizer or shutting down upstream production units, to be taken
in a controlled manner and is important for the safety of both upstream
production plants and the vents treatment unit itself. The diversion system
should also be used when starting up plants to avoid surges or unsteady
operation which could cause problems with the oxidizer.
Diversions can be to existing local discharge stacks or to a new bypass
stack or stacks. Flashback protection (e.g. flame arresters) may be
needed in the diversion system as for the thermal oxidizer.
5.6.2 Discharge
The diversion system can either redirect the vent gases for direct
discharge to atmosphere at a safe location (usually at high level) or else to
a flare stack.

A flare stack may be necessary if the vent gas is toxic, strongly odorous or
if the size of flammable plume could be hazardous. If discharge is to a
flare stack, then similar precautions will need to be taken in the event of a
flammable mixture being present in the system as for when the gas is
being sent to a thermal oxidizer (i.e. flashback prevention using flame
arresters). A flare has a lower destruction efficiency than a thermal
oxidizer and it may cause visual or audible nuisance. There may also be a
thermal radiation hazard and additional support fuel costs for the pilot
flame system.
In the event of an ignition occurring on a plain vent stack (e.g. due to
lightning) it is normal to provide a means of snuffing the flame out. This
can be done using a high flow of steam or another inert gas (typically
nitrogen or carbon dioxide).
If discharge is via a plain vent stack, then it should be located such that
the plume will not cause a hazard such as fire or vapor cloud explosion
(VCE). If the concentration of VOCs in the vent gas is above the LFL, then
there is a chance that the plume could become flammable on dilution with
air. In particular, the location of other ignition sources and the ground level
concentration of the vent gas are important. Modeling of the gas
dispersion of the plume can be done to determine ground level
concentration of vent gases, distance to LFL, radiation from an ignition,
explosion overpressure etc..
5.6.3 Period of Operation
Statutory authorities will generally place a restriction on the amount of time
for which a thermal oxidizer is allowed to be off line during any period.
This is a topic for negotiation based on the projected availability of the
thermal oxidizer and associated equipment and the environmental impact
of the emissions.
The amount of down time allowed for the thermal oxidizer will determine
the level of installed spare equipment, maintenance effort required and
also the level of spares needed in stores.
The effect of unscheduled down time on the upstream plants should not
be underestimated in terms of disruption and consequential losses.

5.6.4 Design
When designing a diversion system, the reliability of the equipment and
the speed of action are important. Any equipment which relies on starting
up an electric or other type of motor should be avoided as there is a risk
that after a long period of idleness the motor will not start. This is of
particular importance in collection systems which rely on suction from a
fan or fans located near the treatment unit. Equally important are the
phasing and sequencing of operations such as the opening of diversion
valves and closing of isolation valves.
Where equipment such as valves or electric motors are critical to the
operation of the diversion system, sufficient instrumentation should be
installed to allow for monitoring of valve positions, operation of motors
etc..
5.6.5 Emissions from Diversion System
If the vents are diverted to a stack or stacks without flaring, consideration
should be given to the expected ground level concentrations of toxic,
odorous or flammable materials. The possibility of ignition of the plume by
external sources (e.g. lightning), should also be considered. If the plume
from a vent stack is ignited, then the thermal radiation effects need
to be considered as for a flare stack. The effects may include personnel
hazard, heat damage to surrounding equipment and ignition hazard for
other vents.
Grouping together of untreated vents may cause the concentration of
controlled materials to go above the allowable concentration or flow rate
for a single vent. This is a topic for discussion with the local statutory
authorities. If the diversion stack is not a flare stack, then the flammability
of the gas mixture should be considered. A schematic of a diversion
system is shown in Figure 3.

FIGURE 3 SCHEMATIC OF DIVERSION SYSTEM
5.7 Snuffing Systems
Snuffing systems are designed to extinguish flames in vent header systems or
stacks. They work by injecting a large quantity of inert gas (or occasionally
powder) into the header. This inert gas takes the composition in the system
below the LFL thus extinguishing the flame.
Nitrogen, carbon dioxide or steam are commonly used for snuffing. The
inert gas injected forms a "slug" of non-combustible material.
The snuffing system may only have a limited inventory of gas. Once this
inventory is exhausted, the composition in the header may become
flammable again. The snuffing system provides time to correct the
problem or shut down the system safely.

6 SAFE DESIGN OF THERMAL OXIDISERS
6.1 Introduction
Information on some of the different types of thermal oxidizers, catalytic
oxidizers, heat and mass balances, recuperative and regenerative heat
recovery, suppliers, etc. Although there is some overlap between the two
guides, this guide concentrates on issues concerning safety and
environmental control and those issues not covered by.
6.2 Design Basis
6.2.1 Capacity
The flowsheet duty is determined from the collection and assessment of
process flow data for the vents to be treated (see Section 4). It should be
noted that this duty should include any base load of inerts or fuel gas to
ensure operation of the vent header(s) outside the flammable region.
In many cases, the maximum duty that is specified initially is determined
by the need to treat a high flow rate of vent gas for short periods of time
from, for example, peak batch operation flows or relief streams. These
specific needs should be examined very carefully because:
 The size and hence capital cost of the thermal oxidizer and its
downstream flue gas handling plant would have to be increased to
cope with these peak flow rates;
 The maximum turn-down on a thermal oxidizer is typically about 5:1
and often as little as 3:1, hence it would be necessary to use large
amounts of excess combustion air and consume associated large
amounts of support fuel outside the peak flow rate periods which would
be a large proportion of the time;
 There will be a limit on the maximum rate of change of flow rate or
calorific value of the incoming waste gas stream with which the control
system on the thermal oxidizer can cope.

Therefore, wherever possible, peak flow rates of waste gas to be treated
should be attenuated and relief streams should not, as a general rule, be
fed to thermal oxidizer systems. Flare stacks may be used to cope with
high flow rates due to relief conditions but their destruction efficiency is
lower.
6.2.2 Destruction Efficiencies and Emission Limits
6.2.2.1 Destruction Efficiencies
The key design parameters for high efficiencies of destruction of organic
materials fed to thermal oxidizers are Temperature, Time, Turbulence
(often known as the 3 Ts) and oxygen concentration. A minimum
temperature of 850°C to 900°C is required to destroy most organics.
However, if halogenated materials are present, the statutory authorities
are likely to require a minimum temperature of 1100°C to 1200°C in order
to avoid the formation of halogenated dioxins and furans (see below).
It is generally accepted that the destruction of organics is so fast at
thermal oxidizer temperatures that reaction kinetics are not limiting.
However, minimum residence times of about 2 seconds after the last
injection point of combustion air are often required by statutory authorities
to ensure full and adequate mixing which is of paramount importance [Ref.
8]. The geometry of the combustion chamber and the orientation of the
main burner nozzle and air inlet ports are very important in order to ensure
high radial turbulence without any unmixed or cold spots. Residence time,
temperature and destruction efficiency will be specified on the permit to
operate in the US and Canada and a number of other countries.
A minimum oxygen concentration at the exit from the oxidizer of about 3%
v/v, without correction for water vapor, is generally required to ensure a
very high level of oxidation of organics. Monitoring of the excess oxygen in
the flue gas is mandatory in many countries to ensure complete
combustion.
Carbon monoxide (CO) is a good surrogate indicator of other products of
incomplete combustion (PICs) and, as such, it is customary to specify a
maximum limit of about 100 mg/m³ CO in the flue gas. This concentration
is often referenced to certain standard conditions, such as 1 atmos
pressure, 0°C, 11% v/v oxygen and dry gas in Europe. Also, the
period of time over which the concentration is averaged, such as 1 hour,
should be specified.

The performance of thermal oxidizers is usually specified in terms of
maximum allowable pollutant concentrations, as in Europe, or in terms of
the destruction and removal efficiency (DRE), as in the USA. As with CO,
maximum allowable pollutant concentrations are usually referenced to
certain standard conditions. DRE is defined as:
where POHC = mass flow rate of principal organic hazardous constituent.
6.2.2.2 Acid gases
Acid gases, such as hydrogen chloride (HCl) and oxides of sulfur (SOx),
may have to be removed from the thermal oxidizer flue gas by scrubbing
(see Section 6.5.1). Treatment of scrubber blowdown liquor will inevitably
be required.
The composition of any support fuel used should be taken into account
when considering pollutant concentrations in the thermal oxidizer flue gas.
For example, a fuel oil containing sulfur could produce more SOx than is
produced from the sulfur content of the waste gases being treated.
6.2.2.3 Particulates
Thermal oxidizer flue gas will contain particulates that may require
abatement if the incoming waste gases contain suspended inorganic dust
or if water containing dissolved or suspended solids has been used for
temperature control. Various methods including liquid scrubbers or
dust filters may be used to remove particulates.
6.2.2.4 NOx
In some countries, the statutory authorities set a maximum permissible
limit on emissions to atmosphere of nitrogen dioxide (NO2) and nitric oxide
(NO) (collectively known as NOx). Typically, the ratio of NO: NO2 in a
thermal oxidizer flue gas stack is about 10:1 with the NO slowly oxidizing
to NO2 in the atmosphere.

However, the emission limit is likely to be total NOx expressed as NO2.
Even if a NOx limit is not specified, consideration of NOx control may be
necessary to avoid a visible brown plume especially if burning organics
containing bound nitrogen or if the combustion temperature is about
1200°C or higher. NO is not visible but NO2 is visible. As a general rule of
thumb, the onset of NO2 visibility is characterized by the following Beer-
Lambert law:
Where D = stack diameter in m.
Organically bound nitrogen tends to form NOx (fuel NOx) in oxidizing
atmospheres at any temperature likely to be found in a thermal oxidizer
combustion chamber. The formation of fuel NOx can be controlled by
staged air combustion (see Section 6.3.3).
Elemental nitrogen in the incoming waste gases or in the combustion air
tends to form NOx (thermal NOx) in oxidizing atmospheres at
temperatures of about 1200°C or higher. The combustion chamber wall
does not normally reach this temperature but higher temperatures
can be generated in the flame.
Low-NOx burners are readily available and are usually employed to
control the formation of thermal NOx. These usually employ a short, highly
turbulent flame with staged air injection into the flame.
6.2.2.5 Dioxins
Polychlorinated dibenzodioxins (PCDDs) and polychlorinated
dibenzofurans (PCDFs) (collectively known as dioxins), which can be
formed in incineration processes if chlorine and hydrocarbons are present,
have attracted considerable attention. However, their formation in
the combustion chamber can be reduced to levels generally acceptable to
statutory authorities provided:
• There is sufficient turbulence within the combustion chamber;
• There is sufficient time for good mixing (generally specified at 2
seconds);

• There is no bypassing or dead spots;
• The temperature is high enough (generally specified at a minimum of
1100°C).
It is known that dioxins can be formed as the flue gases cool downstream
of the combustion chamber. Although the exact chemical reactions are not
understood (de novo reactions), it is believed that the critical temperature
range in which these reactions take place is about 200°C to 450°C and
that certain metals, including copper and iron, act as catalysts. Dioxin
formation has been observed more in old municipal solid waste (MSW)
and clinical waste incinerators where the combustion temperatures were
not well controlled and/or where particulates settled in the tubes of waste
heat boilers. Although current regulatory dioxin emission concentration
limits can be as low as 0.1 ng/m³, this should not be a significant
problem with a modern thermal oxidizer burning gases and producing
essentially no particulates in the flue gas.
6.2.3 Availability
In order to achieve high availability it is most important to properly specify
and install the refractory linings and then to care for them during operation
(see Section 6.4).
As a general rule, it should be possible to achieve an on-line availability of
about 90% to 95% (i.e. typically about 20 to 35 days downtime per year)
provided planned maintenance can be scheduled to coincide with
downtime on the plant(s) producing the vent gases to be treated. If the
thermal oxidizer services several plants that do not have planned
concurrent downtime, the on-line availability of the thermal oxidizer could
be less than 90%.
It is likely that a system to divert the vent gases to a safe place will be
required during thermal oxidizer downtime (see Section 5.6).
6.2.4 Heat and Mass Balance
Combustion air and support fuel requirements should be determined by
means of standard heat and mass balance calculations.

6.2.5 Turn-Down
Thermal oxidizer turn-down is typically of the order of about 5:1 but can be
as low as 3:1. The minimum turn-down can be governed by the main
burner characteristics or by the need to ensure appropriate flow patterns
for adequate turbulence without bypass routes in the combustion
chamber. It should be noted that it may be necessary to use excess
combustion air and support fuel at low flow rates of incoming vent gas in
order to maintain minimum turndown.
6.3 Types of High Temperature Thermal Oxidizer
6.3.1 Conventional Oxidizing Combustion Chamber
A typical simple conventional unit with a vertical combustion chamber is
shown schematically in Figure 4. In this example, the support fuel burner
assembly is housed in a sub-chamber attached to the side of the main
combustion chamber. This arrangement protects the main burner
assembly from the radiant heat in the main combustion chamber and also
makes maintenance of the burner assembly and the refractory lining in the
sub-chamber easier.
The temperature in the main combustion chamber controls the feed rate of
support fuel. The combustion air feed rate is ratio-controlled to the support
fuel feed rate with the ratio control set point adjusted by the concentration
of oxygen in the flue gas.

FIGURE 4 CONVENTIONAL VERTICAL THERMAL OXIDIZER
6.3.2 Conventional Oxidizing Combustion Chamber with Integral Water
Sparger
Lentjes supplies a thermal oxidizer with an integral water bath through
which the feed of waste gas bubbles into the combustion chamber. This
configuration minimizes the risk of flash back into the vent header
especially if the mixture in the vent header can approach the
LFL or the UFL during, say, upset conditions. Care should be taken with
this design to ensure that operation at the maximum gas rate does not
result in a continuous free gas path back through the liquid. This type of
unit is shown in Figure 5.

FIGURE 5 CONVENTIONAL OXIDISER WITH INTEGRAL WATER
SPARGER
6.3.3 Staged-Air Combustion
The formation of fuel-NOx (see Section 6.2.2.4) from organically-bound
nitrogen (e.g. amines) can be minimized by the use of staged-air
combustion as shown diagrammatically in Figure 6.
The reduction chamber operates sub-stoichiometrically on air thereby
converting the organically-bound nitrogen to elemental nitrogen. The
gases leaving the reduction chamber typically contain about 10%
combustibles such as hydrogen and carbon monoxide. In order
to achieve a high destruction efficiency of organics within the reduction
chamber, it is necessary to compensate for the oxygen deficiency by
operating at a high temperature, typically 1100°C to 1300°C.

The temperature of the gases leaving the reduction chamber is reduced in
the quench chamber to achieve a maximum of about 1000°C in the
oxidation chamber so as to minimize the formation of thermal NOx (see
Section 6.2.2.4).
Quenching can be achieved by means of cooled recycle gas as shown in
Figure 6, or by using excess secondary combustion air fed to the oxidation
chamber or by introducing quench water or steam. Cooled recycle gas has
the environmental benefit of producing the least volume of final flue gas
and can be used to raise steam but it is the most capital intensive option.
Cooling water requires an atomizing fluid which could be air but steam
should produce lower NOx. Also, special care is required to avoid the
possibility of water droplets impinging on hot refractory surfaces. Both
water and steam cooling could give rise to a visible steam plume from the
stack. Air is the lowest cost option but may not reach the very low NOx
levels achievable with the other options. It should be noted that NFPA and
Factory Mutual have special safety requirements for the use of recycle
gas. Vendors should comply with these regulations for installations in
North America.
FIGURE 6 THERMAL OXIDIZER WITH STAGED AIR INJECTION
The oxidation chamber operates typically at a minimum of 3% v/v oxygen
in the final flue gas.

6.3.4 Down-Fired Combustion Chamber
Figure 7 shows a vertical combustion chamber down-firing directly into a
water bath. This type of unit is suitable for burning gases containing
fluorinated hydrocarbons which produce highly corrosive hydrogen
fluoride. It is essential that expert advice is sought regarding materials of
construction especially for the down comer insert into the water bath.
The down comer should be cooled with quench water. Additionally, it may
be necessary to irrigate the inside surface of the down comer to prevent
localized overheating.
FIGURE 7 DOWN-FIRED UNIT WITH WATER BATH QUENCH

6.3.5 Flameless Thermal Oxidation
Thermatrix markets a flameless thermal oxidation unit shown
diagrammatically in Figure 8. The unit comprises a slightly conical vertical
upflow reactor packed with ceramic spheres. The bottom layers of spheres
are smaller in diameter than those in the main body of the reactor and
serve to distribute the gases evenly across the cross-section of the unit.
Waste gases, combustion air and support fuel, if required, enter a plenum
chamber at the base of the reactor and then flow upwards through the gas
distribution zone and then into the hot reaction zone where oxidation takes
place. Thermatrix suggest that the gas mixture in the plenum chamber
should not exceed 50% LEL but a more conservative approach would
be 25% of the LEL. The temperature profile can be established at start-up
by feeding hot combustion gases from a standard burner downwards
through the bed.
Flameless thermal oxidation offers the following advantages:
• High destruction efficiencies resulting from homogeneous mixing without
bypass paths of the reactant gases;
• Low thermal NOx due to the absence of high flame temperatures;
• High thermal inertia to cope with varying feed flows and compositions;
• The positioning of the reaction zone is, to some extent, self regulating in
the slightly conical reactor in that the rate of advance of the reaction zone
down the bed equals the upward velocity of the gas stream. Therefore, an
increase in calorific value of the influent gases without an increase in
volumetric flow tends to move the reaction zone down the bed, but this is
countered by an increase in upward velocity of the gas in the narrower
diameter of the bed;
• The lower part of the bed may act, to some extent, as a flame arrester.

FIGURE 8 FLAMELESS THERMAL OXIDATION UNIT
However, the following factors also need to be considered:
• The influent gases mix in the feed line, plenum chamber and lower part
of the reactor. The likelihood, and possible consequences of a flammable
mixture developing, should be considered. However, it should be noted
that this issue also applies to catalytic oxidizers;
• It is not known whether the ceramic spheres provide any active sites for
surface catalysis. Therefore, caution should be exercised for gas mixtures
not yet proven with this technology.
6.3.6 Regenerative Heat Recovery in Multiple Beds
Figure 9 shows a multiple bed thermal oxidizer with regenerative heat
recovery. This type of unit, which is more fully described in PSHEG 15,
operates batch-wise by heating up a matrix by direct contact with hot flue
gas and then uses the hot matrix to pre-heat the incoming
waste gas stream.

FIGURE 9 THERMAL OXIDIZER WITH REGENERATIVE HEAT RECOVERY
6.4 Refractories
There are usually several layers of refractory lining. The inner layer(s)
should be resistant to any chemical attack as well as withstanding the high
radiant and convective temperatures in the combustion chamber. The
principal duty of the outer layer(s) is to provide a thermal insulating barrier
to protect the outer metal shell.
Refractory linings can be cast in situ or built from pre-formed bricks. The
latter necessarily require suitable jointing or grouting material.
It is vitally important to consult a Materials Engineer with regard to the
specification of the refractory materials and the means by which they are
fastened to the shell
After installation, refractory linings have to be dried out and cured before
commissioning. This is normally done using a small flame, ramping the
temperature up slowly. This process may take up to three weeks to
complete for a new lining.
The refractory can be damaged by thermal or mechanical shock. Slugs of
liquid entering the oxidizer can cause localized thermal shock damage.
The refractory can also be damaged by erosion due to flame
impingement.

6.5 Flue Gas Treatment
6.5.1 General
It may be necessary to treat the flue gas before it is discharged to
atmosphere by, for example, scrubbing to remove acid gases such as HCl
or SOx. As stated earlier, it is unlikely that treatment would be required to
remove particulates unless poor quality water is used for quenching or
temperature control or if the thermal oxidizer is also used to burn
particulate forming materials (e.g. TiCl4) or if the gases themselves
contain non-combustible particulates.
6.5.2 Flue Gas Cooling
It will usually be necessary to cool the flue gas leaving the combustion
chamber upstream of any flue gas treatment unit by one or a combination
of the following methods.
 Recuperative heat recovery (see Section 6.5.3.1);
 Adiabatic water quench. Great care should be paid to the choice of
materials of construction and to the mechanical design, especially in
the area of first contact between the hot gas and the quenching water.
For advice on materials, see list of Best Contacts in Appendix B. It
should be noted that it may be possible to combine the duties of an
adiabatic quench and an aqueous scrubber into a single unit;
 Partial water quench. This can be achieved using water sprays into the
hot gas stream. It is likely that a quench vessel would be required
rather than in-line spray quenching in order to avoid impingement of
water droplets onto hot refractory surfaces. Although high pressure
spray nozzles are available that do not require an atomizing medium,
they have
• very limited turn-down and it is more likely that steam or air would be
required for atomization;
• Diluent air cooling. This is more likely to be used to achieve a modest
temperature reduction upstream of a gas-gas recuperator rather than a
massive temperature reduction upstream of a scrubber.

Typically, about 0.3 kg of air at 6 bara atomizing about 1 kg of water would
give a turn-down in the region of 3:1. The amount of atomizing air can be
reduced to 0.1 kg per kg of water if the water pressure is increased to 6-7
bara whilst maintaining the atomizing air pressure about 2 bar above that
of the water. Turn-down ratios greater than 3:1 can be achieved by
isolating a number of nozzles, but great care is needed to prevent the
isolated nozzles from overheating or blocking.
Steam atomization typically requires about 0.5 kg of steam per kg of
water.
Pressure atomizing nozzles using no steam or air can give turn-down
ratios up to 4:1 but very high water pressure is required and some means
of cooling or withdrawing isolated nozzles is normally needed to avoid
damage.
6.5.3 Heat Recovery
6.5.3.1 Recuperative Heat Recovery
Heat can be recovered from the hot flue gas using a recuperative
heat exchanger to generate steam, pre-heat boiler feed water in an
economizer, or to pre-heat combustion air or the incoming waste
gas stream. An economic analysis should be carried out to
determine whether it is viable to use some of the waste heat
against the additional costs of the extra equipment and
maintenance needed.
Attention should be paid to the choice of materials of construction,
especially the upstream tube sheet and tubes which should
withstand not only the high temperature of the incoming gas stream
but also the stresses resulting from differential thermal expansion
between cold shut-down conditions and hot operating conditions.
If halogenated organics are present in the waste gas stream, it may
be necessary to limit the recuperator minimum wall temperature in
order to minimize de novo dioxin formation (see Section 6.2.2). The
acid dew point should also be taken into consideration in order to
prevent corrosion.

If nitrated organics are present in the incoming waste gas stream, a
proportion of the cooled flue gas can be recirculated back to the
combustion chambers for fuel-NOx control (see Section 6.3.3). Where
sulfur oxides are present, temperatures should again be high enough to
prevent the formation of sulfuric acid mist and associated corrosion.
6.5.3.2 Regenerative Heat Recovery
A ceramic or brick matrix, pre-heated by direct contact with the hot flue
gas, can be used to pre-heat the incoming waste gas stream or
combustion air as illustrated in Section 6.3.6.
6.5.4 Plume Suppression
The UK Environment Agency may require steps to be taken to reduce the
size or frequency of a visible atmospheric dew point condensation plume
[Ref. 9].
Dew point condensation plume visibility increases with:
 Increased temperature of flue gas that is saturated with water vapour;
 Increased relative humidity and decreased temperature of the ambient
air.
As a general rule-of-thumb, dew point condensation plume visibility in
Europe should not be major issue if the flue gas relative humidity is
significantly below 100% or if its temperature is below about 40°C.
Dew point condensation plume visibility can be reduced by cooling to
condense out some of the water content, by heating the flue gas or by
diluting it with air. The most common approach is to dilute with hot air. As
a general rule-of-thumb, diluting hot flue gas that is saturated with water
vapor at about 70°C with an equal amount of hot air to achieve a
mixed gas temperature of about 110°C will avoid plume visibility issues in
Europe.

6.6 Control and Safety Systems
6.6.1 Control System (Burner Management System)
One of the principal challenges for the design and operation of vent gas
collection and thermal systems is that of control. Most of the problems that
are experienced with these systems result from the variable flow rate and
composition of the gaseous feed streams. It is generally not possible to
measure these variables for use in a feed forward control system.
Consequently, most control systems rely on feedback of the combustion
chamber temperature and flue gas composition as shown in the simplified
diagram in Figure 4. Sometimes it is appropriate, as in the case of staged
air combustion (see Section 6.3.3), to use temperature difference across a
combustion chamber as one of the measured control parameters. It
should be noted that in some parts of the USA, analysis of the excess
oxygen in the flue gas is mandatory. A burner management design guide
has been produced [Ref. 10] which gives advice on design and
specification of thermal oxidizer control systems. Design of control
systems in North America is governed by NFPA 86. In Canada, fuel gas
trains should conform to the Canadian Gas Association Code and oil
burner trains to NFPA 86. Also in Canada, only approved fittings may be
used, therefore the use of local engineering contractors is strongly
recommended. Advice on control systems should be sought from the
corporate insurer who is the "Authority with Jurisdiction" referred to in the
NFPA codes.
As feedback control is typical, it is important to study the process and
control dynamics of the system and to ensure that, for example, rates of
change of flow rate or composition of the incoming waste gases can be
accommodated safely.
It may be appropriate, in some cases, to install a relatively simple control
system and use, for example, larger than normal amounts of excess
support fuel and combustion air.
Dynamic simulation may be used to model the operation of the control
system. For advice on this topic see list of Best Contacts in Appendix B.
Burner management systems may be complex, with large numbers of
inputs and outputs. All control systems should be thoroughly tested at the
factory as it may be difficult to carry out simulations of control conditions
on site.

6.6.2 Burner Design, Burner Management System and Flame Detection
A typical burner assembly comprises a pilot burner, a main burner and a
burner management system (BMS). These should be designed to ensure
that it is not possible for a large volume of unburned flammable gas to
develop in the combustion chamber at any time. This is achieved by the
following provisions:
 Flame detectors, also known as "fire eyes" or "magic eyes", should
monitor the presence of flames on the pilot and main burners. Flame
detectors normally rely on the optical detection of specific wavelengths
of light. Two detectors, set at different wavelengths, can be used to
provide redundancy. The detectors should be set to "fail safe" with shut
down being initiated only if both units indicate failure;
• Flame detectors are generally reliable once set up and operating correctly;
however, they do require cleaning periodically. Soot and other dirt can
collect on the quartz glass lens sometimes giving a false "flame out"
reading. Careful consideration should be given to the location of the flame
detectors since exposure to the full flame temperature can result in
damage and rapid failure of the unit. Flame detectors therefore should be
located out of the flame in the side of the chamber, preferably adjacent to
the burner nozzle. It is advisable to make provision to withdraw the
detector head for maintenance without having to shut the burner down;
 Self checking detectors should be used if possible. These rely on
detecting the difference between light and dark due to the rotation of
an internal shutter. If, for any reason, the regular flickering is not
detected, then an alarm can be generated. This could be due to either
a flame failure or a detector failure;
 Typically two detectors per burner are used, infra red for pilot flames
and UV for the main flame. The detectors may be air purged to keep
the detector window clear;
 The burner management system should cut off the fuel to the pilot
burner if the pilot burner flame is not established after a pre-set time,
so as not to produce an unacceptably large volume of flammable gas
mixture in the combustion chamber;

 The burner management system should cut off the fuel to the main
burner if the main burner flame is not established after a pre-set time,
so as not to produce an unacceptably large volume of flammable gas
mixture in the combustion chamber (as above);
 The burner management system should include a timed delay after
automatic shut-down before it is possible to admit fuel to the pilot
burner, fuel to the main burner or vent gases to the combustion
chamber in order to avoid the possibility of hot refractory igniting a
flammable mixture in the combustion chamber. This is intended to
allow time for complete purging of the chamber but does allow for hot
restarting;
 The burner management system should ensure that the combustion
chamber is purged with air or inert gas equal to at least five times the
volume of the combustion chamber before attempting to re-light the
pilot burner. Air dampers should be opened fully to ensure the best
purge rate;
 Appropriate isolation standards should be used for fuel supplies. In
particular, for fuel gases at high pressure or those containing
significant quantities of hydrogen, the provision of double block and
bleed isolations should be considered. Even with liquid fuels, tight shut
off of the fuel supply is critical to the safety of the unit;
 A hard wired or non-reprogrammable logic system should be included
to take care of emergency trips and shut-down systems;
 A detailed Hazard Study of the logic on both the normal control system
and the non-reprogrammable system is essential. This should be done
by a person conversant with programmable electronic systems and
their hazard study.
NFPA 85C gives requirements for installation of flame detection devices,
burner management systems, trip systems, purging and fuel systems as
applicable to multiple burner boiler - furnaces. NFPA 86 gives guidance on
burner management systems. BMS in the US and Canada should be
approved by Factory Mutual or Underwriters Laboratories. Whilst it is
possible to gain approval for a custom programmed plc, it does not usually
make economic sense to do this.

One of the approved commercial devices such as those produced by Fireye or
Honeywell should be used. Protective devices such as flame detectors, pilot light
and start-up and shut-down sequence are all governed by NFPA codes and
should be strictly adhered to in the USA.
6.6.3 Start-up & Shut-down
The hazards associated with start-up can be minimized by the use of a
proper procedure and the presence of suitable trips and alarms. The vents
system should not be started up direct to the destruction unit. Vent flows
should be established to the "cold stack" or bypass vent route prior to
bringing the destruction system on line. Vent deviations at start-up can be
a major cause of problems with destruction systems, due to variations in
flow and the possibility of getting liquid in the header; therefore it is better
to establish flows to the header prior to bringing the thermal oxidizer on
line.
A simplified sequence, which may be controlled wholly or in part by the
burner management system, would be as follows:
 Establish vent flows to diversion system or cold stack;
 Purge burner chamber with at least five volumes of air or inert gas to
ensure that any VOCs or other fuel gas is swept out;
 Check that fuel gas is available to pilot/igniter;
 Check combustion air flow;
 Establish the presence of a spark or equivalent to the igniter;
 Initiate the pilot burner gas flow;
 Check for presence of the pilot flame;
 Establish a flow of support fuel to the main burner at low rate;
 Confirm the presence of the main flame;
 Turn off the pilot flame;
 Ramp up the main burner gas flow;

 Bring on the vent gas flow at low rate to oxidizer;
 Ramp up the vent gas flows to normal operating conditions and shut
off flows to diversion system.
The failure to establish any step successfully should abort the start-up and
initiate a purge of the burner chamber. It should be noted that after the fuel
to the thermal oxidizer has been shut off, the refractory material may
remain hot for a long period. If the VOC vent flow or support fuel is re-
introduced during this period, then an explosion may result due to delayed
ignition from the hot refractory. A hot restart should not, therefore, be
attempted if the temperature of the refractory is above the AIT unless the
burner management system is specifically designed to cope with hot
restarts. The burner management system should have provision for hot
restart unless there are exceptional circumstances which would make this
event hazardous.
Factory Mutual 6-11 [Ref. 13] states that there should be a mandatory pre-
ventilation period to purge the combustion chamber, intake and exhaust
systems of any fumes or fuel which may have accumulated during
shutdown periods. It should be noted that in the US and Canada the start-
up and shut-down sequences should be handled by the BMS as described
above. This is also strongly recommended for installations in other
countries. The purge should be proven by suitable interlocks or monitoring
instruments. At least three volume changes of air are required by Factory
Mutual 6-11 and the period should be timer controlled to prevent
actuation of fuel valves or ignition devices during the purge. Dampers
should be set to the open position during the purge and, if necessary,
provided with interlocks or indicators to prove them open.
The pilot flame is established using small flows of natural gas (or fuel gas)
and air. The flow of gas to the pilot flame is small enough that it would
take a significant time to fill the combustion chamber with sufficient
quantity of unburned gas to cause damage to the chamber shell in the
event of a delayed ignition. Factory Mutual 6-11 [Ref. 13] states that
interlocks should be provided on fuel supply pressure (high and low as
required) and also atomizing air or steam pressure where used. NFPA
85C also contains recommendations on purging and fuel shut off.
The pilot flame is usually turned off after the main flame has been
established. This minimizes erosion of the spark ignition electrodes and
wear and tear on the pilot burner itself. Operation of the pilot and ignition
system can normally be checked with the thermal oxidizer on-line.

Controlled shut-down of the thermal oxidizer should also be done in
sequence, typically:
 Shut off waste flows to oxidizer and send to diversion system;
 Shut off fuel to main, auxiliary and pilot burners;
 Purge with air or inert gas (at least five combustion chamber volumes).
On shut-down, the burner chamber should be purged thoroughly to
remove any unburned gas. Temperature measurement should not be
taken from the exit gas but from the refractory lining itself. Measuring the
exit gas temperature is likely to give a false (unsafe) reading considerably
below the lining temperature. An automatic or manual quench flow (low
pressure air or steam) may be incorporated to cool the refractory lining.
6.6.4 Trips and Emergency Shut-down
The number of trips and shut-down systems should be kept to a minimum,
commensurate with acceptable safety, as over-complicating the unit could
result in the system being difficult to start up and operate. Typically, trips
will be needed for the following:
 Support fuel high / low pressure;
 Combustion air low pressure/flow;
 Instrument air low pressure;
 Combustion chamber high temperature;
 Electrical power failure to ignition spark generator (during start-up);
 Flame failure;
 High temperature on flame arresters;
 Low oxygen concentration or high carbon monoxide concentration in
the flue gas;
 High oxygen in fuel-rich or inerted header or low oxygen in fuel-lean
header.

It is likely that the statutory authority would require the flow of waste gases
into the combustion chamber to be shut off automatically in the event of
the last event on the above list occurring.
Subject to the complexity of the system, other trips may also be required.
There may be some degree of redundancy in the control system (e.g. use
of a supervisory module to back up critical trips and sequence steps).
6.6.5 Explosion Relief
It is unlikely that it would be practicable to fit suitably sized explosion relief
panels to the oxidizer unit for the following reasons:
 It would be difficult to make a good seal in the refractory lining around the
explosion relief panel due to high temperature;
 The large volume of the combustion chamber would generally require
comparatively large explosion relief panels which are likely to be impractical
to fit due to space limitations;
 The large size of the explosion relief panels and the high temperatures in the
combustion chamber are likely to present problems of warping and gas
leakage.
The basis of safety is, therefore, to prevent any significant explosion over-
pressure from occurring by ensuring that there is insufficient unburned fuel gas in
the system to generate a hazardous pressure in the event of an ignition. A small
"pop" on ignition, involving only a small volume of flammable gas, is considered
normal for many combustion units.
6.7 Project Program
6.7.1 Outline Program
The length of time taken to execute a project of this nature should not be
underestimated. An outline project program for the design and
construction of a typical new vent collection and destruction system is
shown below in Figure 10.

It can be seen that a total of about 4 years may be needed with about half of this
required for pre-sanction work including, in particular, vent identification,
characterization and quantification. Delays to the project may be caused by
external factors including matters such as statutory approvals. The possibility of
additional clarification being needed in order to obtain planning permission etc.
should not be discounted.
VCDS projects may be completed in less time than this if some level of
parallel engineering is done. This may entail taking a number of financial
and design risks in order to procure long delivery items. Obviously, the
level of parallel engineering which can be tolerated also depends on the
complexity of the project. There have been examples where the project
time scale has been reduced to under two years by parallel engineering
on new plants.
FIGURE 10 TYPICAL PROJECT PROGRAM

6.7.2 Personnel
As with all project program, it is important to appoint the appropriate
personnel, including the more senior members of the commissioning
team, early in the project program. Late appointments, temporary
appointments or the use of part time personnel will most likely lead
to difficulties including extended commissioning program and the
possibility of an unsafe design.
It is particularly important that members of the commissioning team are fully
acquainted with the design and have a complete understanding of the operation
of the system. Members of the plant operating team should also be involved in
the design.
6.7.3 Preliminary and Detailed Enquiries
Normally, a preliminary enquiry is sent to about 6 to 12 suppliers which usually
enables about 3 to be selected for the detailed enquiry. The changes necessary
to a vendor’s proposal in order to meet GBHE standards should be considered.
Some suppliers may not have experience of units which are required to operate
24 hours per day for long periods, or units which are left for long periods without
operator supervision.
Preparation of detailed enquiries should involve all the relevant personnel,
including a Materials Engineer and a Furnaces & Boilers Engineer.
As part of the bid analysis on the detailed proposals, visits should be made to
selected operating units in order to confirm the suppliers' claims. Valuable
information on the design, construction and operation of the unit can be gained in
this way.
6.8 Commissioning
As with all commissioning program, it is essential that all systems are
thoroughly checked out in a proper manner. Items should be tested
against a checklist to ensure that there are no omissions. An outline
checklist is as follows:
• Check that all mechanical equipment including process and service pipe
work has been installed according to final approved drawings and
specifications;
• Ensure that all slip plates and temporary blank flanges or line blinds are
removed;

• Where appropriate de-scale/condition the inside surfaces of metallic
vent collection pipe work;
• Ensure that all drain lines and overflows from lute pots etc. are free
flowing (i.e. not blocked);
• Check liquid top up supply to lute pots is flowing and that the correct
fluid level is present;
• Check lute pot maintenance schedule and that frost prevention systems
are operable;
• Blow out process, utility and instrument air lines to ensure no
obstructions in the system which might block flame traps, instruments,
burner nozzles etc.;
• Physically check flame arresters (crimped metal and other in-line types)
for integrity, physical damage or blockage;
• Pressure/leak test all equipment where required or practicable;
• Dead check all cable/wiring loops;
• Live check all cable/wiring loops;
• Check operation of fans, pumps etc.;
• Stroke check all valves including, where appropriate, that they move to
their intended failure positions;
• Zero and span check measuring systems;
• Field test gas analyzers including response times;
• Check emergency shut-down and voting systems (e.g. on gas analysis
equipment and flame failure devices);
• Full functional loop tests on all control and emergency shut-down
systems;

• Check start-up sequence control system including safety interlocks (e.g.
delay timers on repeat start ups);
• Check all emergency shut-down initiation systems (e.g. high
temperature and flame failure devices);
• Check change over to emergency back up supplies (e.g. electrical
power);
• Start-up and check all equipment and process units down-stream of
combustion chamber(s);
• Test isolation of support fuel, confirm by gas sampling and analysis;
• Purge vent headers and combustion chamber with air for fuel-lean
systems and inert gas for fuel-rich systems with at least five volume
changes;
• Check ignition and operation of pilot flame visually via inspection port;
• Dry out refractory lining according to supplier’s specifications;
• Test stability of main flame throughout operating regime;
• Ensure all interlocks and trip systems function properly by deliberately
simulating shut-down situations in a controlled fashion (e.g. fan trips, low
support fuel pressure, flame failure etc.);
• Full load and control test on support fuel before admitting any process
streams;
• Combustion efficiency compliance tests.

6.9 Operational and Maintenance Management
6.9.1 Fundamental Principles
It is most important that vent collection and thermal oxidizer systems are treated
with the same high respect as afforded to production units. They should not be
treated as service units of secondary importance, otherwise there could be
serious safety, environmental and loss of production risks. The operational and
maintenance management of these systems should be integrated into
management arrangements for the associated production units. If a vent
collection and thermal oxidizer system treats waste gases from a number of
production units under the control of different groups of personnel, special
arrangements should be made to interface the management systems of the
production units and of the thermal oxidizer.
6.9.2 Communications
If it is not possible for the thermal oxidizer to be operated by the same team of
personnel that operates the associated production unit, then it is essential that
good communications are established between the two teams. Each team should
ensure that the other team has an awareness of the status of their plant,
especially with regard to unusual or unexpected situations.
6.9.3 Maintenance
Wherever possible, maintenance on a vent collection and thermal oxidizer
system should be planned to coincide with downtime on associated production
plants and vice versa.
Major repairs to thermal oxidizer refractory lining can take up to about three
weeks taking into account drying and curing time. Steps should be taken,
therefore, to safeguard the integrity of the refractories and to carry out inspection
and preventative maintenance accordingly.
The critical steps are:
• Proper specification;
• Careful installation;
• Drying out slowly;
• Minimizing temperature cycling by minimizing the frequency of shut-down and
by avoiding rapid temperature changes;
• Avoiding flame impingement;

• Avoiding impingement of quench water that may be used;
• Regular inspection through viewing ports;
• Regular inspection of outer shell for hot spots;
• Expeditious minor repairs.
Where a thermal oxidizer is connected to a number of upstream plants via a
complex header system, robust management procedures should be in place for
maintenance activities. Full understanding of the scope of maintenance work and
the effects on both upstream plants and the VCDS is essential to the safety of
the operation.
NFPA 85C requires that a formal maintenance training program be in place and
that procedures should be established to cover routine and special techniques.
6.9.4 Management of Change
Changes to equipment specifications or operating conditions should be managed
in the same way as a normal operating plant in order to minimize the risk of
hazards. All changes should be documented and a hazard analysis or hazard
study carried out as appropriate.

APPENDIX A GLOSSARY
AIT Autoignition temperature. The temperature at which a
flammable gas mixture will ignite without the presence of a
flame or spark.
BACT Best available control technology.
BATNEEC Best available techniques not entailing excessive cost.
BMS Burner management system.
BPEO Best practicable environmental option.
CHC Chlorinated hydrocarbon.
Deflagration A sub-sonic explosion where the flame front is preceded by
the pressure wave. A deflagration will produce a pressure of
up to 8 times the initial pressure.
Detonation A supersonic explosion where the flame front and pressure
wave are coincident. A detonation will produce a pressure of
up to 20-40 times the initial pressure.
Detonation arrester Similar to a flame arrester but designed to higher standards
of pressure and generally with larger arrester elements.
Designed to prevent the propagation of a detonation.
DRE Destruction and removal efficiency.
Flame arrester A device fitted in or at the end of a pipeline which is
designed to prevent the propagation of a flame. See also
"detonation arrester".
FM Factory Mutual Insurance. US industrial insurance company.
Fuel-rich Above the upper flammable (explosive) limit for the mixture.
Fuel-lean Below the lower flammable (explosive) limit for the mixture.
GRP Glass reinforced plastic, also known as FRP.

LFL (or LEL) Lower flammable limit or lower explosive limit. The lowest
percentage (v/v) of fuel which will just sustain combustion.
Specified at a particular temperature and pressure.
Inerted A gas mixture in which the amount of inerts is sufficient to
keep the composition out of the flammable region.
KO pot A knock-out pot designed to remove liquid or solid particles
from gas streams.
MIE Minimum ignition energy. The minimum amount of energy
required to ignite a flammable gas mixture. Generally spark
energy.
MOC Minimum oxidant concentration. The percentage (v/v) of
oxidant below which the mixture will not burn.
MSW Municipal solid waste.
NFPA National Fire Protection Association (of America).
Oxidant Generally oxygen but other possible oxidants include
chlorine and oxides of nitrogen.
PCDDs Polychlorinated dibenzodioxins.
PCDFs Polychlorinated dibenzofurans.
PICs Products of incomplete combustion.
POCP Photochemical ozone creation potential. Generally
referenced on a scale relative to ethylene.
POHC Mass flow rate of principal organic hazardous constituent.
Quenching diameter The diameter of orifice through which a flame will not
propagate. Measured at set pressure and temperature, this
parameter is used in the specification of flame arresters.
QRA Quantified risk assessment.

UFL (or UEL) Upper flammable limit or upper explosive limit. The
maximum percentage (v/v) of fuel which will just sustain
combustion. Specified at a particular temperature and
pressure.
UL Underwriters Laboratories. US testing and insurance
company.
UNECE United Nations Economic Commission for Europe
USCG United States Coast Guard. A regulatory body which has
issued a specification for the design and testing of flame
arresters.
VCDS Vent collection and destruction system. The collective
headers, destruction unit and discharge stack system.
VOC Volatile organic compound.
WHO World Health Organization.

APPENDIX B FLAMMABILITY
B.1 GENERAL
For combustion to occur, three things are necessary: fuel, oxidant and an
ignition source. Many substances, particularly hydrocarbons, burn readily
within certain composition limits. The limits, known as the LFL and UFL
have been determined under standard conditions for a variety of
substances. The flammable limits vary with temperature, pressure and
composition and can be plotted on a flammability diagram. A typical
diagram is shown in Figure 11. The flammable region is shown hatched. It
should be noted that the diagram is triangular and, as such, the sum of the
compositions at any point always adds up to 100%. Many diagrams
originating in the USA are shown on rectangular plots with fuel and
oxidant as the axes, the percentage of inert being 100 minus the sum of
the other two.
Figure 11 shows the major features of a typical flammability diagram. The
"air line" shows the composition change on diluting 100% fuel with air.
Where the air line crosses the bottom axis, the composition is 21%
oxygen, 79% nitrogen. Some flammability diagrams show only the region
to the right of the air line where the only oxidant available in the system is
air. This section is then expanded to fill the full diagram. This is often done
where only the composition of the gas when mixed with air is important.
FIGURE 11 TYPICAL FLAMMABILITY DIAGRAM

The line to the right of the air line and just touching the nose of the
flammable region shows the minimum oxidant concentration for
combustion (MOC). The composition on the oxidant axis is known as the
MOC on a fuel-free basis. This is the concentration of oxidant, below
which no combustion can occur.
The stoichiometry line shows the range of compositions which would
result in complete combustion of the fuel and oxidant. Compositions just
above the stoichiometric line produce the largest temperatures and
pressures on ignition.
The LFL is essentially constant for the majority of hydrocarbons and
hence is shown on the diagram as a horizontal line. The UFL is different in
air and pure oxygen, hence it is important to know whether the UFL is in
air or oxygen. Although for many hydrocarbons this line is essentially
straight, there is, however, a significant number of substances which
exhibit non-typical behavior. Where there is any doubt as to the
characteristics of the diagram, experimental determination of the
flammability limits is strongly recommended.
Oxidants other than air/oxygen can also be expressed on this type of
diagram (e.g. chlorine, NOx etc.).
The flammability diagram should be annotated with the temperature and
pressure at which the data were measured. Increasing the temperature or
pressure tends to increase the size of the flammable region although it
generally has a small effect for pressures greater than atmospheric. The
size of the flammable region is also affected by the ignition source, e.g.
whether hot wire, spark or fuse head. The more powerful the ignition
source, the larger the flammable region. Most flammability diagrams are
measured using a powerful spark or other ignition source and hence are
applicable to vent gas collection systems where the typical ignition energy
available is a few milliJoules. For detailed interpretation of flammability
diagrams, ignition energies, flammability data etc.
The lower flammability limit for a mixture of gases may be calculated using
Le Chatelier’s law:

Where:
L = Lower flammable limit of mixture
Cx = Percentage of component in mixture (v/v)
Lx = Lower flammable limit of each component
Flammability limits for many substances can be found in Reference 12. Le
Chatelier’s law can also be used to calculate the UFL of a mixture.
Although this formula is generally reliable for calculating LFLs, it is less
reliable when calculating UFLs due to the more complex reactions which
take place in a fuel-rich mixture.
B.2 CHANGES OF COMPOSITION
The triangular diagram can be used to ascertain the effect of changes in
composition on the flammability of a particular mixture. If the operating
point is plotted on the diagram and gas of another composition is added,
the composition moves in a straight line from the original operating point
towards the composition of the gas being added.
Figure 12 shows the example of a particular mixture being diluted with air.
The initial composition on this diagram is:
Fuel 43%
Oxidant 32%
Inert 25%
As the initial composition is diluted with air, the operating point moves in a
straight line towards the composition of air. At approximately 17% fuel, the
operating point enters the flammable region. Figure 13 shows the effect of
various amounts of diluent air on an initial volume of 100 Rm³ of gas of the
above composition. From Figure 13 it can be seen that the composition
enters the flammable region when 150 Rm³ of air has been injected.

FIGURE 12 EFFECT OF DILUTION WITH AIR
As the composition moves further away from the original operating point,
an increased amount of diluent is needed to change the composition by
the same percentage. This can be seen again in Figure 13 by considering
the amount of air needed to dilute the now flammable mixture to a point
below the flammable limit. The LFL on the diagram is 3% and by
calculation a total of 1,350 Rm³ of air is needed to reach this limit. A total
of 2,300 Rm³ of diluent air is needed to reach a level of 60% of the LFL
and 5,700 Rm³ to reach 25% of the LFL (0.75% fuel).
Thus it can be seen that the strategy for treating vents should be
considered carefully in order to provide a safe and economic solution.

FIGURE 13 EFFECT OF DILUTION WITH AIR ON 100 Rm³ OF
FLAMMABLE GAS

APPENDIX C EXAMPLE PROFORMA
Operating plant name and unit number
Vent title and description

APPENDIX D REFERENCES

DOCUMENTS REFERRED TO IN THIS PROCESS SHE GUIDE
This Process Engineering Guide makes reference to the following documents:
NATIONAL FIRE PROTECTION ASSOCIATION
NFPA 30 Flammable and Combustible Liquids Code (referred to in 5.2.5.4)
NFPA 69 Explosion Prevention Systems (referred to in 5.2.3.2 and 5.2.3.3)
NFPA 85C Prevention of Furnace Explosions & Implosions in Multiple Burner
Boiler Furnaces (referred to in 6.6.2, 6.6.3 and 6.9.3)
NFPA 86 Ovens and Furnaces (referred to in 6.6.1 and 6.6.2)
PROCESS SAFETY / SHE GUIDES
GBHE-PEG-008 Pressure Relief (referred to in 1.2 and 1.3)
GBHE-PEG-015 Practical Guide on the reduction of Discharges to
Atmosphere of Volatile Organic Compounds (VOCs)
(referred to in 1.2, 3.1, 3.2, 6.1 and 6.3.6)

DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS

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DESIGN OF VENT GAS COLLECTION AND DESTRUCTION SYSTEMS