QRA Presentation

Agenda

Introduction to Risk Assessment Introduction to PHAST: Inputs-Scenario, Weather, Material, Map Discharge Modelling Flammable & Toxic Gas Dispersion Parts Count and Failure Frequency Input to PHAST Risk (Population, Ignition Sources, Inserting Map) Risk Estimation (Individual and Societal Risk, Risk Criteria)

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Common DefinitionsHazard: A state or condition having the potential to cause a deviation

from uniform or intended behaviour which, in turn, may result in damage to property, people or environment

Incident: Actual Realization of Hazard i.e. an event or chain of events, which has caused or could have caused personal injury, damage to property or environment

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What is QRA?

Means of making a systematic analysis of the risks from hazardous activities, and forming a rational evaluation of their significance, in order to provide input to a decision-making process.

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Objective of QRA

To identify and quantify the major process hazards associated with in the facility.

Assess the acceptability of the risks to people against internationally recognised criteria.

Identify the main risk contributors in order to establish potential risk reduction measures.

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Why Risk Assessment required?

Legal requirement ?

To understand risks and manage them effectively

Insurance requirement

Improve the morale of operating staff

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Why Risk Assessment required?

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Risk Management Workflow

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Hazard Identification

A qualitative review of Possible incidents that may occur, based on previous accident, experience or engineering judgement, creating synthetic situations where necessary.

Failure cases are usually derived by breaking the process system down into sub- systems when required

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Hazard Identification Needs

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Types of materials handled, stored and used

Hazard potential based on material properties and conditions

Types of Hazard Events e.g. Vessel Leak, Line rupture, equipment failure, over-

pressurisation and high temperature

Plant and process information/ Operating conditions

Safety systems provided

Learning from Previous incidents / accidents

Previous Risk analysis if done

Isolatable sections

The limit of Isolatable sections boundaries are defined by location of the following:

Shutdown Valve

Blowdown Valve

Pressure Safety Valve

Normally closed valve with positive isolation

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Inventory Analysis

Inventory that could potentially be released from each isolatable sections.

Total released inventory = Static Inventory + Dynamic Inventory

Static inventory was the amount of material within the isolatable section's vessels

and piping, prior to a leak

Dynamic Inventory was calculated based on the pumped-in flow rate and the isolation

time by:

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I (T ) = Total potential inventory released (kg)I ( S ) = Static inventory (kg)

rL = Leak rate (kg/s)rP = Process flow rate (kg/s)t = Release duration (s)

I (T ) = I ( S ) + MIN (rL, rP) • t

Meteorological Conditions

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Maximum Ambient Temperature 39.4ºC

Average Barometric Pressure 1.010 bar

Relative Humidity 61%

Solar Radiation 1 kW/m2

Surface Roughness Coefficient 0.1m

Stability Class DefinitionA Very UnstableB UnstableC Slightly UnstableD NeutralE Slightly StableF Stable

The Pasquill Atmospheric Stability Classes

Population Data

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Onsite Population

Offsite population around 200

Worker Category No. of person Hour per day (A) Day per Week (B) Week Per year (C) (Note 1)

No. of Hours Spent per year

( A x B x C)

A shift 45 8 7 52 2,912B shift 45 8 7 52 2,912

General shift 15 9 5 52 2,340

RELEASE SECTIONS

The key factors in the selection of these representative sections are:

Material / phase released (pressurized gas/atmospheric liquid etc.)

Release condition (whether the driver for the release is the inventory of a vessel

upstream, pressure, etc.)

Process conditions (temperature and pressure)

Release location (the area in which the release occurs, including the height)

Isolation

RELEASE (HOLE) SIZES

For each of the sections containing process equipment / piping / pipeline, three representative release sizes are considered:

Large leaks(50-150) – 100 mm diameter hole

Medium leaks(10-50) – 25 mm diameter hole

Small leaks (3-10) – 5 mm diameter hole

Failure Frequency Analysis

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Once the potential hazards have been identified, failure frequency estimates how likely it is for the accidents to occur, based on the type and number of equipment components included in the defined failure cases

Parts count of equipment, flanges, valves, etc.The component failure frequencies to be used are usually derived from an analysis

of historical accident experience (Failure databases) UK HSE Failure Frequencies Database OGP (Oil and Gas procedure) for frequencies of failure

Consequence Analysis

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In parallel with the frequency analysis, consequence modelling evaluates the resulting effects if the accidents occur, and their impact on man, machinery and environment.

Estimation of the consequences of each possible scenario by latest software modelling.

Consequence analysis requires the modelling of a number of distinctive phases, i.e. Discharge rate, evaporation, dispersion of flammable and toxic gases , fires and explosions etc.

Classification of Consequence

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Toxic / Flammable Gas release

Fire (Jet Fire, Flash Fire, Pool Fire) leading to injury, fatality or property damage

Vapour Cloud Explosion leading to injury, fatality or property damage

Toxic

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Dispersion of toxic material in the atmosphere is a function of source strength, relative density of toxic cloud, weather conditions etc. The severity of harm depends on the exposed concentration level and exposure time

For the purpose of analysis we consider fatality as the concentration of interest, as the data for fatality can be available from the history while it is difficult to get data for injury accurately.

NIOSH (National Institute Of Safety & Health) have defined different Threshold concentrations levels (TWA, STEL, ERPG, IDLH etc) based on exposure time.

time-weighted average (twa)short term exposure limit(STEL)Emergency Response Planning Guidelines (ERPGs) Immediately Dangerous to Life or Health Concentrations (IDLH)

Fire

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A release of flammable substance results in various events depending upon direct or delayed ignition. Different types of fire are envisaged based on its shape such as jet fire, pool fire etc. The shape of fire again depends on storage condition, release condition, ignition etc.

Personal injury, death or damage to properties may occur either by direct flame or the radiation which goes around the flame.

Jet Fire

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Release of a flammable material at high pressure due to hardware failure (leak) may lead to formation of jet, which may cause jet fire on immediate availability of ignition.

The Jet Fire could damage the neighboring vessels / tanks by direct flame impingement. The thermal radiations may as well affect surrounding population.

Flash Fire

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A Flash Fire is low-intensity combustion without explosion, whose effect zone is up to the LFL (Lower Flammable Limit) region of the cloud. A flash fire may occur if the gas cloud reaches a source of ignition and rapidly burns back to the source of release. Due to the short duration of a flash fire, only people within the fire path will be affected.

Pool Fire

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A pool fire is a turbulent diffusion fire burning above a horizontal pool of vaporizing hydrocarbon fuel where the fuel has zero or low initial momentum.

Fires in the open will be well ventilated (fuel-controlled), but fires within enclosures may become under-ventilated (ventilation-controlled).

Vapour Cloud Explosion

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Continuous release of flammable material over a period may lead to formation of vapour cloud due to non-availability of ignition source.

This may lead to Vapour Cloud Explosion (VCE), if this cloud gets ignition before it is diluted to the concentration below the Lower Explosive Limit (LEL) in air. The explosion may cause overpressure resulting into damage to the surrounding buildings and equipment.

Vapour Cloud Explosions (VCE) are the most dangerous and destructive explosions in the chemical process industries.

Damage Criteria

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Heat RadiationKW/m2

Damage to Equipment Damage to People

4 ---- Causes pain if duration is longer than 20 seconds. Blistering is unlikely.

12.5 Minimum energy to ignite wood with a flame; Melts plastic tubing.

1% Fatality in 20 sec, 30% Fatality in 30 seconds.

37.5 Severe damage to plant

100 % Fatality

Explosion Over Pressure Impacts

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OverpressureBar

Damage to Equipment

Damage to People

0.2 - 0.5 (3-8psi)

Heavy damage to plant & structure

Fatality probability = 1 for humans indoor as well as outdoor

> 50% eardrum damage > 50% serious wounds from flying objects

0.15 - 0.2(2-3psi)

Repairable damage

1% death > 1% eardrum damage

> 1% serious wounds from flying objects0.02 - 0.15 (0.3-2psi)

Major glass damage/10% glass

damage

Slight injury from flying glass

Risk Calculations

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When the frequencies and consequences of each modelled scenario have been estimated, they can be combined to produce Over all Risk Results for the site.

Various forms of risk presentation may be used, commonly grouped as follows: Individual riskSocietal risk - Often expressed in terms of frequency distribution of multiple 

casualty events  (F N curve.) 

Risk Assessment

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Risk Assessment is the process of comparing the level of risk against a set criteria as well as the identification of major risk contributors

Criteria is used as per UK HSE Guidelines

Risk Assessment Guidelines

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Base Data Source Used Worldwide From: UK HSE Hydrocarbon Release Statistics, HSR 2001 002 Purple (Guideline for quantitative risk assessment) Yellow (Methods for the calculation of physical effects) Green (Methods for the determination of possible damage to people

and objects resulting from releases of hazardous materials) Red Book (Methods for determining and process probabilities)

Risk Tolerability Criteria - ALARP

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Individual Risk- IR Contour

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Consequence Contours

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LPG storage bullet Over pressure - BLEVE (2F)

Societal Risk- FN Curve

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Societal Risk- FN Curve

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Conclusions

BLEVE in storage LPG bullet; The resulting over pressure of 0.03 bar reaches the maximum distance of 389m and 0.1 bar reaches the maximum distance of 155m , 0.3 bar reaches the maximum distance of 84 m . Based on the results, BLEVE potentially impacts Pump house areas.

The societal risk (F-N Curve) from facility shows that the societal risk falls in the ALARP region as per the UKHSE Acceptance Criteria with the fatality of 11 persons.

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Recommendations

It is preferable to have fire rated doors at least for 2 hours for office building and canteen.

Consider providing fire proof wall for office building and Cylinder storage area.Safety valves must be tested regularly. The block valves before safety valve must

always be kept in open condition when safety valves are in position. Vehicular traffic as well as entry of personnel inside the plant area must be

restricted.The water sprinkler system, heat detectors and remote operated valves must be

checked regularly for their timely actuation.The DG(Diesel Generators) sets must be periodically tested on load to ensure that

it remains always in operating condition.Safety Audits must be regularly done as per norms and recommendations of OISD.

Risk Analysis Study in future shall be required if there is any change in the plant facility.

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