RISK ASSESSMENT OF LIQUEFIED PETROLEUM GAS STORAGE FACILITY

A DISSERTATION Submitted In partial fulfillment of the

requirements for the award of the degree of

MASTER OF TECHNOLOGY In

CHEMICAL ENGINEERING (With Specialization In Industrial Safety and Hazards Management)

IC,.

LOO Data

By /,

141RUFAMA

DEPARTMENT OF CHEMICAL ENGINEERING INDIAN INSTITUTE OF TECHNOLOGY ROORKEE

ROORKEE -247 SIT (INDIA) JUNE, 2010

CANDIDATE'S DECLARATION

I hereby declare that the work, which is being presented by me in this project report titled "RISK ASSESSMENT OF LIQUEFIED PETROLEUM GAS STORAGE FACILITY", in partial fulfilment of the requirements for the award of the degree of Master of Technology in Chemical Engineering with specialization in "Industrial Safety and Hazards Management", is an authentic record of the work carried out by me under the guidance of Dr. Shishir Sinha, Associate Professor and Dr. Prakash Biswas Assistant Professor, Department of Chemical Engineering, Indian Institute of Technology Roorkee. The matter embodied in this work has not been submitted for the award of any other degree.

Date: t_ 16 I-WO

Place: Indian Institute of Technology Roorkee (NIRUPAMA)

CERTIFICATE

This is to certify that the above statement made by the candidate is correct to the best of my knowledge.

Dr. Shishir Sinha Dr. Prakash Biswas Associate Professor Assistant Professor Department of Chemical Engineering Department of Chemical Engineering Indian Institute of Technology Roorkee Indian Institute of Technology Roorkee

. ACKNOWLEDGEMENT

Nothing worth more than praising and acknowledgement at the way outset, the eternal and immense benevolence of the Almighty God who endowed with the power of intellect and convergence. It is His will which has enabled me to accomplish my work which otherwise would not have been possible.

It is with a deep sense of gratitude that I profoundly acknowledge my indebtness to my guide Dr. Shishir Sinha, Associate Professor and co-guide Dr. Prakash Biswas, Assistant Professor, Chemical Engineering Department for their perceptive suggestions and comments that helped me to remain motivated and enthused in compiling my dissertation work. Their infallible supervision and guidance has made this work a more rewarding experience for me.

I would like to thank Dr. I D Mall, Head of the Chemical Engineering Department _ for providing various facilities during the course of this project work. I would like to thank all the teaching and non teaching staff members who have contributed directly or indirectly in successful completion of my project work. Thanks are due to CAD Centre facilities.

I am greatly indebted to all my friends for their enthusiastic support, encouragement and help during dissertation work.

I would also like to express my gratitude to the authors and publishers of textbook, magazines, journals and websites from where I have collected the materials and information for this report.

Last but not the least, it is all owed to the blessings of my parents which helped me to complete the work in due time.

NIRUPAMA

ABSTRACT

Storage tanks are very important for containing small to large volumes of flammable and

hazardous chemicals at elevated temperature and pressure in chemical process industries.

There are various organizations and engineering societies who published strict engineering

guidelines and standards for the construction, material selection, design and safe management

of storage tanks and their accessories. Even after following these standards and guidelines in

the design, construction and operation, tank accidents do happen. By preventing equipment

failure rate, the frequency of risk reduces but its impact is still high. Therefore, scope of risk

assessment in the process industries widen to cover hazards related to workers and property

in and near the plant.

Liquefied Petroleum Gas (LPG) is very common fuel for domestic and industrial

purpose. LPG storage tank has severe degree of hazard. A simple risk assessment method was

used to find the risk associated with the three LPG storage tanks having 100 tonnes each of a

LPG bottling plant. Fault tree analysis was used to identify the hazards and its frequency.

Four main events were identified which can lead to fire and explosion: overfilling,

overpressure, mal-operation and mechanical failure/damage which can further lead to open

vapour cloud explosion and catastrophic failure of the tank. Impact of various accident

scenarios (pool fire, fireball and vapour cloud explosion), was calculated in terms of

individual risk to the society. It was found out that pool fire contributes less hazards to people

but it can cause accident in the nearby equipment or property due to its thermal radiation. The

largest contribution to the individual risk is due to vapour cloud explosion. Impact of worst

case scenario was calculated and found that risk near 600 m from the LPG storage tank

location was acceptable.

PAPER COMMUNICATED

RISK ASSESSMENT OF LIQUEFIED PETROLEUM GAS STORAGE FACILITY•

Submitted to Chemical Sciences Journal on May 19th 2010, authored by Nirupama and Shishir Sinha.

A

rARJeY Y 1►1~~;

PAGE NO.

CANDIDATE'S DECLARATION I

ACKNOWLEDGEMENT II

ABSTRACT III

CONTENTS V

LIST OF FIGURES VII

LIST OF TABLES VIII

NOMENCLATURE IX

1. CHAPTER 1

INTRODUCTION 1

1.1 Risk Assessment 2

1.2 Acceptable Risk

2

1.3 Liquefied Petroleum Gases 3

1.4 LPG Related Hazard

3

1.5 Aim and Objective 5

2. CHAPTER 2

LITERATURE REVIEW

3

3. CHAPTER 3

RISK ASSESSMENT METHODOLOGY 20

3.1 Some Important Definitions Related to Risk Assessment 20

3.2 Risk Assessment 20

4. CHAPTER 4

DEFINITION OF THE PROBLEM 29

4.1 Storage Tank Accessories

30

4.2 Properties of LPG 30

4.3 Sources for creating Hazardous scenarios 30

5. CHAPTER 3

RESULT AND DISCUSSION

33

5.1 Hazard Survey 33

V

5.2 Hazard identification 34

5.3 Frequency Estimation 41

5.4 Consequences Analysis 43

5.5 Measure of Risk

50

CONCLUSION 51

REFERENCES 53

APPENDIX 57

VI

LIST OF FIGURES

S. No. Figure Description Page No.

1 4.1 LPG Bottling Plant 29

2 4.2 Schematic Diagram of an LPG Storage Tank 32

3 5.1 Fault tree for fire/ explosion due to release of LPG to the 35

atmosphere

4 5.2 Fault tree for Overfilling 36

5 5.3 Fault tree for Overpressure 37

6 5.4 Fault tree for Mal-operation 38

7 5.5 Fault tree for Mechanical failure/damage 39

8 5.6 Fault tree for catastrophic failure of tank 40

9 5.7 Conditional probability of human death vs human death 44

for fireball due to catastrophic failure of one LPG storage

tank

10 5.8 Conditional probability of human death vs human death 45

for fireball due to catastrophic failure of two LPG storage

tanks

11 5.9 Conditional Probability of Human Death Vs Human 45

Death for Fireball Due to Catastrophic Failure of All the

Three LPG Storage Tanks

12 5.10 Conditional probability of human death vs human death 47

for VCE due to catastrophic failure of one LPG storage

tank

13 5.11 Conditional probability of human death vs human death 48

for VCE due to catastrophic failure of two LPG storage

tanks

14 5.12 Conditional probability of human death vs human death 48

for VCE due to catastrophic failure of three LPG storage

tanks

15 5.13 The Individual Risk Contours 50

VII

LIST OF TABLES

S. No. Table Description Page No.

1 2.1 Summary of Literature Review 9

2 3.1 Dow Fire &Explosion Index (Dow F&EI) Degrees of 21

Hazards

3 3.2 Boolean logic functions used for fault tree 22

4 3.3 Basic probability relation 23

5 3.4 The conditional probabilities of a human death in 24

dependence on the probit-function Pr Values

6 5.1 Result of Dow fire & explosion (F&EI) 33

7 5.2 Impact distance for percentage death/injury of human 46

due to different scenario of fireball

8 5.3 Damage distances for different thermal heat radiation 46

in case worst case

9 5.4 Impact distance for percentage death of human due to 49

different scenario of VCE

NOMENCLATURE

Fl General process hazards

F2 Special process hazards

F3 Process unit hazards factor

Pr Probit-function

t An effective exposure time, s

q Thermal radiation intensity, kW/m2

to Characteristic time of a fire detection,s

x Distance from human location to safe zone, m

u Velocity of a human motion to safe zone, m/s

Ef Mean flame surface radiation intensity, kW/m2 Fq Angle coefficient

i Coefficient of atmosphere transparency

r Distance from geometric pool fire centre to an object, m

d Effective pool diameter, m

H Flame height, m

m' Specific mass of burning liquid, kg/m2s

Pb Ambient air density, kg/m3

g Gravitational constant, m/s2 Ds effective fireball diameter, m

m Mass of a combustible substance participating in the fireball formation, kg

Hb height of the fireball centre over the ground surface, m

rb Distance from an object to a point on the ground surface under fireball centre, m

V ( 17500 8-a + r 290 1 0 ' 3

l AP ) AP Maximum pressure in a shock wave, Pa

i Impulse of a shock wave

Po Atmospheric pressure, Pa

re Distance from a geometric centre of a vapour cloud to an object, m

ms Specific mass of a combustible vapour in cloud, kg

Qc specific heat of a vapour combustion, J/kg

Q, Constant equals to 4.52 x 106 J/kg IX

Z Coefficient of a vapour participating in an explosion

m,, Vapour mass in the cloud, kg

n Number of event or scenario

Qdi Conditional probability of a human death which is given in probit-function

Q(A1) A year probability of realization of the i-th event per year

X

CHAPTER 1

INTRODUCTION

Storage tanks are very important for containing small to large volumes of flammable and

hazardous chemicals at elevated temperature and pressure in chemical process industries.

These storage tanks should designed and handled properly as even a small accident can lead

to damage to the property, community, structure, service, geographical area (Demichela et al.

2004). If these accidents are large then these results in lawsuits, stock devaluation or

company bankruptcy. Therefore various organizations and engineering societies, such as

American Petroleum Institute (API), American Institute of Chemical Engineers (AIChE),

American Society of Mechanical Engineers (ASME) and National Fire Protection

Association (NFPA) published strict engineering guidelines and standards for the

construction, material selection, design and safe management of storage tanks and their

accessories (Lees et al.,1996).Even after following these standards and guidelines in the

design, construction and operation, tank accidents do happen.

With the rise in the growth of process industries and storage tank requirements, the

workers employed in the plant have also increased. Thus, the general public living in the

vicinity of the plant could be endangered because an accident involving these chemicals may

cause injury or loss, of life or damage to property including adverse effect on environment or

disruption in and around the industry.

Accidents in the storage tank can occur either by mechanical failure, mal-operation or

external impact, which sometimes turns to catastrophic consequences. The most gruesome

example is the Bhopal gas tragedy in 1984 in which more than 2000.0 people were killed.

There have been many other accidents in the past like Flixborough (1974), Basel (1986),

Panipat (1993), Mumbai (1995), etc. (Kletz, 1989; Khan et al., 1990; Lees et al., 1996;

Marshall et al., 1987). It is very common to find industrial areas where a number of industries

are situated in close proximity to each other. Such an industrial area gives rise to the `chain of

accidents'. An accident in one industry may cause another accident in the neighbouring

industry which may trigger another accident and so on. Some past experiences like Mexico

(1984), Antwerp (1987), Pasadena (1989), Vishakhapatnam (1997), Jaipur (2010).

Safety has been a prime concern in the design, construction and operation of liquid

hydrocarbon storage facilities. By preventing equipment failure rate, the frequency of risk

1

reduces but its impact is still high. Therefore, scope of risk assessment in the process

industries widen to cover hazards related to workers and property in and near the plant. This

led to the rise of Risk Assessment as a powerful tool in the process industries.

1.1 RISK ASSESSMENT

Risk assessment is a step in a risk management process and very important tool for analysing

the systems and proposing cost effective mitigation measures thus making industries safer

and more reliable. Risk assessment includes incident identification and consequence analysis.

Incident identification describes how an accident occurs. It frequently includes an analysis of

the probabilities. Consequence analysis describes the expected damage. This includes loss of

life, damage to the environment or capital equipment and days outage. Risk assessment is the

determination of quantitative or qualitative value of risk and to find the hazards associated.

Quantitative risk assessment is the calculation of two components of risk, first is the

magnitude of the potential loss and second is the probability of the occurrence of the loss.

Dealing with these hazards following questions have to be answered:

• to what extent can a technological hazard be considered acceptable?

• to what extent should the hazards that are considered conditionally acceptable be

reduced in the light of societal and economic factors?

1.2 ACCEPTABLE RISK

The goal of risk assessment is to reduce risks to an acceptable or tolerable level. A zero risk

level is not possible. The basis of design is ALARP which stands for `As Low As Reasonably

Practicable'. Principle of ALARP is associated with the design and development of safety

systems and captures the notion that the risk to individuals, society and the environment

should be low as reasonably practicable. A risk should be low enough such that attempting to

make it lower would actually be more costly than any cost likely to come from the risk itself.

This is called tolerable risk. Risk reduction involves cost which increases exponentially with

the reduction measures implemented and thus, infinite time, effort and money will be

required to reduce a risk to zero. It should not be understood as simply a quantitative measure

of benefit against detriment. A major factor that comes into the ALARP principle is the cost

of assessing the improvement gained in an attempted risk reduction. In extremely complex

systems, this can be very high and could be the limiting factor in practicality of risk reduction

(Lees et al., 1996). It has now become an essential part in any industry to identify the

hazards, assess the associated risks and to bring the risks to a tolerable level.

1.3 LIQUEFIED PETROLEUM GASES (LPG)

Liquefied petroleum gas is one of the most common and an alternative fuels used in the world

today. Liquefied petroleum gas is also called as LPG, LP Gas, or Auto gas. LPG is obtained

as a by product when refining crude oil or natural gas and is primarily composed of propane

and butane with smaller amounts of propylene and butylenes. LPG is widely used and stored

for domestic, leisure, commercial and industrial purpose and is widely transported by road,

rail and through pipelines. LPG is kept in containers ranging in capacity from a few grams to

thousands of tonnes and in both fixed and mobile or portable systems.

For economical transportation, petroleum gases are liquefied. These liquefied gases

are stored in either pressure, semi-pressure or fully refrigerated condition. The vessels are

surrounded with a bund in case of any leakage due to failure of vessel. Small quantity Iike

100 tonnes is mostly stored in pressure vessels and very large quantity in semi-pressure or

fully refrigerated (Buschmann, 1974).Pressure vessel is suitable for storing LPG and stored at

petroleum storage terminals, chemical works and many non chemical factories (Lees et al.,

1996).

1.4 LPG RELATED HAZARD

The fire and explosion hazard of LPG storage vessel is extremely high (Shebeko et al.,

1995a). There are a number of possible hazardous scenarios which can occur in the LPG

installation, irrespective of its size (Melchers et al., 2001).

These hazard are well documented in literatures, some of the most significant hazards are:

1.4.1 Boiling Liquid Expanding Vapor Explosion (BLEVE): BLEVE is defined as a major

container failure, into two or more pieces when the contained liquid is at a temperature well

above its boiling point at atmospheric pressure. The rupture of LP-gas containers is one

particular type of BLEVE that has drawn much attention due to the destructive and

spectacular effects of the failure (Abbasi et al. 2007). LP-gas containers are steel pressure

vessels equipped with pressure relief devices set to maintain sufficient pressure to keep the

LP-gas as liquid but to relieve any pressure greater than the container is designed to

withstand. Heat radiation is the main hazard associated with BLEVE (Ruj et al. 2006).

1.4.2 Fireball: When superheated liquid like LPG release, if immediately ignited, may burn

as a fireball. The fireball grows larger and moves upward continuously because of buoyancy.

The duration of the fireball is small but the radiation levels are intense. Within the radius of

the fireball there will be severe damage to process equipment and buildings. Beyond this, the

3

danger is mainly for the people that may be affected by the radiation. Therefore, the fireball

radius is defined as the domino effects radius.

1.4.3 Unconfined Vapor Cloud Explosions (UVCE): When LPG release from the tank and

form clouds of vapour without ignition taking place, it ignited later at a considerable distance

from the release point. Overpressure is important in terms of consequences to human and

property. The effects have ranged from minor damage to structures to major damage to the

entire plant..

1.4.4 Flash Fire: This is a lesser form of UVCE due to less amount of gas involved. Heat

radiation is more significant than overpressure in terms of consequences.

1.4.5 Jet Fire: A high pressure release of vapour or vapour/aerosol into free space, if ignited,

will burn as a flame jet. Any equipment on which that flame jet impinges would be subjected

to very high thermal loads, often exceeding the capacity of fixed water sprays. Outside the

flame jet, radiation hazards are very small.

1.4.6 Pool Fire: Pool fires can occur when a significant quantity of liquid is released and

immediately ignited. These can be confined, e.g. in case of releases in containment dikes or

unconfined, e.g. in case of releases from LPG storage tank (Mudan, 1984). There will be less

problems if plants have proper layout and drainage.

In the present work, risk assessment of Liquefied Petroleum Gas (LPG) storage

facilities was done. Risk analysis for LPG storage facilities is described in many papers

(Drysdale et al., 1979/80; Roberts et al., 1981; Pietersen et al., 1988; Crocker et al., 1988;

Ames et al., 1988; Aydemir et al. 1988; Davenport et al. 1988; Khan, 1990; Pietersen et al.,

1990; Shebeko et al.,1995; Melchers et al., 2001; Robert et al., 2001; Stawczyk et al., 2003;

Demichela et al., 2004; Ruj et al. 2006; Park et al., 2006; Bubbico et al., 2008; Razus et al.,

2009). Most of the papers are based on hazard analysis of accidental release of LPG from

storage facility and during refuelling. Some papers analyse the hazards on the basis of

experiments and some are from the past accidents. In present work simple risk assessment

methodologies are used to quantify the risk associated with the LPG storage facilities.

4

1.5 AIM AND OBJECTIVE

To assess risk imposed by LPG bottling plant with focus on LPG storage tank by the

following methodology:

1. Characterizing and analysing hazard using Dow fire and explosion index.

2. Determination of Individual Risk of LPG storage tank by the following steps:

i. Identification of major process hazards using fault tree analysis and

calculation of overall hazard frequency.

ii. Calculation. of impact of thermal radiation and overpressure for various

accident scenarios such as pool fire, fire ball and vapour cloud explosion.

3. Critically examine the result obtained and suggest risk reduction if needed.

5

CHAPTER 2

LITERATURE REVIEW

Flixborough accident (1974) which was shortly followed by Beek (1975), Bhopal gas tragedy

(1984), Mexico city (1984) (Lees et al., 1996) shook the chemical process industries (CPI)

and forced it towards the massive revamp with special attention to the safety and loss

prevention. With the increasing public concern and awareness and government initiatives,

risk reduction became an integral part. This led to the development of the special field of

safety engineering with entire gamut of techniques and extensive studies about hazards and

risks posed by the chemical process industries (Crowl et al., 1990).

The Loss Prevention is very important to countless companies, municipalities and

governments around the world, because of the trend for processing plants to become larger

and often be situated in or close to densely populated areas, thus increasing the hazard of loss

of life or property. Lees et al., 1996 gave a complete collection of information on the theory,

practice, design elements, equipment, regulations and laws covering the field of process

safety. He discussed about the hazard assessment which is the important tool for decision-

making on hazards. The main focus was process industries, but the process aspects of related

industries, notably nuclear power, oil and gas platform are briefly touched. Guidelines of

CCPS/AIChE were followed and cover all the aspects of hazard, fire, explosion and toxicity

in a very fine manner. Reliability engineering, design relief valve system and transportation

of the chemicals were also included.

Dow fire and explosion index hazard classification guide designed to help the user to

quantify the expected damage of potential fire and explosion incidents in realistic terms,

identify the equipment likely to contribute to the creation or escalation of an incident and

communicate the fire and explosion risk potential to management. It was first published in

1964 as a tool for plant engineers to give relative value to the risk of individual process unit

losses due to fires and explosions and to communicate these risk to management in terms

easily understood, i.e., potential of financial losses due to lost production and damage to plant

facilities. This index is still widely used and has been upgraded seven times. This index

estimates the hazards of a single process unit based on chemical properties and inventories,

and then uses plant construction cost or replacement cost to estimate the potential risk in

dollar terms. This communicate to develop an index, which is a measure of risk and takes

0

into account risk reduction measures implemented or proposed for the plant unit, such as

process control systems, material isolation systems and fire protection systems. Thus as the

F&EI rates the hazards, the proposed index rates the risk. The index has been adjusted based

on both internal and external data as well as qualitative and quantitative analysis. The main

aim of this tool is to communicate the risk to management in such a way that management

may take appropriate actions to reduce the risk. The purpose is not to rate a given facility as

safe or unsafe, but to give a relative ranking of hazards and risks within an organization. The

current version of the guide is available from AIChE (1994), and is referred to as the F&EI

Guide in the remainder of this communication.

Crowl et al., 1990 described and combined academic theory with practical industry

experience, updated to include the latest regulations and references, covers hazard

identification, risk assessment and inherent safety. They also included case studies and

problems to enhance learning. He gave primary focus on technical fundamentals of chemical

process safety which provides a solid groundwork for understanding with full coverage of

both prevention and mitigation measures. In addition, he also included an overview of

government regulations, dispersion modeling, source modeling, flammability

characterization, fundamentals of static electricity and explosion venting. He also has given

extensively new case histories, as well as important material from the American Institute of

Chemical Engineers' Center for Chemical Process Safety. Firstly he has given overview of

the risks of chemical processes and then the nature of accidents leading approaches to

chemical engineering safety and the ethics of chemical engineering. Next, he reviewed the

fundamentals of toxicology, showing how toxicants enter and are eliminated from biological

organisms: their impact, the dose vs response relationship and other key issues. Coverage

includes industrial hygiene identification, evaluation, and control source models of liquid

flow, toxic release and dispersion models, the nature and prevention of fires and explosions,

the use of reliefs, the identification of hazards and more.

Khan et al. 1999 briefly recapitulated some of the major accidents in chemical process

industries which occurred during 1926-1997. In these case studies they had analysed with a

view to understand the damage potential of various types of accidents, and the common

causes or errors which have led to disasters. An analysis of different types of accidental

events such as fire, explosion and toxic release had also been done to assess the damage

potential of such events. Finally they concluded that vapour cloud explosion (VCE) poses the

greatest risk of damage. Their study highlights the need for accident forecasting, consequence

7

assessment, and development of up-to date emergency preparedness and disaster

management plans in the chemical process industries.

Chang et al., 2006 reviews 242 accidents of storage tanks that occurred in industrial

facilities over last 40 years. The causes and the contributing failures that led to accidents were

expressed with a fishbone diagram in a systematic way. Corrective actions are also provided

to help operating engineers handling similar situations in the future. They had shown that

74% of accidents occurred in petroleum refineries, oil terminals or storage. Fire and

explosion account for 85% of the accidents. There were 80 accidents (33%) caused by

lightning and 72 (30%) caused by human errors including poor operations and maintenance.

Other causes were equipment failure, sabotage, crack and rupture, leak and line rupture, static

electricity, open flames, etc. Most of those tank accidents would have been avoided if good

engineering in design, construction, maintenance and operation has been practiced and safety

management program has been implemented and executed.

8

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2.1 Conclusion from the Literature Review

Following conclusion can be derived from the above literature survey:

• Risk Assessment is vital in planning for emergency situations encountered in process

industries. This can be qualitative as well as quantitative measures.

• There is no possibility of eliminating all the hazards completely and the concept of

tolerable risk is important.

• Guidelines for risk assessment are available from the source such as Centre For

Chemical Process Safety (CCPS), British Health safety Executive (HSE), SEVESO

Directive etc.

• Strict engineering guidelines and standards for the construction, material selection,

design and safe management of storage tanks and their accessories available in

various organizations and engineering societies such as American Petroleum Institute

(API), American Institute of Chemical Engineers (AIChE), American Society of

Mechanical Engineers (ASME) and National Fire Protection Association (NFPA).

• Safety has been a prime concern in the design, 'construction and operation of liquid

hydrocarbon storage facilities.

• Main causes of accident in process industries are due to the poor maintenance,

inadequate instrumentation system, mal-operation and poor safety management

system.

• Quantitative Risk Assessment is a tool that is being used increasingly to help prevent

rare but potentially catastrophic events. Expressed in terms of absolute risk which can

be compared to established levels of unacceptability.

• Quantitative Risk Assessment increasingly but there is always a danger in using its

results blindly without sound engineering and management judgement.

• All explosion parameters are strongly dependent on initial pressure of fuel—air

mixture and on fuel/air ratio.

• Unconfined vapour cloud explosions, BLEVE, confined explosions and fires are the

incidents of LPG storage.

19

CHAPTER 3

RISK ASSESSMENT METHODOLOGY

3.1 SOME IMPORTANT DEFINITIONS RELATED TO RISK ASSESSMENT

GIVEN IN CENTRE FOR CHEMICAL PROCESS SAFETY (CCPS):

Hazard: Hazard is a physical or chemical condition that has potential to damage the people,

property and environment. This is also defined as phenomena that cause thread to people,

structure or economic assets.

Accident: An accident is an unplanned or uncontrolled event, which has led to damage, ill

health or injury.

Consequence: A consequence can be simply defined as an outcome of an accident.

Severity: The nature and extent of the consequence is defined as severity.

Likelihood or frequency: This is the chance or probability that a hazard may result in an

accident and consequence.

Risk: Risk is the likelihood that a hazard will give rise to a consequence with a particular

severity in terms of damage, ill health, injury etc.

Individual Risk: The risk to a person in the vicinity of a hazard. This includes the nature of

the injury to the individual, the likelihood of the injury occurring, and the time period over

which the injury might occur.

Societal risk: A measure of risk to a group of people. It is most often expressed in terms of

the frequency distribution of multiple casualty events.

3.2 RISK ASSESSMENT METHODOLOGY:

• Process screening

• Hazard identification

• Frequency estimation

• Consequences analysis

• Measure of risk

20

3.2.1 Process screening

Dow fire and explosion index (F&EI) is a quick and systematic procedure for ranking

processes by their intrinsic hazards given in Dow Fire and Explosion Index hazard

Classification Guide. The Dow Fire and Explosion Index system is a useful tool to identify

the hazardous equipment (i.e. equipment that would be likely to create or escalate an

incident) and make engineers aware of the potential losses in each process area. It is based on

historic loss data, the energy potential of the processed materials in the chemical plants and

the current application of loss prevention practices. The fire and explosion index determines

the realistic maximum loss occurring under the most adverse operating conditions and is

applicable to processes where flammable, combustible or reactive materia 's stored or

processed.

3.2.1.1 Factors in Dow F&EI

• Material Factor (MF)

• General process hazards (F1 )

• Special process hazards (F2)

GENTS G' Ca 2oz~i

ZACC No ................

Date.............

R00vk'~~~,

• Process unit hazards factor (F3)

The Unit Hazard Factor for process unit is the product of general and special process hazards.

F3 =F1 xF2 (3.1)

Dow Fire and Explosion Index (F&EI)

Dow F&EI = MF x F3 (3.2)

Table 3.1 Dow Fire &Explosion Index (Dow F&EI) Degrees of Hazards

F&EI Degrees of Hazards

1-60 Light

61 —96 Moderate

97 — 127 Intermediate

128-158 Heavy

159 up Severe

The further use of the Dow F&EI is to undertake a process risk analysis, covering property

damage and business interruption. The Dow F&EI used to obtain the exposure radius (ER)

21

and hence area of exposure. The value of the area of exposure (VAE) is obtained from are of

exposure and capital density. Then damage factor (DF) is calculated from F&EI- and the

process unit hazard factor. Then base maximum probable property damage (MPPD) is

calculated my multiplying DF and VAE (Lees et al., 1996).

3.2.2 Hazard identification

The main objective of hazard identification to answer the question: what can go wrong? This

is the most important step because hazards that are not identified will not be quantified. Some

of the techniques used for hazard identification include HAZard and Operability (HAZOP)

Analysis, Failure Mode and Effects Analysis (FMEA), Fault Tree, 'What If Analysis,

Preliminary Hazard Analysis (PHA) and Checklist Analysis (Lees et al., 1996).

Fault tree method was used in present work for risk assessment. Fault tree is a

deductive method for identifying ways using Boolean symbols in which hazards can lead to

accident. There are AND and OR gates to name only the mostly used Boolean logic

functions. The approach starts with well defined accidents, or top event and work backward

toward the various scenarios that can cause accident (Crowl et al., 1990).

Table 3.2 Boolean logic functions used for fault tree (Crowl et al., 1990; Lees et al., 1996)

AND gate The output event occurs only when all the input fl events exist simultaneously

OR gate The output event occurs if any of the input events occur

INTERMEDIATE A fault event that result from the interactions of

Event other fault events that are developed through logic gates such as those defined above Intermediate

BASIC Event A component failure that required no further development. A basic event is the lowest level of resolution in a fault tree

EXTERNAL or A condition or an event that is assumed to exit as a

fl HOUSE Event boundary condition for the fault tree

TRANSFER This symbol indicates that the fault tree is developed

Symbols further on another page and is labelled further using number code. Transfer symbol are often used to avoid repairing identical logic in several places in a fault tree model

22

3.2.3 Frequency estimation

The objective of frequency estimation is to calculate the frequency/probability of each

accident scenario. Two common sources of this information are historical data and fault tree

analysis. Once fault tree drawn minimal cut set is determined. The minimal cut sets are the

various sets of events that could lead to top event. Then fault tree is used for quantitative

calculation to determine the probability of top event using basic probability relations. The

frequency or probability of failure of the basic events is estimated from reliability data which

is based on accidental study and by engineering judgements. After the failure data of all basic

events have been determined, the frequency of the TOP event can be calculated (Khan et al.,

1998).

Table 3.3 Basic probability relation (Lees et al., 1996)

Logic System Boolean algebra Probability Relation

A

A = BC P(A) = P(B)P(C)

B C

A

A = B + C P(A)=P(B) + P(C)- P(A)P(B)

3.2.4 Consequences analysis:

The objective of consequence analysis is to quantify the negative impact of an event.

Consequences are normally measured in terms of the number of fatalities, although they

could also be measured in the number of injuries or in property loss. The three types of

effects that are normally considered are thermal radiation, explosion overpressure and toxic

exposure (Khan et al., 1998).

For the present study LPG storage tank was taken which is not toxic but it can lead to

fire and explosion hazards. Hazards related to LPG are pool fire, torch fire, BLEVE, VCE,

fireball, jet fire. Consequences of these scenarios are calculated using probit-function. The

conditional probability of human death is found by means probit-function given in Table 3.4

23

Table 3.4 The conditional probabilities of a human death in dependence on the probit-

function Pr Values

The Conditional Probability of Human Death, %

Probit-function Values

0 1 2 3 4 5 6 7 8 9

0 - 2.67 2.95 3.12 3.25 3.36 3.45 3.52 3.59 3.66 10 3.72 3.77 3.82 3.90 3.92 3.96 4.01 4.05 4.08 4.12 20 4.16 4.19 4.23 4.26 4.29 4.33 4.36 4.39 4.42 4.45 30 4.48 4.50 4.53 4.56 4.59 4.61 4.64 4.67 4.69 4.72 40 4.75 4.77 4.80 4.82 4.85 4.87 4.90 4.92 4.95 4.97 50 5.00 5.03 5.05 5.08 5.10 5.13 5.15 5.18 5.20 5.23 60 5.25 5.28 5.31 5.33 5.36 5.39 5.41 5.44 5.47 5.50 70 5.52 5.55 5.58 5.61 5.64 5.67. 5.71 5.74 5.77 5.81 80 5.84 5.88 5.92 5.95 5.99 6.04 6.08 6.13 6.18 6.23 90 6.28 6.34 6.41 6.48 6.55 6.64 6.75 6.88 7.05 7.33 - 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70 0.8 0.9 99 7.33 7.37 7.41 7.46 7.51 7.58 7.65 7.75 7.88 8.09

3.2.4.1 Methods for Calculation of Impact Parameters and Probit-Function Values for

Different Scenarios:

3.2.4.1.1 Pool Fire

The probit-function value for pool fire can be calculated by the formula (Marshall, 1987;

Shebeko et al., 1995b)

P r = - 14 .9 + 2.56 ln( tq 4/3) (3.3)

Where

t is an effective exposure time, s

q is the thermal radiation intensity, kW/m2 The exposure time is calculated-by:

t = t o + x (3.4)

M

24

Where

to is a characteristic time of a fire detection (expected to be 5s),

x is a distance from human location to safe zone (q lower than 4 kW/m2)

u is a velocity of a human motion to safe zone (expected to be 5m/s)

Thermal radiation intensity can be calculated by the formula (Lees et al., 1996)

q = E f .F q .z

(3.5)

Where

Ef is a mean flame surface radiation intensity, kW/m2 (for LPG it is 100 kW/m2)

Fq is an angle coefficient

i is the coefficient of atmosphere transparency

The angle coefficient for pool fire is determined by formula (Lees et al., - 1996)

F q = F 2 + F h2 (3.6)

B--1/S 1 (B+1).(S--1) x an 1 B 2 -1 (B -1).(S'+1 )

FI, = —. (3.7) 7r (A-1/S) 1 (A+1).(S-1) A2_1 - x

A = (h 2 + S 2 + 1)/(2.S) (3.8)

B = (1+ S 2 )l(2.S) (3.9)

S = 2r/d (3.10)

h=2H/d (3.11)

Where

r is the distance from geometric pool fire centre to an object, m

d is the effective pool diameter, m

H is the height (m) of the flame determine by formula (Lees et al., 1996) 0.61

H = 42 . d . m d (3.12) Pb

25

Where

m is a specific mass of burning liquid, (kg/mzs) (0.1 kg/m2s for LPG)

Pb is an ambient air density, kg/m3

g is the gravitational constant, m/s2

The coefficient of the atmosphere transparency is determined by the expression:

r = exp[ —7.0 x 10 - 4 (r — 0.5d)] (3.13)

3.2.4.1.2 Fireball

The probit-function value for fireball can be calculated by formula (1). The effective fireball

diameter (m) can be calculated by formula (Lees et al., 1996)

D 9 = 5.33

(3.14)

Where

m is the mass of a combustible substance participating in the fireball formation, kg

The effective exposure time can be calculated by formula:

= 6 .48 m 0.325 (3.15)

Thermal radiation q can be determined by equ. 3.5. The E£ value is 450 kW/m2 (Shebeko et

al., 1995b)

Fq can be calculated by following formula (Lees et al., 1996):

Fq = ( HbID,+0.5) (3.16) q

3/2 4 [(H b IDS +0.5)]

2 + ( rb IDS )

Z

Where

Hb is height of the fireball centre over the ground surface (m) which is equal to D/2

rb rb is a distance fron an object to a point on the ground surface immediately under fireball

centre,m

i can be determine by formula:

= exp[ —7.0x10 -4 x( rb2 +Hb —D 5 /2)] (3.17)

►z.

3.2.4.1.3 Vapour Cloud Explosion (VCE)

For vapour cloud explosion, the probit-function is calculated by formula (Shebeko et al.,

1995b; Lees et al., 1996)

Pr = 5 — 0.26 • In V (3.18)

Where

V = 17500 8.4 + 290 0.3 OP i

(3.19)

AP is the maximum pressure in a shock wave, Pa

i is the impulse of a shock wave AP calculated by formula:

[O.8m33 3mo.66 5m=S + ? + 3S

J (3.20)

~e re re

Where

P,, is atmoshpheric pressure, Pa

re is a distance from a geometric centre of a vapour cloud to an object, m

ms is the specific mass of a combustible vapour in cloud (kg) which is calculated by formula:

ms=(Q,/Q0).m.Z'

(3.21)

Where

Q,, is a specific heat of a vapour combustion, J/kg

Qo is a constant equals to 4.52 x 106 J/kg

Z is a coefficient of a vapour participating in an explosion, which is expected to be 0.1

m is the vapour mass in the cloud, kg

The impulse of a shock wave i can be calculated by formula:

i = 123 e m 0.66 /re (3.22)

3.2.5 Measure of risk

Last step of risk assessment is to calculate actual risk. A number of accident scenarios have

been identified. Frequency and consequences have been calculated for each and risk can now

be determined. Quantitative risk assessment is plotted in terms of risk contour where

27

individual risk is measured instead of societal risk. Individual risk (IR) can be calculated by

formula:

IR = Q di •Q (A~) (3.23)

Where

n is the number of event or scenario

Qdi is a conditional probability of a human death which is given in probit-function shown in

Table 3.4

Q(A1) is a year probability of realization of the i-th event (pool fire, fireball, vapour cloud

explosion) per year

Population distribution is not taken into account with individual risk. The contour is still very

useful because particular point can be shown at the plant fence line.

28

CHAPTER 4

DEFINITION OF THE PROBLEM

For the present study, an LPG bottling plant has been chosen for the risk analysis. LPG

bottling plant receives bulk LPG by road tankers and stored in horizontal pressure vessels

also called bullets then the LPG is bottled in small cylinders for consumers. The present work

is focussed on Risk Assessment of storage tanks.

I.PG ROM( T(IOk I PO $tome ioln

L%C- BC b Gx

Fig. 4.1 LPG Bottling Plant

I have visited one of the LPG bottling plant of Indian Oil Corporation Limited

(IOCL). Because of confidentiality arrangement I am not disclosing the location of the plant.

The plant is located in an industrial area with 100 employees in a shift. The whole area of the

plant is about 13 acres with three horizontal types of LPG storage vessels. Each vessel stores

about 100 tonnes at ambient temperature. LPG storage tanks are refilled by road tanker

having capacity of 18 tonnes to the horizontal storage tank using flexible hose. Number of

road tankers depends on the requirement, but mostly 12 road tankers are used per day. For

safety, they are stored in three vessels of 300 kL each. The tank contents are drawn down

gradually for refilling to small cylinders for end users. About 18000 cylinders per day are

refilled for domestic and industrial purposes.

LPG storage tank is about 9 m long and 4 m high. Because of high coefficient of

expansion of LPG in its liquid phase, the tanks are never completely filled with liquid. Tanks

are filled to approximately 85% of their water capacity, the remaining space being taken up

with vapour which is called as vapour space. As LPG gas (vapour) is drawn from the tank,

the vapour pressure in the tank falls and the liquid boils, producing more vapour and restoring

the pressure. The storage tank is equipped with pressure relief devices to overcome over pressure.

29

4.1 STORAGE TANK ACCESSORIES

• LPG transfer pump or compressor

• LPG vaporizer

• Emergency shut of valve

• Gas leak detection system

• Water sprinkler system

• Excess flow check valve

• Pressure and temperature gauge

• High and low level alarm

4.2 PROPERTIES OF LPG (MSDS)

• LPG forms a flammable mixture with air in concentrations of between 2% and 10%.

• LPG is approximately twice as heavy as air when in gas form and will tend to sink to

the lowest possible level and may accumulate in cellars, pits, drains, etc.

• LPG in liquid form can cause severe cold burns to the skin owing to its rapid

vaporisation.

• Vaporisation can cool equipment so that it may be cold enough to cause cold burns.

• It can create fire and explosion hazard if stored or used incorrectly.

• Vapour/air mixtures arising from leakage may be ignited. some distance from the point

of escape and the flame can travel back to the source of the leak.

• At very high concentrations when mixed with air, vapour is an anaesthetic and

subsequently an asphyxiant by diluting the available oxygen.

• A vessel that has contained LPG is nominally empty but may still contain LPG vapour

and be potentially dangerous. Therefore treat all LPG vessels as if they were full.

4.3 SOURCES FOR CREATING HAZARDOUS SCENARIOS

4.3:1 Catastrophic Failure: Sudden failure of the tank may occur due to metal fatigue and

fracture, overfilling followed by excessive pressure build-up and fracture, weakening of the

vessel due to metal corrosion, etc.

4.3.2 Flame Impingement: Ignition of LPG released due to rupture/crack of vessel, pipe-

work, hose, relief valve, etc.

30

4.3.3 External Impact: Rupture of hoses or pipe due to external impact, mostly from vehicles.

4.3.4 Mal-operation: This includes operator error during the loading and unloading or

sampling of LPG.

4.3.5 Poor Maintenance: Includes hose wear and tear, corrosion in valves, pump seal failure,

connection seats, etc.

4.3.6 Sabotage: During assessing of LPG via the drain valve.

31

N M

w

N

y

CHAPTER 5

RESULT AND DISCUSSION

Risk assessment was used to determine the events that can produce an accident, frequency of those events and consequences.

For the present work a flied study on LPG storage tank of LPG bottling plant was

performed for the risk assessment. Risk assessment is - done considering the worst case

scenario. Necessary data for the risk assessment is taken from the literature available.

For risk assessment following methodology was considered:

5.1 Hazard Survey (Hazard characterization and analysis)

5.2 Hazard Identification (Identification of accident scenarios)

5.3 Frequency Estimation (Estimation of frequency of each scenario)

5.4 Consequences analysis (Consequences analysis of each scenario)

5.5 Measure of Risk (Estimation of total risk)

5.1 HAZARD SURVEY

Hazard survey is done using Dow fire and explosion index (F&EI). Following results obtained from Dow F&EI calculation:

Table 5.1 Result of Dow fire & explosion (F&EI)

1. General process hazards factor 2

2. Special process hazards factor 4.55 3. Process unit hazards factor 9.1 4. Fire and explosion index 191.1 5. Loss of Credit factor 0.31 6. Exposure radius 53 m 7. Area of exposure 8829 m2

8. Value of exposure 197.77Crores

9. Damage factor 1

10. Base MPPD 197.77Crores 11. Actual MPPD 61.31 Crores

*Maximmum allowable F&EI is 168 (Lees et al., 1996)

*Assuming cost of the plant to be 2.24 lakh/ m2 (Gupta, 2003)

33

Dow F&EI obtained 191.1 which means its degree of hazard is severe.

The complete calculation for above are shown in Appendix I.

5.2 HAZARD IDENTIFICATION

Fault tree analysis method is used for hazard identification for the possible causes of failure

of LPG tank leading to leak or release of large volume of LPG. Four events listed below are

identified as a prime causes that could do severe harm to tank safety:

• Overfilling and spillage from the storage tank

• Over-pressurization of the storage tank

• Leakage or release due to human error

• Mechanical failure/damage of the storage tank

5.2.1 Overfilling

Overfilling can lead to a large release of LPG from tank. LPG storage tank subjected high

level when the tank was being filled and level indicator fails to indicate true level to operator

or operator fails to correctly observe level indicator. At same time if high level alarms fail to

actuate or the operator fails to take correct action after recognizing the high level alarms, then

tank level increases continuously until inlet valve was not closed on unloading line. Fig. 5.2

shows a fault tree for overfilling scenario.

5.2.2 Overpressure

Tank can rupture by overpressure and release large amount of LPF from the tank. Pressure

rise in LPG tank can be occurred due to blockage of safety valve, or sudden drop in

barometric pressure, or rollover. At the same time, if high pressure alarm fails to actuate or

the operator fails to take correct action after recognizing the high pressure alarm, the pressure

in the tank continuously increases. This pressure deviation rises and if relief valve fails to

actuate then it can damage the tank. Loss of instrument air is another cause for overpressure

scenario. Fig. 5.3 shows a fault tree for overpressure scenario.

5.2.3 Mal-operation

LPG can release from storage tank due to careless operators and the malfunctions of the

equipments. If by fault left valve open after doing cleaning the tank, opening of wrong valve

during sampling operation, filling line not coupled to hose properly, operator not vigilant to

34

F

on

iH :

vi on

Q

0

~ F 0

E 0 C)

K.

0

V 4:

w U

+O U V •1~' a~"•+ OO o

~ N

bA

H

0 M

E

0

U

L

p

•4 o~ bA w L=~

N m

A

0

ts. 0

vi

Co m

E

~ k 0 c

eo

W

rfl

O III:Y

U

O C a° .5

Q

O ~ A

O

I I

~

Q rJ-

R I

O 0

U o

Ep 'er

Ho

0 lqr

E

alarm and at the same time pump not stopped can led to large release of LPG from the

tank. Fig. 5.4 shows a fault tree for mal-operation scenario.

5.2.4 Mechanical failure/damage Due to failure/damage of LPG can release from tank. Failure of equipment like flange

failure, valve seating failure, hose failure, gasket failure or crack/rupture of tank led to

LPG release and if pump is not stopped. Fig.5.5 shows a fault tree for mechanical

failure/damage scenario.

These events if not identified and controlled then they can lead to following events:

• A large leakage of LPG followed by catastrophic explosion of a flammable

vapour cloud in the open. Fault tree for this is shown in Fig. 5.1.

• A fire causing sudden rupture of the tank and a large fireball; Fault tree for shown

in Fig.5.6. Sudden rupture of vessel due to overpressure and weakening of

material burst open to the atmosphere called (boiling liquid expanding vapour

cloud explosion) BLEVE. As a result of expansion and flashoff a pressure wave

occurs and a fireball formed.

5.3 FREQUENCY ESTIMATION

Using frequency data from literature, frequency is estimated for each scenario. Frequency

data used for find frequency of top event is given in appendix II.

Minimal cut sets is determined for all the fault trees by applying Boolean algebra.

Minimal cut sets is the smallest combination, if they all occur, will cause the tope event

to occur. The calculation of probability of triggering of top event due to particular

minimal cut set calculated below:

Overfilling

Minimal cut set for overfilling (Fig. 5.2)

P(T1) = A1'YI

= A1 (Y2'Y3)

=A1 (B1'Y4), (C1+D1)

= Al , (B1' (E1+ F1))'(Cl+D1)

41

= Al - (Bl El+B1-Fl)-(CI+D1)

=A1B1C1E1 + AIB1D1E1+ AIB1C1F1+ A1B1D1F1

There are four minimal cut sets obtained: Al B 1 C 1 E 1, A 1 B 1 D 1 E 1, A 1 B 1 C 1 F 1 and

A1B1DIF1

Probability calculation for each minimal cut sets:

P(A1B1C1E1) = P(A1): • P(Bl) - P(Cl) - P(El)

=0.1 X 10-6 X 10-3 X 10 3 =10-13/y

Similarly

P(A1B1DIE1) = 5 x 10-12/y

P(A1BIC1F1) = 10"'°/y

P(A1BICIE1) = 5 x 10'9/y

The probability of top event:

P(T1) = P(A1B1CIE1) + P(A1B1D1E1)+ P(A1B1C1F1)+ P(A1B1DIF1)

=10-3/y

Similar calculation is done for all the fault trees and result is shown in appendix III.

From above calculation frequency of occurrence of overfilling is 10-3 per year which means once in 1 000years.

Similar calculation for frequency of other scenario obtained as: Overpressure (Fig.5.3)

Mal-operation (Fig.5.4)

Mechanical failure/damage (Fig. 5.5)

Fire/Explosion (Fig. 5.1)

Catastrophic failure of tank (Fig.5.6)

5.6 x 10-8 per year

Once in a 1.8 x 107 years 3 x 10-4 per year Once in a 3.3 x 103 years 3.1 x 10-5 per year Once in a 3.2 x 104 years

3.85 x 10-5 per year Once in a 2.5 x 104 years

1.6 x 10-6 per year Once in a 6.25 x 105 years

42

These frequencies of these events can be reduced by Proper design, construction,

maintenance and operation of storage tank; increasing the reliability of the relief valve;

by avoiding valves, flanges, sampling points or any other potential location for leakage

beneath the tank; trained/skilled labours; use of appropriate fire fighting systems;

implementation of safety management program and regular safety audit of the plant.

5.4 CONSEQUENCES ANALYSIS

Following events can occur in case of any leakage/release of LPG from LPG storage tank

• Pool fire

• Fireball

• Torch

• Flash fire

• Vapour cloud explosion

• Vapour cloud without ignition

For the present study three events are considered they are pool fire, fireball and

vapour cloud explosion. Flash fire was considered as vapour cloud explosion and torch

fire as pool fire. Conditional probability of human death for these three events is

calculated by considering worst case scenario.

5.4.1 Pool fire

Accidental release of LPG can result in pool fire. Thermal radiation hazards from pool

fires depend on the composition, the size and shape of the fire, the duration of the fire, the

distance to the object, and the characteristics of the object exposed to thermal radiation.

Consequences due to pool fire were calculated using etus 3.3-3.14. Pool fire pool

diameter was taken from the literature i.e 20 m (Mudan et al., 1984), this from large pool

fire experiment. For this diameter flame height 36.87 metre and effective time of

exposure 14 second was obtained.

Thermal radiation intensity, probit function value and conditional probability of

human death with distance are shown in Appendix IV.

From the result shown in Appendix IV it seen that the probability of human death

is very less only 7 % upto 15 m. Risk to human death is very less but thermal radiation

43

100

90 44 o 80

70

oc 60

50

40 0 x 30

U 20

10

0

—4--Conditional probability of human death vs distance

from pool can act as a trigger for the nearby storage tank or property and cause injury to

people. If proper distance is maintained between two tanks then its risk can be eliminated.

5.4.2 Fireball

When superheated LPG released and immediately ignited may burn as a fireball. The

fireball grows larger and moves upward continuously because of buoyancy. For

consequences analysis equations 3.3 and 3.15 - 3.18 and Table 3.4 were used.

Thermal radiation intensity, probit function value and conditional probability of

human death with distance are shown in Appendix V

Graphs are plotted between conditional probability of human death vs. distance for three

worst accidents.

• Catastrophic failure of one tank (Fig.5.7)

• Catastrophic failure of two tanks (Fig.5.8)

• Catastrophic failure of all three tanks (Fig.5.9)

0 200 400 600

Distance (m)

Fig.5.7 Conditional probability of human death vs human death for fireball due to

catastrophic failure of one LPG storage tank

44

100 90

80

70

60 o c a q 50 —4—Conditional probability of human

0 40 death vs distance

•30

U 20

10

0

0 200 400 600

Distance (m)

Fig.5.8 Conditional probability of human death vs human death for fireball due to

catastrophic failure of two LPG storage tanks

100

90

0 80

if

70

60

a Q 50 —0—Conditional Probability of

40 Human Death Vs Distance

30

U 20

10

0

0 200 400 600 800

Distance (m)

Fig.5.9 Conditional probability of human death vs human death for fireball due to

catastrophic failure of all the three LPG storage tanks

45

From graphs between conditional probability of human death and distance it was

observed that probability of death depends on the amount of LPG release. Probability of

human death increases with LPG release due to increase in thermal radiation intensity

and effective exposure time.

Observation from Fig.5.7, Fig.5.8, Fig.5.9 and Appendix V.

Table 5.2 Impact distance for percentage death/injury of human due to different scenario

of fireball:

Consequences on

Human

Impact Distance (radius in m)

Failure of one Failure of two Failure of three

tank tank tank

100% death probability 72 133 180

99% death probability 128 192 240

50 % death probability 450 335 397

1%death probability 363 473 553

Human injury 800 980 1080

Table 5.3 Damage distances for different thermal heat radiation in case worst case:

Thermal radiation intensity Effect Damage distance

(kW/m2) (radius in m)

37.5* Sufficient to cause damage to 378

process equipment

12.5* Minimum energy required for

piloted ignition of wood and 618

melting of plastic tubing

4* Sufficient cause pain to personnel 916

within 20 s

* Heat flux value for different experience (Ruj et al., 2006)

46

5.1.4.3 Vapour cloud explosion (VCE)

When LPG release and form cloud of vapour without ignition taking place, it ignited later

to a considerable distance from the release point. Three worst case scenarios were

considered for calculation vapour cloud formation due to release of total LPG present in

one tank, two tank and all the three tanks. For calculation for consequences due to VCE equs 3.19-3.22 and Table 3.4 were used.

Maximum overpressure, probit function value and conditional probability of human death with distance are shown in appendix VI.

Graphs are plotted between conditional probability of human death vs distance for three worst accidents.

1. Open vapour Cloud explosion of one tank (Fig.5.10) 2. Open vapour Cloud explosion of two tanks (Fig.5.11) 3. Catastrophic failure of all three tanks (Fig.5.12)

120

100

O

Via° 80

O n a 60 ~p C C ~ o E r 40

0 U

{

—4—conditional probability of human death vs diastance

1 1/ 111 171,

Distance (m)

Fig.5.10 Conditional probability of human death vs human death for VCE due to catastrophic failure of one LPG storage tank

47

120

0 500 1000

Distance (m)

—4—conditional probability of human death vs distance

1500

w, 100 0

80

o 1 a b 60

40

0 V 20

U

Fig.5.11 Conditional probability of human death vs human death for VCE due to

catastrophic failure of two LPG storage tanks

120

100

80

o c o..-b 60

40 c ~ U 20

IC

conditional probability of human death vs distance

0 500 1000 1500

Distance (m)

Fig.5.12 Conditional probability of human death vs human death for VCE due to

catastrophic failure of three LPG storage tanks

48

Observation from Fig. 5.10, Fig. 5.11, Fig. 5.12 and APPENDIX V.

Table 5.4 Impact distance for percentage death of human due to different scenario of VCE:

Consequences on

Human Damage Distance (radius in m)

Failure of one Failure of two Failure of three tank tank tank

100% death probability 146 182 207 99% death probability 172 220 251 50 % death probability 336 425 485 1%death probability 755 950 1050

From the study of various scenarios i.e. pool fire, fireball and VCE it is observed that

impact distance for human death and probability of occurrence of VCE than fireball.

10, VCE Fireball

— Total Risk

105

co -o

C

Iw £VV JUV 4w ODU ODU ((Al t5(U 990 1000 Distance m

10'

o L 0

Fig. 5.13 The Individual Risk Contours

49

5.5 MEASURE OF RISK

This was the last step of risk assessment to calculate actual risk. Individual risk contour

was plotted for fireball, VCE and total risk.

5.1.5.1 Individual Risk

Individual risk was calculated for the identified scenario with its frequency and

consequences. Individual risk is calculated using formula (.22). Calculated risk of

fireball, VCE and total risk is given in Appendix VII.

In Fig.5.13 the individual risk contours are shown. From the graph it is observed

that the largest contribution to the individual risk is the scenario with vapour cloud

explosion. This is due to two reasons first due to large distance travelled by cloud and

second is due to high conditional probability of occurrence. Fireball has less contribution

to total risk due to less frequency of occurrence catastrophic failure of tan contribution to

the individual risk due to less condition probability of VCE.

The HSE quotes 1 x 10-6 per year as a risk of fatality that regarded broadly as

acceptable and 1 x 10-4per year as boundary between tolerable and unacceptable for the

public (Jo et al., 2005). From the fig.15 distance near 600 m from the LPG storage

observed as acceptable risk.

50

CONCLUSION

A simple risk assessment method was performed for three LPG storage tanks having 100

tonnes capacity each considering worst case scenarios. Conclusions drawn from the risk assessment are as follow:

• LPG storage facilities have severe degree of hazard.

• Fault tree analysis method was used to identify the hazards related to LPG storage

facilities and found four main events which can lead to severe hazard to plant or

nearby community. These events were overfilling and spillage from the storage

tank, over-pressurization of the storage tank, leakage or release due to human

error and mechanical failure/damage of the storage tank. These can further lead to

open vapour cloud explosion and catastrophic failure of the storage tank.

• Frequencies of each event were calculated and it was found that overfilling of the

tank has higher frequency of occurrence. Frequency of catastrophic failure of the

tank is less than the vapour could explosion which is once in a 6.25 x 105 years. • The frequency events which can contribute to hazardous scenarios can be reduced

by properdesign, construction, maintenance and operation of storage tank; proper

separation distance between two equipments; increasing the reliability of the relief

valve; by avoiding valves, flanges, sampling points or any other potential location

for leakage beneath the tank; trained/skilled labours; use of appropriate fire

fighting systems; implementation of safety management program and regular safety audit of the plant.

• Consequences analysis was calculated for three main scenarios pool fire, fireball

and vapour cloud explosion. It was found out that pool fire contributes less

hazards to people but it can be the cause accident in the nearby equipment or

property due to its thermal radiation. The largest contribution to the individual

risk is due to vapour cloud explosion.

• Impact of worst case scenario was calculated and it was found that risk near 600m

from the LPG storage tank location was acceptable.

51

FUTURE PLAN OF THE STUDY

The present study is focused on the risk assessment of LPG storage facility of a small

bottling plant. This study can be extended to assessing the risk of large bottling plant and

also to other chemical storage facilities. For more detailed safety analysis, risk

assessment should be carried out at each stage of the bottling operation that happens at

the LPG bottling plant.

52

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[18] Khan F.I., Abbasi S.A., 1998. Techniques and methodologies for risk analysis in

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[23] Marshall V.C., 1989. Major chemical hazards. Chichester: Ellis Horwood.

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55

[34] Shebeko Y.N., Korolchenko A.Y., Shevchuk A.P., Kolosov V.A., Smolin I.M., 1995. Fire and explosion risk assessment for LPG storage. Fire Science & Technology, 15 (1, 2): 37-45 (b).

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56

APPENDIX I

FIRE AND EXPLOSION INDEX

Plant: LPG bottling Processes unit: Evaluated by: Reviewed by: storage tank Nirupama

Material - 300 tonnes of Liquefied petroleum gas (LPG) Material Factor: 21 Base Factor: 1 1. General Process Hazards Penalty Factor Penalty Factor

Range Used A Exothermic Chemical Reactions 0.3-1.25 B Endothermic processes 0.2-0.4 C Material handling and transfer 0.25-1.05 D Enclosed and indoor process units 0.25-0.9 E Access 0.2-0.35 F Drainage and spill control 0.25-0.5 0.5

General process Hazards Factor(F1) 2 2. Special Process Hazards A Toxic material(s) 0.2-0.8 N B Sub atmospheric pressure(<500mm Hg) C Operation in or near flammability range

1.Tank farms storage flammable liquids 0.5 0.5 2.Process upset and purge failure 3.Allways in flammable range 0.8

D Dust explosion E Pressure 1.3 F Low temperature G Quantity of flammable/unstable material

1.liquid or gas in process 2.Liquid or gas in storage 0.85 3.Combustible solid in storage, dust in process

H Corrosion and Erosion 0.1-0.75 .1 I Leakage joint and Packing N J Use of fired equipments N K Hot oil heat exchange system N L Rotating Equipment N

Special Process Hazards Factor(F2) 4.55 3. Process Unit Hazards Factor(F1* F2)=F3 9.1 4. Fire And Explosion Index(F3*MF=F&EI) M F 21(LPG) 191.1

57

Process control (C1) Emergency power 0.98 Cooling 0.97 Explosion control 0.84 Cl 0.7

Material isolation (C2) Remote control valve 0.96 Drainage 0.91

C2 0.8

Fire protection (C3)

Leak detection 0.94 Structural steel 0.95 Water supply 0.94 Special system 0.91

C3 0.51

Emergency shutdown 0.96 Other hazard analysis 0.91

Interlock 0.98

Sprinkler system 0.74 Water curtains 0.97 Hand extinguisher 0.93

5. Credit factor (CF)= C) x C2 x C3 = 0.31 6. Exposure radius = 0.84 X F&EI/3.028 = 53 m 7. Area of exposure = 8829 m2 8. Value of area of exposure (VAE) (2.24 lakh/m2) = 197.77 Crores 9. Damage factor (DF) = 1

10. Base MPPD =VAE x DF =197.77crores 11. Actual MPPD = Base MPPD x CF = 61.31 Crores

58

APPENDIX II

Failure probability data from literatures (Drysdale et al., 1979/80; Lees et al., 1996) Factor Frequency per year

Instrumentation system fails to actuate 10"

Operator fails to observe level indicator correctly iO 3

Operation fails to respond on instrumentation system 5 x 102

Indicator fails to indicate true level to operator 10"

Filling tank 0.1

Fails to actuate to close unloading line 10

Tank pressure rises due to rollover 10

Pressure control valve fails to open 10"

Tank pressure rises due to increase in barometric pressure 02

Operator fails to open safety valve 10.2

Human fault 10"

Pump did not stop 10"

Tank cleaning 0.1

Flange failure 4 x 10"

Gasket failure 5 x 10

Hose failure 4 x 10

Valve seating failure 3 x10 2

Vehicle impact 10"

Crack on the tank 10"

Vibration 10

Repairing while operating 10-

Corrosion 10

Excess pressure 10"

Fatigue 10

Creep 10"

Sabotage 2 x 10"

Weather condition ignition within plant to give open

flammable cloud explosion

0.3

Effective ignition source 2.5 x 10°

59

Water supply fails 10

Brigade slow to act 10

Flame impinge on vessel above liquid level 0.5

Water shut off 10 2

Activation fails 2 x 10-

System frozen 5 x 10

Low main pressure 3 x 10

Sprinkler system damage 10

Pipe completely blocked iO 4

Pipe partially blocked 3 x 10

Sprinkler system head damage 8 x 10

60

A4J4 10

A4K4 10"

A4M4 10"

A4N4 108

E. Minimum cut set for fire at the storage tank Cut set Oder of frequency

ABCT1 7.5 x 10"

ABCT2 4.2 x 10"

ABCT3 3.1 x 10`

ABCT4 9x10 9

F. Minimum cut set for catastrophic failure of tank Cut set Oder of frequency a 5.6 x 10"

be 10

deh 1.9 x 10"

dei 3.8x10 7

dej 9x10 9 "

dek 10 ,

del 5.7 x 10"

dem 1.9x10'°

fgn 5x109

fgo •5.7x 10"

fgp 1.5x108

62

APPENDIX IV

POOL FIRE

Distance (m) Thermal Radiation Intensity (kW/m2)

Probit Function Value Conditional Probability of Human Death (%)

15 30.67246 3.541073 7

20 18.81781 1.873451 0

25 12.54756 0.490102 0

30 8.776268 -0.73008 0

35 6.347933 -1.83573 0

40 4.710043 -2.85438 0

45 3.57142 -3.79897 0

50 2.761583 -4.67674 0

55 2.17054 -5.49876 0

60 1.73809 -6.25717 0

65 1.404858 -6.9837 0

70 1.150644 -7.66505 0

75 0.945964 -8.33363 0

80 0.79031 -8.94727 0

85 0.664198 -9.54066 0

90 0.567323 -10.0788 0

95 0.48054 -10.6455 0

100 0.422525 -11.0846 0

63

APPENDIX V

FIREBALL

A. Fireball due to catastrophic failure of one LPG storage tank Distance (m)

Effective exposure time (s)

Effective fireball diameter (m)

Thermal Radiation Intensity (kW/m2)

Probit Function Value

Conditional Probabilities of Human Death (%)•

50 17 273.26 106.4156 8.284255 100

100 17 273.26 91.06298 7.752455 99.7

150 17 273.26 72.3485 6.967195 98

200 17 273.26 54.90373 6.025411 85

250 17 273.26 40.7298 5.006116 50

300 17 273.26 30.01071 3.963665 15

350 17 273.26 22.17841 2.931353 3

400 17 273.26 16.52826 1.927671 0

450 17 273.26 12.45449 0.961729 0

500 17 273.26 9.498953 0.037059 0

550 17 273.26 7.333719 -0.84597 0

600 17 273.26 5.729317 -1.68867 0

650 17 273.26 4.526224 -2.49322 0

700 17 273.26 3.613298 -3.26213 0

750 17 273.26 2.912567 -3.998 0

800 17 273.26 2.368817 -4.70335 0

850 17 273.26 1.942529 -5.38055 0

900 17 273.26 1.605107 -6.03182 0

950 17 273.26 1.335623 -6.65916 0

1000 17 273.26 1.118596 -7.26443 0

64

B. Fireball due to catastrophic failure of first and second LPG storage tank Distance (m) Effective

exposure time (s)

Effective fireball diameter (m)

Thermal Radiation Intensity kW/m2)

Probit Function Value

Conditional Probabilities of Human Death (%)

50 20.3567 342.302 108.4494 ' 8.810179 100

100 20.3567 342.302 .97.62761. 8.451357 100

150 20.3567 342.302 83.09415 7.901175 99.8

200 20.3567 342.302 67.89398 7.21159 99

250 20.3567 342.302 54.02463 6.431618 92

300 20.3567 342.302 42.35654 5.601096 73

350 20.3567 342.302 33.00256 4.749341 40

400 20.3567 342.302 - 25.70544 3.896408 13

450 20.3567 342.302, 20.09068 3.055203 3

500 20.3567 342.302 15.7924 2.233523 0

550 20.3567 342.302 12.50077 1.435694 0

600 20.3567 342.302 9.970603 0.663771 0

650 20.3567 342.302 8.014545 -0.08164 0

700 20.3567 342.302 6.491908 -0.80084 0

750 20.3567 342.302 5.297823 -1.49463 0

800 20.3567 342.302 4.354238 -2.16415 0

850 20.3567 342.302 3.602933 -2.81064 0

900 20.3567 342.302 3.000286 -3.43541 0

950 20.3567 342.302 2.513424 -4.03978 0

1000 20.3567 342.302 2.117411 -4.62501 0

65

C. Fireball due to catastrophic failure of all three LPG storage tank Distance (m) Effective

exposure time (s)

Effective fireball diameter (m)

Thermal Radiation Intensity (kW/m2)

Probit Function Value

Conditional Probabilities of Human Death (%)

50 22.92 390.5165 109.3061 9.140653 100

100 22.92 390.5165 100.5648 8.856151 100

150 22.92 390.5165 88.30966 8.412578 100

200 22.92 390.5165 74.78265 7.845065 100

250 22.92 390.5165 61.70603 7.189008 98

300 22.92 390.5165 50.06524 6.475435 93

350 22.92 390.5165 40.23313 5.729157 77

400 22.92 390.5165 32.1982 4.968734 49

450 22.92 390.5165 25.76061 4.207341 24

500 22.92 390.5165 20.65827 3.453914 6

550 22.92 390.5165 16.63341 2.714236 2

600 22.92 390.5165 13.46066 1.99183 0

650 22.92 390.5165 10.95463 1.288655 0

700 22.92 390.5165 8.967872 0.605598 0

750 22.92 390.5165 7.385222 -0.05716 0

800 22.92 390.5165 6.117652 -0.6999 0

850 22.92 390.5165 5.096599 -1.32319 0

900 22.92 390.5165 4.269299 -1.92777 0

950 22.92 390.5165 3.595066 -2.51448 0

1000 22.92 390.5165 3.042426 -3.08419 0

APPENDIX VI

VAPOUR CLOUD EXPLOSION (VCE)

A. Vapour cloud explosion (VCE) from one LPG storage tank Distance (m) Maximum

Overpressure (Pa) Probit Function

Value Conditional

Probabilities of Human Death (%)

50 734015.5 11.87492 100

100 149904.4 9.65657 100

150 67051.56 7.931844 100

200 40116.95 6.811484 97

250 27764.11 6.007881 84

300 20923.52 5.390156 65

350 16660 4.892529 46

400 13779.73 4.47799 30

450 11717.41 4.123922 19

500 10174.71 3.815608 12

550 8980.731 3.542996 7

600 8031.213 3.298945 4

650 7259.197 3.078217 3

700 6619.866 2.876867 2

750 6082.165 2.691851 1

800 5623.934 2.52078 0

850 5228.964 2.361746 0

900 4885.134 2.213198 0

950 4583.211 2.073866 0

1000 4316.04 1.942692 0

67

B. Vapour cloud explosion (VCE) from two LPG storage tank Distance (m) Maximum

Overpressure (Pa) Probit Function

Value Conditional

Probabilities of Human Death (%)

50 1310385 12.98195 100

100 246843.7 10.74788 100

150 104539.2 8.902507 100

200 60215.79 7.698689 100

250 40548.06 6.835143 87

300 29938.19 6.172645 78

350 23464.24 5.640506 74

400 19166.53 5.19866 58

450 16134.07 4.822506 43

500 13893.65 4.495997 31

550 12178.07 4.208157 21

600 10826.28 3.951188 15

650 9736.033 3.719372 10

700 8839.562 3.508406 7

750 8090.339 3.314976 5

800 7455.438 3.136485 3

850 6910.95 2.970858 2

900 6439.123 2.816417 2

950 6026.517 2.671786 1

1000 5662.779 2.535822 0

68

C. Vapour cloud explosion (VCE) from all three LPG storage tanks Distance (m) Maximum

Overpressure (Pa) Probit Function Value

Conditional Probabilities of Human Death (%)

50 1857667 13.62938 100

100 335105.8 11.41303 100

150 137545.7 9.501966 100

200 77442.6 8.248236 100

250 51269.02 7.347524 99

300 37364.05 6.656572 95

350 28986.45 6.102101 87

400 23483.93 5.642346 - 74

450 19636.36 5.251555 60

500 16815.74 4.912889 46

550 14670.39 4.614807 35

600 12989.89 4.349103 23

650 11641.56 4.109758 19

700 10537.96 3.892238 13

750 9619.419 3.693057 10

800 8843.901 3.509479 7

850 8181.032 3.339324 5

900 7608.351 3.180827 3

950 7108.924 3.032543 2

1000 6669.752 2.893274 2

1050 6280.708 2.762015 1

1100 5933.787 2.63792 0

1150 5622.585 2.520266 0

1200 5341.922 2.408432 0

APPENDIX VII

INDIVIDUAL RISK

Individual risk for worst scenario Distance (m) Vapour Cloud Fireball Total Risk

Explosion

50 3.85E-05 1.6E-06 0.0000401

100 3.85E-05 1.6E-06 0.0000401 150 3.85E-05 1.6E-06 0.0000401 200 3.85E-05 1.6E-06 4.01 E-05 250 3.81E-05 1.58E-06 3.97E-05 300 3.66E-05 1.49E-06 3.806E-05 350 3.35E-05 1.23E-06 3.473E-05 400 2.85E-05 7.84E-07 2.927E-05 450

2.31E-05 3.84E-07 2.348E-05 500 1.77E-05 9.6E-08 1.781E-05 550

1.35E-05 3.2E-08 1.351E-05 600

8.86E-06 0 8.855E-06 650

7.32E-06 0 7.315E-06 700

5.01E-06 0 5.005E-06 750 3.85E-06 0 3.85E-06 800

2.7E-06 0 2.695E-06 850

1.93E-06 0 1.925E-06 900

1.16E-06 0 1.155E-06 950 7.7E-07 0 7.7E-07 1000 7.7E-07

0 7.7E-07

70