ASSESSMENT OF HAZARDS: CONSEQUENCE ANALYSIS OF LOSS SCENARIOS

Dr. Asit K Patra Disaster Management Institute Bhopal

Logic Diagram for Consequence /Risk Analysis

Release of Hazardous Substance

Discharge & Dispersion Models

Explosion & Fire Models

Consequence Models

Risk Calculation

Flammable Release

Toxic Release

Mitigation Factors

Components of Consequence Analysis

1. Discharge Models : Loss of containment/Release scenarios.

2. Dispersion Models : Transport and dispersion of released flammable/Explosive/toxic chemicals.

3. Consequence/Effect models: Effect on people, property and environment.

Typical Release Scenarios

(A)Liquid Discharges

* Hole in atmospheric storage tank or other atmospheric storage vessel or pipe under liquid head.* Hole in vessel or pipe containing pressurized liquid below its normal boiling point.

(B)Gas Discharges* Hole in equipment (pipe, vessel) containing gas

under pressure * Relief valve discharge (of vapour only)

Contd..

Typical Release Scenarios

* Evaporation from liquid pool (e.g., naptha, heavy cut etc.) * Relief valve discharge from top of pressurized storage tank * Generation of toxic combustion products as a result of fire

(C) Two-Phase Discharges

* Hole in pressurized storage tank or pipe containing a liquid above its normal boiling point.

Scenario Identification

Pressure Relief Devices Relieving Directly to the Atmosphere:

Scenario Identification

Vessels : Rupture based on largest diameter process pipe attached to the vessel

Scenario Identification

Tank Overflows and Spills

……. …….. ……… …. .. .

+++++++++++++++++++++++++++++++++++++++ ++++++++++++++++

Leak

Aerosol

TBOILING POINT < T AMBIENT

Pool Spread

Boiling Pool

Flash

Example: Super-heated release (Release of LPG/Liquid Chlorine)

Tank with liquid

EXAMPLE – RELEASE OF PRESSURISED GAS

Release

Jet fire

No ignitionSafe dispersion

Ignition

Delayedignition

Immediate ignition

Open atmosphere

Congested atmosphere Vapour cloud explosion

Flash fire

EXAMPLE – RELEASE OF A REFRIGERATED LIQUEFIED GAS

Release

Pool fire

No ignitionSafe dispersion

Ignition

Delayedignition

Immediate ignition

Open atmosphere

Congested atmosphere Vapour cloud explosion

Flash fire

EXAMPLE – EVENT TREE FOR RELEASING OF LIQUEFIED GAS UNDER PRESSURE

Release

No ignitionSafe dispersion

Ignition

Delayedignition

Open atmosphere

Congested atmosphereVCE

Flash fire

Immediateignition

No cooling

CoolingJet fire

BLEVE

Scenario: Jet Fire Scenario of Natural Gas while being transported through pipeline

Pipe Diameter: 20 inches Pipe Length: 10 KmPipe Press: 500 psia Pipe Temperature: 35° CWind: 5 meters/second Ground Roughness: Industrial Air Temperature: 35° C Stability Class: DRelative Humidity: 70% *******************************************************THREAT ZONE: Threat Modeled: Thermal radiation from jet fire 4.5 Kw/m2 (Blue Zone) 168 12.5 Kw/m2 (Orange Zone) 99 37.5 Kw/m2 (Red Zone) 45 meters

Consequence Analysis using PHAST RISK

Scenario: Catastrophic rupture of a Naptha storage tank: Pool Fire Chemical details :  * Capacity of tank = 950 m3. * Density at 20OC = 0.93 g/ml.* Dyke Dimension: 40.5 m x 36.25 m* Storage Conditions: Atmospheric.* LFL/LEL = 1% by volume of air.

Table 1 : Maximum affected distances (in meter) for Pool fire scenarios under D class with wind speed of 5.0 m/s

Thermal radiation Affected Distance in Metre from Pool levels for Fire 4.5 Kw/m2 (Blue Zone) 69 12.5 Kw/m2 (Orange Zone) 24 37.5 Kw/m2 (Red Zone) Not Reached

Consequence Analysis using PHAST RISK

RADIATION FLUX LEVEL AND EXPLOSION OVERPRESSURE LEVELS

Wind speed & direction :Dilutes the released chemical and then it spreads along the wind direction. Thermal stability : Stable (E - F), Neutral(D) and Unstable (A-C)

Temperature Inversion : Suppress the movement of toxic cloud.

Factors Affecting Transport and

Dispersal of toxic chemicals

Stability Class (Pasquill, 1961).

(i) A Class : Very Unstable. Imagine summer noon time.(ii) B Class : Unstable.

(iii) C Class : Slightly Unstable.

(iv) D Class : Neutral. Slightly cloudy.

(v) E Class : Stable.

(vi) F Class : Very stable. Imagine Winter Night.

Contd.

Scenario: Release of Chlorine from a tonner through hole with various sizes (Inventory = 900 Kg).

Table 1 : Maximum Downwind Effect Distance (km) of Chlorine Vapour Cloud under various atmospheric conditions

Hole Diameter Atmospheric Stability Class and Wind Speed (mm) F ; 1.5 m/s D; 3.0 m/s A; 2 m/s

5.0 0.77 0.20 0.05 *10.0 1.61 0.44 0.11 *15.0 2.70 0.71 0.15 20.0 3.65 0.95 0.23

Scenario: Catastrophic Failure of a Chlorine Tonner (Contains 900 kg)

Concentration Maximum Downwind Distance (m) (ppm) under the following atmospheric conditions B ; 2.0 m/s D; 4.0 m/s F; 1.5 m/s

1000 (Fatal) 206 286 195 35 668 1432 2957 10 (IDLH) 1208 2858 7516 3 (STEL) 2109 5350 16260

Consequence Analysis using Phast Risk

FOOT-PRINT OF CHLORINE GAS CLOUD

Dispersion Modelling of Chlorine gas after being released from CWPH in NTPC, SingorliInventory = 900 kg.)

Outcome of Consequence Analysis

(a) Maximum loss scenarios

(b) Consequences in terms of Heat radiation, Over pressure and intoxication.

(c) Identification of vulnerable zones and classification of units which have the potential for creating an off-site emergency.

(d)Identification of important facilities which are coming into the vulnerable zones.

(e) Selection of assembly point/escape route/Adm. Building etc.

. Complete Inventory of hazardous processes, chemicals, details of storage (T, P, type of vessel etc.) etc.

. Expert professionals having thorough understanding of the process thermodynamics and atmospheric dispersion.

. Meteorological data and knowledge of stability classes.

. Relevant Software to predict vulnerable zones

DMI Infrastructures required for Consequence/Risk Analysis

For Further Information, Please contact:asitbangra@rediffmail.com

Thanks