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Hazardous Gas Dispersion Modelling of Ammonia

Collating key data and using a sophisticated modelling technique to determine the risks and impact to the local environment of an ammonia gas escape or explosion from a food manufacturing facility

Regulatory Authority demands risk assessment of hazardous gas dispersion

A major food manufacturing brand in Ireland was instructed by the HSA (Health & Safety Authority) to undertake an assessment of the risks and potential impact to the local surrounding area of a major ammonia gas leak at their manufacturing site. The resultant report had to be made available to both the HSA and the Fire Authorities to enable them to plan for the risks and ensure contingency measures were in place to deal with the impact. Ammonia is a very common gas in food manufacturing plants as it is a highly effective refrigerant and is quite cost effective. However as a hazardous gas, the risks of storage and use of the gas have to be managed effectively to minimise risk not just for the safety of production operators and staff onsite but to the wider community.

Sophisticated software modelling techniques using local data

Malone Group were engaged to manage the development of a comprehensive risk assessment report with recommendations for the plant that could be shared with the HSA and Fire Authorities.

This involved a Consequential Modelling Review to understand the impact of a gas escape ranging from a small leak to a catastrophic failure as well as a boiling liquid expanding vapor explosion (BLEVE). Sophisticated modelling techniques were used for Hazardous Gas Dispersion Modelling which integrated local site data to model different scenarios of gas escaping to the atmosphere. A specialist provider was engaged to use the PHAST hazard analysis software application to model the behavior of the gas, given the local topography, climate and the social infrastructure of the area i.e. residential housing/population, schools, hospitals, care homes, farms and industry.

The analysis examines different scenarios, i.e. a localised or contained gas leak within the pipe network of the plant; a significant gas escape to the atmosphere and a catastrophic escape for instance as a result of a BLEVE (an explosion caused by the rupture of a vessel containing a liquid at high pressures). These scenarios are modelled with different threshold concentration levels of hazardous gas for defined periods of time from the epicenter to a radius of up to several kilometres to determine zones of impact to either people, animals or the environment. The impacts are assessed in terms of transient impact to long term impact. At the extreme catastrophe level the risks are analagous to a bomb explosion, hence the European ATEX directive applies to ensure protective systems are in place to improve the health and safety protection to workers.

Safety recommendations onsite and local response plans

The final report from an independent authority was a key requirement for the customer to fulfil their responsibility to the HSA. More importantly, it provided the essential information for the Fire Service to start the process of planning to deal with the risks to the local area. Finally, the analysis and modelling carried out on this project enabled a series of recommendations to be made to the customer for upgrading key plant and equipment, installation of continuous monitoring of gas levels and alarms to protect and warn operators of any hazardous gas escapes. These upgrades will become part of a range of capital projects which will be implemented in the plant.