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RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

of 36

Template No. 5-0000-0001-T2 Rev. 1 Copyrights EIL ¬ All rights reserved

PREFACE

NRL is in the process of implementing 0.7 MMTPA Diesel Hydrotreater Unit (DHT) & other

supporting units and utilities for production of BS IV & BS V grade HSD. Engineers India Limited

(EIL), New Delhi, has been entrusted by M/s Numaligarh Refinery Limited, Assam (NRL) as

Consultant for this Project.

In this perspective, NRL has appointed EIL to carry out the Rapid Risk Analysis Study of the units

under scope of DHT Project.

Rapid Risk Analysis study identifies the hazards associated with the facility, analyses the

consequences, draws suitable conclusions and provides necessary recommendations to mitigate

the hazard/ risk.

This Rapid Risk Analysis study is based on the information made available at the time of this

study and EIL’s own data source for similar plants. EIL has exercised all reasonable skill, care

and diligence in carrying out the study. However, this report is not deemed to be any undertaking,

warrantee or certificate.

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TABLE OF CONTENTS

1 EXECUTIVE SUMMARY ....................................................................................................... 6

1.1 INTRODUCTION ............................................................................................................ 6

1.2 APPROACH METHODOLOGY ....................................................................................... 6

1.3 MAJOR FINDINGS AND RECOMMENDATIONS ........................................................... 6

2 INTRODCUTION ................................................................................................................. 10

2.1 STUDY AIMS AND OBJECTIVE ................................................................................... 10

2.2 SCOPE OF WORK ....................................................................................................... 10

3 SITE CONDITION ................................................................................................................ 11

3.1 GENERAL .................................................................................................................... 11

3.2 SITE, LOCATION AND VICINITY ................................................................................. 11

3.3 METEOROLOGICAL CONDITIONS ............................................................................. 11

4 HAZARDS ASSOCIATED WITH THE FACILITIES .............................................................. 15

4.1 GENERAL .................................................................................................................... 15

4.2 HAZARDS ASSOCIATED WITH FLAMMABLE MATERIALS........................................ 15

4.2.1 LIQUIFIED PETROLEUM GAS .............................................................................. 15

4.2.2 HYDROGEN .......................................................................................................... 15

4.2.3 NAPHTHA AND OTHER HEAVIER HYDROCARBONS ........................................ 16

4.3 HAZARDS ASSOCIATED WITH TOXIC MATERIALS .................................................. 17

4.3.1 HYDROGEN SULPHIDE ....................................................................................... 17

4.3.2 AMMONIA ............................................................................................................. 17

5 HAZARD IDENTIFICATION ................................................................................................. 18

5.1 GENERAL .................................................................................................................... 18

5.2 MODES OF FAILURE ................................................................................................... 18

5.3 SELECTED FAILURE CASES ...................................................................................... 19

6 CONSEQUENCE ANALYSIS ............................................................................................... 21

6.1 GENERAL .................................................................................................................... 21

6.2 CONSEQUENCE ANALYSIS MODELLING .................................................................. 21

6.2.1 DISCHARGE RATE ............................................................................................... 21

6.2.2 DISPERSION ......................................................................................................... 21

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6.2.3 FLASH FIRE .......................................................................................................... 21

6.2.4 JET FIRE ............................................................................................................... 22

6.2.5 POOL FIRE ........................................................................................................... 22

6.2.6 VAPOR CLOUD EXPLOSION ............................................................................... 22

6.2.7 TOXIC RELEASE .................................................................................................. 22

6.3 SIZE AND DURATION OF RELEASE ........................................................................... 22

6.4 DAMAGE CRITERIA ..................................................................................................... 23

6.4.1 LFL OR FLASH FIRE ............................................................................................. 23

6.4.2 THERMAL HAZARD DUE TO POOL FIRE, JET FIRE AND FIRE BALL ................ 23


6.4.4 TOXIC HAZARD .................................................................................................... 24

6.5 CONSEQUENCE ANALYSIS FOR UNITS.................................................................... 24

6.5.1 DHT ....................................................................................................................... 25

6.5.2 SRU ....................................................................................................................... 28

6.5.3 EXISTING ARU & SWS ......................................................................................... 28

6.5.4 OFFSITES ............................................................................................................. 29

7 MAJOR FINDINGS & RECOMMENDATIONS ..................................................................... 30

8 GLOSSARY ......................................................................................................................... 34

9 REFERENCES .................................................................................................................... 36

ANNEXURE-I: CONSEQUENCE ANALYSIS HAZARD DISTANCES

ANNEXURE-II: FIGURES FOR CONSEQUENCE ANALYSIS

LIST OF TABLES

Table 1: New Proposed Process Facilities under DHT Project .................................................... 10

Table 2: Existing Process Facilities under DHT Project ............................................................... 10

Table 3: Offsite facilities –Storage tanks ..................................................................................... 10

Table 4: Atmospheric Parameter ................................................................................................. 12

Table 5: Average Mean Wind Speed (m/s) .................................................................................. 12

Table 6: % Number of Days Wind From ...................................................................................... 12

Table 7: Pasquill Stability Classes ............................................................................................... 13

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Table 8: Weather Conditions ....................................................................................................... 14

Table 9: Hazardous Properties of LPG ........................................................................................ 15

Table 10: Hazardous Properties of Hydrogen ............................................................................. 16

Table 11: Hazardous Properties of Naphtha ............................................................................... 16

Table 12: Toxic Effects of Hydrogen Sulphide ............................................................................. 17

Table 13: Toxic Effects of Ammonia ............................................................................................ 17

Table 14: Size of Release ........................................................................................................... 22

Table 15: Damage Due to Incident Thermal Radiation Intensity .................................................. 23

Table 16: Damage Effects of Blast Overpressure ........................................................................ 24

LIST OF FIGURES

Figure 1: NRL Site ...................................................................................................................... 11

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1 EXECUTIVE SUMMARY

1.1 INTRODUCTION

NRL, a subsidiary of M/s Bharat Petroleum Corporation Limited presently operates a 3.0 MMTPA

refinery at Numaligarh, Golaghat, Assam. The Refinery presently produces Euro-III and part Euro-

IV grade HSD. NRL is in the process of implementing 0.7 MMTPA Diesel Hydrotreater Unit (DHT)

& other supporting units and utilities for production of BS IV & BS V grade HSD in line with the

AFYP 2025 requirements. EIL has been selected as consultant for the DHT Project.

In this context, NRL has also entrusted EIL to carry out the Rapid Risk Analysis Study of the units

under scope of DHT Project.

This executive summary covers major findings arising out of the Rapid Risk Analysis study and

recommendations for the facilities under DHT Project, NRL-Assam. The detailed analysis is given

in Section –6.

1.2 APPROACH METHODOLOGY

RRA study evaluates the consequences of potential failure scenarios, assess extent of damages,

based on damage criteria’s and suggest suitable measures for mitigating the Hazard.

RRA involves identification of various potential hazards & credible failure scenarios for various

units and other facilities including off-site storages & pumping, etc., based on their frequency of

occurrence & resulting consequence. Basically two types of scenarios are identified spanning

across various process facilities; Cases with high chance of occurrence but having low

consequence, e.g., Instrument Tapping Failure and cases with low chance of occurrence but

having high consequence, e.g., Catastrophic Rupture of Pressure Vessels / Large Hole on the

outlet of Pressure Vessels. Effect zones for various outcomes of failure scenarios (Flash Fire, Jet

Fire, Pool Fire, Blast overpressure, toxic release, etc.) are studied and identified in terms of

distances on plot plan. Based on effect zones, measures for mitigation of the hazard/risk are

suggested.

1.3 MAJOR FINDINGS AND RECOMMENDATIONS

The detailed consequence analysis of release of hydrocarbon in case of major credible scenarios

are modeled in terms of release rate, dispersion, flammability and toxic characteristics, which

have been discussed in detail in the report. The major findings and recommendations arising out

of the Rapid Risk analysis study for units under DHT Project are summarized below:

Consequence modeling of low frequency failure scenarios for DHT is carried out and it was

observed that in the event of realization of failure scenarios (Large Hole in Feed Surge Drum,

HP Cold Separator, LP Cold Separator, Product Stripper, Naphtha Stabilizer), radiation &

explosion effect zones may get extended up to adjacent Hydro-finishing unit on eastern side

& Fire station, Lab Building, Medical Center on western side.

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Further, in the event of Large Hole failure scenarios in HP Cold Separator, LP Cold

Separator, Product Stripper, 5 & 3 psi blast waves may extend beyond the Refinery

compound wall on southern side depending upon the prevalent weather condition and

presence of ignition source at the time of release.

Since, these are low frequency credible failure scenarios, outcomes of the above to be

utilized for updation of the existing Disaster Management Plan (DMP) & Emergency

Response Plan (ERP). Adequate number of hydrocarbon detectors to be ensured at strategic

locations within the DHT unit for early leak detection and inventory isolation.

High frequency credible failure scenarios are also modeled for DHT unit and their explosion &

radiation effects are analyzed. It is observed that, in the event of the Instrument tapping

failure at feed charge pump, LFL may get extended up to a distance of 76 m, covering SRR-1

and sub-station area present on the western side of the unit.

The Jet fire radiation intensity of 37.5 & 12.5 kW/m2 may be realized up to a distance of 60 m

& 73 m respectively and may affect SRR-1 & sub-station.

5 & 3 psi blast overpressure wave may reach up to a distance of 88 m & 95 m respectively.

These pressure waves may cause damage to SRR-1, Substation and Caustic solution tank.

On the western side, 2 psi blast overpressure wave may reach up to existing fire station last

bay, depending upon the prevalent weather condition and presence of ignition source at the

time of release.

Last bay (maintenance bay) of existing Fire Station is under affect zone of high frequency

credible scenario (Instrument tapping failure at Feed charge pump). Being a maintenance

bay, no Fire Tender is expected to be present regularly in this bay. However, it is

recommended to ensure that Fire tender is not stationed on permanent basis in the last bay.

Toxic gas dispersion modelling is also carried out for various high frequency credible failure

scenarios for DHT (Instrument Tapping Failure at HP Cold Separator Overhead, Recycle Gas

Compressor, Stripper Reflux Drum Overhead & Flange Leakage at Stripper Reflux Pump)

and it is observed that H2S IDLH hazard distance may extend beyond DHT unit battery limit.

However, the affect zone is confined within the Refinery Compound wall.

Hence, it is recommended to locate the toxic gas detectors within the DHT unit at strategic

locations and utilize the outcomes of these failure scenarios in the updation of the existing

Disaster Management Plan (DMP) & Emergency Response Plan (ERP).

Toxic gas credible failure scenarios are modeled for SRU. It is observed that in the event of

Instrument tapping failure at Acid Gas KOD & Sour Gas KOD under 1F weather condition,

H2S IDLH concentration may extend up to a distance of 283 m & 307 m respectively, from the

leak source, depending upon the prevailing wind conditions at the time of release. H2S IDLH

concentration may extend beyond the Refinery compound wall on southern side & may affect

the nearby personnel’s.

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Hence, it is recommended to install toxic gas detectors within SRU at strategic locations and

utilize the outcome of these failure scenarios in the updation of the existing Disaster

Management Plan (DMP) & Emergency Response Plan (ERP). Moreover, due to close

proximity of another Sulphur Block, it is recommended to carry out Quantitative Risk Analysis

(QRA) of the refinery, so as to assess the effect of the H2S IDLH concentration in terms of

Individual & Societal Risk on the nearby buildings / operator cabins / populations.

a) Recommendations for Construction Safety during execution of the DHT Project

Adequate barricading of the proposed unit to be done from existing running process units

during construction phase. Hydrocarbon / toxic detectors to be placed along the

barricading suitably to detect any hydrocarbon / toxic gas in vicinity of construction area.

Also, adequate firefighting & toxic gas handling arrangement are to be ensured in the

construction area. Ensure training of persons associated with construction activities for

response during fire & toxic gas release.

Proper material movement path within the Refinery shall be identified during the

construction phase of the project.

Detailed HSE Plan & HSE Philosophy to be developed by contractors during construction

phase of the project, in line with client’s safety requirements.

It is suggested to carry out HAZID, SIMOPS studies during pre-execution phase of the

DHT project to get a detailed overview of the possible hazards during construction phase

and action plan to prevent / mitigate the same.

b) General Recommendations

No operator cabin shall be located inside battery limits of proposed units under DHT

project. Also, operator cabin, if any located in the vicinity of unit shall be made of Blast

Resistant Construction with Positive Pressurization for protection of individuals inside

operator cabin from Explosion, Radiation & Toxic effects of various leak scenarios.

Proper checking of contract people for smoking or inflammable materials to be ensured at

entry gates to avoid presence of any unidentified source of ignition.

Ensure vehicles entering the Refinery are fitted with spark arrestors, as a mandatory item.

In order to prevent secondary incident arising from any failure scenario, it is recommended

that sprinklers and other protective devices provided on the tanks are regularly checked to

ensure these are functional.

Mock drills to be organized at organization level to ensure preparation of the personnel’s

working in Refinery for handling any hazardous situation.

For positively pressurized building, both Hydrocarbon & Toxic detectors need to be placed

at suction duct of HVAC. HVAC to be tripped automatically in event of the detection of any

Hydrocarbon / toxic material by detector.

Ensure usage of safer oxidizing agents (Chlorine free) in Cooling Water circuit.

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c) Mitigating Measures

Mitigating measures are those measures in place to minimize the loss of containment event and,

hazards arising out of Loss of containment. These include:

Early detection of an undesirable event (HC leak, Toxic gas leak, Flame etc.) and

development of subsequent quick isolation mechanism for major inventories.

Measures for controlling / minimization of Ignition sources inside the Refinery complex.

Active and Passive Fire Protection for critical equipment’s and major structures

Effective Emergency Response plans to be in place

d) Ignition Control

Ignition control will reduce the likelihood of fire events. This is the key for reducing the risk

within facilities processing flammable materials. As part of mitigation measure it strongly

recommended to consider minimization of the traffic movement within the Refinery.

e) Escape Routes

Ensure sufficient escape routes from the site are available to allow redundancy in escape

from all areas.

Ensure sufficient number of windsocks throughout the site to ensure visibility from all

locations. This will enable people to escape upwind or crosswind from flammable / toxic

releases.

Provide sign boards marking emergency/safe roads to be taken during any exigencies.

f) Preventive Maintenance for Critical Equipment’s

In order to reduce the failure frequency of critical equipment’s, the following are

recommended:

a. High head pumps and Compressors, which are in flammable / toxic services, are

needed to be identified.

i. Their seals, instruments and accessories are to be monitored closely

ii. A detailed preventive maintenance plan to be prepared and followed.

b. Surge Drums & Reflux drums and high inventory vessels whose rupture may lead

to massive consequences are needed to be identified and following to be ensured:

i. Monitoring of vessel internals during shut down.


g) Others

Ensure removal of hammer blinds from the process facilities, if any.

Closed sampling system to be considered for pressurized services like LPG, Propylene

etc.

Recommended to use portable HC detector during sampling and maintenance etc.

Provide breathing apparatus at strategic locations inside Refinery.

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2 INTRODCUTION

2.1 STUDY AIMS AND OBJECTIVE

The objectives of the Rapid Risk Analysis study are to identify and quantify all potential failure

modes that may lead to hazardous consequences and extent. Typical hazardous consequences

include fire, explosion and toxic releases.

The Rapid Risk analysis will also identify potential hazardous consequences having impacts on

population and property in the vicinity of the facilities, and provides information necessary in

developing strategies to prevent accidents and formulate the Disaster Management Plan.

The Rapid Risk Analysis includes the following steps:

a) Identification of failure cases within the process and off-site facilities

b) Evaluate process hazards emanating from the identified potential accident scenarios.

c) Analyze the damage effects to surroundings due to such incidents.

d) Suggest mitigating measures to reduce the hazard / risk.

The Risk analysis study has been carried out using the risk assessment software program

‘PHAST ver. 7.11 developed by DNV Technica.

2.2 SCOPE OF WORK

The study addresses the hazards that can be realized due to operations associated with the

proposed facilities under DHT Project. It covers the following facilities of Refinery:

Table 1: New Proposed Process Facilities under DHT Project

S. No. Description Remarks

1. DHT

2. SRU

Table 2: Existing Process Facilities under DHT Project


1. SWS Revamp

2. ARU Revamp

Table 3: Offsite facilities –Storage tanks

S. No. Facility Remarks

1. DHT Feed Storage Tanks

2. DHT Feed Pump

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3 SITE CONDITION

3.1 GENERAL

This chapter describes the location of Numaligarh Refinery complex and meteorological data,

which have been used for the Rapid Risk Analysis study.

3.2 SITE, LOCATION AND VICINITY

Numaligarh Refineries Limited (NRL) presently operates a 3 MMTPA refinery at Golaghat, Assam,

India.

Figure 1: NRL Site

3.3 METEOROLOGICAL CONDITIONS

The consequences of released toxic or flammable material are largely dependent on the

prevailing weather conditions. For the assessment of major scenarios involving release of toxic or

flammable materials, the most important meteorological parameters are those that affect the

atmospheric dispersion of the escaping material. The crucial variables are wind direction, wind

speed, atmospheric stability and temperature. Rainfall does not have any direct bearing on the

results of the risk analysis; however, it can have beneficial effects by absorption / washout of

released materials. Actual behavior of any release would largely depend on prevailing weather

condition at the time of release.

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For the Risk Analysis study, Meteorological data of Tezpur have been taken from the

Climatological Tables of Observatories in India (1961-1990) published by Indian Meteorological

Department, Pune.

Atmospheric Parameters

The Climatological data which have been used for the Risk Analysis study is summarized below:

Table 4: Atmospheric Parameter

Sl. No. Parameter Average Value Considered For Study

1. Ambient Temperature (OC) 28

2. Atmospheric Pressure (mm Hg) 760

3. Relative Humidity (%) 75

4. Solar Radiation flux (kW/m2) 0.70

Wind Speed and Wind Direction

Based on the Meteorological data provided in IMD table, it is observed that average wind speed of

magnitude of around 1 m/s blows for around 60% of the time, in a year. And for rest of time period

average wind speed is around 1-2 m/s. Hence predominant wind speed for Refinery complex

considered for RRA Study is 1 & 2 m/s.

Table 5: Average Mean Wind Speed (m/s)

Jan Feb Mar April May June July Aug Sep Oct Nov Dec

0.91 1.16 1.75 1.88 1.2 0.94 0.8 0.72 0.8 0.8 1.05 0.9

Table 6: % Number of Days Wind From

N NE E SE S SW W NW Calm

N 1 43 12 4 1 6 1 0 32

D 1 28 6 2 1 7 2 1 52

Weather Category

One of the most important characteristics of atmosphere is its stability. Stability of atmosphere is

its tendency to resist vertical motion or to suppress existing turbulence. This tendency directly

influences the ability of atmosphere to disperse pollutants emitted into it from the facilities. In most

dispersion scenarios, the relevant atmospheric layer is that nearest to the ground, varying in

thickness from a few meters to a few thousand meters. Turbulence induced by buoyancy forces in

the atmosphere is closely related to the vertical temperature gradient.

Temperature normally decreases with increasing height in the atmosphere. The rate at which the

temperature of air decreases with height is called Environmental Lapse Rate (ELR). It will vary

from time to time and from place to place. The atmosphere is said to be stable, neutral or unstable

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according to ELR is less than, equal to or greater than Dry Adiabatic Lapse Rate (DALR), which is

a constant value of 0.98°C/100 meters.

Pasquill stability parameter, based on Pasquill – Gifford categorization, is such a meteorological

parameter, which decreases the stability of atmosphere, i.e., the degree of convective turbulence.

Pasquill has defined six stability classes ranging from `A' (extremely unstable) to `F' (stable). Wind

speeds, intensity of solar radiation (daytime insulation) and nighttime sky cover have been

identified as prime factors defining these stability categories. Below Table indicates the various

Pasquill stability classes.

Table 7: Pasquill Stability Classes

Surface Wind Speed

(meter/s)

Day time solar radiation Night time cloud cover

Strong Medium Slight Thin < 3/8 Medium 3/8 Overcast >4/5

< 2 A A – B B - - D

2 – 3 A – B B C E F D

3 – 5 B B – C C D E D

5 – 6 C C – D D D D D

> 6 C D D D D D

Legend: A = Very unstable, B = Unstable, C = Moderately unstable, D = Neutral, E = Moderately

stable, F = stable

When the atmosphere is unstable and wind speeds are moderate or high or gusty, rapid

dispersion of pollutants will occur. Under these conditions, pollutant concentrations in air will be

moderate or low and the material will be dispersed rapidly. When the atmosphere is stable and

wind speed is low, dispersion of material will be limited and pollutant concentration in air will be

high. In general worst dispersion conditions (i.e. contributing to greater hazard distances) occur

during low wind speed and very stable weather conditions, such as that at 1F weather condition

(i.e. 1 m/s wind speed and Pasquill Stability F).

Stability category for the present study is identified based on the cloud amount and wind speed.

For risk analysis the representative average annual weather conditions are assessed based on

the following:

Average Wind speed in order of 1 m/s would be experienced for more than 60% of time in a year

and Wind speed of 1-2 m/s can be realized in the month of Feb to May. Based on weather

analysis, predominant weather stability of “F”& “D” was selected with wind speed 1 m/s & 2 m/s

for consequence analysis, respectively.

The consequence results are reported in tabular form for all the weather conditions and are

represented graphically for worst weather condition.

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Table 8: Weather Conditions

Wind Speed Pasquill Stability

1 F

2 D

Note: For RRA Study Plot Plan (Doc. No.: A774-000-17-44-0001 Rev N) has been used. 1

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4 HAZARDS ASSOCIATED WITH THE FACILITIES

4.1 GENERAL

Refinery complex handles a number of hazardous materials like LPG, Hydrogen, Naphtha and

other hydrocarbons which have a potential to cause fire and explosion hazards. The toxic

chemicals like Ammonia and Hydrogen sulfide are also handled in the Refinery. This chapter

describes in brief the hazards associated with these materials.

4.2 HAZARDS ASSOCIATED WITH FLAMMABLE MATERIALS

4.2.1 LIQUIFIED PETROLEUM GAS

LPG is a colorless liquefied gas that is heavier than air and may have a foul smelling odorant

added to it. It is a flammable gas and may cause flash fire and delayed ignition.

LPG is incompatible to oxidizing and combustible materials. It is stable at normal temperatures

and pressure. If it is released at temperatures higher than the normal boiling point it can flash

significantly and would lead to high entrainment of gas phase in the liquid phase. High

entrainment of gas phase in the liquid phase can lead to jet fires. On the other hand negligible

flashing i.e. release of LPG at temperatures near boiling points would lead to formation of pools

and then pool fire. LPG releases may also lead to explosion in case of delayed ignition.

Inhalation of LPG vapors by human beings in considerable concentration may affect the central

nervous system and lead to depression. Inhalation of extremely high concentration of LPG may

lead to death due to suffocation from lack of oxygen. Contact with liquefied LPG may cause

frostbite. Refer to below table for properties of LPG.

Table 9: Hazardous Properties of LPG

Sl. No. Properties Values

1. LFL (%v/v) 1.7

2. UFL (%v/v) 9.0

3. Auto ignition temperature (°C) 420-540

4. Heat of combustion (Kcal/Kg) 10960

5. Normal Boiling point (°C) -20 to –27

6. Flash point (°C) - 60

4.2.2 HYDROGEN

Hydrogen (H2) is a gas lighter than air at normal temperature and pressure. It is highly flammable

and explosive. It has the widest range of flammable concentrations in air among all common

gaseous fuels. This flammable range of Hydrogen varies from 4% by volume (lower flammable

limit) to 75% by volume (upper flammable limit). Hydrogen flame (or fire) is nearly invisible even

though the flame temperature is higher than that of hydrocarbon fires and hence poses greater

hazards to persons in the vicinity.

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Constant exposure of certain types of ferritic steels to hydrogen results in the embrittlement of the

metals. Leakage can be caused by such embrittlement in pipes, welds, and metal gaskets.

In terms of toxicity, hydrogen is a simple asphyxiant. Exposure to high concentrations may

exclude an adequate supply of oxygen to the lungs. No significant effect to human through dermal

absorption and ingestion is reported. Refer to below table for properties of hydrogen.

Table 10: Hazardous Properties of Hydrogen


1. LFL (%v/v) 4.12

2. UFL (%v/v) 74.2

3. Auto ignition temperature (°C) 500


5. Normal Boiling point (°C) -252

6. Flash point (°C) N.A.

4.2.3 NAPHTHA AND OTHER HEAVIER HYDROCARBONS

The major hazards from these types of hydrocarbons are fire and radiation. Any spillage or loss of

containment of heavier hydrocarbons may create a highly flammable pool of liquid around the

source of release.

If it is released at temperatures higher than the normal boiling point it can flash significantly and

would lead to high entrainment of gas phase in the liquid phase. High entrainment of gas phase in

the liquid phase can lead to jet fires. On the other hand negligible flashing i.e. release at

temperatures near boiling points would lead to formation of pools and then pool fire.

Spillage of comparatively lighter hydrocarbons like Naphtha may result in formation of vapor

cloud. Flash fire/ explosion can occur in case of ignition. Refer to below table for properties of

Naphtha.

Table 11: Hazardous Properties of Naphtha

S. No. Properties Values

1. LFL (%v/v) 0.8

2. UFL (%v/v) 5.0

3. Auto ignition temperature (°C) 228

4. Heat of combustion (Kcal//Kg) 10,100

5. Normal Boiling point (°C) 130 -155

6. Flash point (°C) 38 - 42

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4.3 HAZARDS ASSOCIATED WITH TOXIC MATERIALS

4.3.1 HYDROGEN SULPHIDE

Hydrogen sulfide is a known toxic gas and has harmful physiological effects. Accidental release of

hydrocarbons containing hydrogen sulfide poses toxic hazards to exposed population. Refer to

below table for hazardous properties of Hydrogen Sulphide.

Table 12: Toxic Effects of Hydrogen Sulphide

Sl. No. Threshold Limits Concentration (PPM)

1. Odor threshold 0.0047

2. Threshold Limit Value(TLV) 10

3. Short Term Exposure Limit (STEL)(15 Minutes) 15

4. Immediately Dangerous to Life and Health (IDLH) level (for 30

min exposure) 100

4.3.2 AMMONIA

Ammonia is likely to be present in sour gas produced from Sour water stripper unit (SWSU). The

hazard associated with ammonia is both toxic and flammable hazards. Toxic hazards being more

pronounced. Vapors of ammonia may cause severe eye or throat irritation and permanent injury

may result. Contact with the liquid freezes skin and produces a caustic burn. Below table indicates

the toxic properties of ammonia.

Table 13: Toxic Effects of Ammonia

Sl. No. Threshold Limits Concentration (PPM)

1. Threshold Limit Value (TLV) 25

2. Short Term Exposure Limit (STEL)(15 Minutes) 35

3. Immediately Dangerous to Life and Health (IDLH) level

(for 30 min exposure) 300

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5 HAZARD IDENTIFICATION

5.1 GENERAL

A classical definition of hazard states that hazard is in fact the characteristic of

system/plant/process that presents potential for an accident. Hence all the components of a

system/plant/process need to be thoroughly examined in order to assess their potential for

initiating or propagating an unplanned event/sequence of events, which can be termed as an

accident.

In Risk Analysis terminology a hazard is something with the potential to cause harm. Hence the

Hazard Identification step is an exercise that seeks to identify what can go wrong at the major

hazard installation or process in such a way that people may be harmed. The output of this step is

a list of events that need to be passed on to later steps for further analysis.

The potential hazards posed by the facility were identified based on the past accidents, lessons

learnt and a checklist. This list includes the following elements.

Catastrophic Rupture of Pressure vessel

Large hole on outlet of process vessel

“Guillotine-Breakage” of pipe-work

Small hole, cracks or small bore failure (i.e. instrument tapping failure, drains/vents failure

etc.) in piping and vessels.

Flange leaks.

Storage Tank on fire

Leaks from pump glands and similar seals.

5.2 MODES OF FAILURE

There are various potential sources of large leakage, which may release hazardous chemicals

and hydrocarbon materials into the atmosphere. These could be in form of gasket failure in

flanged joints, bleeder valve left open inadvertently, an instrument tubing giving way, pump seal

failure, guillotine failure of equipment/ pipeline or any other source of leakage. Operating

experience can identify lots of these sources and their modes of failure. A list of general

equipment and pipeline failure mechanisms is as follows:

Material/Construction Defects

Incorrect selection or supply of materials of construction

Incorrect use of design codes

Weld failures

Failure of inadequate pipeline supports

Pre-Operational Failures

Failure induced during delivery at site

Failure induced during installation

Pressure and temperature effects

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Overpressure

Temperature expansion/contraction (improper stress analysis and support design)

Low temperature brittle fracture (if metallurgy is incorrect)

Fatigue loading (cycling and mechanical vibration)

Corrosion Failures

Internal corrosion (e.g. ingress of moisture)

External corrosion

Cladding/insulation failure (e.g. ingress of moisture)

Cathodic protection failure, if provided

Failures due to Operational Errors

Human error

Failure to inspect regularly and identify any defects

External Impact Induced Failures

Dropped objects

Impact from transport such as construction traffic

Vandalism

Subsidence

Strong winds

Failure due to Fire

External fire impinging on pipeline or equipment

Rapid vaporization of cold liquid in contact with hot surfaces

5.3 SELECTED FAILURE CASES

A list of selected failure cases was prepared based on process knowledge, engineering judgment,

experience, past incidents associated with such facilities and considering the general

mechanisms for loss of containment. A list of cases has been identified for the consequence

analysis study based on the following.

Cases with high chance of occurrence but having low consequence: Example of such

failure cases includes two-bolt gasket leak for flanges, seal failure for pumps, instrument

tapping failure, etc. The consequence results will provide enough data for planning routine

safety exercises. This will emphasize the area where operator's vigilance is essential.

Cases with low chance of occurrence but having high consequence (The example includes

Large hole on the outlet of pressure vessels, Catastrophic Rupture of Pressure Vessels,

etc.)

This approach ensures at least one representative case of all possible types of accidental

failure events, is considered for the consequence analysis. Moreover, the list below

includes at least one accidental case comprising of release of different sorts of highly

hazardous materials handled in the refinery. Although the list does not give complete failure

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incidents considering all equipment’s, units, but the consequence of a similar incident

considered in the list below could be used to foresee the consequence of that particular

accident.

For selected credible failure scenarios and likely consequences for units of Numaligarh Refinery

under DHT Project, refer Section-6.

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6 CONSEQUENCE ANALYSIS

6.1 GENERAL

Consequence analysis involves the application of the mathematical, analytical and computer

models for calculation of the effects and damages subsequent to a hydrocarbon / toxic release

accident.

Computer models are used to predict the physical behavior of hazardous incidents. The model

uses below mentioned techniques to assess the consequences of identified scenarios:

Modeling of discharge rates when holes develop in process equipment/pipe work

Modeling of the size & shape of the flammable/toxic gas clouds from releases in the

atmosphere

Modeling of the flame and radiation field of the releases that are ignited and burn as jet fire,

pool fire and flash fire

Modeling of the explosion fields of releases which are ignited away from the point of release

The different consequences (Flash fire, pool fire, jet fire and Explosion effects) of loss of

containment accidents depend on the sequence of events & properties of material released

leading to the either toxic vapor dispersion, fire or explosion or both.

6.2 CONSEQUENCE ANALYSIS MODELLING

6.2.1 DISCHARGE RATE

The initial rate of release through a leak depends mainly on the pressure inside the equipment,

size of the hole and phase of the release (liquid, gas or two-phase). The release rate decreases

with time as the equipment depressurizes. This reduction depends mainly on the inventory and

the action taken to isolate the leak and blow-down the equipment.

6.2.2 DISPERSION

Releases of gas into the open air form clouds whose dispersion is governed by the wind, by

turbulence around the site, the density of the gas and initial momentum of the release. In case of

flammable materials the sizes of these gas clouds above their Lower Flammable Limit (LFL) are

important in determining whether the release will ignite. In this study, the results of dispersion

modeling for flammable materials are presented LFL quantity.

6.2.3 FLASH FIRE

A flash fire occurs when a cloud of vapors/gas burns without generating any significant

overpressure. The cloud is typically ignited on its edge, remote from- the leak source. The

combustion zone moves through the cloud away from the ignition point. The duration of the flash

fire is relatively short but it may stabilize as a continuous jet fire from the leak source. For flash

fires, an approximate estimate for the extent of the total effect zone is the area over which the

cloud is above the LFL.

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6.2.4 JET FIRE

Jet fires are burning jets of gas or atomized liquid whose shape is dominated by the momentum of

the release. The jet flame stabilizes on or close to the point of release and continues until the

release is stopped. Jet fire can be realized, if the leakage is immediately ignited. The effect of jet

flame impingement is severe as it may cut through equipment, pipeline or structure. The damage

effect of thermal radiation is depended on both the level of thermal radiation and duration of

exposure.

6.2.5 POOL FIRE

A cylindrical shape of the pool fire is presumed. Pool-fire calculations are then carried out as part

of an accidental scenario, e.g. in case a hydrocarbon liquid leak from a vessel leads to the

formation of an ignitable liquid pool. First no ignition is assumed, and pool evaporation and

dispersion calculations are being carried out. Subsequently late pool fires (ignition following

spreading of liquid pool) are considered. If the release is bunded, the diameter is given by the size

of the bund. If there is no bund, then the diameter is that which corresponds with a minimum pool

thickness, set by the type of surface on which the pool is spreading.

6.2.6 VAPOR CLOUD EXPLOSION

A vapor cloud explosion (VCE) occurs if a cloud of flammable gas burns sufficiently quickly to

generate high overpressures (i.e. pressures in excess of ambient). The overpressure resulting

from an explosion of hydrocarbon gases is estimated considering the explosive mass available to

be the mass of hydrocarbon vapor between its lower and upper explosive limits.

6.2.7 TOXIC RELEASE

The aim of the toxic risk study is to determine whether the operators in the plant, people occupied

buildings and the public are likely to be affected by toxic substances. Toxic gas cloud e.g. H2S,

chlorine, Benzene etc. was undertaken to the Immediately Dangerous to Life and Health

concentration (IDLH) limit to determine the extent of the toxic hazard Created as the result of loss

of containment of a toxic substance.

6.3 SIZE AND DURATION OF RELEASE

Leak size considered for selected failure cases are listed below1.

Table 14: Size of Release

Failure Description Leak Size

Pump seal failure 6 mm hole size

Flange gasket failure 10 mm hole size

Instrument tapping failure 20 mm hole size

Large Hole in the Piping 50 mm, complete rupture of 2” drain line at the Process

vessel outlet

1 Refer to Guideline for Quantitative Risk assessment ‘Purple Book’.

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Catastrophic Rupture Complete Rupture of the Pressure Vessels

The discharge duration is taken as 10 minutes for continuous release scenarios as it is

considered that it would take plant personnel about 10 minutes to detect and isolate the leak2.

6.4 DAMAGE CRITERIA

In order to appreciate the damage effect produced by various scenarios, physiological/physical

effects of the blast wave, thermal radiation or toxic vapor exposition are discussed.

6.4.1 LFL OR FLASH FIRE

Hydrocarbon vapor released accidentally will spread out in the direction of wind. If a source of

ignition finds an ignition source before being dispersed below lower flammability limit (LFL), a

flash fire is likely to occur and the flame will travel back to the source of leak. Any person caught

in the flash fire is likely to suffer fatal burn injury. Therefore, in consequence analysis, the distance

of LFL value is usually taken to indicate the area, which may be affected by the flash fire.

Flash fire (LFL) events are considered to cause direct harm to the population present within the

flammability range of the cloud. Fire escalation from flash fire such that process or storage

equipment or building may be affected is considered unlikely.

6.4.2 THERMAL HAZARD DUE TO POOL FIRE, JET FIRE AND FIRE BALL

Thermal radiation due to pool fire, jet fire or fire ball may cause various degrees of burn on human

body and process equipment. The damage effect due to thermal radiation intensity is tabulated

below.

Table 15: Damage Due to Incident Thermal Radiation Intensity

Incident Radiation Intensity

(Kw/M²) Type of Damage

37.5 Sufficient to cause damage to process equipment

32.0 Maximum flux level for thermally protected tanks containing flammable

liquid

12.5 Minimum energy required for piloted ignition of wood, melting of plastic

tubing etc.

8.0 Maximum heat flux for un-insulated tanks

4.0 Sufficient to cause pain to personnel if unable to reach cover within 20

seconds. However blistering of skin (1stdegree burns) is likely.

2 Release duration is based on Chemical Process Quantitative Risk Analysis, CCPS.

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The hazard distances to the 37.5 kW/m2, 32 kW/m2, 12.5 kW/m2, 8 kW/m2 and 4 kW/m2 radiation

levels, selected based on their effect on population, buildings and equipment were modeled using

PHAST.


In the event of explosion taking place within the plant, the resultant blast wave will have damaging

effects on equipment, structures, building and piping falling within the overpressure distances of

the blast. Tanks, buildings, structures etc. can only tolerate low level of overpressure. Human

body, by comparison, can withstand higher overpressure. But injury or fatality can be inflicted by

collapse of building of structures. The damage effect of blast overpressure is tabulated below.

Table 16: Damage Effects of Blast Overpressure

Blast Overpressure (PSI) Damage Level

5.0 Major structure damage

3.0 Oil storage tank failure

2.5 Eardrum rupture

2.0 Repairable damage, pressure vessels remain intact, light

structures collapse

1.0 Window pane breakage possible, causing some injuries

The hazard distances to the 5 psi, 3 psi and 2 psi overpressure levels, selected based on their

effects on population, buildings and equipment were modeled using PHAST.

6.4.4 TOXIC HAZARD

The inhalation of toxic gases can give rise to effects, which range in severity from mild irritation of

the respiratory tract to death. Lethal effects of inhalation depend on the concentration of the gas

to which people are exposed and on the duration of exposure. Mostly this dependence is

nonlinear and as the concentration increases, the time required to produce a specific injury

decreases rapidly.

The hazard distances to Immediately Dangerous to Life and Health concentration (IDLH) limit is

selected to determine the extent of the toxic hazard Created as the result of loss of containment of

a toxic substance.

6.5 CONSEQUENCE ANALYSIS FOR UNITS

This section discusses the consequences of selected failure scenarios for units whose affect

zones crosses the respective unit’s B/L and causes worst consequences. The consequence

distances are reported in tabular form for all weather conditions in Annexure-I and are

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represented graphically in Annexure-II for the all failure scenarios in a unit for worst weather

conditions.

6.5.1 DHT

NOTE: Refer Figures 6.5.1.1 to 6.5.1.13 in Annexure-II

Large hole on Bottom Outlet of Feed Surge Drum: From the consequence analysis of the selected

failure scenario it is observed that LFL hazard distance is extended up to a distance of 153 m and

may cover the hydro-finishing unit on the eastern side, SRR-1, sub – station and can also extend

up to the new and existing fire station on the western side. LFL may also spread up to the medical

center & lab building present on the north western side of the unit and cooling towers present on

the northern side. The Jet Fire Radiation Intensity of 37.5 & 12.5 kW/m2 can extend up to a

distance of 69 m & 84 m respectively. Jet fire radiation may cause damage to the SRR-1,

substation and caustic solution tanks. The Pool Fire Radiation Intensity of 37.5 kW/m2 is not

realized in this failure case & 12.5 kW/m2 can extend up to a distance of 35 m and may be

restricted in the vicinity of the unit. The 5 & 3 psi blast wave may spread up to a distance of 174 m

& 188 m respectively and can affect the cooling tower, medical center, lab building and new &

existing fire station on the western side and hydro- finishing unit on the eastern side.

Instrument Tapping Failure at Feed Charge Pumps: From the consequence results and graphs of

the selected credible scenario, it can be concluded that LFL may be extended up to a distance of

76 m. LFL may cover the SRR-1 and sub-station area present on the western side of the unit.

The Jet Fire radiation intensity of 37.5 & 12.5 kW/m2 would spread up to a distance of 60 m & 73

m respectively and may engulf the SRR-1 and sub-station. The 5 & 3 psi blast overpressures

travel up to a distance of 88 m & 95 m respectively. These pressure waves can cause damage to

the SRR-1, sub – station and caustic solution tank. The 2 psi blast overpressures may travel up to

existing fire station bays on the western side.

Large hole on Bottom Outlet of HP Cold Separator - Toxic: From the incident outcome analysis of

the selected failure scenario it is observed that LFL hazard distance is extended up to 211 m and

can spread up to the new proposed SRU under DHT project covering hydro-finishing unit

completely. LFL may also extend up to fire station area, medical center, lab building, wax control

room and cooling water area. The Jet Fire radiation intensity of 37.5 & 12.5 kW/m2 would spread

up to a distance of 98 m & 120 m respectively. The Jet Fire radiation can affect the fire station

facilities, sub – station, SRR-1, caustic solution tanks and facilities at hydro-finishing unit. The 5 &

3 psi blast waves may reach up to a distance of 257 m & 275 m and effects the above mentioned

facilities. In addition, these blast over pressure waves may also affect the Boiler facilities under

DHT project, DM tanks, SRU Block. These waves may also affect the CL/FC near SRU block and

Wax control room and sub – station near wax CR. The 5 & 3 psi blast waves may also cross the

Refinery compound wall on southern side, depending upon the prevalent weather condition and

ignition source encountered at the time of release. The H2S IDLH concentration can travel a

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distance of 61 m depending upon the prevailing weather conditions at the time of release and may

affect the personnel present in the effect zone.

Instrument Tapping Failure at HP Cold Separator Overhead - Toxic: From the event outcome of

the selected failure scenario it can be observed that LFL may be extended up to a distance of 34

m and can cross the B/L of the unit on the western side. The Jet Fire radiation intensity of 37.5

kW/m2 may not be realized in this failure scenario & 12.5 kW/m2 can reach up to the distance of

18 m. The 5 & 3 psi blast waves may reach up to a distance of 38 m & 41 m respectively,

producing localized damages. The H2S IDLH concentration may not reach the ground level

depending upon the elevation of the release/equipment. However, H2S IDLH concentration may

reach up to a downwind distance of 27 m at the height of 9 m, affecting any individual if present in

this area.

Large hole on Bottom Outlet of LP Cold Separator - Toxic: From the consequence analysis of the

selected failure scenario it is observed that LFL hazard distance is extended up to a distance of

204 m and may cover the hydro-finishing unit & new proposed SRU on the eastern side, SRR-1,

sub – station and can also extend up to the fire station on the western side. LFL may also cover

medical center & lab building present on the north western side of the unit, cooling towers present

on the northern side and Wax control room. The Jet Fire Radiation Intensity of 37.5 & 12.5 kW/m2

can extend up to a distance of 103 m & 125 m respectively. Jet Fire radiation may cause damage

to the SRR-1, substation, fire station facilities, hydro-finishing unit facilities, SRU block facilities

and caustic solution tanks. The 5 & 3 psi blast wave may spread up to a distance of 256 m and

277 m respectively and may affect the cooling tower on northern side, medical center & lab

building on the north western side and fire station on the western side and hydro-finishing unit on

the eastern side. These waves may also affect existing nitrogen facilities, wax control room, sub-

station near wax C/R and field cabin & CL near SRU block. The 5 & 3 psi blast waves may also

cross the Refinery compound wall on southern side, depending upon the prevalent weather

condition and ignition source encountered at the time of release. The H2S IDLH concentration can

travel a distance of 63 m from the leak source depending upon the prevailing wind conditions at

the time of the release and can extend up to SRR-1 and sub- station on the western side.

Large Hole on Bottom Outlet of Product Stripper: From the consequence modeling of the selected

failure scenario, it can be observed that LFL may be spreading up to a distance of 212 m from

leak source and can spread up to the new proposed SRU under DHT project covering whole

hydro-finishing unit. LFL values can also extend up to fire station area, medical center, lab

building, wax control room and cooling water area. The Jet Fire Radiation Intensity of 37.5 kW/m2

& 12.5 kW/m2 would extend up to a distance 82 m & 102 m respectively. Jet Fire radiation may

cause damage to sub – station, SRR-1, caustic solution tanks and facilities at hydro-finishing unit.

The Pool Fire radiation intensity of 12.5 kW/m2 would extend up to a distance 41 m and may

cross the B/L of the unit on the western side if realized & 37.5 kW/m2 Pool Fire radiation intensity

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would not be realized. The 5 & 3 psi blast waves for this leakage scenario would be extended up

to a distance of 248 m & 263 m respectively. These waves can affect the existing fire station, new

fire station, Boiler facilities under DHT project, DM tanks, SRU Block and sub-station & SRR-1.

The blast waves may also affect the existing nitrogen facilities, CL/FC near SRU block and wax

control room and sub – station near by wax CR. The 5 & 3 psi blast waves may also cross the

Refinery compound wall on southern side, depending upon the prevalent weather condition and

ignition source encountered at the time of release.

Instrument Tapping Failure at Stripper Reflux Drum Overhead - Toxic: From the consequence

results and graphs of the selected credible scenario, it can be concluded that LFL may be

extended up to a distance of 7 m. The Jet Fire radiation intensity of 37.5 & 12.5 kW/m2 would not

be realized. The H2S IDLH concentration may not reach the ground level depending upon the

elevation of the release/equipment. However, H2S IDLH concentration may reach up to a

downwind distance of 85 m at the height of 10 m, affecting any individual if present in this area.

Flange Leakage at Stripper Reflux Pump - Toxic: From the consequence results and graphs it can

be concluded that LFL may travel up to a distance of 39 m. The Jet Fire Radiation of intensity

37.5 & 12.5 kW/m2 may reach up to a distance of 25 m & 30 m respectively. The Pool Fire

Radiation of intensity 37.5 & 12.5 kW/m2 may reach up to a distance of 17 m & 26 m respectively.

The 5 & 3 psi blast wave can extend up to a distance of 38 m & 41 m respectively. The hazardous

affects arising out of this failure scenario can damage the equipment within the unit. The H2S

IDLH concentration may travel up to a distance of 107 m depending upon the prevailing wind

conditions at the time of release and may affect the nearby personnel.

Large hole on Bottom Outlet of Naphtha Stabilizer: From the incident outcome analysis of the

selected failure scenario it is observed that LFL hazard distance is extended up to 132 m and may

affect new & existing fire station, SRR-1 & sub-station on the western side, hydro finishing unit on

the eastern side and cooling tower area on the northern side. The Jet Fire radiation intensity of

37.5 & 12.5 kW/m2 would extend up to a distance of 86 m & 103 m respectively and may engulf

SRR-1, sub-station and cooling water facilities. The 5 & 3 psi blast waves may reach up to a

distance of 163 m & 175 m and can cause damage to the above mentioned facilities. In addition

to the above mentioned facilities, blast waves can also affect the DM tanks and nitrogen plant

facilities.

In addition to the above mentioned failure scenarios, Instrument Tapping Failure at Recycle Gas

Compressor, Instrument Tapping Failure at Make-up Gas Compressor, Seal Failure at Diesel

Product Pump and Catastrophic Rupture of Stripper Reflux Drum were also modeled & analyzed.

For Instrument Tapping Failure at Recycle Gas Compressor and Instrument Tapping Failure at

Make-up Gas Compressor, hazard effect zones may cross the B/L of the unit on eastern side. For

Seal Failure at Diesel Product Pump and Catastrophic Rupture of Stripper Reflux Drum

hazardous effect zone for these failure scenario might be restricted within the B/L’s of unit

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depending upon the prevailing weather conditions at the time of release, mostly producing

localized damages within the unit.

6.5.2 SRU

NOTE: Refer Figures 6.5.2.1 to 6.5.2.3 in Annexure- II

Instrument Tapping Failure at Acid Gas KOD - Toxic: From the consequence modeling for this

failure event, it is observed that LFL will extend only up to a distance of 2 m and Jet Fire Radiation

of intensity 37.5 & 12.5 kW/m2 is not realized. Under 1 F weather condition IDLH concentration of

H2S may spread up to a distance of 283 m from the leak source depending upon the prevailing

wind direction at the time of release and may cross the Refinery compound wall on southern side.

Instrument Tapping Failure at NH3 Rich Gas KOD – Toxic: From the incident outcome analysis of

selected failure scenario, it is observed that LFL for this event will not go beyond 0.5 m from the

leak source and Jet Fire Radiation of intensity 37.5 & 12.5 kW/m2 is not realized. The NH3 IDLH

concentration may spread up to a distance of 51 m from the leak source, depending prevalent

weather conditions at the time of release.

Instrument Tapping Failure at Sour Gas KOD - Toxic: From the consequence results and graphs it

can be concluded that LFL may travel up to a distance of 2 m only and Jet Fire Radiation of

intensity 37.5 & 12.5 kW/m2 is not realized. The H2S IDLH concentration may travel up to a

distance of 307 m from leak source, depending upon the prevailing wind conditions at the time of

release and may cross the Refinery compound wall on southern side and may affect the nearby

personnel’s.

6.5.3 EXISTING ARU & SWS

NOTE: Refer Figures 6.5.3.1 to 6.5.3.3 in Annexure- II

Instrument Tapping Failure at 1st Stage Sour Water Stripper Overhead – Toxic: From the

consequence modeling results it can be seen that LFL for this failure case will go up to a distance

of 3 m from the leak source. The Jet Fire Radiation of intensity 37.5 & 12.5 kW/m2 is not realized

in this failure scenario. The IDLH H2S concentration may not reach to the ground but it can spread

up to a distance of 290 m from the leak source at height of 30 m from the ground.

Instrument Tapping Failure at 2nd Stage Sour Water Stripper – Toxic: From the incident outcome

analysis of selected failure scenario, it is observed that LFL for this event will be limited to a

distance of 1 m and Jet Fire Radiation of intensity 37.5 & 12.5 kW/m2 is not realized. The H2S

IDLH concentration may not reach to the ground but it can spread up to a distance of 90 m at

cloud height of 41 m from the ground.

Instrument Tapping Failure at Acid Gas Line from Amine Regenerator: From the consequence

modeling results it can be seen that LFL for this failure case will go up to a distance of 2 m from

the leak source. The Jet Fire Radiation of intensity 37.5 & 12.5 kW/m2 is not realized in this failure

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scenario. The IDLH H2S concentration may not reach to the ground but it can spread up to a

distance of 145 m from the leak source at height of 32 m from the ground.

6.5.4 OFFSITES

6.5.4.1 Tank on Fire

NOTE: Refer Figures 6.5.4.1.1 in Annexure-II

DHDT Feed Tank on Fire: From the consequence results and graphs of the selected credible

scenario, it can be concluded that Pool Fire radiation intensity of 8 kW/m2 may not affect the

adjacent DHT Feed Tank. The Pool Fire radiation intensity of 32 kW/m2 is not realized in this

case.

6.5.4.2 2” Leak at Tank Manifold


2 inch Leak at DHDT Feed Tank Manifold: From the incident outcome analysis of the selected

failure scenario it is observed that LFL may be travelling up to a distance of 54 m. The Jet Fire

radiation intensity of 32 & 8 kW/m2 would extend up to a distance of 21 m & 16 m respectively.

The Pool Fire radiation intensity of 32 kW/m2 is not realized and 8 kW/m2 would extend up to a

distance of 85 m. The 5 & 3 psi blast overpressures travel up to a distance of 67 m & 74 m

respectively depending upon the location of the release.

6.5.4.3 Offsite Pump


Instrument Tapping Failure at DHT Feed Pumps: From the event outcome of the selected failure

scenario it can be observed that LFL may be extended up to a distance of 26 m. The Jet Fire

Radiation Intensity of 32 & 8 kW/m2 can extend up to a distance of 43 m & 57 m respectively &

may damage the vacuum distillate tanks. The Pool Fire Radiation Intensity of 32 kW/m2 is not

realized & 8 kW/m2 can extend up to a distance of 54 m and may affect the vacuum distillate tank.

The 5 & 3 psi blast wave may reach up to a distance of 28 m & 31 m respectively.

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7 MAJOR FINDINGS & RECOMMENDATIONS




of the Rapid Risk analysis study for units under DHT Project are summarized below:

Consequence modeling of low frequency failure scenarios for DHT is carried out and it was

observed that in the event of realization of failure scenarios (Large Hole in Feed Surge Drum,

HP Cold Separator, LP Cold Separator, Product Stripper, Naphtha Stabilizer), radiation &

explosion effect zones may get extended up to adjacent Hydro-finishing unit on eastern side

& Fire station, Lab Building, Medical Center on western side.

Further, in the event of Large Hole failure scenarios in HP Cold Separator, LP Cold

Separator, Product Stripper, 5 & 3 psi blast waves may extend beyond the Refinery

compound wall on southern side depending upon the prevalent weather condition and

presence of ignition source at the time of release.

Since these are low frequency credible failure scenarios, outcomes of the above to be utilized

for updation of the existing Disaster Management Plan (DMP) & Emergency Response Plan

(ERP). Adequate number of hydrocarbon detectors to be ensured at strategic locations within

the DHT unit for early leak detection and inventory isolation.

High frequency credible failure scenarios are also modeled for DHT unit and their explosion &

radiation effects are analyzed. It is observed that, in the event of the Instrument tapping

failure at feed charge pump, LFL may get extended up to a distance of 76 m, covering SRR-1

and sub-station area present on the western side of the unit.

The Jet fire radiation intensity of 37.5 & 12.5 kW/m2 may be realized up to a distance of 60 m

& 73 m respectively and may affect SRR-1 & sub-station.

5 & 3 psi blast overpressure wave may reach up to a distance of 88 m & 95 m respectively.

These pressure waves may cause damage to SRR-1, Substation and Caustic solution tank.

On the western side, 2 psi blast overpressure wave may reach up to existing fire station last

bay, depending upon the prevalent weather condition and presence of ignition source at the

time of release.

Last bay (maintenance bay) of existing Fire Station is under affect zone of high frequency

credible scenario (Instrument tapping failure at Feed charge pump). Being a maintenance

bay, no Fire Tender is expected to be present regularly in this bay. However, it is

recommended to ensure that Fire tender is not stationed on permanent basis in the last bay.

Toxic gas dispersion modelling is also carried out for various high frequency credible failure

scenarios for DHT (Instrument Tapping Failure at HP Cold Separator Overhead, Recycle Gas

Compressor, Stripper Reflux Drum Overhead & Flange Leakage at Stripper Reflux Pump)

1 1

RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

of 36


and it is observed that H2S IDLH hazard distance may extend beyond DHT unit battery limit.

However, the affect zone is confined within the Refinery Compound wall.

Hence, it is recommended to locate the toxic gas detectors within the DHT unit at strategic

locations and utilize the outcomes of these failure scenarios in the updation of the existing

Disaster Management Plan (DMP) & Emergency Response Plan (ERP).

Toxic gas credible failure scenarios are modeled for SRU. It is observed that in the event of

Instrument tapping failure at Acid Gas KOD & Sour Gas KOD under 1F weather condition,

H2S IDLH concentration may extend up to a distance of 283 m & 307 m respectively, from the

leak source, depending upon the prevailing wind conditions at the time of release. H2S IDLH

concentration may extend beyond the Refinery compound wall on southern side & may affect

the nearby personnel’s.

Hence, it is recommended to install toxic gas detectors within SRU at strategic locations and

utilize the outcome of these failure scenarios in the updation of the existing Disaster

Management Plan (DMP) & Emergency Response Plan (ERP). Moreover, due to close

proximity of another Sulphur Block, it is recommended to carry out Quantitative Risk Analysis

(QRA) of the refinery, so as to assess the effect of the H2S IDLH concentration in terms of

Individual & Societal Risk on the nearby buildings / Operator cabins / populations.

a) Recommendations for Construction Safety during execution of the DHT Project

Adequate barricading of the proposed unit to be done from existing running process units

during construction phase. Hydrocarbon / toxic detectors to be placed along the

barricading suitably to detect any hydrocarbon / toxic gas in vicinity of construction area.



response during fire & toxic gas release.






DHT project to get a detailed overview of the possible hazards during construction phase

and action plan to prevent / mitigate the same.


No operator cabin shall be located inside battery limits of proposed units under DHT

project. Also, operator cabin, if any located in the vicinity of unit shall be made of Blast

Resistant Construction with Positive Pressurization for protection of individuals inside

operator cabin from Explosion, Radiation & Toxic effects of various leak scenarios.

RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

of 36













Ensure usage of safer oxidizing agents (Chlorine free) in Cooling Water circuit.






Measures for controlling / minimization of Ignition sources inside the Refinery complex.



d) Ignition Control


within facilities processing flammable materials. As part of mitigation measure it strongly


e) Escape Routes


from all areas.



releases.




recommended:




RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

of 36



b. Surge Drums & Reflux drums and high inventory vessels whose rupture may lead

to massive consequences are needed to be identified and following to be ensured:

i. Monitoring of vessel internals during shut down.


g) Others



etc.


Provide breathing apparatus at strategic locations inside Refinery.

RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

of 36


8 GLOSSARY

CASUALTY Someone who suffers serious injury or worse i.e. including fatal

injuries. As a rough guide fatalities are likely to be half the total

casualties. But this may vary depending on the nature of the event.

HAZARD A chemical or physical condition with the potential of causing

damage.

FLAMMABILITY LIMITS In fuel-air systems, a range of compositions exists inside which a

(UFL – LFL) flame will propagate substantial distance from an

ignition source. The limiting fuel concentrations are termed as

Upper flammability or explosives limit (Fuel concentrations

exceeding this are too rich) and Lower flammability or explosives

limit (Fuel concentrations below this are too lean).

FLASH FIRE The burning of a vapor cloud at very low flame propagation speed.

Combustion products are generated at a rate low enough for

expansion to take place easily without significant overpressure

ahead or behind the flame front. The hazard is therefore only due to

thermal effects.

OVERPRESSURE Maximum pressure above atmosphere pressure experiences during

the passage of a blast wave from an explosion expressed in this

report as pounds per square inch (psi).

EXPLOSION A rapid release of energy, which causes a pressure discontinuity or

shock wave moving away from the source. An explosion can be

produced by detonation of a high explosive or by the rapid burning

of a flammable gas cloud. The resulting overpressure is sufficient to

cause damage inside and outside the cloud as the shock wave

propagation into the atmosphere beyond the cloud. Some authors

use the term deflagration for this type of explosion

DOMINO EFFECT The effect that loss of containment of one installation leads to loss

of containment of other installations

EVENT TREE A logic diagram of success and failure combinations of events used

to identify accident sequences leading to all possible consequences

of a given initiating event.

TLV “Threshold limit value” is defined as the concentration of the

substance in air that can be breathed for five consecutive 8 hours

work day (40 hours work week) by most people without side effect.

STEL “Short Term Exposure Limit” is the maximum permissible average

exposure for the time period specified (15 minutes).

RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

of 36


IDLH “Immediate Dangerous to Life and Health” is the maximum

concentration level from which one could escape within 30 minutes

without any escape impairing symptoms.

PASQUILL CLASS Classification to qualify the stability of the atmosphere, indicated by

a letter ranging from A, for very unstable, to F, for stable.

FREQUENCY The number of times an outcome is expected to occur in a given

period of time.

RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

of 36


9 REFERENCES

1. Classification of hazardous locations, A. W. Cox, F. P. Lees and M. L. Ang, Published by

the Institute of Chemical engineers, U. K.

2. The reference manual, Volume-II, Creemer & Warner Ltd. U. K. (Presently Entec).

3. Risk analysis of six potentially hazardous industrial objects in the Rijnmond area; A pilot

study. A report to the Rijnmond Public Authority. D. Riedel publishing company, U. K.

4. Loss prevention in the process industries, Hazard identification, Assessment and Control,

Frank. P. Lees (Vol. I, II & III), Published by Butterworth-Heinemann, U. K.

5. AICHE, CCPS, Chemical process Quantitative Risk Analysis

6. Guideline for Quantitative Risk assessment, ‘Purple book’.

RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

Page 1 of 4


ANNEXURE-I

CONSEQUENCE ANALYSIS HAZARD DISTANCES

Temp.

(OC)

Press.

(Kg/cm2g)4 KW/m2 12.5 KW/m2 37.5 KW/m2 4 KW/m2 12.5 KW/m2 37.5 KW/m2 2 psi 3 psi 5 psi

1 F 153 109 84 69 64 35 NR 203 188 174 -

2 D 107 104 78 63 71 35 NR 140 130 122 -

1 F 76 94 73 60 - - - 103 95 88 -

2 D 73 88 67 54 - - - 101 93 87 -

1 F 207 173 134 110 - - - 297 273 253 H2S - 61

2 D 211 158 120 98 - - - 296 275 257 H2S - 58

1 F 34 27 18 NR - - - 45 41 38 H2S - NR

2 D 30 27 19 NR - - - 32 29 27 H2S - NR

1 F 204 163 125 103 - - - 303 277 256 H2S - 63

2 D 209 150 113 92 - - - 286 265 247 H2S - 58

1 F 35 30 22 16 - - - 47 43 39 H2S - 13

2 D 30 30 23 17 - - - 45 42 38 H2S - 14

1 F 32 29 21 14 - - - 48 43 40 -

2 D 28 30 23 16 - - - 34 31 28 -

1 F 207 142 109 89 61 38 NR 281 261 244 -

2 D 212 137 102 82 68 41 NR 280 263 248 -

1 F 12 22 17 15 26 20 15 17 15 14 -

2 D 10 21 16 13 27 22 16 16 14 13 -

1 F 18 - - - - - - 70 55 42 -

2 D 20 - - - - - - 70 55 42 -

1 F 7 NR NR NR - - - - - - H2S - NR

2 D 7 NR NR NR - - - - - - H2S - NR

1 F 39 39 30 25 38 26 17 45 41 38 H2S - 107

2 D 33 37 28 23 37 27 17 43 40 37 H2S - 77

1 F 132 133 103 86 - - - 190 175 163 -

2 D 133 127 96 79 - - - 182 169 158 -

1 F 2 NR NR NR - - - - - - H2S - 283

2 D 2 NR NR NR - - - - - - H2S - 89

1 F 0.5 NR NR NR - - - - - - NH3 - 44

2 D 0.5 NR NR NR - - - - - - NH3 - 51

1 F 2 NR NR NR - - - - - - H2S - 307

2 D 2 NR NR NR - - - - - - H2S - 80

1 F 3 NR NR NR - - - - - - H2S - NR

2 D 3 NR NR NR - - - - - - H2S - NR

1 F 0.5 NR NR NR - - - - - - NH3 - NR

2 D 0.5 NR NR NR - - - - - - NH3 - NR

Blast Over Pressure (m)

WeathersFlash Fire

(m)Failure Case

0.06

Jet Fire (m)

0.8 0.1Instrument Tapping Failure - Toxic

Instrument Tapping Failure - Toxic

40

40

Large Hole on bottom outlet 166

Pool Fire (m)

1.5

55

124 83

Flange Leakage - Toxic

Existing SWS &

ARU

Ist Stage Sour Water Stripper

(08-CC-00-001)Instrument Tapping Failure - Toxic 84

3

IInd Stage Sour Water Stripper

(08-CC-00-002)90 0.7

Sour Gas KOD

1

2 Instrument Tapping Failure - Toxic


EquipmentLeak Rate

Kg/sSl No.

Large Hole on bottom outlet - Toxic


8

6

Unit

Feed Charge Pump

(21-PA-101 A/B)165 83

Large Hole on bottom outlet 248

HP Cold Separator Overhead

(21-VV-103)

Operating Conditions

83

Make-Up Gas Compressor 2nd Stage (21-

KA-102 A/B)

Feed Surge Drum (21-VV-102) 165

Instrument Tapping Failure

IDLH Conc

Distance

(m)

NOTE

Stripper Reflux Drum (21-VV-201) Catastrophic Rupture 40

1

2

3

18.2

21.5

7.9

4

5

1.5

38.6

Recycle Gas Compressor

(21-KA-101)

Large Hole on bottom outlet


1.2

12

35.6

HP Cold Separator (21-VV-103) 55 67.5 130.3

7.1

Large Hole on bottom outlet - Toxic


7

Product Stripper (21-CC-201)

0.47.1

-

DHT

10

12.1 0.7

LP Cold Separator (21-VV-104) 56.8 36 95

67.5

82

1.3

Diesel Product Pump

(21-PA-202 A/B)Seal Failure9

13 Naphtha Stabilizer (21-CC-202)

2

2 NH3 Rich Gas KOD

11 40

7

84

248

Instrument Tapping Failure - Toxic 90

1

7.65

12

0.7 0.06

Acid Gas KOD

Stripper Reflux Pump

(21-PA-201 A/B)

Stripper Reflux Drum Overhead

(21-VV-201)

0.8

0.5

0.09

Numaligarh Refinery Ltd.- DHT Project Consequence Analysis Hazard Distances

SRU

Page 2 of 4

Temp.

(OC)

Press.

(Kg/cm2g)4 KW/m2 12.5 KW/m2 37.5 KW/m2 4 KW/m2 12.5 KW/m2 37.5 KW/m2 2 psi 3 psi 5 psi


WeathersFlash Fire

(m)Failure Case

Jet Fire (m) Pool Fire (m)

1.5

EquipmentLeak Rate

Kg/sSl No.Unit


Feed Surge Drum (21-VV-102) 165

IDLH Conc

Distance

(m)

NOTE

1 18.2Large Hole on bottom outlet

DHT

Numaligarh Refinery Ltd.- DHT Project Consequence Analysis Hazard Distances

1 F 2 NR NR NR - - - - - - H2S - NR

2 D 2 NR NR NR - - - - - - H2S - NR

Existing SWS &

ARU

3

Acid Gas Line from Amine Regenerator

Column

(11-CC-00-002)

Instrument Tapping Failure - Toxic 113 0.95 0.1

Page 3 of 4

Temp.

(OC)

Press.

(Kg/cm2g)4 KW/m2 8 KW/m2 32 KW/m2 4 KW/m2 8 KW/m2 32 KW/m2 2 psi 3 psi 5 psi

1 F - - - - 36 22 NR - - -

2 D - - - - 41 26 NR - - -

1 F 54 25 21 16 132 85 NR 81 74 67

2 D 33 24 20 15 143 90 NR 49 45 41

1 F 22 65 55 43 66 48 NR 35 31 28

2 D 26 69 57 43 74 54 NR 34 31 28

8.43 DHT Feed Pump Instrument Tapping Failure 30 10.5

-

2 DHT Feed Tank 2" Leak in Manifold Amb. 0.9 15.9

1 DHT Feed Tank Tank On Fire Amb.

Offsite

Numaligarh Refinery Ltd. - DHT Project :Offsite Consequence Analysis Hazard Distances

Unit Sl No. Equipment Failure Case

Operating ConditionsLeak Rate

Kg/sWeathers

Flash Fire

(m)

Jet Fire (m) Pool Fire (m) Blast Over Pressure (m)

NOTE

Atm.

1

Page 4 of 4

RRA Study of

DHT Project, NRL,

Assam

Doc No: A800-17-43-RA-0001

Rev. No.: 1

Page 1 of 74


ANNEXURE-II

FIGURES FOR CONSEQUENCE ANALYSIS

1

Figure 6.5.1.1 A: DHT: Large Hole on bottom outlet of Feed Surge Drum; Flash Fire Distances (m)

Page 2 of 74

Figure 6.5.1.1 B: DHT: Large Hole on bottom outlet of Feed Surge Drum; Jet Fire Distances (m)

Page 3 of 74

Figure 6.5.1.1 C: DHT: Large Hole on bottom outlet of Feed Surge Drum; Over Pressure Distances (m)

Page 4 of 74

Figure 6.5.1.1 D: DHT: Large Hole on bottom outlet of Feed Surge Drum; Pool Fire Distances (m)

Page 5 of 74

Figure 6.5.1.2 A: DHT: Instrument Tapping Failure at Feed Charge Pump; Flash Fire Distances (m)

Page 6 of 74

Figure 6.5.1.2 B: DHT: Instrument Tapping Failure at Feed Charge Pump; Jet Fire Distances (m)

Page 7 of 74

Figure 6.5.1.2 C: DHT: Instrument Tapping Failure at Feed Charge Pump; Over Pressure Distances (m)

Page 8 of 74

Figure 6.5.1.3 A: DHT: Large Hole on bottom outlet of HP Cold Separator; Flash Fire Distances (m)

Page 9 of 74

Figure 6.5.1.3 B: DHT: Large Hole on bottom outlet of HP Cold Separator; Jet Fire Distances (m)

Page 10 of 74

Figure 6.5.1.3 C: DHT: Large Hole on bottom outlet of HP Cold Separator; Over Pressure Distances (m)

Page 11 of 74

Figure 6.5.1.3 D: DHT: Large Hole on bottom outlet of HP Cold Separator; H2S IDLH Hazard Distances (m)

Page 12 of 74

Figure 6.5.1.3 E: DHT: Large Hole on bottom outlet of HP Cold Separator; H2S IDLH Hazard Distances (m)

Page 13 of 74

Figure 6.5.1.4 A: DHT: Instrument Tapping Failure at HP Cold Separator Overhead; Flash Fire Distances (m)

Page 14 of 74

Figure 6.5.1.4 B: DHT: Instrument Tapping Failure at HP Cold Separator Overhead; Jet Fire Distances (m)

Page 15 of 74

Figure 6.5.1.4 C: DHT: Instrument Tapping Failure at HP Cold Separator Overhead; Over Pressure Distances (m)

Page 16 of 74

Figure 6.5.1.4 D: DHT: Instrument Tapping Failure at Cold HP Separator Overhead; H2S IDLH Hazard Distances (m)

Page 17 of 74

Figure 6.5.1.5 A: DHT: Large Hole on bottom outlet of LP Cold Separator; Flash Fire Distances (m)

Page 18 of 74

Figure 6.5.1.5 B: DHT: Large Hole on bottom outlet of LP Cold Separator; Jet Fire Distances (m)

Page 19 of 74

Figure 6.5.1.5 C: DHT: Large Hole on bottom outlet of LP Cold Separator; Over Pressure Distances (m)

Page 20 of 74

Figure 6.5.1.5 D: DHT: Large Hole on bottom outlet of LP Cold Separator; H2S IDLH Hazard Distances (m)

Page 21 of 74

Figure 6.5.1.5 E: DHT: Large Hole on bottom outlet of LP Cold Separator; H2S IDLH Hazard Distances (m)

Page 22 of 74

Figure 6.5.1.6 A: DHT: Instrument Tapping Failure at Recycle Gas Compressor; Flash Fire Distances (m)

Page 23 of 74

Figure 6.5.1.6 B: DHT: Instrument Tapping Failure at Recycle Gas Compressor; Jet Fire Distances (m)

Page 24 of 74

Figure 6.5.1.6 C: DHT: Instrument Tapping Failure at Recycle Gas Compressor; Over Pressure Distances (m)

Page 25 of 74

Figure 6.5.1.6 D: DHT: Instrument Tapping Failure at Recycle Gas Compressor; H2S IDLH Hazard Distances (m)

Page 26 of 74

Figure 6.5.1.7 A: DHT: Instrument Tapping Failure at Make-up Gas Compressor 2nd Stage; Flash Fire Distances (m)

Page 27 of 74

Figure 6.5.1.7 B: DHT: Instrument Tapping Failure at Make-up Gas Compressor 2nd Stage; Jet Fire Distances (m)

Page 28 of 74

Figure 6.5.1.7 C: DHT: Instrument Tapping Failure at Make-up Gas Compressor 2nd Stage; Over Pressure Distances (m)

Page 29 of 74

Figure 6.5.1.8 A: DHT: Large Hole on bottom outlet of Product Stripper; Flash Fire Distances (m)

Page 30 of 74

Figure 6.5.1.8 B: DHT: Large Hole on bottom outlet of Product Stripper; Jet Fire Distances (m)

Page 31 of 74

Figure 6.5.1.8 C: DHT: Large Hole on bottom outlet of Product Stripper; Over Pressure Distances (m)

Page 32 of 74

Figure 6.5.1.8 D: DHT: Large Hole on bottom outlet of Product Stripper; Pool Fire Distances (m)

Page 33 of 74

Figure 6.5.1.9 A: DHT: Seal Failure at Diesel Product Pump; Flash Fire Distances (m)

Page 34 of 74

Figure 6.5.1.9 B: DHT: Seal Failure at Diesel Product Pump; Jet Fire Distances (m)

Page 35 of 74

Figure 6.5.1.9 C: DHT: Seal Failure at Diesel Product Pump; Overpressure Fire Distances (m)

Page 36 of 74

Figure 6.5.1.9 D: DHT: Seal Failure at Diesel Product Pump; Pool Fire Distances (m)

Page 37 of 74

Figure 6.5.1.10 A: DHT: Catastrophic Rupture of Stripper Reflux Drum; Flash Fire Distances (m)

Page 38 of 74

Figure 6.5.1.10 B: DHT: Catastrophic Rupture of Stripper Reflux Drum; Overpressure Distances (m)

Page 39 of 74

Figure 6.5.1.11 A: DHT: Instrument Tapping Failure at Stripper Reflux Drum Overhead; Flash Fire Distances (m)

Page 40 of 74

Figure 6.5.1.11 B: DHT: Instrument Tapping Failure at Stripper Reflux Drum Overhead; H2S IDLH Hazard Distances (m)

Page 41 of 74

Figure 6.5.1.12 A: DHT: Flange Leakage at Stripper Reflux Pump; Flash Fire Distances (m)

Page 42 of 74

Figure 6.5.1.12 B: DHT: Flange Leakage at Stripper Reflux Pump; Jet Fire Distances (m)

Page 43 of 74

Figure 6.5.1.12 C: DHT: Flange Leakage at Stripper Reflux Pump; Over Pressure Distances (m)

Page 44 of 74

Figure 6.5.1.12 D: DHT: Flange Leakage at Stripper Reflux Pump; Pool Fire Distances (m)

Page 45 of 74

Figure 6.5.1.12 E: DHT: Flange Leakage at Stripper Reflux Pump; H2S IDLH Hazard Distances (m)

Page 46 of 74

Figure 6.5.1.12 F: DHT: Flange Leakage at Stripper Reflux Pump; H2S IDLH Hazard Distances (m)

Page 47 of 74

Figure 6.5.1.13 A: DHT: Large Hole on the Bottom Outlet of Naphtha Stabilizer; Flash Fire Distances (m)

Page 48 of 74

Figure 6.5.1.13 B: DHT: Large Hole on the Bottom Outlet of Naphtha Stabilizer; Jet Fire Distances (m)

Page 49 of 74

Figure 6.5.1.13 C: DHT: Large Hole on the Bottom Outlet of Naphtha Stabilizer; Overpressure Fire Distances (m)

Page 50 of 74

Figure 6.5.2.1 A: SRU: Instrument Tapping Failure at Acid Gas KO Drum; Flash Fire Distances (m)

Page 51 of 74

Figure 6.5.2.1 B: SRU: Instrument Tapping Failure at Acid Gas KO Drum; H2S IDLH Hazard Distances (m)

Page 52 of 74

Figure 6.5.2.1 C: SRU: Instrument Tapping Failure at Acid Gas KO Drum; H2S IDLH Hazard Distances (m)

Page 53 of 74

Figure 6.5.2.2 A: SRU: Instrument Tapping Failure at NH3 Rich Gas KO Drum; Flash Fire Distances (m)

Page 54 of 74

Figure 6.5.2.2 B: SRU: Instrument Tapping Failure at NH3 Rich Gas KO Drum; H2S IDLH Hazard Distances (m)

Page 55 of 74

Figure 6.5.2.2 C: SRU: Instrument Tapping Failure at NH3 Rich Gas KO Drum; H2S IDLH Hazard Distances (m)

Page 56 of 74

Figure 6.5.2.3 A: SRU: Instrument Tapping Failure at Sour Gas KO Drum; Flash Fire Distances (m)

Page 57 of 74

Figure 6.5.2.3 B: SRU: Instrument Tapping Failure at Sour Gas KO Drum; H2S IDLH Hazard Distances (m)

Page 58 of 74

Figure 6.5.2.3 C: SRU: Instrument Tapping Failure at Sour Gas KO Drum; H2S IDLH Hazard Distances (m)

Page 59 of 74

Figure 6.5.3.1 A: Existing SWS: Instrument Tapping Failure at Ist Stage Sour Water Stripper Overhead; Flash Fire Distances (m)

Page 60 of 74

Figure 6.5.3.1 B: Existing SWS: Instrument Tapping Failure at Ist Stage Sour Water Stripper Overhead; H2S IDLH Hazard Distances (m)

Page 61 of 74

Figure 6.5.3.2 A: Existing SWS: Instrument Tapping Failure at IInd Stage Sour Water Stripper Overhead; NH3 IDLH Hazard Distances (m)

Page 62 of 74

Figure 6.5.3.3 A: Existing ARU: Instrument Tapping Failure at Acid Gas Line from Amine Regenerator; Flash Fire Distances (m)

Page 63 of 74

Figure 6.5.3.3 B: Existing ARU: Instrument Tapping Failure at Acid Gas Line from Amine Regenerator; H2S IDLH Hazard Distances (m)

Page 64 of 74

Figure 6.5.4.1.1 A: Offsite: DHDT Feed Tank on Fire; Pool Fire Distances (m)

Page 65 of 74

Figure 6.5.4.1.1 B: Offsite: DHDT Feed Tank on Fire; Pool Fire Distances (m)

Page 66 of 74

Figure 6.5.4.2.1 A: Offsite: 2 inch Leak at DHDT Feed Tank; Flash Fire Distances (m)

Page 67 of 74

Figure 6.5.4.2.1 B: Offsite: 2 inch Leak at DHDT Feed Tank; Jet Fire Distances (m)

Page 68 of 74

Figure 6.5.4.2.1 C: Offsite: 2 inch Leak at DHDT Feed Tank; Over Pressure Distances (m)

Page 69 of 74

Figure 6.5.4.2.1 D: Offsite: 2 inch Leak at DHDT Feed Tank; Pool Fire Distances (m)

Page 70 of 74

Figure 6.5.4.3.1 A: Offsite: Instrument Tapping Failure at DHT Feed Pump; Flash Fire Distances (m)

Page 71 of 74

Figure 6.5.4.3.1 B: Offsite: Instrument Tapping Failure at DHT Feed Pump; Jet Fire Distances (m)

Page 72 of 74

Figure 6.5.4.3.1 C: Offsite: Instrument Tapping Failure at DHT Feed Pump; Overpressure Distances (m)

Page 73 of 74

Figure 6.5.4.3.1 D: Offsite: Instrument Tapping Failure at DHT Feed Pump; Pool Fire Distances (m)

Page 74 of 74

RRA Study of

LPG Mounded Bullets &

Upgradation of LPG Bottling

Plant, NRL- Assam

Doc No: A816-17-43-RA-0001

Rev. No.: 0

Page 2 of 28


PREFACE

Numaligarh Refinery Limited, (NRL) presently operates a 3 MMTPA refinery at Numaligarh,

District Golaghat, Assam. NRL is planning to implement a project of installing 4 nos. LPG

mounded bullets and upgradation of the existing LPG Bottling Plant.

Engineers India Limited (EIL), New Delhi, has been entrusted by M/s Numaligarh Refinery

Limited, Assam (NRL) as an EIA consultant for this Project. As a part of the project, RRA is being

carried out for the facilities

Rapid Risk Analysis study identifies the hazards associated with the facility, analyses the

consequences, draws suitable conclusions and provides necessary recommendations to mitigate

the hazard/ risk.

This Rapid Risk Analysis study is based on the information made available at the time of this

study and EIL’s own data source for similar plants. EIL has exercised all reasonable skill, care

and diligence in carrying out the study. However, this report is not deemed to be any undertaking,

warrantee or certificate.

RRA Study of



Plant, NRL- Assam

Doc No: A816-17-43-RA-0001

Rev. No.: 0

Page 3 of 28


TABLE OF CONTENTS

1 EXECUTIVE SUMMARY ....................................................................................................... 5

1.1 INTRODUCTION ............................................................................................................ 5

1.2 APPROACH METHODOLOGY ....................................................................................... 5

1.3 MAJOR FINDINGS AND RECOMMENDATIONS ........................................................... 5

2 INTRODCUTION ................................................................................................................... 9

2.1 STUDY AIMS AND OBJECTIVE ..................................................................................... 9

2.2 SCOPE OF WORK ......................................................................................................... 9

3 SITE CONDITION ................................................................................................................ 10

3.1 GENERAL .................................................................................................................... 10

3.2 SITE, LOCATION AND VICINITY ................................................................................. 10

3.3 METEOROLOGICAL CONDITIONS ............................................................................. 10

4 HAZARDS ASSOCIATED WITH THE FACILITIES .............................................................. 14

4.1 GENERAL .................................................................................................................... 14

4.2 HAZARDS ASSOCIATED WITH FLAMMABLE MATERIALS........................................ 14

4.2.1 LIQUIFIED PETROLEUM GAS .............................................................................. 14

5 HAZARD IDENTIFICATION ................................................................................................. 15

5.1 GENERAL .................................................................................................................... 15

5.2 MODES OF FAILURE ................................................................................................... 15

5.3 SELECTED FAILURE CASES ...................................................................................... 16

6 CONSEQUENCE ANALYSIS ............................................................................................... 18

6.1 GENERAL .................................................................................................................... 18

6.2 CONSEQUENCE ANALYSIS MODELLING .................................................................. 18

6.2.1 DISCHARGE RATE ............................................................................................... 18

6.2.2 DISPERSION ......................................................................................................... 18

6.2.3 FLASH FIRE .......................................................................................................... 18

6.2.4 JET FIRE ............................................................................................................... 19

6.2.5 POOL FIRE ........................................................................................................... 19


RRA Study of



Plant, NRL- Assam

Doc No: A816-17-43-RA-0001

Rev. No.: 0

Page 4 of 28


6.3 SIZE AND DURATION OF RELEASE ........................................................................... 19

6.4 DAMAGE CRITERIA ..................................................................................................... 20

6.4.1 LFL OR FLASH FIRE ............................................................................................. 20

6.4.2 THERMAL HAZARD DUE TO POOL FIRE, JET FIRE AND FIRE BALL ................ 20


6.5 CONSEQUENCE ANALYSIS FOR UNITS.................................................................... 21

6.5.1 LPG Mounded Bullet .............................................................................................. 21

6.5.2 LPG Bottling Plant ................................................................................................. 22

7 MAJOR FINDINGS & RECOMMENDATIONS ..................................................................... 23

8 GLOSSARY ......................................................................................................................... 26

9 REFERENCES .................................................................................................................... 28

ANNEXURE-I: CONSEQUENCE ANALYSIS HAZARD DISTANCES

ANNEXURE-II: FIGURES FOR CONSEQUENCE ANALYSIS

LIST OF TABLES

Table 1: New Proposed Facilities under Project ............................................................................ 9

Table 2: Existing Facilities under Project ....................................................................................... 9

Table 3: Atmospheric Parameter ................................................................................................. 11

Table 4: Average Mean Wind Speed (m/s) .................................................................................. 11

Table 5: % Number of Days Wind From ...................................................................................... 11

Table 6: Pasquill Stability Classes ............................................................................................... 12

Table 7: Weather Conditions ....................................................................................................... 13

Table 8: Hazardous Properties of LPG ........................................................................................ 14

Table 9: Size of Release ............................................................................................................. 19

Table 10: Damage Due to Incident Thermal Radiation Intensity .................................................. 20

Table 11: Damage Effects of Blast Overpressure ........................................................................ 21

LIST OF FIGURES

Figure 1: NRL Site ...................................................................................................................... 10

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1 EXECUTIVE SUMMARY

1.1 INTRODUCTION

NRL, a subsidiary of M/s Bharat Petroleum Corporation Limited presently operates a 3.0 MMTPA

refinery at Numaligarh, Golaghat, Assam. The Refinery presently produces Euro-III and part Euro-

IV grade HSD. NRL is planning to implement a project of installing 4 nos. LPG mounded bullets

and upgradation of the existing LPG Bottling Plant.

The project envisages construction of 4 nos. of LPG mounded bullets of capacity 1760.56 m3

(1000MT) each and upon successful completion / construction of mounded bullets, existing 03

nos. of LPG Spheres and 01 LPG bullet will be dismantled up to ground level.

NRL has been also operating a LPG bottling plant adjacent to its Refinery location at Numaligarh,

Assam since the year 2002 and producing packed LPG getting distributed to retail consumers

through Oil marketing Companies. The proposed Upgradation of LPG bottling plant would

envisage replacement of the old semi-automatic system by a fully automatic electronic system.

The total capacity of the upgraded plant will be 42000 MT per annum along with improved safety

efficiency performance as against 10000 MT per annum existing plant.

EIL has been selected as an EIA consultant for the Project. As a part of the project Rapid Risk

Analysis Study of the facilities under scope of Project is being carried out.

This executive summary covers major findings arising out of the Rapid Risk Analysis study and

recommendations for the facilities under this Project. The detailed analysis is given in Section –6.

1.2 APPROACH METHODOLOGY

RRA study evaluates the consequences of potential failure scenarios, assess extent of damages,

based on damage criteria’s and suggest suitable measures for mitigating the Hazard.

RRA involves identification of various potential hazards & credible failure scenarios for various

units and other facilities including off-site storages & pumping, etc., based on their frequency of

occurrence & resulting consequence. Basically two types of scenarios are identified spanning

across various process facilities; Cases with high chance of occurrence but having low

consequence, e.g., Instrument Tapping Failure and cases with low chance of occurrence but

having high consequence, e.g., Catastrophic Rupture of Pressure Vessels / Large Hole on the

outlet of Pressure Vessels. Effect zones for various outcomes of failure scenarios (Flash Fire, Jet

Fire, Pool Fire, Blast overpressure, toxic release, etc.) are studied and identified in terms of

distances on plot plan. Based on effect zones, measures for mitigation of the hazard/risk are

suggested.

1.3 MAJOR FINDINGS AND RECOMMENDATIONS



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of the Rapid Risk analysis study for facilities under Project are summarized below:

Consequence modeling of failure scenarios for LPG Mounded Bullets & associated pumping

system are modelled & studied. It is observed that in the event of realization of 20 & 10 mm

leak scenarios for LPG transfer pumps of Gantry & Bottling Plant, radiation & explosion effect

zones may be limited to leak vicinities, depending upon the prevalent weather condition &

ignition sources encountered at the time of the release and may not affect any nearby

facilities.

Adequate number of hydrocarbon detectors to be ensured at strategic locations within the

LPG pump house for early leak detection and inventory isolation and suitable sprinklers

system as per OISD requirements. It is recommended that road on northern, western &

eastern side of the LPG pump house to be classified for traffic movement.

Consequence failure modeling of leak scenarios in LPG Bottling Plant is carried out &

studied. In the event of realization of 20 & 10 mm leak scenarios in the LPG line inside

Bottling Plant, radiation & explosion effect zones may engulf & affect the whole LPG Bottling

plant, depending upon the location of the release, prevalent weather condition & ignition

sources encountered at the time of the release and may affect the personnel’s working inside

the LPG Bottling Plant.

It is recommended to ensure the hydrocarbon & fire detectors inside the Bottling Plant at

appropriate locations and suitable active/passive fire protection measures inside the Bottling

Plant. These scenarios to be also used for updation of existing Disaster Management Plan &

Emergency Response Procedures.

In the event of fire scenario inside LPG Bottling Plant, the escalation effects need to be

evaluated further, as projectiles from LPG cylinder bursting may affect & damage the other

nearby facilities / buildings in the radius of the 460 m.

a) Recommendations for Construction Safety during execution of the Project

Adequate barricading of the proposed facilities to be done from existing running facilities

during construction phase. Hydrocarbon detectors to be placed along the barricading

suitably to detect any hydrocarbon gas in vicinity of construction area.



response during fire.





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project to get a detailed overview of the possible hazards during construction phase and

action plan to prevent / mitigate the same.


No operator cabin shall be located in the vicinity of the LPG Mounded Bullet Pump House

and LPG Bottling Plant.



















d) Ignition Control


within facilities processing flammable materials. As part of mitigation measure it is strongly


e) Escape Routes


from all areas.



releases.


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recommended:





g) Others



etc.


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2 INTRODCUTION

2.1 STUDY AIMS AND OBJECTIVE

The objectives of the Rapid Risk Analysis study are to identify and quantify all potential failure

modes that may lead to hazardous consequences and extent. Typical hazardous consequences

include fire, explosion and toxic releases.

The Rapid Risk analysis will also identify potential hazardous consequences having impacts on

population and property in the vicinity of the facilities, and provides information necessary in

developing strategies to prevent accidents and formulate the Disaster Management Plan.

The Rapid Risk Analysis includes the following steps:

a) Identification of failure cases within the process and off-site facilities

b) Evaluate process hazards emanating from the identified potential accident scenarios.

c) Analyze the damage effects to surroundings due to such incidents.

d) Suggest mitigating measures to reduce the hazard / risk.

The Risk analysis study has been carried out using the risk assessment software program

‘PHAST ver. 7.11 developed by DNV Technica.

2.2 SCOPE OF WORK

The study addresses the hazards that can be realized due to operations associated with the

proposed facilities under Project. It covers the following facilities of Refinery:

Table 1: New Proposed Facilities under Project


1. LPG Mounded Bullets 4 nos.

Table 2: Existing Facilities under Project


1. Upgradation of LPG Bottling Plant

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3 SITE CONDITION

3.1 GENERAL

This chapter describes the location of Numaligarh Refinery complex and meteorological data,

which have been used for the Rapid Risk Analysis study.

3.2 SITE, LOCATION AND VICINITY

Numaligarh Refineries Limited (NRL) presently operates a 3 MMTPA refinery at Golaghat, Assam,

India.

Figure 1: NRL Site

3.3 METEOROLOGICAL CONDITIONS

The consequences of released toxic or flammable material are largely dependent on the

prevailing weather conditions. For the assessment of major scenarios involving release of toxic or

flammable materials, the most important meteorological parameters are those that affect the

atmospheric dispersion of the escaping material. The crucial variables are wind direction, wind

speed, atmospheric stability and temperature. Rainfall does not have any direct bearing on the

results of the risk analysis; however, it can have beneficial effects by absorption / washout of

released materials. Actual behavior of any release would largely depend on prevailing weather

condition at the time of release.

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For the Risk Analysis study, Meteorological data of Tezpur have been taken from the

Climatological Tables of Observatories in India (1961-1990) published by Indian Meteorological

Department, Pune.

Atmospheric Parameters

The Climatological data which have been used for the Risk Analysis study is summarized below:

Table 3: Atmospheric Parameter

Sl. No. Parameter Average Value Considered For Study

1. Ambient Temperature (OC) 28

2. Atmospheric Pressure (mm Hg) 760

3. Relative Humidity (%) 75

4. Solar Radiation flux (kW/m2) 0.70

Wind Speed and Wind Direction

Based on the Meteorological data provided in IMD table, it is observed that average wind speed of

magnitude of around 1 m/s blows for around 60% of the time, in a year. And for rest of time period

average wind speed is around 1-2 m/s. Hence predominant wind speed for Refinery complex

considered for RRA Study is 1 & 2 m/s.

Table 4: Average Mean Wind Speed (m/s)

Jan Feb Mar April May June July Aug Sep Oct Nov Dec

0.91 1.16 1.75 1.88 1.2 0.94 0.8 0.72 0.8 0.8 1.05 0.9

Table 5: % Number of Days Wind From

N NE E SE S SW W NW Calm

N 1 43 12 4 1 6 1 0 32

D 1 28 6 2 1 7 2 1 52

Weather Category

One of the most important characteristics of atmosphere is its stability. Stability of atmosphere is

its tendency to resist vertical motion or to suppress existing turbulence. This tendency directly

influences the ability of atmosphere to disperse pollutants emitted into it from the facilities. In most

dispersion scenarios, the relevant atmospheric layer is that nearest to the ground, varying in

thickness from a few meters to a few thousand meters. Turbulence induced by buoyancy forces in

the atmosphere is closely related to the vertical temperature gradient.

Temperature normally decreases with increasing height in the atmosphere. The rate at which the

temperature of air decreases with height is called Environmental Lapse Rate (ELR). It will vary

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from time to time and from place to place. The atmosphere is said to be stable, neutral or unstable

according to ELR is less than, equal to or greater than Dry Adiabatic Lapse Rate (DALR), which is

a constant value of 0.98°C/100 meters.

Pasquill stability parameter, based on Pasquill – Gifford categorization, is such a meteorological

parameter, which decreases the stability of atmosphere, i.e., the degree of convective turbulence.

Pasquill has defined six stability classes ranging from `A' (extremely unstable) to `F' (stable). Wind

speeds, intensity of solar radiation (daytime insulation) and nighttime sky cover have been

identified as prime factors defining these stability categories. Below Table indicates the various

Pasquill stability classes.

Table 6: Pasquill Stability Classes

Surface Wind Speed

(meter/s)

Day time solar radiation Night time cloud cover

Strong Medium Slight Thin < 3/8 Medium 3/8 Overcast >4/5

< 2 A A – B B - - D

2 – 3 A – B B C E F D

3 – 5 B B – C C D E D

5 – 6 C C – D D D D D

> 6 C D D D D D

Legend: A = Very unstable, B = Unstable, C = Moderately unstable, D = Neutral, E = Moderately

stable, F = stable

When the atmosphere is unstable and wind speeds are moderate or high or gusty, rapid

dispersion of pollutants will occur. Under these conditions, pollutant concentrations in air will be

moderate or low and the material will be dispersed rapidly. When the atmosphere is stable and

wind speed is low, dispersion of material will be limited and pollutant concentration in air will be

high. In general worst dispersion conditions (i.e. contributing to greater hazard distances) occur

during low wind speed and very stable weather conditions, such as that at 1F weather condition

(i.e. 1 m/s wind speed and Pasquill Stability F).

Stability category for the present study is identified based on the cloud amount and wind speed.

For risk analysis the representative average annual weather conditions are assessed based on

the following:

Average Wind speed in order of 1 m/s would be experienced for more than 60% of time in a year

and Wind speed of 1-2 m/s can be realized in the month of Feb to May. Based on weather

analysis, predominant weather stability of “F”& “D” was selected with wind speed 1 m/s & 2 m/s

for consequence analysis, respectively.

The consequence results are reported in tabular form for all the weather conditions and are

represented graphically for worst weather condition.

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Table 7: Weather Conditions

Wind Speed Pasquill Stability

1 F

2 D

Note: For RRA Study Plot Plan (Doc. No.: A774-000-17-44-0001 Rev R) has been used.

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4 HAZARDS ASSOCIATED WITH THE FACILITIES

4.1 GENERAL

Refinery complex handles a number of hazardous materials like LPG, Naphtha and other

hydrocarbons which have a potential to cause fire and explosion hazards. This chapter describes

in brief the hazards associated with these materials.

4.2 HAZARDS ASSOCIATED WITH FLAMMABLE MATERIALS

4.2.1 LIQUIFIED PETROLEUM GAS

LPG is a colorless liquefied gas that is heavier than air and may have a foul smelling odorant

added to it. It is a flammable gas and may cause flash fire and delayed ignition.

LPG is incompatible to oxidizing and combustible materials. It is stable at normal temperatures

and pressure. If it is released at temperatures higher than the normal boiling point it can flash

significantly and would lead to high entrainment of gas phase in the liquid phase. High

entrainment of gas phase in the liquid phase can lead to jet fires. On the other hand negligible

flashing i.e. release of LPG at temperatures near boiling points would lead to formation of pools

and then pool fire. LPG releases may also lead to explosion in case of delayed ignition.

Inhalation of LPG vapors by human beings in considerable concentration may affect the central

nervous system and lead to depression. Inhalation of extremely high concentration of LPG may

lead to death due to suffocation from lack of oxygen. Contact with liquefied LPG may cause

frostbite. Refer to below table for properties of LPG.

Table 8: Hazardous Properties of LPG


1. LFL (%v/v) 1.7

2. UFL (%v/v) 9.0

3. Auto ignition temperature (°C) 420-540


5. Normal Boiling point (°C) -20 to –27

6. Flash point (°C) - 60

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5 HAZARD IDENTIFICATION

5.1 GENERAL

A classical definition of hazard states that hazard is in fact the characteristic of

system/plant/process that presents potential for an accident. Hence all the components of a

system/plant/process need to be thoroughly examined in order to assess their potential for

initiating or propagating an unplanned event/sequence of events, which can be termed as an

accident.

In Risk Analysis terminology a hazard is something with the potential to cause harm. Hence the

Hazard Identification step is an exercise that seeks to identify what can go wrong at the major

hazard installation or process in such a way that people may be harmed. The output of this step is

a list of events that need to be passed on to later steps for further analysis.

The potential hazards posed by the facility were identified based on the past accidents, lessons

learnt and a checklist. This list includes the following elements.

Catastrophic Rupture of Pressure vessel

Large hole on outlet of process vessel

“Guillotine-Breakage” of pipe-work

Small hole, cracks or small bore failure (i.e. instrument tapping failure, drains/vents failure

etc.) in piping and vessels.

Flange leaks.

Storage Tank on fire

Leaks from pump glands and similar seals.

5.2 MODES OF FAILURE

There are various potential sources of large leakage, which may release hazardous chemicals

and hydrocarbon materials into the atmosphere. These could be in form of gasket failure in

flanged joints, bleeder valve left open inadvertently, an instrument tubing giving way, pump seal

failure, guillotine failure of equipment/ pipeline or any other source of leakage. Operating

experience can identify lots of these sources and their modes of failure. A list of general

equipment and pipeline failure mechanisms is as follows:

Material/Construction Defects

Incorrect selection or supply of materials of construction

Incorrect use of design codes

Weld failures

Failure of inadequate pipeline supports

Pre-Operational Failures

Failure induced during delivery at site

Failure induced during installation

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Pressure and temperature effects

Overpressure

Temperature expansion/contraction (improper stress analysis and support design)

Low temperature brittle fracture (if metallurgy is incorrect)

Fatigue loading (cycling and mechanical vibration)

Corrosion Failures

Internal corrosion (e.g. ingress of moisture)

External corrosion

Cladding/insulation failure (e.g. ingress of moisture)

Cathodic protection failure, if provided

Failures due to Operational Errors

Human error

Failure to inspect regularly and identify any defects

External Impact Induced Failures

Dropped objects

Impact from transport such as construction traffic

Vandalism

Subsidence

Strong winds

Failure due to Fire

External fire impinging on pipeline or equipment

Rapid vaporization of cold liquid in contact with hot surfaces

5.3 SELECTED FAILURE CASES

A list of selected failure cases was prepared based on process knowledge, engineering judgment,

experience, past incidents associated with such facilities and considering the general

mechanisms for loss of containment. A list of cases has been identified for the consequence

analysis study based on the following.

Cases with high chance of occurrence but having low consequence: Example of such

failure cases includes seal failure for pumps, instrument tapping failure, etc. The

consequence results will provide enough data for planning routine safety exercises. This

will emphasize the area where operator's vigilance is essential.

Cases with low chance of occurrence but having high consequence (The example includes

Large hole on the outlet of pressure vessels, Catastrophic Rupture of Pressure Vessels,

etc.)

This approach ensures at least one representative case of all possible types of accidental

failure events, is considered for the consequence analysis. Moreover, the list below

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includes at least one accidental case comprising of release of different sorts of highly

hazardous materials handled in the refinery. Although the list does not give complete failure

incidents considering all equipment’s, units, but the consequence of a similar incident

considered in the list below could be used to foresee the consequence of that particular

accident.

For selected credible failure scenarios and likely consequences for facilities of Numaligarh

Refinery under LPG Mounded Bullets & upgradation of LPG Bottling Plant Project, refer

Section-6.

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6 CONSEQUENCE ANALYSIS

6.1 GENERAL

Consequence analysis involves the application of the mathematical, analytical and computer

models for calculation of the effects and damages subsequent to a hydrocarbon / toxic release

accident.

Computer models are used to predict the physical behavior of hazardous incidents. The model

uses below mentioned techniques to assess the consequences of identified scenarios:

Modeling of discharge rates when holes develop in process equipment/pipe work

Modeling of the size & shape of the flammable/toxic gas clouds from releases in the

atmosphere

Modeling of the flame and radiation field of the releases that are ignited and burn as jet fire,

pool fire and flash fire

Modeling of the explosion fields of releases which are ignited away from the point of release

The different consequences (Flash fire, pool fire, jet fire and Explosion effects) of loss of

containment accidents depend on the sequence of events & properties of material released

leading to the either toxic vapor dispersion, fire or explosion or both.

6.2 CONSEQUENCE ANALYSIS MODELLING

6.2.1 DISCHARGE RATE

The initial rate of release through a leak depends mainly on the pressure inside the equipment,

size of the hole and phase of the release (liquid, gas or two-phase). The release rate decreases

with time as the equipment depressurizes. This reduction depends mainly on the inventory and

the action taken to isolate the leak and blow-down the equipment.

6.2.2 DISPERSION

Releases of gas into the open air form clouds whose dispersion is governed by the wind, by

turbulence around the site, the density of the gas and initial momentum of the release. In case of

flammable materials the sizes of these gas clouds above their Lower Flammable Limit (LFL) are

important in determining whether the release will ignite. In this study, the results of dispersion

modeling for flammable materials are presented LFL quantity.

6.2.3 FLASH FIRE

A flash fire occurs when a cloud of vapors/gas burns without generating any significant

overpressure. The cloud is typically ignited on its edge, remote from- the leak source. The

combustion zone moves through the cloud away from the ignition point. The duration of the flash

fire is relatively short but it may stabilize as a continuous jet fire from the leak source. For flash

fires, an approximate estimate for the extent of the total effect zone is the area over which the

cloud is above the LFL.

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6.2.4 JET FIRE

Jet fires are burning jets of gas or atomized liquid whose shape is dominated by the momentum of

the release. The jet flame stabilizes on or close to the point of release and continues until the

release is stopped. Jet fire can be realized, if the leakage is immediately ignited. The effect of jet

flame impingement is severe as it may cut through equipment, pipeline or structure. The damage

effect of thermal radiation is depended on both the level of thermal radiation and duration of

exposure.

6.2.5 POOL FIRE

A cylindrical shape of the pool fire is presumed. Pool-fire calculations are then carried out as part

of an accidental scenario, e.g. in case a hydrocarbon liquid leak from a vessel leads to the

formation of an ignitable liquid pool. First no ignition is assumed, and pool evaporation and

dispersion calculations are being carried out. Subsequently late pool fires (ignition following

spreading of liquid pool) are considered. If the release is bunded, the diameter is given by the size

of the bund. If there is no bund, then the diameter is that which corresponds with a minimum pool

thickness, set by the type of surface on which the pool is spreading.


A vapor cloud explosion (VCE) occurs if a cloud of flammable gas burns sufficiently quickly to

generate high overpressures (i.e. pressures in excess of ambient). The overpressure resulting

from an explosion of hydrocarbon gases is estimated considering the explosive mass available to

be the mass of hydrocarbon vapor between its lower and upper explosive limits.

6.3 SIZE AND DURATION OF RELEASE

Leak size considered for selected failure cases are listed below1.

Table 9: Size of Release


Pump seal failure 6 mm hole size

Flange Leakage 10 mm hole size

Instrument tapping failure 20 mm hole size

Large Hole in the Piping 50 mm, complete rupture of 2” drain line at the Process

vessel outlet

Catastrophic Rupture Complete Rupture of the Pressure Vessels

The discharge duration is taken as 10 minutes for continuous release scenarios as it is

considered that it would take plant personnel about 10 minutes to detect and isolate the leak2.

1 Refer to Guideline for Quantitative Risk assessment ‘Purple Book’. 2 Release duration is based on Chemical Process Quantitative Risk Analysis, CCPS.

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6.4 DAMAGE CRITERIA

In order to appreciate the damage effect produced by various scenarios, physiological/physical

effects of the blast wave, thermal radiation or toxic vapor exposition are discussed.

6.4.1 LFL OR FLASH FIRE

Hydrocarbon vapor released accidentally will spread out in the direction of wind. If a source of

ignition finds an ignition source before being dispersed below lower flammability limit (LFL), a

flash fire is likely to occur and the flame will travel back to the source of leak. Any person caught

in the flash fire is likely to suffer fatal burn injury. Therefore, in consequence analysis, the distance

of LFL value is usually taken to indicate the area, which may be affected by the flash fire.

Flash fire (LFL) events are considered to cause direct harm to the population present within the

flammability range of the cloud. Fire escalation from flash fire such that process or storage

equipment or building may be affected is considered unlikely.

6.4.2 THERMAL HAZARD DUE TO POOL FIRE, JET FIRE AND FIRE BALL

Thermal radiation due to pool fire, jet fire or fire ball may cause various degrees of burn on human

body and process equipment. The damage effect due to thermal radiation intensity is tabulated

below.

Table 10: Damage Due to Incident Thermal Radiation Intensity

Incident Radiation Intensity

(Kw/M²) Type of Damage

37.5 Sufficient to cause damage to process equipment

32.0 Maximum flux level for thermally protected tanks containing flammable

liquid

12.5 Minimum energy required for piloted ignition of wood, melting of plastic

tubing etc.

8.0 Maximum heat flux for un-insulated tanks

4.0 Sufficient to cause pain to personnel if unable to reach cover within 20

seconds. However blistering of skin (1stdegree burns) is likely.

The hazard distances to the 32 kW/m2, 8 kW/m2 and 4 kW/m2 radiation levels, selected based on

their effect on population, buildings and equipment were modeled using PHAST.


In the event of explosion taking place within the plant, the resultant blast wave will have damaging

effects on equipment, structures, building and piping falling within the overpressure distances of

the blast. Tanks, buildings, structures etc. can only tolerate low level of overpressure. Human

body, by comparison, can withstand higher overpressure. But injury or fatality can be inflicted by

collapse of building of structures. The damage effect of blast overpressure is tabulated below.

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Table 11: Damage Effects of Blast Overpressure

Blast Overpressure (PSI) Damage Level

5.0 Major structure damage

3.0 Oil storage tank failure

2.5 Eardrum rupture

2.0 Repairable damage, pressure vessels remain intact, light

structures collapse

1.0 Window pane breakage possible, causing some injuries

The hazard distances to the 5 psi, 3 psi and 2 psi overpressure levels, selected based on their

effects on population, buildings and equipment were modeled using PHAST.

6.5 CONSEQUENCE ANALYSIS FOR UNITS

This section discusses the consequences of selected failure scenarios for units whose affect

zones crosses the respective unit’s B/L and causes worst consequences. The consequence

distances are reported in tabular form for all weather conditions in Annexure-I and are

represented graphically in Annexure-II for the all failure scenarios in a unit for worst weather

conditions.

6.5.1 LPG MOUNDED BULLET


Instrument Tapping Failure at LPG Transfer Pumps - Loading Gantry (45-PA-CF-101 A/B): From

the incident outcome analysis of this failure scenario it is observed that LFL for this failure

scenario may extend upto 61 m. The Jet fire radiation intensity of 32 & 8 kW/m2 may extend upto

51 m & 64 m respectively. The 5 & 3 psi blast waves may reach upto 75 & 80 m respectively.

Instrument Tapping Failure at LPG Transfer Pumps - Bottling Plant (45-PA-CF-102 A/B): From the

consequence analysis of this failure scenario it is observed that LFL for this failure scenario may

extend upto 64 m. The Jet fire radiation intensity of 32 & 8 kW/m2 may extend upto 53 m & 66 m

respectively. The 5 & 3 psi blast waves may reach upto 75 & 81 m respectively.

LPG Mounded Bullet Inlet Line (At the top of the bullet) - 50 mm Leak: From the consequence

modelling of this failure scenario it is observed that LFL for this failure scenario may extend upto

83 m. The Jet fire radiation intensity of 32 & 8 kW/m2 may extend upto 88 m & 114 m respectively.

The 5 & 3 psi blast waves may reach upto 102 & 110 m respectively.

LPG Mounded Bullet Inlet Line (At the top of the bullet) - 20 mm Leak: From the consequence

modelling of this failure scenario it is observed that LFL for this failure scenario may extend upto

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30 m. The Jet fire radiation intensity of 32 & 8 kW/m2 may extend upto 32 m & 47 m respectively.

The 5 & 3 psi blast waves may reach upto 38 & 42 m respectively.

In addition to above failure scenario, Seal Failure at LPG Transfer Pumps - Loading Gantry (45-

PA-CF-101 A/B) and Seal Failure at LPG Transfer Pumps - Bottling Plant (45-PA-CF-102 A/B) are

also modelled & evaluated, it is observed that for these failure scenarios the affect zone may be

limited to vicinity of leakage.

6.5.2 LPG BOTTLING PLANT


Instrument Tapping Failure at LPG Line: From the incident outcome analysis of this failure

scenario it is observed that LFL for this failure scenario may extend upto 62 m. The Jet fire

radiation intensity of 32 & 8 kW/m2 may extend upto 52 m & 64 m respectively. The 5 & 3 psi blast

waves may reach upto 75 & 80 m respectively.

Catastrophic Rupture of LPG Cylinder: From the consequence analysis of this failure scenario it is

observed that LFL for this failure scenario may extend upto 18 m. The 5 & 3 psi blast waves may

reach upto 11 & 15 m respectively.

Flange Leakage at LPG Line: From the event outcome analysis of this failure scenario it is

observed that LFL for this failure scenario may extend upto 25 m. The Jet fire radiation intensity of

32 & 8 kW/m2 may extend upto 28 m & 34 m respectively. The 5 & 3 psi blast waves may reach

upto 26 & 28 m respectively.

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7 MAJOR FINDINGS & RECOMMENDATIONS




of the Rapid Risk analysis study for facilities under Project are summarized below:

Consequence modeling of failure scenarios for LPG Mounded Bullets & associated pumping

system are modelled & studied. It is observed that in the event of realization of 20 & 10 mm

leak scenarios for LPG transfer pumps of Gantry & Bottling Plant, radiation & explosion effect

zones may be limited to leak vicinities, depending upon the prevalent weather condition &

ignition sources encountered at the time of the release and may not affect any nearby

facilities.

Adequate number of hydrocarbon detectors to be ensured at strategic locations within the

LPG pump house for early leak detection and inventory isolation and suitable sprinklers

system as per OISD requirements. It is recommended that road on northern, western &

eastern side of the LPG pump house to be classified for traffic movement.

Consequence failure modeling of leak scenarios in LPG Bottling Plant is carried out &

studied. In the event of realization of 20 & 10 mm leak scenarios in the LPG line inside

Bottling Plant, radiation & explosion effect zones may engulf & affect the whole LPG Bottling

plant, depending upon the location of the release, prevalent weather condition & ignition

sources encountered at the time of the release and may affect the personnel’s working inside

the LPG Bottling Plant.

It is recommended to ensure the hydrocarbon & fire detectors inside the Bottling Plant at

appropriate locations and suitable active/passive fire protection measures inside the Bottling

Plant. These scenarios to be also used for updation of existing Disaster Management Plan &

Emergency Response Procedures.

In the event of fire scenario inside LPG Bottling Plant, the escalation effects need to be

evaluated further, as projectiles from LPG cylinder bursting may affect & damage the other

nearby facilities / buildings in the radius of the 460 m.

a) Recommendations for Construction Safety during execution of the Project

Adequate barricading of the proposed facilities to be done from existing running facilities

during construction phase. Hydrocarbon detectors to be placed along the barricading

suitably to detect any hydrocarbon gas in vicinity of construction area.



response during fire.

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project to get a detailed overview of the possible hazards during construction phase and

action plan to prevent / mitigate the same.


No operator cabin shall be located in the vicinity of the LPG Mounded Bullet Pump House

and LPG Bottling Plant.













Mitigating measures are those measures in place to minimize the loss of containment event

and, hazards arising out of Loss of containment. These include:





d) Ignition Control


within facilities processing flammable materials. As part of mitigation measure it is strongly


e) Escape Routes


from all areas.

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Doc No: A816-17-43-RA-0001

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releases.




recommended:





g) Others



etc.


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8 GLOSSARY

CASUALTY Someone who suffers serious injury or worse i.e. including fatal

injuries. As a rough guide fatalities are likely to be half the total

casualties. But this may vary depending on the nature of the event.

HAZARD A chemical or physical condition with the potential of causing

damage.

FLAMMABILITY LIMITS In fuel-air systems, a range of compositions exists inside which a

(UFL – LFL) flame will propagate substantial distance from an

ignition source. The limiting fuel concentrations are termed as

Upper flammability or explosives limit (Fuel concentrations

exceeding this are too rich) and Lower flammability or explosives

limit (Fuel concentrations below this are too lean).

FLASH FIRE The burning of a vapor cloud at very low flame propagation speed.

Combustion products are generated at a rate low enough for

expansion to take place easily without significant overpressure

ahead or behind the flame front. The hazard is therefore only due to

thermal effects.

OVERPRESSURE Maximum pressure above atmosphere pressure experiences during

the passage of a blast wave from an explosion expressed in this

report as pounds per square inch (psi).

EXPLOSION A rapid release of energy, which causes a pressure discontinuity or

shock wave moving away from the source. An explosion can be

produced by detonation of a high explosive or by the rapid burning

of a flammable gas cloud. The resulting overpressure is sufficient to

cause damage inside and outside the cloud as the shock wave

propagation into the atmosphere beyond the cloud. Some authors

use the term deflagration for this type of explosion

DOMINO EFFECT The effect that loss of containment of one installation leads to loss

of containment of other installations

EVENT TREE A logic diagram of success and failure combinations of events used

to identify accident sequences leading to all possible consequences

of a given initiating event.

TLV “Threshold limit value” is defined as the concentration of the

substance in air that can be breathed for five consecutive 8 hours

work day (40 hours work week) by most people without side effect.

STEL “Short Term Exposure Limit” is the maximum permissible average

exposure for the time period specified (15 minutes).

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IDLH “Immediate Dangerous to Life and Health” is the maximum

concentration level from which one could escape within 30 minutes

without any escape impairing symptoms.

PASQUILL CLASS Classification to qualify the stability of the atmosphere, indicated by

a letter ranging from A, for very unstable, to F, for stable.

FREQUENCY The number of times an outcome is expected to occur in a given

period of time.

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9 REFERENCES

1. Classification of hazardous locations, A. W. Cox, F. P. Lees and M. L. Ang, Published by

the Institute of Chemical engineers, U. K.

2. The reference manual, Volume-II, Creemer & Warner Ltd. U. K. (Presently Entec).

3. Risk analysis of six potentially hazardous industrial objects in the Rijnmond area; A pilot

study. A report to the Rijnmond Public Authority. D. Riedel publishing company, U. K.

4. Loss prevention in the process industries, Hazard identification, Assessment and Control,

Frank. P. Lees (Vol. I, II & III), Published by Butterworth-Heinemann, U. K.

5. AICHE, CCPS, Chemical process Quantitative Risk Analysis

6. Guideline for Quantitative Risk assessment, ‘Purple book’.

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Page 1 of 2


ANNEXURE-I

CONSEQUENCE ANALYSIS HAZARD DISTANCES

Temp.

(OC)

Press.

(Kg/cm2g)4 KW/m2 8 KW/m2 32 KW/m2 4 KW/m2 8 KW/m2 32 KW/m2 2 psi 3 psi 5 psi

1 F 61 73 64 51 - - - 87 80 75

2 D 57 69 59 46 - - - 73 67 62

1 F 13 24 21 17 - - - 16 14 13

2 D 11 23 20 16 - - - 15 14 13

1 F 64 76 66 53 - - - 88 81 75

2 D 59 72 61 48 - - - 74 68 63

1 F 13 25 22 18 - - - 16 15 13

2 D 11 24 21 16 - - - 16 14 13

1 F 83 133 114 88 - - - 120 110 102

2 D 65 125 106 80 - - - 94 86 79

1 F 30 56 47 32 - - - 46 42 38

2 D 28 53 44 28 - - - 33 30 27

1 F 62 74 64 52 - - - 87 80 75

2 D 58 70 60 47 - - - 73 67 63

1 F 15 - - - - - - 20 15 11

2 D 18 - - - - - - 20 15 11

1 F 25 40 34 28 - - - 31 28 26

2 D 22 37 32 25 - - - 30 28 26

Mounded Bullets

18

Equipment

Bottling Plant

LPG Line Flange Leakage

0.8


WeathersFlash Fire

(m)Failure Case

Jet Fire (m) Pool Fire (m)

16.6

Amb.

Unit

LPG Transfer Pumps - Loading Gantry

(45-PA-CF-101 A/B)Amb. 16.6

LPG Transfer Pumps - Bottling Plant

(45-PA-CF-102 A/B)


Sl No.


Catastrophic Rupture2

NOTE

1

2

3

8.2

0.7

Leak Rate

Kg/s

LPG Transfer Pumps - Loading Gantry

(45-PA-CF-101 A/B)Amb.

20.48

Amb.Saturation

pressure

5

4

1

-LPG Cylinder


Seal Failure

2.14

LPG Transfer Pumps - Bottling Plant

(45-PA-CF-102 A/B)Amb. 20.48 9.1Instrument Tapping Failure

Seal Failure

3

LPG Line Amb. 18 8.6

Amb.

Numaligarh Refinery Ltd.- LPG Mounded Bullets & LPG Bottling Plant Consequence Analysis Hazard Distances

LPG Mounded Bullet Inlet Line

(At the top of the bullet)50 mm Leak Amb. 6 30.9

6LPG Mounded Bullet Inlet Line

(At the top of the bullet)20 mm Leak Amb. 6 4.94

B511

Text Box

Page 2 of 2

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Doc No: A816-17-43-RA-0001

Rev. No.: 0

Page 1 of 27


ANNEXURE-II

FIGURES FOR CONSEQUENCE ANALYSIS

Figure 6.5.1.1 A: Mounded Bullet: Instrument tapping failure at LPG Transfer Pumps - Loading Gantry; Flash Fire Distances (m)

Page 2 of 27

Figure 6.5.1.1 B: Mounded Bullet: Instrument tapping failure at LPG Transfer Pumps - Loading Gantry; Jet Fire Distances (m)

Page 3 of 27

Figure 6.5.1.1 C: Mounded Bullet: Instrument tapping failure at LPG Transfer Pumps - Loading Gantry; Over Pressure Distances (m)

Page 4 of 27

Figure 6.5.1.2 A: Mounded Bullet: Seal failure at LPG Transfer Pumps - Loading Gantry; Flash Fire Distances (m)

Page 5 of 27

Figure 6.5.1.2 B: Mounded Bullet: Seal failure at LPG Transfer Pumps - Loading Gantry; Jet Fire Distances (m)

Page 6 of 27

Figure 6.5.1.2 C: Mounded Bullet: Seal failure at LPG Transfer Pumps - Loading Gantry; Over Pressure Distances (m)

Page 7 of 27

Figure 6.5.1.3 A: Mounded Bullet: Instrument tapping failure at LPG Transfer Pumps – Bottling Plant; Flash Fire Distances (m)

Page 8 of 27

Figure 6.5.1.3 B: Mounded Bullet: Instrument tapping failure at LPG Transfer Pumps - Bottling Plant; Jet Fire Distances (m)

Page 9 of 27

Figure 6.5.1.3 C: Mounded Bullet: Instrument tapping failure at LPG Transfer Pumps - Bottling Plant; Over Pressure Distances (m)

Page 10 of 27

Figure 6.5.1.4 A: Mounded Bullet: Seal failure at LPG Transfer Pumps – Bottling Plant; Flash Fire Distances (m)

Page 11 of 27

Figure 6.5.1.4 B: Mounded Bullet: Seal failure at LPG Transfer Pumps - Bottling Plant; Jet Fire Distances (m)

Page 12 of 27

Figure 6.5.1.4 C: Mounded Bullet: Seal failure at LPG Transfer Pumps - Bottling Plant; Over Pressure Distances (m)

Page 13 of 27

Figure 6.5.1.5 A: Mounded Bullet: LPG Inlet line to Mounded Bullet-50 mm Hole at Top of the Mounded Bullet; Flash Fire Distances (m)

Page 14 of 27

Figure 6.5.1.5 B: Mounded Bullet: LPG Inlet line to Mounded Bullet-50 mm Hole at Top of the Mounded Bullet; Jet Fire Distances (m)

Page 15 of 27

Figure 6.5.1.5 C: Mounded Bullet: LPG Inlet line to Mounded Bullet-50 mm Hole at Top of the Mounded Bullet; Over Pressure Distances (m)

Page 16 of 27

Figure 6.5.1.6 A: Mounded Bullet: LPG Inlet line to Mounded Bullet-20 mm Hole at Top of the Mounded Bullet; Flash Fire Distances (m)

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Figure 6.5.1.6 B: Mounded Bullet: LPG Inlet line to Mounded Bullet-20 mm Hole at Top of the Mounded Bullet; Jet Fire Distances (m)

Page 18 of 27

Figure 6.5.1.6 C: Mounded Bullet: LPG Inlet line to Mounded Bullet-20 mm Hole at Top of the Mounded Bullet; Over Pressure Distances (m)

Page 19 of 27

Figure 6.5.2.1 A: Bottling Plant: Instrument Tapping failure at LPG Line; Flash Fire Distances (m)

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Figure 6.5.2.1 B: Bottling Plant: Instrument Tapping failure at LPG Line; Jet Fire Distances (m)

Page 21 of 27

Figure 6.5.2.1 C: Bottling Plant: Instrument Tapping failure at LPG Line; Over Pressure Distances (m)

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Figure 6.5.2.2 A: Bottling Plant: Catastrophic Rupture of LPG Cylinder; Flash Fire Distances (m)

Page 23 of 27

Figure 6.5.2.2 B: Bottling Plant: Catastrophic Rupture of LPG Cylinder; Over Pressure Distances (m)

Page 24 of 27

Figure 6.5.2.3 A: Bottling Plant: Flange Leakage (10mm) at LPG Line; Flash Fire Distances (m)

Page 25 of 27

Figure 6.5.2.3 B: Bottling Plant: Flange Leakage (10mm) at LPG Line; Jet Fire Distances (m)

Page 26 of 27

Figure 6.5.2.3 C: Bottling Plant: Flange Leakage (10mm) at LPG Line; Over Pressure Distances (m)

Page 27 of 27

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