to uv student

1. Process Risk Management

2. Objectives:
 Introduce the concept of hazard identification and
 Risk assessment methodologies

3.  It is essential to identify the hazards and reduce the risk well
in advance of an accident.
 Unfortunately, a hazard is not always identified until an accident
occurs
 The process risks to be identified, reduced, and managed are
those associated with potential releases of dangerous toxicant
, flammable, explosive, and reactive materials.

4.  For each process in a chemical plant the following questions must
be asked:
1. What are the hazards?
2. What can go wrong and how?
3. What are the chances?
4. What are the consequences?
• Hazard identification and risk assessment are sometimes
combined into a general category called hazard evaluation.
• After a description of the process is available, the hazards are
identified.
• The various scenarios by which an accident can occur are then
determined.
.

5. Hazards identification and risk assessment procedure

6. • This information is assembled into a final risk assessment.
• If the risk is acceptable, then the study is complete and the process is
operated.
• If the risk is unacceptable, then the system must be modified and the
procedure is restarted.
• Hazards identification and risk assessment studies can be performed at
any stage during the initial design or ongoing operation of a process.
• If the study is performed with the initial design, it should be done as
soon as possible. This enables modifications to be easily incorporated
into the final design.
• Hazard identification can be performed independent of risk
assessment. However, the best result is obtained if they are done
together.

7. Hazard identification methods
1. Process hazards checklists:
 This is a list of items and possible problems in the process that must be
checked.
 A checklist can be used during the design of a process to identify design
hazards, or it can be used before process operation.
Example:-
an automobile checklist that one might review before driving
away on a vacation.
This checklist might contain the following items:
 Check oil in engine.
 Check air pressure in tires,
 Check fluid level in radiator,
 gasoline level in tank etc…

8. 2. What if
• "What if" analysis: This less formal method of identifying
hazards applies the words "what if" to a number of areas of
investigation.
• For instance, the question might be, What if the flow stops?
The analysis team then decides what the potential
consequences might be and how to solve any problems.
• To identify potential failures and their associated risks it
evolves the generation of what if equations ,assessing
answers to those questions along with probability of
occurrence and consequence and develop recommendation
based on the assessment

9. 3. Hazards and Operability (HAZOP)
• The HAZOP study is a formal procedure to identify hazards
in a chemical process facility.
• Hazard and operability (HAZOP) studies provide
information on how a particular accident occurs. This is a
form of incident identification.
• The procedure is effective in identifying hazards and is well
accepted by the chemical industry.
• The basic idea is to let the mind go free in a controlled
fashion in order to consider all the possible ways that process
and operational failures can occur.

10. • Before the HAZOP study is started, detailed information on the
process must be available.
 Up-to-date process flow diagrams (PFDs),
 Process and instrumentation diagrams (P&IDs),
 Detailed equipment specifications, materials of construction,
 Mass and Energy balances.
• The full HAZOP study requires a committee composed of
experienced plant,
laboratory,
technical, and safety professionals.
• One individual must be a trained HAZOP leader and serves as the
committee chair.
• a complete HAZOP study requires a large investment in time and
effort, but the value of the result is well worth the effort.
• Time and cost of a HAZOP are directly related to the size and
complexity of the plant being analyzed.

11. The HAZOP procedure uses the following steps to complete an
analysis:
1. Begin with a detailed flow sheet. Break the flow sheet into a
number of process units. Thus the reactor area might be one
unit, and the storage tank another. Select a unit for study.
2. Choose a study node (vessel, line, operating instruction).
3. Describe the design intent of the study node. For example,
vessel V-1 is designed to store the benzene feedstock and
provide it on demand to the reactor.

12. 4. Pick a process parameter: flow, level, temperature, pressure,
concentration, pH, viscosity, state (solid, liquid, or gas), agitation,
volume, reaction, sample, component, start, stop, stability, power,
inert.
5. Apply a guide word to the process parameter to suggest possible
deviations. A list of guide words is shown in Tables 10-3 and 10-6
for process lines and vessels.
6. If the deviation is applicable, determine possible causes and note
any protective systems.
7. Evaluate the consequences of the deviation (if any).
8. Recommend action (what? by whom? by when?)
9. Record all information.

14. The item is represented by numbering system used is a number-letter combination. Thus
the designation "1 A" would designate the first study node and the first guide word.

15. Example :- Exothermic Reactor
Consider the reactor system shown in below. The reaction is
exothermic, so a cooling system is provided to remove the
excess energy of reaction. In the event that the cooling function
is lost, the temperature of the reactor would increase. This
would lead to an increase in reaction rate, leading to additional
energy release. The result would be a runaway reaction with
pressures exceeding the bursting pressure of the reactor vessel.

16. Perform a HAZOP study on this unit to improve the safety of
the process.
Use as study nodes the cooling coil (process parameters: flow
and temperature) and the stirrer (process parameter:
agitation).
Solution
The guide words are applied to the study node of the cooling
coils and the stirrer with the designated process parameters.
The HAZOP results are shown in Table below, which is
only a small part of the complete

18. The potential process modifications resulting from this study (the above
Example) are the following:
1. install a high-temperature alarm to alert the operator in the event of
cooling function loss,
2. install a high-temperature shutdown system (this system would
automatically shut down the process in the event of a high reactor
temperature; the shutdown temperature would be higher than the alarm
temperature to provide the operator with the opportunity to restore
cooling before the reactor is shutdown),
3. install a check valve in the cooling line to prevent reverse flow (a check
valve could be installed both before and after the reactor to prevent the
reactor contents from flowing upstream and to prevent the backflow in
the event of a leak in the coils),
periodically inspect the cooling coil to ensure its integrity,

19. 5. study the cooling water source to consider
possible contamination and interruption of
supply,
6. install a cooling water flow meter and low-flow
alarm (which will provide an immediate
indication of cooling loss).

20. Risk Assessment Techniques
• Occasionally an incident occurs that results in a common
mode failure.
• focus on determining the frequency of accident scenarios
• When working with control systems, one needs to
deliberately design the systems to minimize common cause
failures.
• The most applicable techniques
 Event Trees Analysis and
 Fault Trees Analysis

21. Event Tree Analysis (ETA)
• Event trees begin with an initiating/accidental event and work
toward a final result.
• This approach is inductive. The method provides information
on how a failure can occur and the probability of occurrence.
• When an accident occurs in a plant, various safety systems
come into play to prevent the accident from propagating.
• These safety systems either fail or succeed.
• The event tree approach includes the effects of an event
initiation followed by the impact of the safety systems

22. The typical steps in an event tree analysis are
1. identify an initiating event of interest,
2. identify the safety functions designed to deal with the
initiating event,
3. construct the event tree, and
4. describe the resulting accident event sequences.

23. • An event tree analysis (ETA) is an inductive procedure that
shows all possible outcomes resulting from
• an accidental (initiating) event, taking into account whether
installed safety barriers are functioning or not, and
additional events and factors.

24. Example:-1 – Consider the chemical reactor system shown in
Figure. To prevent loss-of-coolant in reactor, the reactor has
been installed with high-temperature alarm and temperature
controller. Develop a event tree to assess process safety
systems come into play to prevent the accident.

25. Four safety functions are identified.
• The first safety function is the high-temperature alarm.
• The second is the operator noticing the high reactor
temperature during normal inspection.
• The third is the operator reestablishing the coolant flow by
correcting the problem in time.
• The final is invoke /call by the operator performing an
emergency shutdown of the reactor.

26. Assuming that:
 a loss-of-cooling event occurs once a year.
 Alarm fails 1% of time placed in demand This is a failure
rate of 0.01 failure in demand.
 Also assume that the operator will notice the high reactor
temperature 3 out of 4 times and that 3 out of 4 times the
operator will be successful at reestablishing the coolant
flow. Both of these cases represent a failure rate of 1 time
out of 4, or 0.25 failure in demand.
 Finally, it is estimated that the operator successfully shuts
down the system 9 out of 10 times. This is a failure rate of
0.10 failure in demand.

27. • A line is drawn from the initiating event to the first safety
function. At this point the safety function can either
succeed or fail.

28. • The lettering notation in the sequence description column is
useful for identifying the particular event. The letters indicate
the sequence of failures of the safety systems. The initiating
event is always included as the first letter in the notation.
• Such as ADE represents initiating event A followed by failure
of safety functions D and E.

29. What is wrong with the logic of this
example analysis?
Answer: If the operator fails to notice the high
temperature after the alarms fails, then he/she will
never restart the cooling.

30. Fault Tree Analysis (FTA)
• The approach starts with a well-defined accident, or top event,
and works backward toward the various scenarios that can cause
the accident.
• Fault trees are a deductive method for identifying ways in which
hazards can lead to accidents.
• Graphical representation of the logical structure displaying the
relationship between an undesired potential event (top event) and
all its probable causes
– top-down approach to failure analysis
– starting with a potential undesirable event - top event
– determining all the ways in which it can occur
– mitigation measures can be developed to minimize the
probability of the undesired event

32. Example 1:- a flat tire of an automobile
is caused by two possible events.
In one case the flat is due to driving over debris on the road,
such as a nail. The other possible cause is tire failure. The
flat tire is identified as the top event.
Solution
The two contributing causes are either
basic (cannot be defined further) or
intermediate events(can defined further)
For this example, driving over the road debris is a basic event
because no further definition is possible.
The tire failure is an intermediate event because it results from
either a defective tire or a worn tire

34. Accident and Loss Statistics
• Accident and loss statics are
important measures of the
effectiveness of safety programs.
• Many statistical methods are
available to characterize accident
and loss performance.
• Methods report the number of
accidents and/or fatalities for a
fixed number of workers during a
specified period.
1. OSHA incidence rate,
2. fatal accident rate (FAR), and
3. fatality rate, or deaths per person
per year.

35. 1. OSHA Incident Rate
OSHA stands for the Occupational Safety and Health Administration
of the United States government.
• OSHA is responsible for ensuring that workers are provided with a
safe working environment.
• The OSHA incidence rate is based on cases per 100 worker years.
• A worker year is assumed to contain 2000 hours (50 work
weeks/year X 40 hours/week). The OSHA incidence rate is
therefore based on 200,000 hours of worker exposure to a hazard.
• The OSHA incidence rate is calculated from the number of
occupational injuries and illnesses and the total number of
employee hours worked during the applicable period.

36. OSHA incidence rate (based on injuries and illness) =
Number of injuries and illnesses X 200,000
Total hours worked by all employees during period
covered
• Occupational injury: Any injury such as a cut, sprain, or
burn that results from a work accident.

37. • Occupational illness : Any abnormal condition or disorder,
other than one resulting from an occupational injury, caused
by exposure to environmental factors associated with
employment.
• It includes acute and chronic illnesses or diseases that may
be caused by inhalation, absorption, ingestion, or direct
contact.

38. 2. Fatal Accident Rate (FAR)
• The FAR is used mostly by the British chemical industry. The
FAR report the number of fatalities based on.
 1000 employees working their entire lifetime.
 The employees are assumed to work a total of 50 years.
 A worker year is assumed to contain 2000 hours (50
weeks/year *40hr/week).
2000*50 year = 108 working hrs Thus the FAR is based on 108
working hours.

39. FAR = Number of fatalities X 108/ Total hours worked by all
employees during period covered.

40. 3. Fatality Rate (FR)
• The last method considered is the fatality rate(FR) or deaths
per person per year.
• This system is independent of the number of hours actually
worked and reports only the number of fatalities expected
per person per year.
FR= Number of fatalities per year/ Total number of people in
applicable population.
• Both the OSHA incidence rate and the FAR depend on the
number of exposed hours.

41. • A FAR can be converted to a fatality rate (or vice versa) if
the number of exposed hours is known.
• The OSHA incidence rate cannot be readily converted to a
FAR or fatality rate because it contains both injury and
fatality information

42. Accident Facts, 1985 ed. (Chicago: National Safety Council, 1985), p.
"rank P. Lees, Loss Prevention in the Process Industries
(London: Butterworths, 1986), p. 177.

43. Fire and Explosion
• The major distinction between fires and explosions is the rate of
energy release.
• Fires release energy slowly, whereas explosions release energy
rapidly.
• Fires can also result from explosions, and explosions can result from
fires
Example: energy release rate for automobile tire of how the affects
consequences of an accident
• The compressed air within the tire contains energy.
• If the energy is released slowly through the nozzle, the tire is
harmlessly deflated.
• But, If the tire ruptures suddenly explosion will occurs

45.  Combustion or fire: is a chemical reaction in which a substance
combines with an oxidant and releases energy.
Elements for combustion are: fuel, an oxidizer, and an ignition
source.

46. Fuels
 Liquids: gasoline, acetone, ether, pentane
 Solids: plastics, wood dust, fibers, metal particles
 Gases: acetylene, propane, carbon monoxide, hydrogen
Oxidizers
Gases: oxygen, fluorine, chlorine
Liquids: hydrogen peroxide, nitric acid, per chloric acid
Solids: metal peroxides, ammonium nitrite
Ignition sources
Sparks, flames, static electricity, heat

47. • The combustion always occurs in the vapor phase; liquids are
volatized and solids are decomposed into vapor before combustion
• Vapor-air mixtures will ignite and burn only over a well-specified
range of compositions (Flammability limits).
• The mixture will not burn when the composition is lower than the
lower flammable limit (LFL); the mixture is too lean for
combustion.
• The mixture is also not combustible when the composition is too
rich; that is, when it is above the upper flammable limit (UFL).
• A mixture is flammable only when the composition is between the
LFL and the UFL. Commonly used units are volume percent fuel
(percentage of fuel plus air).

49. Flammability characteristics of liquids and vapors
• The flash point temperature is one of the major quantities
used to characterize the fire and explosion hazard of liquids.
• Flash point (FP): The flash point of a liquid is the lowest
temperature at which it gives off enough vapor to form an
ignitable mixture with air.
• The flash point temperatures for pure materials correlate well
with the boiling point of the liquid.
Where: Tf, is the flash point temperature (K), a, b, and c are constants provided in Table 6-1
(K), and Tb, is the boiling point temperature of the material (K).

51. • Flash points can be estimated for multi-component mixtures
if only one component is flammable and if the flash point of
the flammable component is known.
• In this case the flash point temperature is estimated by
determining the temperature at which the vapor pressure of
the flammable component in the mixture is equal to the pure
component vapor pressure at its flash point

52. Vapor Mixtures
Frequently LFLs and UFLs (lower and upper flammable limit)
for mixtures are needed. These mixture limits are computed
using the Le Chatelier equation:
where
LFLi and UFLi is the lower and upper flammable limit for
component i (in volume %) of component i in fuel and air,
yi is the mole fraction of component i on a combustible basis, and
n is the number of combustible species.

53. The derivation shows that the following assumptions are
inherent in this equation:
 The product heat capacities are constant.
 The number of moles of gas is constant.
 The combustion kinetics of the pure species is independent
and unchanged by the presence of other combustible species.
 The adiabatic temperature rise at the flammability limit is the
same for all species.

56. Flammability Limit Dependence on Temperature
In general, the flammability range increases with
temperature.
The following empirically derived equations are available
for vapors:
where
 ∆Hc is the net heat of combustion (kcal/mole) and
 T is the temperature (0C).
 LFL is lower flammability limit
 UFL is upper flammability limit

57. Flammability Limit Dependence on Pressure
• Pressure has little effect on the LFL except at very low
pressures (40 mm Hg absolute), where flames do not
propagate.
• The UFL increases significantly as the pressure is increased,
broadening the flammability range.
• An empirical expression for the UFL for vapors as a
function of pressure is available.
where
 P is the pressure (mega pascals absolute) and
 UFL is the upper flammable limit (volume % of fuel plus air at 1 atm).
 LFL is lower flammability limit (volume % of fuel plus air at 1 atm)

58. Estimating Flammability Limits
• For some situations it may be necessary to estimate the
flammability limits without experimental data
Jones found that for many hydrocarbon vapors the LFL and
the UFL are a function of the Stoichiometric concentration
(Cst) of fuel:
The stoichiometric concentration for most organic compounds is
determined using the general combustion reaction.

59. It follows from the stoichiometry that
w
where z has units of moles O2/mole fuel.
Additional stoichiometric and unit changes are required to
determine Cst ,as a function of z:

60. Another method that correlates the flammability limits as a function
of the heat of combustion of the fuel. A good fit was obtained for 30
organic materials containing carbon, hydrogen, oxygen, nitrogen,
and sulfur. The resulting correlations are:

61. where
• LFL and UFL are the lower and upper flammable limits (vol.
% fuel in air), respectively, and
• ∆Hc, is the heat of combustion for the fuel (in kJ/mol).
• NB: Equation for UFL is applicable only over the UFL range
of 4.9-23%. If the heat of combustion is provided in
kcal/mol, it can be converted to kJ/mol by multiplying by
4.184.

62. Toxic Release and Dispersion
A hazard is a situation that poses a level of danger to life, health,
property, or environment.
Chemical plant contain a large variety of Hazards
a. Mechanical hazards: moving equipment and falling
b. Chemical hazard (Reactivity , corrosiveness)
c. Fire and explosion hazard
d. Toxic hazard
During an accident, process equipment can release toxic materials
quickly and in significant enough quantities to spread in dangerous
clouds throughout a plant site and the local community.
examples
explosive rupture of a process vessel as a result of excessive
pressure caused by a runaway reaction,

63.  rupture of a pipeline containing toxic materials at high
pressure,
 rupture of a tank containing toxic material Stored above its
atmospheric boiling point, and
 rupture of a train or truck transportation tank following an
accident.
An excellent safety program strives to identify problems before they
occur.
engineers must understand all aspects of toxic release to prevent the
existence of release situations and to reduce the impact of a release
if one occurs.
Parameters Affecting Dispersion:- Dispersion describe the airborne
transport of toxic materials away from the accident site and into
the plant and community.
• After a release the airborne toxic material is carried away by the
wind in a characteristic plume, or a puff.

65. A wide variety of parameters affect atmospheric dispersion of toxic
materials:
 wind speed,
 atmospheric stability,
 ground conditions (buildings, water, trees),
 height of the release above ground level,
 momentum and buoyancy of the initial material released.
Wind speed
As the wind speed increases, the plume becomes longer and narrower;
the substance is carried downwind faster but is diluted faster by a
larger quantity of air.

66. Atmospheric stability:- Atmospheric stability relates to vertical
mixing of the air.
 During the day, the air temperature decreases rapidly with height,
encouraging vertical motions.
 At night the temperature decrease is less, resulting in less vertical motion.

67. • Ground conditions:- affect the mechanical mixing at the surface
and the wind profile with height. Trees and buildings increase
mixing, whereas lakes and open areas decrease it.

68. Height of the release above ground level

69. • The buoyancy and momentum of the material released change
the effective height of the release. The momentum of a high-
velocity jet will carry the gas higher than the point of release,
resulting in a much higher effective release height.
Momentum and buoyancy of the initial material released

70. • If the gas has a density less than air, the released gas will initially
be positively buoyant and will lift upward.
• If the gas has a density greater than air, then the released gas will
initially be negatively buoyant and will slump toward the ground.
• For all gases, as the gas travels downwind and is mixed with fresh
air, a point will eventually be reached where the gas has been
diluted adequately to be considered neutrally buoyant. At this
point the dispersion is dominated by ambient turbulence.