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2011 2nd International Conference on Environmental Science and Development
IPCBEE vol.4 (2011) © (2011) IACSIT Press, Singapore
Consequence Analysis for probable accidents of filter separators installed in Gas
Pressure Reduction Stations
Mohammad Sadegh Yousefzadegan*, Amir
Masoud Masoudi, Yaser Kazemi Ashtiani, Masoud
Kambarani
Seyed Iman Pishbin
Research and Technology Department
Khorasan Razavi province Gas Company
Mashhad, Iran
[email protected]
Chemical Processes Design Group
ACECR (Tehran University branch)
Tehran, Iran
[email protected]
Abstract—Natural gas undergoes a number of processes as it is
transported from wellhead to end-user, and many of these
steps require filtration of the product. The filtration process
removes debris and condensation from natural gas. Filter
cartridges need to be periodically cleaned in order to perform
efficiently and statistics show that the risk of fire in the
cartridge cleaning process is high. As cartridge cleaning is
performed during maintenance process, technicians and
employees working near filter may encounter serious harms in
case of fire together with environmental aspects. As a result,
consequence assessment of the mentioned accident would be
helpful in taking quicker actions and mitigating the harmful
effects of fire for employees, equipments and environment. In
this paper natural gas filter of Abbasabad gate station near
Mashhad in NW of Iran is studied as a case. In order to predict
the consequence of this accident, DNV Consequence Modeling
software package is used for simulation. Necessary information
for simulating the scenario is collected from reliable sources.
Different weather categories comprised of cool, very cold,
warm, very hot and windy weathers are defined to consider all
meteorological conditions in which the accident might occur.
Information on meteorological condition is gathered from the
province meteorological organization. The hazardous accident
is simulated by the software and the results of the simulation
are analyzed. The severity of probable accidents is assessed
consequently, and the safety distances around filter could be
determined accordingly.
Keywords- Natural Gas; City Gate Station; Filter Separator;
Consequence Modeling;
I.
INTRODUCTION
Natural gas produces less carbon dioxide when it is
burned than does either coal or petroleum. This has led
governments into replacing other fossil fuels with natural gas.
Statistics show that natural gas consumption increases by an
average of 1.6 percent per year and that it will reach to 153
trillion cubic feet in 2030. In Iran, natural gas provides a
great part of the nation's energy demand and its consumption
increases by the rate of 12% per year which is much greater
than the world's rate of increase. It is obvious that the
increase in the mentioned energy supply consumptions is
followed by the development of gas transmission and
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distribution systems. Like any other process, environmental
and safety issues are of great importance in these systems.
As natural gas consumption increases in the world, the
importance of safety considerations in this field gets more
highlighted. Management systems such as engineering codes,
checklists and process safety management provide layers of
protection against accidents. However, the potential for
serious incidents cannot be totally eliminated. AIChE
provides a quantitative method to evaluate risk and to
identify areas for cost- effective risk reduction. Consequence
analysis is an important stage in Chemical Process
Quantitative Risk Analysis which evaluates the impacts of
potential hazards [1].
Several valuable researches are performed on
consequence and risk analysis of natural gas transmission
and distribution systems. Spyros Sklavounos et al estimated
the safe distance in the vicinity of fuel gas pipelines by using
event tree analysis method and BREEZE software package
[2]. Z.Y. Han and W.G. Weng worked on an integrated
quantitative risk analysis method for natural gas pipeline
network [3]. Their method is composed of the probability
assessment of accidents, the analysis of consequences and
the evaluation of risks. Many other works have been done on
natural gas pipelines to improve safety (Krueger & Smith,
2003; Metropolo & Brown, 2004; Jo & Ahn, 2005; Jo &
Crowl, 2008; Suardin, McPhate, & Sipkema 2009).
Reviewing the papers in the field of safety in natural gas
transmission and distribution systems show that all
researches have only considered safety in gas pipelines while
there are lots of other equipments that play important roles in
the system and their failures could cause serious harms.
There was no previous work on consequence modeling of
probable accidents in gas pressure reduction stations. As
most of these stations are located near cities, it is very
important for gas distribution companies to assess the
impacts of their facilities on nearby inhabited areas, thought.
To make it clear, distributers are generally willing to know
the impact of probable incidents upon the nearby towns and
environment with more precision in that inhabited areas are
very close to city gate stations.
identified. The next step is accident scenario modeling. In
the present study, DNV (Det Norske Veritas) consequence
modeling software, PHAST 6.53.1 (Process Hazard Analysis
Software Tool), is used to complete this step. After the
second step is performed successfully, the results are
analyzed in the third step and the production loss, human
health, environmental aspects and safety loss together with
assets loss will be estimated accordingly.
A. Accident Scenario Selection
An accident scenario is a description of an expected
situation. It contains single events or combinations of them.
The expectation of a scenario does not mean it will indeed
occur, but that there is a reasonable probability that it would
occur. A credible accident is defined as ‘the accident which
is within the criteria of possibility and has a propensity to
cause significant damage’. There may be a type of accident
which can occur very frequently but would cause little
damage. And there are other types of accident which may
cause great damage but would have very low probability of
occurrence in reverse. Both are not ‘credible’. But accidents
which have appreciable probability of occurrence as well as
significant damage potential (as quantified above) come
under the category of ‘credible accidents’ [6].
As the primary step of consequence modeling, scenario
selection plays an important role in the reliability of results.
This paper investigates the consequences of the worst case
scenario in a filter separator. This worst case scenario was
selected in accordance with the identified hazards in the
HAZOP study and considering the accident records of other
Iranian cities gate stations. Accordingly, it is found out that
flash fire and jet fire are likely incidents that are followed by
the worst consequences. One of the most probable events
that could lead to the fire is the back flow of natural gas in
the rehabilitation time. Because the filter’s cap is opened
during maintenance to allow the natural gas being released,
oxygen in the air can enter the filter and react with Iron
Sulfide accumulated on filter cartridge. As this reaction is
exothermic, it can be a good start point for fire by providing
the necessary initial heat.
Figure 1. Typical Filter separator used in city gate stations
Filter separators are inseparable parts of the natural gas
processing system. Solids and/or fluids are removed from the
gas in the filter/separator section. Investigations on past
industrial accidents show that failure in filters could result in
extreme financial losses and injuries. In 1996, failure in the
closure of a 40-inch diameter filter separator resulted in an
estimated $25 million in damages [4]. Similar accidents may
occur in all filter separators. In this article Abbasabad gate
station near Mashhad in North West of Iran is selected as a
case and different steps of consequence modeling are
performed on it in order to identify probable hazardous
accidents of the filter and evaluate the undesirable
consequences of these accidents.
II.
GAS FILTRATION SYSTEM
Natural gas filter units are installed at each station to
remove any entrained liquids and solids from the gas stream.
The filters may comprise cyclonic elements to centrifuge
particles and liquids to the sides of the enclosing pressure
vessel. These particles and liquids will then drop down for
collection in a sump, which can be drained periodically [5].
Depending on the gas quality and requested efficiency, a
filter separator will be selected. A wide range of filter
separators can be used, including a dust filter, a baffle plate /
coalescer separator, cyclone separators, cyclone / coalescer
separators and cyclone/cellulose separators. Each filter has a
specific application area, separation efficiencies and
operating range. The filter separators can be vertically
positioned, or have horizontal configurations. Installed filter
separators in Abbasabad gate station like most of filter
separators used in Iranian Gas reduction stations are
horizontal type (Fig. 1), so our calculation for probable
accidents is based on this type of filters.
III.
B. Accident Scenario Modeling
After the accident scenario is selected, it is modeled by
PHAST vr.6.53.1 to identify the possible consequences. In
order to simulate the accident scenario and identify the
consequences of the accident, PHAST requires some input
data including process conditions at which the accident is
CONSEQUENCE MODELING
TABLE I.
Consequence analysis is an integral part of risk
assessment process which gives an estimation of the
damages that a probable accident may bring to the properties
and human beings. The consequence estimation scheme that
is followed in this study involves three steps:
1. accident scenario selection
2. accident scenario modeling
3. accident impact assessment
The first step is selection of an accident scenario where
possible accidents leading to hazardous consequences are
Parameters
Scenario
Filter
Flash Fire /
Filter Jet
Fire
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BASIC INFORMATION OF ACCIDENT CONDITION
Type of
scenario
Physical
state of
released
material
Material
released
Pressure
Temperature
-
-
(m3)
(psig)
(º C)
Fixed
time
release
Gas
3333
3
20
TABLE IV.
EFFECT ZONE DISTANCES OF FILTER FIX TIME RELEASE
Jet fire affected distance
(m)
Meteorolo
gical
condition
Warm
Very hot
Cool
Windy
TABLE II.
FLAMMABILITY LIMITS OF NATURAL GAS
Critical
Vapor
limits Molecular
Mass % Density LFL% UFL%
Weight
(air=1)
Material
Methane
16.04
96.20
0.6
5
15
Ethane
30.1
0.98
1.05
3
12.5
0.15
1.52
2
9.5
Propane
44.1
0.05
2.06
1.8
8.4
i-Butane
58.12
n-Butane
58.12
0.10
2.05
1.5
9
n72.15
2.5
1.3
8
Pentane
0.19
n3
1
7.7
86.18
0.32
Hexane
CO2
44.01
1.10
1.53
2.80E1.19
4.3
45.5
H2S
34.08
06
N2
28.01
0.91
0.97
-
Maximum distance
affected by flash fire
LFL
Radiation Intensity
kW/m2
4
kW/m2
12.5
kW/m2
37.5
42
44
40
40
32
33.5
34
37
25
25.5
29
35
½ LFL
(m)
28
28
26
13
52
52
50
33
occurred (pressure, temperature etc), the physical state of the
released material (gas, liquid or both), physical, chemical and
thermodynamic properties of the released material and
topographical and meteorological conditions of the region.
Although provision of all this information is a time
consuming process, the more exact this information is the
more realistic the results will be.
In this research all necessary information is collected
from reliable sources. Table I shows the conditions at which
the accident is occurred. This information is collected from
the unit Process Flow Diagram.
In table II, physical and chemical property of Natural Gas
which is required for modeling is presented.
As ground condition affects the dispersion process of the
released material, it is required to gather information about
the topographical conditions of the plant. This information is
obtained from the satellite photos taken from the plant.
Different topographical conditions are defined in PHAST.
Having studied the satellite photos of the unit, it is found out
that the most relevant definition in PHAST for Abbasabad
gate station is regular large obstacle coverage areas (suburbforest) for which the surface roughness is defined as 1m.
Moreover, the official website of Khorasan Razavi
meteorological organization is used for the relevant
meteorological data. Five different conditions are identified
for warm, very hot, cool, very cold and windy weather. This
TABLE III.
Parameters
Weather
Warm
Very hot
Cool
Very
cold
Windy
METEOROLOGICAL CONDITION OF THE PLANT
Wind
direction
Atmospheric
stability
Relative
humidity
Maximum
wind
speed
(ºC)
%
(m/s)
(º)
-
23
40.6
8
38.42
0
59.17
2.94
2.94
4.97
13
13
13
C
B
E
-21
99
4.97
13
D
23
38.42
23.9
13
D
Ambient
temperature
information is summarized in table III. It should be noticed
that due to harsh condition, maintenance operation would not
be accomplished in very cold weather and this condition is
omitted from the modeling.
Different mathematical models for three hazard
categories including toxic gas dispersion, fires, and
explosions are defined in PHAST. By entering all the
necessary information, PHAST is able to run these models
and to determine consequences of the chosen accidents.
Analyzing the reports of the software, it is concluded that the
most destructive consequences of the filter fixed release
Figure 2. Radiation Intensity graph of Jet Fire in windy condition
117
Figure 5. Effect zone of Flash Fire in hot weather
Figure 3. Effect zone of Jet Fire in windy condition
scenario are related to jet fire and flash fire. Effect zone
distances from the center of the accident are presented in
table IV.
Fig. 2 shows radiation intensity of jet fire in the wind
direction at windy condition. Effect zones diagram for jet fire
which is fitted with the region map is illustrated in fig. 3.
Fig. 4 shows a side view for probable flash fire in hot
weather. Effect zones diagram for the flash fire which is
fitted with the region map is illustrated in fig. 5.
12.5 kW/m2 : it is the minimum radiation required
for firing wood and melting the plastic materials.
• 37.5 kW/m2 : this is adequate radiation to damage
equipments
Flash fire effect zone diagrams show that there will be
flammable concentrations of natural gas in the plant area.
When this flammable bulk of gas reaches to a source of
ignition, there will be flash fire. Flash fire flames will cause
extreme damages to the equipments besides serious injuries
to the employees and in the worst case, especially at the
maintenance time, can claim lives.
It should be also noticed that if fire occurs in the filters of
a gate station, there is possibility of explosion or other kinds
of accidents in nearby equipments which could create a
catastrophic condition by domino effect.
•
C. Accident Impact Assessment
Jet fire in the filters does not exceed the plant boundary,
but can harm people in the plant. It should be mentioned that
jet fire is extended at the wind direction. Although regular
wind direction has been shown in the map, we should
consider the affected area in all directions.
Radiation intensity of jet fire can be more understandable
by the following examples:
• 4 kW/m2 : Staffs feel pain after 20 second. It can
cause blister at the skin.
IV.
CONCLUSION
This paper discusses the procedure for consequence
evaluation of the possible accidents in natural gas filtering
Figure 4. Side view for flash Fire area in hot weather
118
systems. Natural gas may pose major risks to the
surroundings because of its flammability and toxicity.
Making confident predictions about the consequences of the
probable accidents in case of natural gas release could help
us define safety distances around filter separators. In this
paper Abbasabad gate station filter separator is studied as the
case. The results of this study could be summarized as
follows:
• Fixed time release of natural gas from filter is
selected as the worst case scenario both because it
will have the worst consequences and that it is
probable enough to be identified as a credible
scenario.
• Results of the simulation with PHAST vr.6.53.1
indicate that the most significant consequences for
the natural gas release are flash fire and jet fire.
• Comparing the results of the simulation for jet fire in
different meteorological conditions, we realized that
jet fire will travel the furthest distance in windy
condition. This distance is not exceed plant’s
boundary but is considerable as it can harm
personnel and/or affect on other equipments in the
station.
• A valid comparison between the results of
simulation for flash fire in different meteorological
conditions shows that flammable concentrations of
natural gas will encompass the largest area in hot
weather. Moreover, it is concluded that if the
flammable concentrations of natural gas are ignited,
main equipments will be seriously damaged and
employees in the unit could suffer considerable
harms; although nearby farmlands won’t be
extremely destructed.
ACKNOWLEDGMENT
We highly appreciate Khorasan Razavi Gas Co. for their
technical and financial support of this research.
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119
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