7.2 Risk Assessment Studies

7.2 Risk Assessment Studies 7.2.1 Introduction Hazard analysis involves the identification and quantification of the various hazards (unsafe condition...

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Risk Assessment Studies


Introduction Hazard analysis involves the identification and quantification of the various hazards (unsafe conditions) that exist in the proposed power plant operations. On the other hand, risk analysis deals with the recognition and computation of risks, the equipment in the plant and personnel are prone to, due to accidents resulting from the hazards present in the plant. Risk analysis follows an extensive hazard analysis. It involves the identification and assessment of risks the neighboring populations are exposed to as a result of hazards present. This requires a thorough knowledge of failure probability, credible accident scenario, vulnerability of population etc. Much of this information is difficult to get or generate. Consequently, the risk analysis is often confined to maximum credible accident studies. In the sections below, the identification of various hazards, probable risks in the proposed power plant, maximum credible accident analysis, consequence analysis are addressed which gives a broad identification of risks involved in the plant. The Disaster Management Plan (DMP) has been presented.


Approach to the Study Risk involves the occurrence or potential occurrence of some accidents consisting of an event or sequence of events. The risk assessment study covers the following:       


Identification of potential hazard areas; Identification of representative failure cases; Visualization of the resulting scenarios in terms of fire (thermal radiation) and explosion; Assess the overall damage potential of the identified hazardous events and the impact zones from the accidental scenarios; Assess the overall suitability of the site from hazard minimization and disaster mitigation point of view; Furnish specific recommendations on the minimization of the worst accident possibilities; and Preparation of broad Disaster Management Plan (DMP), On-site and Off-site Emergency Plan, which includes Occupational and Health Safety Plan.

Hazard Identification Identification of hazards in the proposed power plant is of primary significance in the analysis, quantification and cost effective control of accidents involving chemicals and process. 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 to assess their potential for initiating or propagating an unplanned event/sequence of events, which can be termed as accident. The following two methods for hazard identification have been employed in the study: 

Identification of major hazardous units based on Manufacture, Storage and Import of Hazardous Chemicals Rules, 1989 of Government of India (GOI Rules, 1989); and

Identification of hazardous units and segments of plants and storage units based on relative ranking technique, viz. Fire-Explosion and Toxicity Index (FE&TI).


Classification of Major Hazardous Units Hazardous substances may be classified into three main classes; namely flammable substances, unstable substances and toxic substances. The ratings for a large number of chemicals based on flammability, reactivity and toxicity have been given in NFPA Codes 49 and 345 M. The major hazardous materials to be stored, transported, handled and utilized within the facility have been summarized in the Table-7.2. The fuel storage details and properties are given in Table-7.3 and Table-7.4 respectively. TABLE-7.2 HAZARDOUS MATERIALS PROPOSED TO BE STORED/TRANSPORTED Materials

Hazardous Properties UN 1203. Dangerous Goods class 3 – Flammable Liquid






No. of Tanks 1

Design Capacity (KL) 10

Classification Non-dangerous Petroleum


TLV MP UEL 7.3.2

Codes/Label Flammable

: : :

TLV 5 mg/m3

Threshold Limit Value Melting Point Upper Explosive Limit


MP °c -



: : :







Final Boiling Point Flash Point Lower Explosive Limit

Identification of Major Hazard Installations Based on GOI Rules, 1989 Following accidents in the chemical industry in India over a few decades, a specific legislation covering major hazard activities has been enforced by Govt. of India in 1989 in conjunction with Environment Protection Act, 1986. This is referred here as GOI Rules 1989. For the purpose of identifying major hazard installations the rules employ certain criteria based on toxic, flammable and explosive properties of chemicals. A systematic analysis of the fuels/chemicals and their quantities of storage has been carried out, to determine threshold quantities as notified by GOI Rules, 1989 and the applicable rules are identified. Applicability of storage rules are summarized in Table7.5. TABLE-7.5 APPLICABILITY OF GOI RULES TO FUEL/CHEMICAL STORAGE Sr. No. 1

Chemical/ Fuel LDO

Listed in Schedule 3(1)

Total Quantity (KL) 10

Threshold Quantity (T) for Application of Rules 5,7-9,13-15 10-12 25 MT 200 MT


Hazard Assessment and Evaluation


Methodology An assessment of the conceptual design is conducted for the purpose of identifying and examining hazards related to feed stock materials, major process components, utility and support systems, environmental factors, proposed operations, facilities, and safeguards.


Preliminary Hazard Analysis (PHA) The process and the materials to be used at thermal power station the following can be considered as major plant sections. Thermal Plant         

Coal Handling Plant; Main Plant (Boiler, Turbo Generator etc.); Water treatment Plant; Hydrogen gas filing station/Turbo –Generator coolers; Switch Yard including sub-stations; Fuel oil handling plant; Anhydrous Ammonia; Cable Galleries; and Stores where hazardous, flammable and explosive materials are stored.

Identification of Major Hazard Potential at NSPCL-Durgapur The nature of hazards that can occur at the thermal power plants of NSPCLDurgapur may be broadly classified in to two categories i.e. Natural and Man-Made Hazards (Chemical Hazards).These are detailed below. Natural Hazards a. Earthquake; b. Lighting Strike; c. Dust Strom/Cyclone; and d. Flash Floods. Major Chemical Hazards that can cause Emergency a. Slow isolated fire; b. Fast spreading fires; c. Explosions; d. Bursting of pipelines vessels; e. Uncontrolled release of toxic/corrosive/flammable gases/dusts; f. Uncontrolled release of topic / flammable liquid; g. Floods; and h. Earth Quake. Each of the above may constitute an emergency but it depends on their nature, scale and speed end impact on environment. Fire Hazard a. In coal handling plant and at conveyors; b. Fuel oil handling area and oil tank in main plant; c. Cables in galleries, and on trays in all plant sections;

d. Transformer oil; and e. Burners area in boilers. Explosion Hazard a. Turbo generator where hydrogen is used for cooling T.G; b. Transformer (oil cooled); c. Boiler (coal / on fired); and d. Coal dust in mills and boilers. Bursting of Pipelines / Vessels a. Steam pipes due to high pressure; b. Water pipe due to high pressure; and c. Compressor Air Receiver. Release of Gases and Dust a. Hydrogen in turbo generator area of main plant; b. Pulverized coal dust from mills and associated piping. Release of Liquids a. Fuel oil tanks in fuel oil handling section and Diesel tanks area; b. Chemical tanks in water treatment plant; and c. Ash dyke (bund failure). Floods Breach of ash Dykes No Smoking Zones         

Fuel oil Handling Plant and Main Oil Tank areas in Main Plant; CHP Conveyor & Crusher House; Store Sheds / Cable storage areas/ Cable galleries; Hydrogen Filling station/storage area; Switch Gear rooms/ MCC rooms; AVR Rooms; Battery Rooms; NR Relay Room; and Control Rooms.

A preliminary hazard analysis is carried out initially to identify the major hazards associated with storages and the processes of the plant. This is followed by consequence analysis to quantify these hazards. Finally, the vulnerable zones are plotted for which risk reducing measures are deduced and implemented. Preliminary hazard analysis for fuel storage area and whole plant is given in Table-7.6. and Table-7.7. TABLE-7.6 PRELIMINARY HAZARD ANALYSIS FOR STORAGE AREAS Unit LDO

Capacity (KL) 1x10

Hazard Identified Fire/Explosion


Environmental factors


Description of Plausible Hazard If there is any leakage and eventuality of source of ignition. Highly inflammable nature of the liquid fuels may cause fire hazard in the storage facility.




All electrical fittings and cables are provided as per the specified standards. All motor starters are flame proof. Fire extinguisher of small size and big size are provided at all potential fire hazard places. In addition to the above, fire hydrant network is also provided.

A well designed fire protection including foam, dry powder, and CO2 extinguisher should be provided.

Fire Explosion and Toxicity Index (FE&TI) Approach Fire, Explosion and Toxicity Indexing (FE & TI) is a rapid ranking method for identifying the degree of hazard. The application of FE & TI would help to make a quick assessment of the nature and quantification of the hazard in these areas. However, this does not provide precise information. The degree of hazard potential is identified based on the numerical value of F&EI as per the criteria given below: Sr. No. 1 2 3 4 5

F&EI Range 0-60 61-96 97-127 128-158 159 and above

Degree of Hazard Light Moderate Intermediate Heavy Severe

By comparing the indices F&EI and TI, the unit in question is classified into one of the following three categories established for the purpose (Table-7.8). TABLE-7.8 FIRE EXPLOSION AND TOXICITY INDEX Category I II III

Fire and Explosion Index (F&EI) F&EI < 65 65 < or = F&EI < 95 F&EI > or = 95

Toxicity Index (TI) TI < 6 6 < or = TI < 10 TI > or = 10

Certain basic minimum preventive and protective measures are recommended for the three hazard categories. Results of FE and TI for Storage/Process Units Based on the GOI Rules 1989, the hazardous fuels used by the proposed power plant were identified. Fire and Explosion are the likely hazards, which may occur due to the fuel storage. Hence, Fire and Explosion index has been calculated for in plant storage. Estimates of FE&TI are given in Table-7.9.



Chemical/ Fuel LDO

Total Capacity (KL) 1x10









Conclusion Results of FE&TI analysis show that the storage of LDO falls into Light category of fire and explosion index with a Nil toxicity index.


Maximum Credible Accident Analysis (MCAA) Hazardous substances may be released as a result of failures or catastrophes, causing possible damage to the surrounding area. This section deals with the question of how the consequences of the release of such substances and the damage to the surrounding area can be determined by means of models. Major hazards posed by flammable storage can be identified taking recourse to MCA analysis. MCA analysis encompasses certain techniques to identify the hazards and calculate the consequent effects in terms of damage distances of heat radiation, toxic releases, vapour cloud explosion etc. A host of probable or potential accidents of the major units in the complex arising due to use, storage and handling of the hazardous materials are examined to establish their credibility. Depending upon the effective hazardous attributes and their impact on the event, the maximum effect on the surrounding environment and the respective damage caused can be assessed. The reason and purpose of consequence analysis are many folds like:      

Part of risk assessment; Plant layout/code requirements; Protection of other plants; Protection of the public; Emergency planning; and Design criteria.

The results of consequence analysis are useful for getting information about all known and unknown effects that are of importance when some failure scenario occurs in the plant and also to get information as how to deal with the possible catastrophic events. It also gives the workers in the plant and people living in the vicinity of the area, an understanding of their personal situation.  Selected Failure Cases The purpose of this listing (refer Table-7.10) is to examine consequences of such failure individually or in combination. It will be seen from the list that a vast range of failure cases have been identified. The frequency of occurrence of failure also varies widely. Damage Criteria The fuel storage and unloading at the storage facility may lead to fire and explosion hazards. The damage criteria due to an accidental release of any hydrocarbon arise from fire and explosion. The vapors of these fuels are not toxic and hence no effects of toxicity are expected.

Tank fire would occur if the radiation intensity is high on the peripheral surface of the tank leading to increase in internal tank pressure. Pool fire would occur when fuels collected in the dyke due to leakage gets ignited. 

Fire Damage

A flammable liquid in a pool will burn with a large turbulent diffusion flame. This releases heat based on the heat of combustion and the burning rate of the liquid. A part of the heat is radiated while the rest is convected away by rising hot air and combustion products. The radiations can heat the contents of a nearby storage or process unit to above its ignition temperature and thus result in a spread of fire. The radiations can also cause severe burns or fatalities of workers or fire fighters located within a certain distance. Hence, it will be important to know beforehand the damage potential of a flammable liquid pool likely to be created due to leakage or catastrophic failure of a storage or process vessel. This will help to decide the location of other storage/process vessels, decide the type of protective clothing the workers/fire fighters, the duration of time for which they can be in the zone, the fire extinguishing measures needed and the protection methods needed for the nearby storage/process vessels. The damage effect on equipment and people due to thermal radiation intensity is given in Table-7.10. Similarly, the effect of incident radiation intensity and exposure time on lethality is given in Table-7.11. TABLE-7.10 DAMAGE DUE TO INCIDENT RADIATION INTENSITIES Sr. No. 1

Incident Radiation (kW/m2) 37.5

Type of Damage Intensity Damage to Equipment Damage to People







Minimum energy required to ignite wood at indefinitely long exposure without a flame Minimum energy required for piloted ignition of wood, melting plastic tubing --




Damage to process equipment

100% lethality in 1 min., 1% lethality in 10 sec. 100% Lethality in 1 min., Significant injury in 10 sec. 1% lethality in 1 min. First degree burns in 10 sec Causes pain if duration is longer than 20 sec, however blistering is un-likely (First degree burns) Causes no discomfort on long exposures

Source: Techniques for Assessing Industrial Hazards by World Bank

TABLE-7.11 RADIATION EXPOSURE AND LETHALITY All values are given in KW/m2 Radiation Intensity (KW/m2) 1.6

Exposure Time (seconds) --

Lethality (%)

Degree of Burns


4.5 4.5 8.0 8.0 8.0 12.0 12.0 12.5 25.0 37.5

20 50 20 50 60 20 50 Inst Inst Inst

0 0 0 <1 <1 <1 8 10 50 100

No Discomfort even after long exposure 1st 1st 1st 3rd 3rd 2nd 3rd ----


Scenarios Considered for MCA Analysis Fuel Storage The details of storages in the proposed power plant are given Table-7.2 above. In case of fuel released in the area catching fire, a steady state fire will occur. Failures in pipeline may occur due to corrosion and mechanical defect. Failure of pipeline due to external interference is not considered as this area is licensed area and all the work within this area is closely supervised with trained personnel. Modeling Scenarios Based on the storage and consumption of various fuels the following failure scenarios for the proposed power plant have been identified for MCA analysis and the scenarios are discussed in Table-7.12. The fuel properties considered in modeling are given in Table-7.13. TABLE-7.12 SCENARIOS CONSIDERED FOR MCA ANALYSIS Sr. No. 1

Fuel/Chemical Failure of LDO tanks

Total Quantity

Scenarios considered


Pool fire


Fuel LDO

Molecular weight (kg/kg mol) 114.24

Boiling Point (C) 400

Density (kg/m3) 880

Pool Fire Models used for MCA Analysis Heat Radiation program ‘RADN’ has been used to estimate the steady state radiation effect from storage of fuel at different distances. The model is based on the equations compiled from various literatures by Prof.J.P.Gupta, Department of Chemical Engineering, IIT Kanpur.


Results and Discussion The results of MCA analysis are tabulated indicating the distances for various damages identified by the damage criteria, as explained earlier. Calculations are done for radiation intensities levels of 37.5, 25, 12.5, 4.5 and 1.6-kW/m2, which are presented in Table-7.14 for different scenarios. The distances computed for various scenarios are from the center of the pool fire. TABLE-7.14 OCCURRENCE OF VARIOUS RADIATION INTENSITIES- POOL FIRE Radiation and Effect

Failure of one LDO tanks of 10 KL

Radiation Intensities (kW/m2)/ Distances (m) 37.5 25.0 19.0 12.5 4.5 1.6 2.9 3.7 4.3 5.4 9.7 17.6

Pool Fire Due to Failure of LDO Storage Tanks (Figure-7.2)

The maximum capacity of storage of LDO tank will be 10 KL. The most credible failure is the rupture of the largest pipe connecting to the storage tank. As the worst case, it is assumed that the entire contents leak out into the dyke forming a pool, which may catch fire on finding a source of ignition. A perusal of the above table clearly indicates that 37.5 kW/m2 (100% lethality) occurs within the radius of the pool which is computed at 2.9 m tank on pool fire. This vulnerable zone will damage all fuel storage equipment falling within the pool radius. Similarly, the threshold limit for first degree burns is 1.6 kW/m2, this vulnerable zone in which the thermal fluxes above the threshold limit for first degree is restricted to 17.6 m in case tank on pool fire. 7.4.9 Conclusions on MCA analysis  LDO Tank Farm There will be one LDO storage tanks each of 10 KL capacity will be provided in the LDO tank farm. The results of MCA analysis indicate that the maximum damage distances for 12.5-Kw/m2 thermal radiations extends up to 5.4 m in the case of 10 KL on fire during worst meteorological conditions. As the fire resistant dyke walls will be created, no cumulative effect of one tank farm on fire to create fire on other tank farm is envisaged. The damage contours for tank on fire of two LDO tanks is shown in Figure-7.2. 7.4.10 Coal Handling Plant - Dust Explosion Coal dust when dispersed in air and ignited would explode. Crusher house and conveyor systems are most susceptible to this hazard. To be explosive, the dust mixture should have:   

Particles dispersed in the air with minimum size (typical figure is 400 microns); Dust concentrations must be reasonably uniform; and Minimum explosive concentration for coal dust (33% volatiles) is 50 gm/m 3.

Failure of dust extraction and suppression systems may lead to abnormal conditions and may increase the concentration of coal dust to the explosive limits. Sources of ignition present are incandescent bulbs with the glasses of bulkhead fittings missing, electric equipment and cables, friction, spontaneous combustion in accumulated dust. Dust explosions may occur without any warnings with Maximum Explosion Pressure upto 6.4 bar. Another dangerous characteristic of dust explosions is that it sets off secondary explosions after the occurrence of the initial dust explosion. Many a times the secondary explosions are more damaging than primary ones. The dust explosions are powerful enough to destroy structures, kill or injure people and set dangerous fires likely to damage a large portion of the Coal Handling Plant including collapse of its steel structure which may cripple the lifeline of the power plant. Stockpile areas shall be provided with automatic garden type sprinklers for dust suppression as well as to reduce spontaneous ignition of the coal stockpiles. Necessary water distribution network for drinking and service water with pumps, piping, tanks, valves etc will be provided for distributing water at all transfer points, crusher house, control rooms etc.

A centralized control room with microprocessor based control system (PLC) has been envisaged for operation of the coal handling plant. Except for locally controlled equipment like traveling tripper, dust extraction/ dust suppression / ventilation equipment, sump pumps, water distribution system etc, all other in-line equipment will be controlled from the central control room but will have provision for local control as well. All necessary interlocks, control panels, MCC’s, mimic diagrams etc will be provided for safe and reliable operation of the coal handling plant. Control Measures for Coal Yards The total quantity of coal shall be stored in separate stockpiles, with proper drains around to collect washouts during monsoon season. Water sprinkling system shall be installed on stocks of coal in required scales to prevent spontaneous combustion and consequent fire hazards. The stock geometry shall be adopted to maintain minimum exposure of stock pile areas towards predominant wind direction. 7.4.11 Identification of Hazards The various hazards associated, with the plant process apart from fuel storage have been identified and are outlined in Table-7.15. TABLE-7.15 HAZARD ANALYSIS FOR PROCESS IN POWER PLANT Sr. No. 1 2 3 4

5 6

Blocks/Areas Coal storage in open yard Coal Handling Plant including Bunker area Boilers

Hazards Identified Fire, Spontaneous Combustion Fire and/or Dust Explosions Fire (mainly near oil burners), Steam Explosions, Fuel Explosions Fires in – a) Lube oil system b) Cable galleries c) Short circuits in: i)Control rooms ii) Switch-gears

Steam Turbine Generator Buildings

Explosion due to leakage of Hydrogen and fire following it. Fire in cable galleries and Switch-gear/Control Room Fire

Switch-yard Control Room LDO Tank Farms HFO Tank Farm

7.4.12 Hazardous Events with Greatest Contribution to Fatality Risk The hazardous event scenarios likely to make the greatest contribution to the risk of potential fatalities are summarized in Table-7.16. ‘Onsite facility’ refers to the operating site at plant site, whereas ‘offsite facility’ refers to transport and handling systems, which are away from the operating site. TABLE-7.16 HAZARDOUS EVENTS CONTRIBUTING TO RISK AT ON-SITE FACILITY Hazardous Event Onsite vehicle impact on personnel Entrapment/struck by Machinery Fall from heights

Risk Rank 3 3

Consequences of Interest Potential for single fatalities, onsite impact only Potential for single fatalities, onsite impact only


Potential for single fatalities, onsite impact only

Hazardous Event Electrocution Storage tank rupture and fire

Risk Rank 3 3

Consequences of Interest Potential for single fatalities, onsite impact only Potential for multiple fatalities, onsite impact only

7.4.13 Risk Assessment Summary The preliminary risk assessment has been completed for the proposed power plant and associated facilities and the broad conclusions are as follows: 

There will be no significant community impacts or environmental damage consequences; and

The hazardous event scenarios and risks in general at this facility can be adequately managed to acceptable levels by performing the recommended safety studies as part of detailed design, applying recommended control strategies and implementing a Safety Management System.







N 19800

N 19700

E 8000

N 19500

E 7900

E 7800

E 7700

E 7600

E 7500

E 7400

N 19400

E 7300

E 7200

N 19600


N 19300 N 19200 N 19100




0 149.0 N 198120.00 E

9 4

N 19000

14 1 13 19



N 18800 N 18700

26 3




5 6



12 15


N 18900




96.00 N 1878828.00 E

N 18600


N 19002.00 E 8173.00

7.4.14 Risk Reduction Opportunities The following opportunities shall be considered as a potential means of reducing identified risks during the detailed design phase: 

Buildings and plant structures shall be designed for cyclone floods and seismic events to prevent structural collapse and integrity of weather (water) proofing for storage of dangerous goods;

Provision for adequate water capacity to supply fire protection systems and critical process water;

Isolate people from load carrying/mechanical handling systems, vehicle traffic and storage and stacking locations;

Installation of fit-for-purpose access ways and fall protection systems to facilitate safe access to fixed and mobile plant;

Provision and integrity of process tanks, waste holding tanks and bunded areas as per relevant standards; Containment of hazardous materials;

 

Security of facility to prevent unauthorized access to plant, introduction of prohibited items and control of onsite traffic; and

Development of emergency response management systems commensurate with site specific hazards and risks (fire, explosion, rescue and first aid).