Hazard analysis is a primarily the most important step and overall procedure for evaluating the hazards, consequences, vulnerabilities, probabilities and risk associated with the presence of hazardous material within any given locality or jurisdiction. The present study has been carried out for MAH (Major Accident Hazard) units identified by chief of factories to estimate the areas of probable threats of key hazards such as toxicity, flammability, thermal radiation and over pressure near a short duration chemical release using ALOHA (Aerial Locations of Hazardous Atmosphere) software developed by National Oceanic and Atmospheric Administration (NOAA) and U.S. Environmental Protection Agency (EPA). Each key hazards or threat zones show an overhead view of the regions in the atmosphere with specific Level of Concerned (LOC) i.e. a threshold value of a hazard above which a threat to people or property may exist. The type of the LOC is depending on the scenario. For toxic gas dispersion scenarios, an LOC is a threshold concentration of the gas at ground level above which a hazard is believed to exist. Each LOC estimates a threat zone where the hazard is predicted to exceed that LOC at some time after a release begins. Toxic LOC also referred as exposure limits, exposure guidelines, or toxic endpoints and expressed by an exposure guidelines designed to help the emergency responders known as Acute Exposure Guideline Levels (AEGLs). AEGLs estimate the concentrations in terms of ppm or percentage at which most people including sensitive individuals such as old, sick, or very young people will begin to experience health effects if they are exposed to a toxic chemical for a specific length of time (duration). Toxic Level of Concern such as Protective Action Criteria (PACs) defined by U. S Department of Energy is also used when AEGLs and ERPGs are not available. A flammable Level of Concern (LOC) is a threshold concentration of a fuel in the air above which a flammability hazard may exist. It is the prediction of flammable area i.e. part of a flammable vapour cloud where the concentration is in the flammable range between Lower and Upper Explosive limits (LEL & UEL) which is also known as Lower and Upper Flammability limits. It is expressed in terms of percentage. A thermal radiation Level of Concern (LOC) is a threshold level of thermal radiation (heat) above which a hazard may exist. This radiation may be from pool fire, jet fire, or BLEVE and expressed in terms of kilowatts per square meter. An overpressure Level of Concern (LOC) is a threshold level of pressure from a blast wave, usually the pressure above which a hazard may exist and expressed in terms of pounds per square inch (psi). Each hazard/threat zone is shown in different colours such as red, orange and yellow. The red zone represents the worst hazard and the orange and yellow zones represent areas of decreasing hazards. An uncertainty dashed line also known as confidence line enclosed each hazard zones if there is a change under atmospheric stability of A, B and C. Vulnerability Assessment includes database preparation- the collection of information on socio-economic and buildings of different use/type, field survey, design of questionnairefor building inventory, ancillary data, etc. Risk assessment in its broad definition is a structured procedure that evaluates qualitatively and/or quantitatively the level of risk imposed by the hazard sources. In this project work risk assessment was done for buildings and population for fire, explosion and toxic. release hazard separately. Risk for the study area was calculated by combining results of hazard and vulnerability assessment and are categorized into ‘very high’, ‘high’, ‘moderate’, ‘low’ and ‘very low’ risk.
In Guwahati City, within the boundary of Guwahati Metropolitan Development Authority, 4 (four) MAH units are identified by the Chief of Factories, Government of Assam namely - IOCL Ltd, Tap off Point, Betkuchi, Guwahati , LPG Bottling Plant, Abhaypur, North Guwahat IOC Ltd., Guwahati Refinery, Noonmati, Guwahati (fig. 4.4), and Aviation Fuel Storage Depot, Borjhar. However, analysis of Aviation Fuel Storage Depot, Borjhar and IOCL Ltd, Tap off Point, Betkuchi, Guwahati has been excluded from the study due to certain constraints. Hindustan Petroleum Corporation Ltd, (HPCL) storage facility (fig. 4.2), Betkuchi, Guwahati is not included in the government record although it has been considered for the study. For the purpose of analysis 21st May, 2011, 14.30 hours recorded by AWS has been considered. At this hour the air temperature recorded was 35.65° C and the prevalent wind speed and direction was 1 meters/second from 358.99° true north. The relative humidity was 46%, cloud cover 3 tenths with atmospheric stability class B.