Analysis and Simulation of Severe Accidents in a Steam Methane Reforming Plant

Abstract

Severe accidents of process industries in Iran have increased significantly in recent decade. This study quantitatively analyzes the hazards of severe accidents imposed on people, equipment and building by a hydrogen production facility. A hazard identification method was applied. Then a consequence simulation was carried out using PHAST 6.54 software package and at the end, consequence evaluation was carried out based on the best-known and different criteria. Most hazardous jet fire and flash fire will be occurred in desulfurization and reformer units respectively. The most dangerous vapor cloud explosion will be caused by a rupture in desorfurizing reactor. This incident with an overpressure of 0.83 bars at a distance of 45 m will kill all people and will destroy all buildings and equipments that are located at this distance. The safety distance determined by TNO Multi-Energy model and according to the worst consequence is equal to 260 m. Vapor cloud explosion will have the longest harmful distance on both human and equipment compared to jet fire and flash fire. Atmospheric condition will have a significant influence on harmful distance, especially in vapor cloud explosion. Therefore, the hydrogen production by natural gas reforming is a high-risk process and should always be accompanied by the full implementation of the safety rules, personal protection and equipment fireproofing and building blast proofing against jet fire and explosions.

Full text

2008-5435/14/63-120-130
INTERNATIONAL JOURNAL OF OCCUPATIONAL HYGIENE
Copyright © 2014 by Ir nian Occupational Health Association (IOHA) a
IJOH 6: 120-130
,
2014
* Corresponding author: Esmaeil Zarei, Email:
E.Zarei
@
umsha.ac.i
r
O
OR
RI
IG
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IN
NA
AL
L
A
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Analysis and Simulation of Severe Accidents in a
Steam Methane Reforming Plant
MOHAMMAD JAVAD JAFARI1, IRAJ MOHAMMADFAM2, and ESMAEIL ZAREI*2
1Department of Occupational Health Engineering, Faculty of Health, Shahid Beheshti University of Medical Sciences,
Tehran, Iran; 2Department of Occupational Health Engineering, School of Public Health and Research Center for
Health
Sciences, Hamadan University of Medical Sciences, Hamadan, Iran.
Received March 21, 2014; Revised May 3, 2014; Accepted May 28, 2014
This paper is available on-line at http://ijoh.tums.ac.ir
ABSTRACT
Severe accidents of process industries in Iran have increased significantly in recent decade. This study
quantitatively analyzes the hazards of severe accidents imposed on people, equipment and building by a
hydrogen production facility. A hazard identification method was applied. Then a consequence simulation
was carried out using PHAST 6.54 software package and at the end, consequence evaluation was
carried out based on the best-known and different criteria. Most hazardous jet fire and flash fire will be
occurred in desulfurization and reformer units respectively. The most dangerous vapor cloud explosion
will be caused by a rupture in desorfurizing reactor. This incident with an overpressure of 0.83 bars at a
distance of 45 m will kill all people and will destroy all buildings and equipments that are located at this
distance. The safety distance determined by TNO Multi-Energy model and according to the worst
consequence is equal to 260 m. Vapor cloud explosion will have the longest harmful distance on both
human and equipment compared to jet fire and flash fire. Atmospheric condition will have a significant
influence on harmful distance, especially in vapor cloud explosion. Therefore, the hydrogen production by
natural gas reforming is a high-risk process and should always be accompanied by the full
implementation of the safety rules, personal protection and equipment fireproofing and building blast
proofing against jet fire and explosions.
Keywords: Hydrogen, Accident Prevention, Chemical Hazard Release, Fires, Explosions
INTRODUCTION
The fast progress of hydrogen technologies and vast
investment on its production, storage and transportation
are accelerating the early transfer to a hydrogen
economy [ 1 , 2 ]. Severe accidents involving hydrogen
utilized in industries as well as in other applications in
the past [ 1 , 3 - 6 ], essentially requires a high level of
safety in hydrogen facilities for preventing such
accidents in the future.
The level of precautions that have been taken or
should be improved to prevent fatality, injury and
destruction of probable accidents in hydrogen process
industries need to be assessed using a reliable technique.
Different techniques have been introduced for such a
purpose. They include qualitative, semi-quantitative and
quantitative methods. Among them, consequence
analysis is a quantitative method that can be used to
assess the hazardous consequences of the accidents in
process industry [ 7 ].
Consequence analysis is an integral part of a risk
assessment process, which gives an estimation of the
damages that a probable accident may bring to the
properties and human beings. This method enables not
only safer design of a hydrogen infrastructure but also
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Analysis and Simulation of Severe Accidents in a Steam Methane Reforming Plant ijoh.tums.ac.ir | 121
Published online: July 9, 2014
early adoption of hydrogen technologies, eliminating
unnecessary additional costs to deploy them [ 7 ]. The
trend towards larger and more complex units has
brought about the need for consequence analysis of
hydrogen process plants.
In the process of consequence analysis, the
consequence modeling is the most important part and
has four steps. The first step is source models, which
provide how materials are discharged from the process.
The source models provide necessary data to describe
the rate, total quantity and the state of discharge. The
state of material discharged from the process may be
liquid, vapor or slash (a combination). In the second
step, dispersion models are subsequently used to
provide how the material is transported downwind and
dispersed to some concentration levels. The third step is
the modeling of the predictable incident outcomes. The
incidents include jet fire, flash fire and vapor cloud
explosion (VCE). In its final step, the application of
these results along with appropriate probit models is
used to evaluate the effects of the studied scenarios on
the exposed environment and human beings [ 7 , 8 ].
In the process of hydrogen production through
Steam Methane Reforming (SMR) in large scales, the
presence of highly explosive and flammable materials
such as methane and hydrogen along with high purity in
large volumes can potentially cause large-scale
incidents that may harm humans, properties or the
environment. To consider these perspectives, the
consequence analysis method applied should identify
and evaluate the hazardous points and incidents of the
SMR plant. Not many consequence analyses have been
applied to the hydrogen production facilities. In 2010,
Zhiyong et al studied the harmful distances of a gaseous
hydrogen refueling station [ 9 ]. The gaseous hydrogen
refueling station seems to have less severe
consequences than hydrogen generators that use natural
gas reforming process.
Process description
In this large plant, the hydrogen is produced using
natural gas reforming method. The process is based on
the catalytic endothermic conversion of methane to give
hydrogen and carbon monoxide. Carbon monoxide is
then converted to carbon dioxide. Finally, hydrogen is
purified by separation (Fig. 1). More details may be
found in Zarei (2012) [ 10] and Jafari et al. (2012) [ 11].
The hydrogen generator with 65 m in length and 25
m in width is located in an industrial plant with 490 m
length and 360 m width. Five vulnerable targets
neighboring the hydrogen plant including workers in
packaging industries, customs warehouses, Paxan Co
and IAC center as well as the vehicles passing the
highway will be exposed to the proposed accidents of
this hydrogen generation facility. In addition, there are
several potential vulnerable targets inside the plant, such
as large vegetable oil storage tanks, office buildings,
vegetable oil transport train, natural gas transferring
pipeline and central restaurant.
The objective of present study was the
comprehensive and quantitative consequence analysis of
severe accident on a steam methane reforming plant in
Tehran.
MATERIALS AND METHODS
The consequence analysis scheme followed in
present study involves four steps [ 7 ] shown in Fig. 2.
They are described in the following.
Identification of hazards and selection of scenarios
The identification of vulnerable areas and specific
hazards is of fundamental importance in consequence
analysis. Different methods are required at different
Pure H
2
C
ESD
Desul
p
hriz
E
Reformer
R
E
E
E
ESD
B
H
D
Purificatio
n
ESD
Purge gas buffer
Purificatio
n
Purificatio
n
Purificatio
n
Feed gas
ESD
Fig 1. The block diagram of hydrogen generation process by steam methane reforming

122 | IJOH | July 2014 | Vol. 6 | No. 3 Jafari et al.
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stages of a project to identify hazards. One of the first
systematic methods of hazard identification used in
chemical industry is HAZID method [ 12]. Scenarios
begin with an incident, which usually result in the
release of containment of material from the process.
Typical incidents might include rupture, break of a
pipeline and a hole in a reactor or pipe [ 12]. For this
purpose, all necessary information for hazard
identification was collected from the production
process. The process hazards were identified by
application of HAZID technique. Finally, after
screening low consequence scenarios the most credible
ones in the selected hydrogen plant were determined as
summarized in Table 1. The pipe diameters used in this
plant were from 150 to 300 mm. On this basis, all
scenarios were categorized in three groups including
small (5 mm) holes, medium (30 mm) holes and Full-
bore rupture (300 mm). A total of 15 scenarios were
modeled and their consequences were quantitatively
assessed based on this categorization. In this study, the
likelihood of these events happening was not
considered.
Consequence simulation
The consequence modeling input data and
assumptions shown in Table 2 were used for the
consequence modeling of the hydrogen generation
facility. The data in Table 2 along with those in Table 1
describe the rate, total quantity and the state of
discharged material in each scenario. The consequence
models employed in the study are those of the Process
Hazard Analysis Software Tool (PHAST) developed by
DNV. PHAST is professional software used for
consequence modeling in chemical process risk
assessments [ 3 , 10- 11]. This software was specifically
validated for the release of hydrogen [ 14].
The representative atmospheric conditions in present
study comprise average wind speed, atmospheric
stability, ambient temperature and humidity. All of the
credible scenarios were modeled in two different
atmospheric conditions corresponding to day (spring-
summer, D5) and night (fall-winter, F2), (Table 3).
Jet fires, flash fires (VCF) and vapor cloud
explosions (VCE) were considered as the major
outcomes of incidents in a hydrogen generation facility.
Table 1. Credible scenarios and their mass flow rate in studied plant
Location Scenario No. Leak size (mm) Mass flow rate (kg/s)
S-1 5 0.06
S-2 30 0.25
Desulfurization reactor S-3 300 225
S-4 5 0.05
S-5 30 0.20
Heat exchanger S-6 300 183
S-7 5 0.07
S-8 30 0.30 Reformer (furnace) S-9 300 268
S-10 5 0.02
S-11 30 0.07
Hydrogen purification
absorbers S-12 300 59
S-13 5 0.02
S-14 30 0.07 Purge gas buffer S-15 300 62
Fig 2. Flow diagram of the procedure used for consequence analysis [ 7 ]

Analysis and Simulation of Severe Accidents in a Steam Methane Reforming Plant ijoh.tums.ac.ir | 123
Published online: July 9, 2014

Table 2. Consequence modeling input data and assumptions
Process condition
Scenario location P(bar) T(°C) Material
composition Molar% Mixture LFL (ppm)
Desulfurization reactor 25 200 NG
CO2
H2
85
0.1
5
40211
Heat exchanger 27 530 NG
H2O
H2
52
46
2
73711
Reformer (furnace) 35 300 CH4
H2
CO2
H2O2
CO
5
60
8
25
2
61124
Hydrogen purification absorbers
(HPA) 15 40 H2 99.99 81517
Purge gas buffer 4 35 CH4
H2
CO2
CO
12
34
40
13
40000
The best-known models were used to estimate the
effects of these outcomes (Table 4) [ 7 - 8 ].
Finally, in last stage by using appropriate probit
models and estimating the population distribution the
number of fatalities was estimated. In a jet and flash
fire, the fatality is caused by the radiation intensity
while in VCE it is caused by the overpressure. For
estimating the number of fatalities from jet fires and
VCEs, probit models were applied but in flash fires it
was assumed that all people exposed to low
flammability limit will die [ 12- 13].
Effect models convert these incident’s specific
results into effects on people (injury or probability of
death) and structures. Probit equations are commonly
used to quantify the expected rate of fatalities of jet fire
for the exposed population. These equations expressed
as [ 7 , 15]:

3/4
21
21 .,56.2,9.14 1)( qtVKK VLnKKp  
Where p is the probit variable, K1 and K2 are
constants and V represents the dose of hazard
(radiation), t is exposure time (sec) that was assumed 20
s for this case and q is radiation power (kW/m2).
One of the most commonly used probit models
which determines the fatalities of outdoor persons from
the blast overpressure is the Hurst, Nussey and Pape
(1989) probit model [ 7 , 16]. The relationship of this
probit variable is generally quoted as:

essureLnp Pr35.147.1  (2)
Where: pressure is in psi. A useful expression for
converting of probit variable (p) to probability of
fatality (P) is given by [ 15]:


















 2
5
5
5
15.0 p
erf
p
p
P (3)
In the case of Flash Fire, the above equation has
only two values of 1 and 0 for the areas in which gas
concentration is above and below flammable
concentrations respectively. Combining the above
equation and population distribution data will give the
number of fatalities in all incident outcomes by using
the following relationship [ 8 , 15]:
 AdAPN (4)
Where N is the number of fatalities, P is uniform
population distribution and A is the area affected by the
incident. In this study, the probability of fatalities was
considered to be 1 in these equations, In other words,
only the area where the probability of death is 1 was
considered.
Consequence assessment
Jet fire
The thermal consequences of Jet fires were assessed
using the radiation intensity of each jet fire from table 5
[ 7 - 8 ].
Flash fire
The consequences of flash fires were determined
using their Lower Flammable Level (LFL) as the
following [ 7 , 12].
 LFL zone: People who are in direct contact
with the flames will die.

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Table 3. Atmospheric conditions corresponding to day and night
Atmospheric parameter Day Night
Wind velocity (m/s) 5 2
Atmospheric stability class D F
Ambient temperature (°C) 28.33 2.77
Relative humidity (%) 19.35 67.27
Table 4. Models used for estimating the harm effect of different incident outcomes [ 7 ]
Incident outcome Model
Flash Fire Eisenberg, Lynch and Breeding model (vulnerability model)
Jet Fire Cone model
VCE TNO Multi-Energy model
Table 5. Effects of thermal radiation from Jet fires (duration 20s) [ 7 , 16]
Radiation intensity (kW/m2) Observed effect
4 Sufficient to cause pain to personnel if unable to reach cover within 20s. However second degree
burns is likely; 0% lethality
12.5 Minimum energy required for piloted ignition of wood, melting of plastic tubing
37.5 Sufficient to cause damage to process equipment
 ½LFL zone: People who are in this zone will
suffer from inhalation effects and diseases
Vapor cloud explosion
The consequences of VCEs were determined using
the overpressure intensity of each VCE according to the
following [ 3 ]:
Persons indoors
The purpose of this model is to determine the fatality
probability of the occupants of buildings subject to blast
loading. This is dependent on the level of blast loading,
the type and construction of the building. The Center for
Chemical Process Safety (CCPS) has published
relationships between the probability of fatality for
occupants and the level of blast overpressure for 5
different types of building [ 17- 18]. In this study, only
primary injury due directly to the blast wave
overpressure was analyzed.
Property damage
This will enable authorities to take the economic
risks to the properties, structures and businesses into
account as part of any land use planning decision.
Explosion overpressure level and its’ damage effect on
structures and corresponding to fatality are shown in
Table 6 [ 16, 18].
RESULTS
Consequence modeling revealed that the main
hazards of hydrogen generation by natural gas
reforming are the vapor cloud explosion (VCE), jet fire
and flash fire, which are mainly due to the physical and
chemical specifications of hydrogen and other material
involved in hydrogen generation cycle. Therefore,
consequences of VCE, flash fires and jet fires for
different scenarios were modeled. The results showed
that the VCE, flash fire and jet fire caused by small and
medium holes size (e.g. 5 & 30 mm holes) would not
have any fatality in day and night. Therefore, the
consequence evaluation results of these scenarios have
not been shown in Table 7. This table shows the
fatality of VCE, jet fire and flash fire caused by a full-
bore rupture at studied units in day and at night.
According to the results, a jet fire and VCE set by a
full-bore rupture at desulfurization reactor will have the
highest fatality of 26 persons among all scenarios. A
flash fire set by a full-bore rupture at reformer will have
the highest fatality of 8 persons in day. The VCE caused
by a full-bore rupture at desulfurization reactor would
have the highest fatality of five persons. The fatality of

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Table 6. Explosion overpressure level and damage effects on structure and people
Pressure (bar) Description of Damage Fatality Outdoor (%) Fatality Indoor (%)
0.01 *Safe Distance - -
0.17 Moderate Damage - 5
0.34 Severe Damage 15 50
0.83 Total Destruction 50 100
*Threshold for glass breakage
Table 7. Lethality of accidents from hydrogen generation facility in day and at night
Jet Fire Flash Fire VCE Consequence
Scenario Location Day Night Day Night Day Night
Desulfurization Reactor 26 10 6 2 5 2
Heat Exchanger 14 6 2 1 2 0
Reformer 15 6 8 3 3 0
Purification Absorbers 20 6 3 1 4 1
Purge Gas Buffer 1 0 0 0 0 0
all incidents caused by a full-bore rupture at night
would be less than them in day (Table 7). Jet fire
Flash fire
In the case of VCF (flash fire) only two values of 1
and 0 are considered for the areas in which gas
concentration is above or below the flammable
concentrations respectively. When the flammable gas
reaches to a source of ignition, there will be flash fire.
Flash fire flames will cause extreme damages to the
equipment’s as well as serious injuries to the
employees. In its worst case, especially at the
maintenance time, it can claim lives. People within the
flash fire envelope (the lower flammable limit, LFL)
will be killed because of extremely radiation doses [ 7 ].
Flash fire effect zone diagrams show that there are
flammable concentrations of material in the plant area.
The analysis of radiated distance of flash fire on
equipment and people showed that the worse case is
related to full-bore rupture of reformer and
desulfurization reactor respectively (Fig. 3). The most
hazardous flash fire will occur in the reformer unit. In
this scenario, the concentration of the material released
in LFL zone (area of 1505 m2) will be from 61125 ppm
down to 40000 ppm as it goes further from the incident
point. The concentration of the material released in this
scenario is high enough to kill all the people (8 people)
in the area (Table 7).
The radiated distance of different intensities from a
jet fire caused by a full-bore rupture at studied units is
shown in Fig. 4. The longest radiated distance of
different intensities belongs to a jet fire caused by a full-
bore rupture in desulfurization reactor. The results show
that if a jet fire is set by a full-bore rupture in
desulfurization reactor then the radiation level at a
distance of 135m will be high enough to cause damage
to process equipment. This is the highest harm full
distance, which has enough radiation intensity to
destroy all equipment in this area.
Vapor cloud explosion
The distance imposed by different levels of
overpressure from a VCE caused by a full-bore rupture
in different units is shown in Fig. 5. The results showed
that desulfurization reactor would impose the largest
area to different overpressures than other units. In case
of a VCE caused by a full-bore rupture in
desulfurization reactor, the safe distance from the
rupture will be 260 m at nights and 250 m during the
days. This distance is safe for other scenarios studied in
present work. The shortest distance imposed by
different levels of overpressure belongs to the VCEs
caused by a full-bore rupture in purge gas buffer.

126 | IJOH | July 2014 | Vol. 6 | No. 3 Jafari et al.
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Fig 3. Distances imposed by flash fire in day and night at different units
Average Harmful Distance
A further analysis was performed based on IGC
harmful criteria to study the harmful distances for the
people and the equipment at different parts of the
hydrogen generation facility. For this purpose, average
harmful distance of three groups of leaks (Table 1) from
different units of hydrogen generation facility was
considered. The results revealed that a VCE caused by a
full bore rupture at desulfurization unit will lead to the
longest average harmful distance both the people (160
m, Fig. 6) and for the equipment (123 m, Fig. 7).
Reformer unit will have the longest harmful distances
for the people and the equipment among all flash fires
caused by a full-bore rupture at different studied units.
Desulfurization unit will have the longest harmful
distances both the people and the equipment when a jet
fire is set by a full bore rupture at different units. The
results showed that all incidents of VCE, flash and jet
fires at all studied units except a flash fire in purge gas
buffer will harm all facilities located in the hydrogen
generation plant’s boundary limit.
The studied hydrogen generation facility was
located in an industrial complex with total area of
176400 m2 and a total number of 1200 workers that
800 of them were working in day and 400 at night. The
average population distribution was 5 and 2.5 persons
per 1000 square meter in the day and night respectively.
The term “Safety distance” will be used in this study to
show the distance from the leaking point that will be
safe. The safety distance was determined according to
worst-case consequence. This indicates that the worst
case may be used as a decisive consequence to
determine the safe distances for hydrogen generation
plant. Worst-case consequence is a VCE caused by a
full-bore rupture at desulfurization reactor. The safety
distance was determined based on IGC criteria (harmful
exposure threshold value to people and equipment e.g.
0.07 bar (160 m, Fig 6) & 0.2 bar (123 m, Fig. 7) of
overpressure respectively) as well as Health and Safety
Authority criteria (0.01bar, Threshold for glass
breakage, Table 5).
Safety distance of hydrogen generation facility is
equal to 260 m according to Fig. 3. This means that the
distance of hydrogen unit’s boundary limit to 0.01bar
overpressure contour is equal to 260 m. Any activity or
construction of any new unit is only allowed further
than this interval. This distance covers not only the
studied hydrogen generation plant but also neighboring
premises.
DISCUSSION
Flash Fire
The results of present study showed that the most
dangerous flash fire will occur in case of a full bore
rupture in reformer unit. In this incident the
concentration of the material released in LFL zone (in
an area of 1505 m2) will be from 61125 ppm down to
40000 ppm and enough to kill all the exposed people (8
persons) in the area. Gas detectors installed in the
hydrogen generation facility will be able to detect the
release of the gases, alarming the operators to take
appropriate actions prior to any combustion. High
process operating temperature (300 OC), the highest
operating pressure (35 bar), high molar rates of
flammable gases (60% H2, 5% CH4 and 2% CO) in
reformer unit and large release hole size in studied
scenario are the main reasons for having the most
dangerous flash fires in this unit.
According to the guidelines for chemical process
quantitative risk analysis issued by AIChE [ 7 ] the

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Fig. 4. Thermally radiated distance from jet fires set by a full-bore rupture at studied units
Fig. 5. Distances imposed by different overpressure levels from VCEs caused by a full-bore rupture
probability of death outside the LFL zone is very low
but flash fire is usually widely distributed and can
reduce oxygen in the environment thus causing
inhalation effects [ 7 ]. The analysis of radiation imposed
on the equipment and people showed that the flash fires
caused by a full bore rupture in reformer and
desulfurization reactor will affect the longest distances
of 182 m and 158 m respectively (Fig. 3).
A valid comparison between the results of flash fire
simulations in different meteorological conditions
conducted by Yousefzadegan et al. in filter separators
installed in gas pressure reduction stations showed that
the flammable concentrations of natural gas will
encompass the larger area in hot weather [ 19] which is
almost consistent with the present study (Fig. 3).
Zhiyong et al. estimated that the harmful distance
due to a flash fire of a hydrogen refueling station would
be 47 m [ 9 ], which is far lower than the results of in
present study. This could be due to large release leak
size in present study (300 mm) compared to Zhiyong
study (15mm).
Jet Fire
Results showed that the jet fire caused by a full-bore
rupture in desulfurization reactor has the highest
lethality (26 people) compared to the similar accident in
other studied units. The jet fire set by a full-bore rupture
in desulfurization reactor will also harm the largest area

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Fig 6. Average harmful distance to people in different incidents caused by a full bore rupture at different Units
Fig 7. Average harmful distance for equipment in different incidents caused by a full-bore rupture at different units
as far as 249 m at night and 242 m during the day (Fig.
4). The longest distance imposed by 37.5 kw/m2
radiation is 28 m [ 7 ]. In the present study, even the
purge gas buffer (with the shortest radiated distance) has
a longer (42 m at night and 44m during the day) harmful
distance for 37.5kw/m2 radiation. The molar rates of
different gases in this unit (e. g. 34% of H2, 12% of CH4
and 13% of CO) is the likely reason for longer harmful
distances compared to the gas refueling station studied
by Zhiyon et al.
According to the results maximum radiation from
the worst jet fire may get up to 350 kw/m2 in warm
weather (e.g. spring & summer) and 370 kw/m2 in cold
weather (e.g. fall & winter), that are much higher than
the sufficient level to cause damage to process
equipment (e.g. 37.5 kw/m2, Table 3), harmful exposure
threshold value to the people (e.g. 9.7 kw/m2) [ 20] and
safe limit of radiation flux (e.g. 0.139 w/cm2) [ 21].
Results showed that the jet fire travels further
(except for desulfurization reactor, Fig 4) in windy
condition (daytime) rather than calm condition
(nighttime), which is consistent to the results of
Zhiyong et al. [ 9 ].
Vapor cloud explosion
Considering the results of VCEs, it may be
concluded that the explosions of vapors in

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Published online: July 9, 2014
desulfurization reactor set by a full-bore rupture is the
worst scenario. This incident has the most lethality rate
(5 people, Table 7) and influences the largest area (Fig.
5). The worst case may be used as a decisive
consequence for determination of safe distances in
hydrogen generation facility. Severe consequence of
VCE in desulfurization reactor is likely because of high
purity of methane gas (85%) content in this unit.
The results also showed that the purge gas buffer
unit has the lowest consequence of VCE caused by a
full-bore rupture in this unit (Fig 5 and Table 7). Low
molar combination of flammable substances (e.g. 12%
CH4, 34% H2 & 13% CO, Table 2) in this unit is likely
the main reason for such a low consequence. Low
process operating pressure (4 bar), and temperature (35
OC) as well as having 40% of CO2 in its material
mixture are other likely reasons for low consequence in
this unit.
Further modeling revealed that the explosions of
vapor set by a full-bore rupture in desulfurization
reactor and purification absorbers will lead to a peak
overpressure of 0.3 bar. The peak overpressure from
these scenarios (purification units and desulfurization
reactor) is much more than the harm exposure threshold
values adopted by IGC for the people (e.g. 0.07 bar) and
the equipment (e.g. 0.2 bar) [ 20]. According to the
results, large leaks expected to be in large size piping
have longer effected distances mainly because of higher
mass of released flammable material. Therefore, smaller
pipe work is expected to be an effective mitigation
measure to reduce the harmful distances.
Fig. 5 shows that the VCEs are more harmful at
night rather than day. It is generally accepted that higher
wind speeds (expected during the day Table 3) will help
the dispersion of hydrogen and other flammable gases,
leading to less harmful VCEs. Lower ambient
temperature, higher relative humidity and stable
atmosphere at night (Table 3) are also expected to help
the hydrogen cloud to travel a longer distance nearby
ground before it rises, leading to a stronger explosion at
night rather than day.
The results of the present study well agree with the
results of Zhiyong et al., which showed that a VCE in
gaseous hydrogen refueling station has the longest
harmful distance among all studied incidents [ 9 ]. Of
course, the effected distances by VCE in present study
are higher than those in Zhiyong et al. study. The
application of suitable ventilation system, smaller
release hole size, new installation, slight shift in
temperature and no chemical reaction between materials
released from a refueling station could had led to lower
harmful distances compared to the present study.
Considering the overpressure of 0.01 bar as a safe
criterion (Table 5), the safety distance of studied
hydrogen generation facility during a vapor explosion is
then 260 m. Any activity or construction of any new
unit is forbidden near this area. This distance covers not
only the studied hydrogen generation plant but also
neighboring premises.
According to this result, the application of gas
detectors and emergency shutdown valve in hydrogen
plant particularly in desulfurization reactor and
reformer, elevating hydrogen piping and
instrumentation as well as preventing from severe
mechanical impacts, are the logical and practical
measures for decreasing the probability and severity of
potential accidents.
CONCLUSION
In present study, a new and comprehensive method
for consequence analysis of probable accidents in a
hydrogen generator facility, which uses natural gas
reforming process, was applied. The main conclusion
can be summarized as follows:
1. Consequence modeling revealed that the main
hazards of hydrogen facility are the vapor cloud
explosion (VCE), jet fire and flash fire, which jet
fire will have most fatality and VCE will have the
longest harmful distances for the people and the
equipment among all accident at different studied
units
2. Reformer unit will have the longest harmful
distances and highest fatality among all flash
fires at different studied units
3. Desulphurizing reactor unit will have the longest
harmful distances and highest fatality among all
jet fires and VCEs at different studied units
4. The jet fire’s harm effect distances, increase with
the growth of wind velocity (day) and in flash
fire released material will encompass the largest
area in hot weather (day), whereas VCE will have
larger harm effect distances at night.
5. Safety distance of hydrogen facility based on the
worst-case consequence is equal to 260 m,
which is outside of hydrogen plant and related
site plant boundary and this is high stakes
imposed on neighborhood and public.
ACKNOWLEDGEMENTS
This paper was extracted from Master's Thesis of
Esmaeil Zarei supervised by MJ Jafari. The authors
wish to greatly thank Shahid Beheshti University of
Medical Science and Behshar Industrial complex for
their supports. No sources of support provided and the
authors declare that there is no conflict of interests.
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