Industrial Fire Journal - Fire & Rescue - Hemming Group Ltd
Demystifying area classification
Published:  04 May, 2018

Research that aimed to shed light on oil mist to control risk of ignition has found significant gaps in existing models and has identified a higher risk from kerosene.

Area classification for oil mists – final report was written by the UK’ Health and Safety Executive and the University of Wales’ Gas Turbine Research Centre with the support of 14 industry sponsors including Maersk Oil North Sea UK, Conoco Phillips and Nexen Petroleum.

The research was carried out because although the UK's Dangerous Substances and Explosive Atmospheres Regulations require risk based ‘area classification’ in places where oil mists can be produced, there is very little information on the conditions in which a mist can be ignited and continue to burn.

The research aims to expand the scientific understanding of mist ignition, in order to assist others to develop better practical guidance on the likelihood of mist formation, and the mitigation measures that may be taken.

Many types of industrial equipment can potentially produce an explosive oil mist if a fault develops; an earlier review had revealed that mist explosion events caused significant number of incidents, a large proportion of which resulted in multiple fatal injuries.

Flammable liquids released under pressure at temperatures below their flash point may form a mist which may be ignited and explode. This is recognised in the ATEX Directives, where the definition of explosive atmospheres includes flammable mists.

In addition to a literature review, the work aimed to develop a potential classification scheme for liquids capable of forming flammable mists; develop practical tests on different fluids at a range of pressures; and to define properties important to area classification using models of leak behaviour with limiting ignition conditions.

Amongst the report’s findings are that the formation of a flammable mist is a more complex phenomenon than was previously recognised, with existing information sparse and sometimes contradictory or inconclusive.

The scientists also found that, as different fluids had significantly different behaviour, different materials should be assessed separately for area classification. In particular, the results suggested that the properties leading to fluid break-up may be more significant than other factors, such as pressure.

One fuel mist in particular, Kerosene, was found to ignite at all the pressures tested, even though it had been previously assumed that below a certain pressure (5 bar or 10 bar) a leak could not produce ignitable droplets. The tests showed that any such limit is below the lowest test pressure of 1.7 bar.

The report concluded that computational fluid dynamics models could provide reasonably good predictions of the droplet sizes and concentrations for atomising sprays. The results from such models were consistent with the existing EI15 guidance, the latter providing methodologies for hazardous area classification around equipment storing or handling flammable fluids.

Significantly, however, the research found that existing models – and therefore the industry guidance that uses those models - only covered a subset of the ways oil leaks can break up to form mists and how readily those mists can then be ignited.

To download the full report, click here.