Risk management in ammonia urea plants

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RISK MANAGEMENT IN AMMONIA/ UREA PLANTS
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RISK MANAGEMENT IN AMMONIA/ UREA
PLANTS
By
Prem Baboo
Former Sr. Manager, National Fertilizers Ltd, India
Abstract-
Risk Management is the Identification, Analysis
and Economic Control of those Risks which can
Threaten the Assets (Property, Human) or the
Earning Capacity of an Enterprise” Risk
management is the process of identifying,
assessing and controlling threats to an
organization's capital and earnings. These risks
stem from a variety of sources including
financial uncertainties, legal liabilities,
technology issues, strategic management errors,
accidents and natural disasters. The article
intended The Fertilizer Plant pose fire, explosion
and toxic hazards due to unwanted and
accidental release of natural gas as well as
process gas containing CO, H2, Methane and
toxic gases like Ammonia and Chlorine. The
effect zones of the fire and explosion hazard are
generally restricted within the plant boundary
limits and near the source of generation itself.
However, effect of accidental release of
Ammonia and other toxic gases may go outside
the factory premises. This section deals with the
failure modes, listing of failure cases leading to
different hazard scenarios, consequence
modeling and the risk evaluation. Consequence
analysis is basically a quantitative study of the
hazard due to various failure scenarios to
determine the possible magnitude of damage
effects and to determine the distances up to
which the damage may be affected using
internationally accepted mathematical models.
Keyword
Ammonia plant, fertilizer production industry,
risk management, Risk, Fire, Explosion,
Introduction
Ammonia is one among the largest volume
inorganic chemicals in the Fertilizers and others
chemicals process industries. About 80% or
more of the ammonia produced is used for
fertilizing agricultural crops. From 1980 to 2021,
the capacity of single stream ammonia plant has
increased drastically in the range from 1500-
3500 TPD. Presently, the largest ammonia plant
has the capacity of 3300 TPD But very soon
will become as the largest single-stream
ammonia plant in the world, which is due on-
stream in the middle of 2022, has the capacity of
4000 TPD. The large volume of Ammonia
storages are also very risky. It is expected that
capacity of single stream may even reach 5000
TPD, considering current pace of development.
But these large plants also pose increased hazard
and risk associated with it. A successful risk
management program helps an organization
consider the full range of risks it faces. Risk
management also examines the relationship
between risks and the cascading impact they
could have on an organization's strategic goals.
Risk management has perhaps never been more
important than it is now. The risks modern
organizations faces have grown more complex,
fueled by the rapid pace of globalization. New

Oct 2021
risks are constantly emerging, often related to
and generated by the now-pervasive use of
digital technology. Climate change has been
dubbed a "threat multiplier" by risk experts.
This article descriptive study which carried out
in observation using a cross sectional design.
Variables in this study included hazard
identification, basic risk analysis, risk control
that has been done, existing risk analysis, and
risk reduction assessment. The tools used for the
data collection were observation sheets,
interview guide sheets, and Job Safety Analysis
sheets. Data that has been obtained through
observation and interviews was processed using
Fine semi quantitative technique. Results: The
results of hazard identification known to have as
many as 6 potential hazards. The assessment
results in the basic risk analysis showed that the
initial risk level consisted of 3 risks with very
high level, 2 risks with a substantial level and 1
risk with priority 3 level. After the risk control
effort was applied, the results of the assessment
in the existing risk analysis showed that the level
of risk has decreased significantly. The main
reason for increase in the capacity of a single
stream to reduce specific production cost
through so-called economy of scale, i.e., if
design output is doubled, the capital cost
increases by only 50- 60%. There are also some
savings on operating costs, particularly in terms
of the thermal economy and labour. Though
ammonia plant has well proven technology;
however, there are problems and failures of
process equipments, machineries, instruments
and control systems etc., many of these are not
reported in the literature. Safety of a plant can be
improved, but cannot be guaranteed.
Hazard Identification
Hazard indices computation helps in ranking
the most vulnerable unit by assigning the
penalties based on the properties of the
chemicals used and type of
installation.Table-1 shows the fire and
explosion toxicity and Mond Indices
Computation of Ammonia plants. The
Toxicity Index is arrived at from fire and
explosion index. The Toxicity Index (TI) is
computed using the health factor (Nh) ,
maximum allowable concentration (MAC)
value ranging between <5,5-50,>50
respectively. Similarly for Nh rage from 0-4
,a corresponding factor , The is assigned.
The TI can be calculated using the following
formula
𝑻𝑰 =
(𝐓𝐡 + 𝐓𝐬)𝐗 𝟏 + 𝐆𝐏𝐇 + 𝐒𝐏𝐇)
𝟏𝟎𝟎
The Degree of hazard is identified based on
FEI and TI range according to the following
criteria.
Sr. No. FEI range Degree of Hazard
1 0-60 Light
2 61-96 Moderate
3 97-127 Intermediate
4 128-158 Heavy
5 159 & abobe Severe
Table-1
Sr. No. TI range Degree of Hazard
1 0-5 Light
2 5-10 Moderate
3 >10 Severe
Table -2
In Fertilizers Complex the ammonia plant
constitutes one of the most hazardous area.It
is therefore of vital importance to collect and
analyze methodically the data based on
accidents.

Oct 2021
Units M.F FEI Index Mond
Index MI
Toxicity of
Hazard
Degree
FEI TI
Primary
Reformer
21 130.4 2531 ---- Heavy ---
Secondary
Reformer
21 128 2579 ------ Heavy -----
Converter
HT
21 128.2 2568 ---- Heavy ------
Converter
LT
21 135.4 2562 ---- Heavy -------
Metanator 21 252 2109 --- severe ----
Amm
Converter
21 166 2553 24.12 Severe High
Seperator 4 22.4 2075 21.2 High High
Amm
Storage
4 23.8 412 13.3 light High
Amm
Tanker
4 23.8 412 14.2 Light high
Table-3
Brief Description of Ammonia Production
The raw material of Ammonia production are
Natural gas, Steam, Air and Power. In old plant
Heavy oil/Naphtha were used which requires
partial oxidation with oxygen. In the plant,
ammonia is produced from synthesis gas
containing hydrogen and nitrogen in the ratio of
approximately 3:1. Besides these components,
the synthesis gas contains inert gases such as
argon and methane to a limited extent. The
source of H2 is demineralized water and the
hydrocarbons in the natural gas. The source of
N2 is the atmospheric air. The source of CO2 is
the hydrocarbons in the natural gas feed. Product
ammonia and CO2 is sent to urea plant. The
main function of the plant is illustrated in the
following sketch (figure-1)

Oct 2021
Fig-1
Brief Description of Process
The process steps necessary for production of
ammonia from the above-mentioned raw
materials are as follows: -
1. The hydrocarbon feed is de sulphurized
to the ppb level in the desulphurization
section.
2. The de sulphurized hydrocarbon feed is
reformed with steam and air into raw
synthesis gas (process gas). The gas
contains mainly hydrogen, nitrogen,
carbon monoxide, carbon dioxide and
steam. –
3. In the gas purification section, the CO is
first converted into CO2. Then the CO2
is removed from the process gas in the
CO2 removal section. –
4. The CO and CO2 residues in the gas
outlet of the CO2 removal unit are
converted into methane by reaction with
H2 (Methanation) before the synthesis
gas is sent to the ammonia synthesis
loop.
5. The purified synthesis gas is compressed
and then routed to the ammonia
synthesis loop, where it is converted into
ammonia. In order to limit the
accumulation of argon and methane in
the loop, a purge stream is taken. The
liquid ammonia product is depressurised
during which the dissolved gases,
letdown gas and inert gas, are flashed
off.

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Fig-2
The natural gas feedstock coming from source
limit contains minor quantities of sulphur
compounds which have to be removed in order
to avoid poisoning of the reforming catalyst in
the primary reformer, and the low temperature
shift catalyst in the CO converter, Particularly
the low temperature shift. Converter
to deactivation by sulphur and sulphur
compounds. Prior to hydrogenation, the feed gas
sulphur absorption catalyst. After
desulphurization, the content of sulphur is less
The natural gas feedstock coming from source
limit contains minor quantities of sulphur
removed in order
to avoid poisoning of the reforming catalyst in
the primary reformer, and the low temperature
shift catalyst in the CO converter, Particularly
shift. Converter, is sensitive
to deactivation by sulphur and sulphur-bearing
compounds. Prior to hydrogenation, the feed gas
is mixed with Hydrogen rich recycle stream
which is coming from syn gas compressor 2
stage discharge. Then the Feed gas is heated in
Heater in the reformer flue gas section. Since the
gas contains organic sulphur compounds, the
desulphurization takes place in two stages. The
organic sulphur compounds are converted to H
by the hydrogenation catalyst,
absorption takes place in the
sulphur absorption catalyst. After
nt of sulphur is less
than 0.1 vol. ppm. A sketch of the
desulphurization section is given in
26th
Oct 2021
te for scientists and researchers.
is mixed with Hydrogen rich recycle stream
which is coming from syn gas compressor 2nd
stage discharge. Then the Feed gas is heated in
Heater in the reformer flue gas section. Since the
c sulphur compounds, the
desulphurization takes place in two stages. The
organic sulphur compounds are converted to H2S
catalyst, and the H2S
absorption takes place in the
than 0.1 vol. ppm. A sketch of the
desulphurization section is given in Figure –3

Fig-3
Hydrogenation The preheated natural gas is fed
to the hydrogenator. The vessel contains
Hydrogenation Catalyst, which is a cobalt
molybdenum based catalyst. catalyzes the
following reactions-
RSH + H2 → RH + H2S
R1SSR2 + 3H2 → R1H + R2H + 2H
R1SR2 + 2H2 → R1H + R2H + H2S
(CH)4S + 4H2 → C4H10 + H2S
COS + H2 → CO + H2S
Where R is hydrocarbon radical.
hydrogenation catalyst must not get into contact
Hydrogenation The preheated natural gas is fed
to the hydrogenator. The vessel contains
which is a cobalt-
molybdenum based catalyst. catalyzes the
H + 2H2S
S
Where R is hydrocarbon radical. The
hydrogenation catalyst must not get into contact
with hydrocarbons without the presence of
hydrogen. The result would be poor conversion
of the organic sulphur compounds causing an
increased sulphur slip to the reforming section.
The temperature also plays an important role
with regard to catalyst activity; at low
temperatures the hydrogenation reactions
progress very slowly and conversion is not
optimal while at high temperatures undesirable
cracking reactions may occur with deactivation
of catalyst due to carbon lay
catalyst itself. The optimum temperature range is
26th
Oct 2021
with hydrocarbons without the presence of
hydrogen. The result would be poor conversion
of the organic sulphur compounds causing an
increased sulphur slip to the reforming section.
lays an important role
with regard to catalyst activity; at low
temperatures the hydrogenation reactions
progress very slowly and conversion is not
optimal while at high temperatures undesirable
cracking reactions may occur with deactivation
e to carbon lay-down on the
catalyst itself. The optimum temperature range is

Oct 2021
between 350 and 400°C. In case natural gas
containing CO and CO2 is fed to the Hydro
generator, the following reactions will take
place.
CO2 + H2 ⇔ CO + H2O
CO2 + H2S ⇔ COS + H2O
Therefore, the presence of CO, CO2 and H2O
influences the sulphur slippage from
downstream the Sulphur absorbers, The catalyst
is oxidized at delivery and resumes its activity
when sulphided. The Catalyst can be sulphided
during initial start-up with natural gas feedstock
at not high temperature and not high H2 flow to
minimize the possibility of the MoO3 being
reduced to MoO2 that means catalyst
irreversible deactivation. In the sulphided state
the catalyst is pyrophoric and it must be not
exposed to air at temperatures above 70°C.
The equilibrium composition for the reaction
between the zinc oxide and hydrogen sulphide is
expressed by the following equation:
K(p) T =PH2S/PH2O=2.5 X 10-6
at 3800
C.
The catalyst is not reacting with oxygen or
hydrogen at any practical temperature. Zinc
sulphide is not pyrophoric and no special care
during unloading is required. Steam operations
should not be carried out in 11-R-202 A/B: the
zinc oxide would hydrate and it would then be
impossible to regenerate the ZnO material in the
reactor.
After desulfurization and scrubbing, the natural
gas is sent to the primary reformer for steam
reforming, where superheated steam is fed into
the reformer with the methane. The gas mixture
passed through reformer tubes which contains
Nickel catalyst and externally heated by the
combustion of fuel, normally natural gas and
purge gas, to approximately 770 0 C in the
presence of a nickel catalyst where methane is
converted into CO/CO2 and H2. At this
temperature, the following endothermic
reactions are driven to the right, converting the
methane to hydrogen, carbon dioxide and small
quantities of carbon monoxide.
CnH2n+2 + 2H2O ⇔ Cn-1H2n + CO2 + 3H2 –
heat
CH4 + 2H2O ⇔ CO2 + 4H2 - heat (39.4
kcal/mol)
CO2 + H2 ⇔ CO + H2O - heat (9.84 kcal/mol)
This gaseous mixture is known as synthesis gas.
Conversion in primary steam reformer is about
70% of the hydrocarbon feed into synthesis gas.
The reactions are endothermic, thus the supply
of heat to the reformer is required to maintain
the desired reaction temperature. The hot flue
gases contain lot of energy and recovered upto
maximum possible extent before releasing to
atmosphere through chimney.

Fig-4
Studies on the prediction of remaining life and
ageing of material for pressurized tubes of
industrial furnace operated at elevated
temperature. The results of mechanical
properties tested at high temperature (800 and
8500
C) had shown that the aged metal’s
mechanical properties improved after the
solution heat treatment. In other words, the
outlet pigtail tubes after being employed to
about 80000 h can be further used continuously,
operated at high temperature, for another design
life (100000 h) by using solution heat treatment
processing based on their proposed methodology
for predicting the remaining life and ageing of
material of furnace tubes. There are numerous
incidents of reformer tube failure and fire in
ammonia plant. The probable causes of fire are
direct impingement of flame on the tube due to
the partial blockage of burner tips is possible.
on the prediction of remaining life and
ng of material for pressurized tubes of
industrial furnace operated at elevated
temperature. The results of mechanical
properties tested at high temperature (800 and
C) had shown that the aged metal’s
mechanical properties improved after the
heat treatment. In other words, the
outlet pigtail tubes after being employed to
about 80000 h can be further used continuously,
operated at high temperature, for another design
life (100000 h) by using solution heat treatment
posed methodology
for predicting the remaining life and ageing of
material of furnace tubes. There are numerous
incidents of reformer tube failure and fire in
ammonia plant. The probable causes of fire are
direct impingement of flame on the tube due to
partial blockage of burner tips is possible.
This may cause overheating of the tubes, which
ultimately led to one tube rupture. Flame
impingement from a nearby leaky tube might
result in overheating and the ultimate rupture of
the other nearby tubes. Sometimes situation may
lead to explosion in reformer furnac
discussed an incident of fire in an ammonia plant
which began with leakage in tube of a natural
gas pre heater ignited a small fire. The small fire
ultimately developed into tube to burst resulti
in large fire and plant shutdown. Reformer tube
can also fail by stress corrosion cracking. During
welding, the steel in the heat affected zone
(HAZ) can get sensitized and this may
subsequently lead to stress corrosion cracking.
The premature failure of a primary reformer tube
in an ammonia plant in which number of catalyst
tubes found to have failed just below the inlet
flange weld within about 2 years in service.
26th
Oct 2021
This may cause overheating of the tubes, which
ultimately led to one tube rupture. Flame
impingement from a nearby leaky tube might
result in overheating and the ultimate rupture of
times situation may
lead to explosion in reformer furnace as
discussed an incident of fire in an ammonia plant
which began with leakage in tube of a natural
heater ignited a small fire. The small fire
ultimately developed into tube to burst resulting
in large fire and plant shutdown. Reformer tube
can also fail by stress corrosion cracking. During
welding, the steel in the heat affected zone
(HAZ) can get sensitized and this may
to stress corrosion cracking.
f a primary reformer tube
ch number of catalyst
found to have failed just below the inlet
flange weld within about 2 years in service.

Oct 2021
Stress corrosion cracking (SCC) may get
aggravated further due improper welding
procedure. Preventing such failure by following
the proper welding procedure is very important.
Therefore it is recommended to cool the steel
below 1000 C after every weld pass, use filler
metal electrodes with low carbon content (such
as the 321
grades) and stabilising the steel grades with
addition of Nb (up to 1%) will help preventing
sensitisation during welding.Reformer tubes
from a fertilizer plant made of modified HK 40
steel which failed after 4 years service during
startup of plant. At that time only 60 burners
(out of 576 burners) were firing in the reformer.
The gas in the catalyst tubes was mainly
hydrogen and steam at low pressure of 3 kg/cm2
only. Seven tubes had ruptured in the bottom
portion in one corner of the radiant chamber.
Their visual observation - the tubes had a rather
black outer surface indicative of high
temperature and the oxide scale was adherent.
There was no indication of any localised damage
in the form of pits.
Categories of Risk associated with the
Fertilizers Complex
The manufacture of anhydrous liquid ammonia
involves processing of hydrocarbons under high
temperature, high pressure conditions in the
presence of various catalysts, chemicals etc.
Typical risks are as follows:
Ammonia Plants
1 Fire / Explosion Risks  Glands/seal leaks in valves, pumps, compressors handling
hydrogen, natural gas, naphtha, synthesis gas etc.
 Hose/pipe failure, leakage from flanged joints carrying
combustible gases, vapours, liquids
2 High / Low
Temperature Exposure
Risks
 Burns due to contact with hot surfaces of pipelines,
equipments, etc. or leaking steam lines, process fluids at high
temperature.
 Frost bite due to contact with anhydrous liquid ammonia at -33
deg. C
 Burns due to contact with pyrophoric catalyst
3 Toxic Chemicals
Exposure Risks
 Asphyxia due to inhalation of simple asphyxiants like CO2 ,
N2, H2, CH4, naphtha etc. and chemical asphyxiants like CO,
NH3, Nickel carbonyl, V2O5, Hydrazine, NOx, SOx, H2S etc.
 Acute toxicity due to inhalation of catalyst dusts containing
heavy metals like Ni, Cr, CO, Mo, Fe, Zn, Alumina etc. and
silica gel molecular sieves, insulation fibers/dusts.
4 Corrosive /
Radioactive Chemicals
Exposure Risk
 Severe burns, damage to eyes, skin and body tissues due to
contact with anhydrous ammonia
Table-4
The risks of process hazards resulting in major
events (fires, explosions and toxic releases) are,
in principle, minimized by good design and the
application of process safety principles. Older
plant was not subjected to the same level of
scrutiny that new plant is today. The adoption of
process safety principles is influenced by the
country legislation, the linkages of the
organization to global companies and the vision
/ policies of the particular organization. There
are a number of models which illustrate the idea
of “Layers of Protection”. The basic unmitigated
risk posed by a process hazard is reduced by a
number of “barriers” which either prevent the
hazard from being materialized or / and mitigate
the effect of the hazard once the event has
happened. A simple popular descriptive model,

is shown below Figure No= 5 to
Fig-5
Fig-6
illustrate the concept.
26th
Oct 2021

The barriers are also known as safeguards,
controls, safety measures or “Layers of
Protection”. Each barrier has a finite probability
of failure so multiple barriers are required to
reduce the risk of the event taking place to an
acceptable level. The barriers need to be fully
independent of each other or else they may be
subject to a form of common mode failure. The
barriers need to be maintained so that their
reliability does not decline. The barriers may be
of three major types. They may be hardware
(e.g. pressure relief valve), systems (e.g.
operating procedure), and people (e.g. training).
The ranking of vulnerable are shown
figure- 6,7
The value of strength of explosion is a function
of the amount of chemical released, the
characteristics of chemicals and the source of
Fig-7
The barriers are also known as safeguards,
controls, safety measures or “Layers of
finite probability
of failure so multiple barriers are required to
reduce the risk of the event taking place to an
acceptable level. The barriers need to be fully
independent of each other or else they may be
subject to a form of common mode failure. The
arriers need to be maintained so that their
reliability does not decline. The barriers may be
of three major types. They may be hardware
(e.g. pressure relief valve), systems (e.g.
operating procedure), and people (e.g. training).
are shown in the
The value of strength of explosion is a function
of the amount of chemical released, the
characteristics of chemicals and the source of
fire. The higher the amount of chemicals
released, the wider the area that has the potentia
to be flammable as well as the greater the
probability of chemical vapors released close to
the source of fire resulting in an explosion. This
type of chemical is also very important to
consider. Some types of chemicals are not
flammable yet some are ver
flammable.
Based on Table 4, it is known that DFEI value
for secondary reformer was 289.74. This value
indicates that the impact caused by the
fire/explosion secondary reformer is classified as
severe. Table 4 This value did not include
trauma treatment for employees, the ability to
rise the company and other non
factors.,
26th
Oct 2021
fire. The higher the amount of chemicals
wider the area that has the potential
to be flammable as well as the greater the
probability of chemical vapors released close to
the source of fire resulting in an explosion. This
type of chemical is also very important to
consider. Some types of chemicals are not
flammable yet some are very volatile and
, it is known that DFEI value
for secondary reformer was 289.74. This value
indicates that the impact caused by the
fire/explosion secondary reformer is classified as
value did not include
ma treatment for employees, the ability to
rise the company and other non-technical

Oct 2021
Fig-8
Urea Plants Risk analysis
All the Ammonia plants product uses in
Urea plant e.g. Ammonia, Carbon Dioxides ,
Hydrogen mixed in carbon Dioxides makes
explosive mixture with Oxygen which is
gives for passivation of stainless steel vessel
& equipments. The Inters are vents in MP
section in Ammonia stripping plants so the
MP section is very risky similarly tin CO2
stripping the HP scrubber is very risky for
explosion. Numbers of explosion are
recoded worldwide.
Fire /
Explosion
Risks
 Ammonia leaks from glands/ pump seals or flanged joints piping resulting in
formation explosive mixtures in air.
 Accumulation of H2 may take place in HP Section in case CO2 purity from
Ammonia Plant is not within allowable limits. Ignition of this accumulated H2
can occur due to dissipation of static charge.
High / Low
Temperature
Exposure Risk
 Burns due to contact with hot surfaces of pipelines, equipments, etc. or leaking
steam lines, process fluids at high temperature.
 Frost bite due to contact with anhydrous liquid ammonia at -33 deg. C
Burns due to contact.
Toxic
Chemicals
Exposure
Risks
 Asphyxia due to inhalation of simple Exposure risk asphyxiants like CO2, N2,
chemical asphyxiant and ammonia. Solution of Urea, Ammonium carbamate
and ammonium carbonate containing high NH3 content.
 Irritation due to inhalation of urea dust.
Corrosive /
Radioactive
Chemicals
Exposure
Risks
Severe burns, damage to eyes, skin and body tissues due to contact with anhydrous
ammonia, conc. Urea and Ammonium carbamate solutions
Table-5

Oct 2021
Fig-9
Major incident occurred in NFL Panipat
detail analysis
An incident of ammonia release occurred on
26th August 1992 in National Fertilizers Ltd.
Panipat (Haryana) India. On the morning of
August 26 some pressure drop was observed in
the discharge end of the ammonia pump
provided before the urea reactor. When search
was made it was observed that the safety valve
provided at the discharge end of the pump was
passing. On the instruction of the shift in charge
the stand-by pump was started and the pump C
was isolated. The pump was depressurized and
flushed. While the safety valve was being
replaced by the maintenance operator with his
helper under the foreman maintenance, the
operator of the urea plant and the shift in charge
of the urea plant, the isolation valve had failed
and the liquefied ammonia start releasing at the
pressure of 26 kg/cm2. The routine job of
replacing the defective safety valve of the
ammonia feed pump at 15 years old urea plant
and began to replacing the valve when the
unthinkable happened. Vaporizing within the
seconds to form suffocating clouds of the deadly
gas. This hit and choked to death eleven people
and injured the ten even as their colleagues
sprung into the action diffuse the gas with water
spray. The ammonia had released into the
atmosphere from the open port of the safety
valve in the form of the spray. Some person was
completely drenched with the liquefied
ammonia. The atmosphere was suddenly filled
with dense cloud of ammonia involving the
persons within it. The rescue and relief operation
were soon started but by the time the victims
taken to the factory hospital 11 persons had died
and 10 others sustained serious injuries. On the
day of accident also the valve had been
hammered to ensure total stoppage of the flow.
The maintenance team had brought another
safety valve. The task involved removing of the
existing valve and fitting another valve at its
place in order to bring the defective valve to the
workshop for repairing and testing. The
condition found after the incident that the
existing had been removed isolation valve had
failed and started releasing the ammonia gas
through the open port of the safety valve.
Immediately after that the area got covered with
foggy fumes. The information of the event
reached to all quickly and rescuers with
selfcontained breathing apparatus entered the
cloud for searching the trapped persons.
Emergency siren was raised, though no one

Oct 2021
acted as per the responsibility distribution in the
onsite emergency plan. As soon as the
information of the ammonia leakage spread in
the plant, the about 25 breathing sets were
brought from safety department to the site of
incident. About 21 breathing sets were used by
the persons involved in the rescue operation. The
fire services of the plant were involved in
spraying of water in the leakage zone. Keeping
the wind direction in view the areas towards
wind ward direction were being evaluated in the
plant. The urea plant was shut down by use of
emergency push button which closes the
activities with shortest possible period. The
nearest fire call point was broken to inform the
fire department about the event. The first fire
tender reached within 3-4 minutes after starting
the event. After that the order fire tender also
joined. There was need to stop the alternate
ammonia pump. The switch was in the area
where the cloud was dense. Nobody could
approach the valve. Then electric department
was asked to switch off the power from the main
supply to the plant. The person who were
rescued reach the factory hospital within 8-10
minutes. After the event has controlled the cause
of the valve failure was checked, it was found
that the valve collar of the globe valve had
broken allowing the spindle of the valve in
upward direction. The person engaged in the
rescue and control operations were also affected
by the ammonia exposure. They were first taken
to the company hospital. But next day such
people were admitted in the hospital. Cause of
valve Failure The subsequent accident
investigations indicated that the cause of the
incident was lifting of valve spindle due to
rupture of the collar of the valve. The careful
investigation of the broken pieces of the valve
indicated that about half of the collar had
developed damage some time back as it has
turned black, whereas the remaining part of the
broken surface was shining white. This indicated
that the crack had taken place some time back
due to repeated hammering of the lever. Lessons
learnt from the event the causes of the frequent
failure of safety valves should be identified and
necessary actions should be taken. The safety
valve and the isolation valve before the safety
valves should be marked with the identified
numbers. The isolation valve before the safety
valves should be locked open. The safety work
permit should be signed by the safety officer
duly countersigned by the issuing authority. The
permit signing authority should ensure that the
precaution indicated in the permit. The isolation
of the machine having toxic or flammable
substances should be dissolved in water by
suitable process and should be lagoon to release
slowly in the atmosphere the designer
instructions regarding type of valve to be used
should be strictly followed. In case the
deviations from the original designed are
required the manufacturer of the machines
should be consulted. Every proposal for change
the process should be critically analyzed by
HAZOP study. Globe valve should be tested for
any crack in the collar by dye penetration test or
other equivalent technique. Condition of the
thread should also be checked to not allow any
slip. The emergency plan should be made a
practical instrument for mitigate the effect of the
events. The escape route from pump platform
should be increased to promote the escape of the
person in case of such events. Provision of waste
spray in ammonia compressor area should be
considered. All the SCBAs should be equipped
with low pressure alarms and working in
continuous positive pressure mode. The SCBA
working only on demand mode should be
marked and levels that these will not use when
there is risk of live or health Quick modification
done in other units of NFL After the Panipat
incident, the motorized valve provided in other
units of NFL so that in case of incident this
ammonia can be protected to vented out as fig
No. 9

Oct 2021
Fig --10
Incident on dated 18/09/2018 failure of one
plunger packing of ammonia reciprocating pump
In NFL Vijaipur unit, one reciprocating plunger
packing failed on date 18/09/2018. A huge
amount dense ammonia cloud observed. The
ammonia pump has five plungers and discharge
pressure is about 250 bar. The plant was running
on full load at about 11.30 hrs. Ammonia
leakage was started due failure of 5th number
plunger. Immediately one stopped the plant and
closed the booster pump motorized suction valve
from control room to immediately control the
situation; this lesson one has learnt from the
Panipat incident. This motorized valve was
provided after Panipat incident. The manual
valve cannot be operated in field because huge
dense ammonia was there. In high pressure
pump house area a water curtains also provided
with control valve which can be operated from
control room. An ammonia sensor also provided
in each pump house and at 50 ppm of ammonia
the alarm appears on central control room.
Immediately water curtains control valve open to
control ammonia.

Oct 2021
Fig- 11

Fig-12
Hazard Identification
There are various modes in which flammable
and toxic chemicals can leak into atmosphere
causing adverse affects. It may be small leaks
from gaskets of the flanged joints, or guillotine
Sr.
No.
Failure Mode Probable Cause
1 Flange / Gasket failure Incorrect gasket Incorrect
installation
2 Weld failure It is normally due to poor quality
of welds
3 Pipe corrosion erosion
or failure due to stress
Sometimes fabrication or
installation leaves stress in the
pipes. Erosion or corrosion also is
sometimes the cause.
There are various modes in which flammable
and toxic chemicals can leak into atmosphere
causing adverse affects. It may be small leaks
from gaskets of the flanged joints, or guillotine
failure of a pipeline of even catastrophic
of the storage tank. Some typical modes of
failures and their possible causes are discussed
below , Table No-6
Probable Cause Remarks
Incorrect gasket Incorrect
installation
Attention to be paid during
selection and installation of
gaskets.
It is normally due to poor quality
of welds
Welding to be done by certified
welders with right quality of
welding rods. Inspection and
radiography must also be done.
Sometimes fabrication or
installation leaves stress in the
pipes. Erosion or corrosion also is
Pipes material of construction
should be selected correctly.
Design should take care of
erosion effects. And installation
of pipes should not leave any
stress
26th
Oct 2021
failure of a pipeline of even catastrophic failure
Some typical modes of
ible causes are discussed
Attention to be paid during
selection and installation of
Welding to be done by certified
welders with right quality of
welding rods. Inspection and
radiography must also be done.
Pipes material of construction
should be selected correctly.
Design should take care of
erosion effects. And installation
pipes should not leave any

Oct 2021
4 Over pressurization of
pipeline
Over pressurization can occur due
to failure of SRV or incorrect
operation.
Necessary procedures should be
there to prevent.
5 Deficient installation of
pipes
Pipes design and installation is
sometimes not as per appropriate
standard.
It must be ensured that
installation is as per correct
standards completely.
6 Leaks from valve Leaks from glands, bonnets or
failures valves spindle is
Right selection of valves and their
maintenance should be ensured.
7 Instruments failure Multifarious instruments are used
for control of process parameters.
Any such instrument failure can
cause mishap.
Reliability of instruments
working must be ensured through
proper selection and maintenance.
8 Failures of protective
system
Protective system like SRV,
bursting discs, vent header, drain
lines etc. are provided to take care
of abnormal conditions.
Reliability of protective system
must be ensured highest through
inspection and proper
maintenance.
9 Operational effort Plant operational parameters
should not be exceeded beyond
the permissible limits.
Operating procedures must be
complete and strictly followed.
10 Other failures There are external other reasons
causing the failures.
Design and operating philosophy
must consider all possible
reasons.
Table-6
Selected failure cases and likely consequences)
outlines the failure cases those selected for
facilities of onshore oil/gas production facilities
at Fertilizer Complex
Sr.
No.
Equipment Failure case Associated hazards
Ammonia Plant
1 NG at B/L Instrument tapping failure Flammable
2 NG Compressor Discharge Instrument tapping failure. Flammable
3 HTS Effluent Large hole in bottom Flammable/ Toxic
4 Converter effluent Catastrophic Failure Flammable/ Toxi
5 HP ammonia scrubber vapour Instrument tapping failure, Flammable/ Toxic
6
Process condensate
Instrument tapping failure Toxic
7 Large hole in bottom, Toxic
8 HP Ammonia scrubber liquid Large hole in bottom. Flammable/ Toxic
9 HP Ammonia scrubber liquid Large hole in bottom, Flammable/ Toxic
Urea Plants
1 Ammonia at B/L Instrument tapping failure Flammable/ Toxic
2 HP Ammonia pum 10 mm failure Flammable/ Toxic
3 Instrument tapping failure, Flammable/ Toxic
4 Reactor Large hole in bottom, Flammable/ Toxic
5 HP Carbamate pump Pump seal failure Flammable/ Toxic
6
Table-7

Oct 2021
Role of Oxygen and stainless steel
The optimum passivation is required if more
than optimum the chances of explosive
mixture and also ammonia losses, i.e. danger
for environment also. Flammable gas
mixtures and the consequences there of on
urea plant safety have been important issues
in the entire history of the urea process
industry. High chromium stainless steel owe
their high resistance to corrosion under
oxidizing condition due to the formation of a
surface-oxide film which is very adherent
and highly impervious; thus the metal is
protected from attack or we say it is passive.
However, if the oxidizing conditions are
lost, the metal is rapidly attacked. The
protective film once formed is not damaged
in normal course. The presence of sulphur
compounds damages this protective film. It
is essential that the protective film is not
damaged during operation and as such
continuous feeding of air has been
incorporated in our process. It was found
from experiments that SS 316L stainless
steel requires 5 ppm of 02 for passivation
and 2RE-69 stainless steel requires 3 ppm.
Although this value is low and in actual
practice as high as 6000 ppm of 02 is being
maintained. Material of higher chromium
content requires less oxygen to remain
passive than low chromium steels. This fact
also points to the suitability of using
stainless steel with a chromium content of
24.5% in HP stripper. Chromium slows
down and nickel accelerates the corrosion of
materials in the active state .This is best
illustrated by corrosion rates of a number of
materials in relation to their Ni contents. At
higher temperature and pressure and where
aggressive condition of urea Carbamate
mixture exists, stainless steel of higher
chromium and nickel contents are found to
be more effective against corrosion. At this
condition, the influence of Nickel is also
apparent. However, the corrosion rate of
2RE-69 stainless steel is about 5 times as
low as that of SS-316L which indicates that
the favorable influence of Chromium is
much larger than the unfavorable influence
of Nickel.
Since the liquid phase in urea synthesis
behaves as an electrolyte, the corrosion is of
an electrochemical nature. Stainless steel in
a corrosive medium owes its corrosion
resistance to the presence of a protective
oxide layer on the metal. As long as this
layer is intact, the metal corrodes at a very
low rate. Passive corrosion rates of
austenitic urea grade stainless steels are
generally between <0.01and (max.) 0.10
mm/a. Upon removal of the oxide layer,
activation and consequently, corrosion set in
unless the medium contains sufficient
oxygen or oxidation agent to build a new
layer. Active corrosion rates can reach
values of 50 mm/a. Stainless steel exposed
to Carbamate containing solutions involved
in urea synthesis can be kept in a passivation
(non corroding) state by a given quantity of
oxygen. If the oxygen content drops below
this limit, corrosion starts after some time
depending upon process conditions and the
quality of the passive layer. Hence, If the
CO2 after the elimination of the air used for
preventing corrosion is as following

Oct 2021
Table-8
The Composition of the vent gases assuming
that all ammonia added and CO2 has been
eliminated , analysis is as following.
Sr. No. Components Percentage
(Volume)
2 Hydrogen 22.6
3 Air 71,7
4 Others 5.7
Table-9
If we neglect others combustible , the
Hydrogen/Total combustible ratio must
equal as unity
𝟏 =
𝐇𝐲𝐝𝐫𝐨𝐠𝐞𝐧
𝐓𝐨𝐭𝐚𝐥 𝐂𝐨𝐦𝐛𝐮𝐬𝐭𝐢𝐛𝐥𝐞 𝐄𝐱𝐩𝐥𝐨𝐬𝐢𝐨𝐧 𝐌𝐢𝐱𝐭𝐮𝐫𝐞
Gas mixture inside the triangle is formed by
the base line and the lines labeled.
The mixture indicated by the star is laying
inside the above mentioned explosive area
and thus explosive as shown in the figure-
13
Sr. No. Components Percentage
(Volume)
1 Carbon Dioxide 94.7
2 Hydrogen 1.2
3 Air 3.8
4 Inerts Gases 0.3

Fig-13
In CO2 Stripping Gas analysis tabulated in Table
Analysis of HP Scrubber Gases
Categories Components
Inflamables
NH3
H2
CH4
C2H6
C3H8
Inertes
N2
Ar
CO2
Oxygen O2
O2+Ar
Total
Table -10
tabulated in Table No-10, and this is found in Explosive range.
Components [%(v/v)] Total % INDIVIDUAL
37.50
48.80
76.84
11.30 23.16
0.00 0.00
0.00 0.00
0.00 0.00
6.06
18.56
100.00
0.00
12.50
32.64 32.64
+Ar 0.00
100.00 100.00
26th
Oct 2021
is found in Explosive range.
% INDIVIDUAL
100.00

Brief Description of High P
The HP scrubber consisting following three
parts. As shown in the 1 6 & 17.
1. Blanketing sphere, which receive
gases coming from reactor.
2. A Heat exchanger part, which is
equipped with central down comer
through which degasified flow down.
A gas distributer with vortex is
installed in bottom.
3. A scrubbing part, in which the
remaining gases are scrubbed with
carbonate solution which is coming
Fig-14
Brief Description of High Pressure Scrubber
The HP scrubber consisting following three
Blanketing sphere, which receive the
gases coming from reactor.
A Heat exchanger part, which is
equipped with central down comer
through which degasified flow down.
A gas distributer with vortex is
A scrubbing part, in which the
remaining gases are scrubbed with
nate solution which is coming
from LP section and where the NH
and CO2 are almost tottaly
condensed.
The synthesis loop is provided with a central
drain line connecting all the
outlet of the HP heat Exchanger then to the
central drain line. The central drain line and
other parts of the HP synthesis section are
connected to the HP flush water source, so
as to enable them to be flushed.
26th
Oct 2021
from LP section and where the NH3
are almost tottaly
The synthesis loop is provided with a central
drain line connecting all the HP vessel to the
outlet of the HP heat Exchanger then to the
central drain line. The central drain line and
other parts of the HP synthesis section are
connected to the HP flush water source, so
as to enable them to be flushed.

Oct 2021
In case that there are some NH3 and CO2 according to the following compositions
Sr.
No.
Components Percentage
(Volume)
1 Hydrogen 11.3
2 Air 35.9
3 Other 2.8
4 Ammonia 37.5
5 Carbon
Dioxide
12.5
Table-11
Then The Hydrogen/Total combustibles are
𝟎. 𝟐𝟑 =
𝐇𝐲𝐝𝐫𝐨𝐠𝐞𝐧
𝐇𝐲𝐝𝐫𝐨𝐠𝐞𝐧 + 𝐀𝐦𝐦𝐨𝐧𝐢𝐚
The above mixture gas mixture will be if it
is situated inside the triangle formed by the
0.23 explosion limits and the base line.
In the operating point marked total
combustibles 48.8%, air 35.9%, this point is
outside the explosion area as shown in the
figure-15
Components [%(v/v)] Total % INDIVIDUAL
Inflammables
NH3 18 49 36.73
H2 31 63.27
CH4 0 0
C2H6 0 0
C3H8 0 0
Inertes N2 28 42 100
Ar 0
CO2 14
Oxygen O2 9 9
O2+Ar 0
Total 100 100
Table-12

Oct 2021
Fig-15
Calculation for Explosively
The hydrogen present in Carbon Dioxide,
hence the composition of vent gases
calculated by carbon Dioxide given to
Stripper.
Carbon Dioxide to HP stripper=37500
Nm3
/hr
Composition of Carbon Dioxide
CO2 =95%
Hydrogen=1.1%
Air=3.6%
Noncombustible material=0.3%
Vented Gas D/S HP Scrubber=3000
Nm3/Hr
Vented Hydrogen=412 Nm3/hr
Vented Air=1350 Nm3/hr
Non Combustible Gases=112 Nm3/hr
Total (Hydrogen +Air +
Noncombustible)=1875Nm3/Hr
Ammonia & Carbon Dioxide in Vent
Gas=3000-1875=1125 Nm3/hr
Then the Vent gas from HP Scrubber

1. Hydrogen=(412*100/3000=13.75%
2. Air=(1350*100/3000)=45%
3. Non
Combustible=112.5*100/3000=3.75
%
4. Carbon Dioxide
37500*0.05=187.5Nm3/hr=281*100/
3000=9.37%
So according to the above figure the point 32.85 is an Explosive mixture
Fig-16
Hydrogen=(412*100/3000=13.75%
Air=(1350*100/3000)=45%
Combustible=112.5*100/3000=3.75
Carbon Dioxide-
37500*0.05=187.5Nm3/hr=281*100/
5. Ammonia=1125-
187.5=937.5Nm3/Hr=937.5*100/300
0=28.13%
6. Combustible Gases are Hydrogen
and Ammonia, so the=
7. 13.75/41.875 = Hydrogen
(Hydrogen + Ammonia
32.8%
So according to the above figure the point 32.85 is an Explosive mixture
26th
Oct 2021
187.5=937.5Nm3/Hr=937.5*100/300
ases are Hydrogen
and Ammonia, so the=
Hydrogen/
Ammonia)

Fig-17
26th
Oct 2021

Calculation for Ammonia Stripping
Process
Where mixtures of two or more
flammable gases are encountered, the
limits of flammability of the mixture
Gases
Explosion Limits
in Air Limits in O
Lower Upper Lower
CH4 5 15 5.5
H2 4 74 4.5
CO 12.5 74 15.5
NH3 15.5 27 13.5
Table-13
The oxygen concentration in the Carbon
dioxide "range from 0.1 to 0.8% of oxygen.
Let ns examine the two above mentioned
concentrations as examples. Normal
compositions of carbon dioxide feed gas at
the battery limits of the urea plant are:
C02- 99.0 % (mole)
H2- 0.6 % (mole)
N2- 0.4 % (mole) including Argon etc.
Example 1_
This mixture is within the explosion limits
as can be seen from figure-18
All of the carbon dioxide gas mixtures with
oxygen concentrations between 0.1 and 0.8
Ammonia Stripping
Where mixtures of two or more
flammable gases are encountered, the
limits of flammability of the mixture
can often be reliably predicted by
using the following formulas
suggested by Le Chatelier:
Where: P1 . . . Pn = volume fractions of
components 1, 2, 3, . . . , n of the mixture
LFL1… LFLn = lower flammable limits of
components 1, 2, 3, .. n of the mixture
UFL1…. UFLn = upper flammable limits of
EXPLOSIVE LIMITS
Explosion
Limits in O2 Minimum O2 Content
Flammable
Lower Upper
5.5 60 12.25
4.5 94 5
15.5 94 6
13.5 79 -
The oxygen concentration in the Carbon
dioxide "range from 0.1 to 0.8% of oxygen.
Let ns examine the two above mentioned
concentrations as examples. Normal
compositions of carbon dioxide feed gas at
the battery limits of the urea plant are:
0.4 % (mole) including Argon etc.
If we need 0.1 % of oxygen in this gas
mixture and we feed this as air the final
composition of the non condensable
components becomes;
Hydrogen 0.6 moles
Nitrogen 0.4 moles
Air 0.1 moles O
+0.4moles N2
Total 1.5 moles
Table-14
This mixture is within the explosion limits
All of the carbon dioxide gas mixtures with
oxygen concentrations between 0.1 and 0.8
% mole will be within the explosive limits
We may thus conclude that each high
efficiency urea process
26th
Oct 2021
can often be reliably predicted by
using the following formulas
suggested by Le Chatelier:
Where: P1 . . . Pn = volume fractions of
components 1, 2, 3, . . . , n of the mixture
LFL1… LFLn = lower flammable limits of
UFL1…. UFLn = upper flammable limits of
LIMITS
Minimum
Flammable
6
4.3
13.75
-
If we need 0.1 % of oxygen in this gas
mixture and we feed this as air the final
composition of the non condensable
40% moles
53% moles
0.1 moles O2 7.0 % moles
100% moles
% mole will be within the explosive limits-.
We may thus conclude that each high
efficiency urea process

Fig-18
will end up with a hazardous gas mixture,
unless provisions are being made to remove
the hydrogen from carbon dioxide and if the
ammonia contains hydrogen, to remove this
hydrogen as well.There are many plants,
running during normal operation or upset
conditions, emitting gas mixtures within the
explosion limits» k great many plants
however have never experienced explo
In those plants all of the conditions required
for an explosion (an explosive gas mixture
and an ignition source simultaneously) did
will end up with a hazardous gas mixture,
unless provisions are being made to remove
de and if the
ammonia contains hydrogen, to remove this
hydrogen as well.There are many plants,
running during normal operation or upset
conditions, emitting gas mixtures within the
explosion limits» k great many plants
however have never experienced explosion.
In those plants all of the conditions required
for an explosion (an explosive gas mixture
and an ignition source simultaneously) did
not occur so far. The potential danger is
there however as practice has shown. In a
few plants Stamicarbon has experi
explosions in the 18 ata purification step of
conventional urea plants as well as in the
140 ata purification step in CO
urea plants in HP scrubber section.
Gas sample analysis
Gas Flow -524.5 NM3/Hr,
CH4-6.78%, Hydrogen 7.45, Ammonia
18.45, Inerts (N2+O2)-67.32,
26th
Oct 2021
not occur so far. The potential danger is
there however as practice has shown. In a
few plants Stamicarbon has experienced
explosions in the 18 ata purification step of
conventional urea plants as well as in the
140 ata purification step in CO2 stripping
urea plants in HP scrubber section.
6.78%, Hydrogen 7.45, Ammonia
67.32,

Oct 2021

Oct 2021
Fig-19
Conclusion
The fundamental method for prevention of vapor
explosion is how to reduce the possibility of
leakage. Vapor explosion is an instantaneous
process of energy release. Once the vapor
explosion takes place, the catastrophic accident
becomes inevitable due to the high speed of
phase transformation. A leakage detection
system should be developed due to impurities in
steam-damaged steels.
The fundamental method for prevention of vapor
explosion is how to reduce the possibility of
leakage. Vapor explosion is an instantaneous
process of energy release. Once the vapor
explosion takes place, the catastrophic accident
becomes inevitable due to the high speed of
phase transformation. A leakage detection
system should be developed due to impurities in
steam-damaged steels. The barrier of having an
effective start-up procedure was totally
ineffective as this not apply to the specific
conditions applicable on the day. Failure of
people undermined the entire risk management
process. Whilst people represent a source of
failure they also are the last barrier to detect
problems and save the day. Safety Model of a

Oct 2021
hazard protected by a series of protective
barriers is not understood by the Fertilizers and
support personnel.
Engineering control that should be conducted
include: periodic maintenance of the gas detector
and provide cleaning tools to clean the oil spills.
In addition, administrative control also need to
be conducted, such as: posting information about
the national Fire protection association (NFPA)
rating of Chemicals which will eventually help
minimize the risk.
References
1. Book “Fertilizers Technology pure
knowledge” by Prem Baboo
published in Notion Press-2021.
2. Section of material in fertilizers
Industries by Prem Baboo,
published in Global scitific journal,
Vol-9 issue -1, January-2021.
3. Operational Experience of De-
hydrogen Reactor at GFGL by
Ashok Agrawal, published in Indian
Journal of Fertilizers,Vol-2, Dec-
2006.
4. Probabilistic risk assessment of
Fertilizers Plants by R.S
olaniya,H.N. Mathurkar,A.W.
deshpandey published in Indian
Journal of chemical Technology,
Vol-3, March 1996.
5. Guidelines for integrated risk
assessment and management in large
industrial areas Jan-98.
Legends
PHA (Process Hazard Analysis), HP-High
Pressure, LS-Low pressure steam, MP- medium
pressure, LP-Low pressure.
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