Understanding mist hazards

SEE THROUGH THE FOG

MISTS • AEROSOL MIST HAZARDS ARE NOT WELL UNDERSTOOD. HSE HAS DELIVERED SOME FINDINGS OF CONCERN AS IT CONTINUES ITS EXPERIMENTS WITH DIESEL MISTS

DIESEL IS A COMPARATIVELY low-hazard substance, at least in terms of its flammability, compared to other fuels. Indeed, some regulations provide specific relief for diesel, prompted in part by its importance in the transport industry and in other sectors.

Under some conditions, however, diesel can be extremely flammable. The UK Health & Safety Executive (HSE) has been investigating the conditions under which mists and sprays of diesel – and other fluids, including hydraulic oils and lubricating oils – at temperatures below their flashpoint can result in jet fires, flash fires or explosions. HSE notes that incidents involving the ignition of mists have caused significant damage and multiple fatalities in the past.

Its investigation centres on diesel since this is perhaps the most widely used example of such fluids and is often transferred under pressure in elevated pipework, an arrangement that can, in the event of a breach of containment, result in a mist being expelled.

Industry manages the hazards by removing or replacing equipment that might cause ignition in areas where a flammable mist might be created. However, HSE says, industry guidance on the hazardous area classification needed to identify these risks is limited, with most such guidance focusing on flammable gases. Nevertheless, in the UK there is a legal requirement under the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) for facilities to classify areas into zones where fire and explosion hazards may occur, which includes those resulting from an aerosol mist.

RESULTS SO FAR In order to get a better understanding of the hazards involved, HSE has partnered with industry in the MISTS2 research project, which aims to generate experimental data on the formation and ignitability of diesel mists so that realistic hazardous area classifications can be developed.

As part of MISTS2, explosive atmospheres and fire safety experts at HSE’s Science and Research Centre undertook large-scale experiments investigating the extent of ignitability in diesel mist sprayed vertically downward. Their results showed that mist generated at 5 bar gauge through a 1 mm orifice remains ignitable for over 4.75 m. Ignition resulted in propagating flash fires, with the intensity of burning decreasing with distance from the release point.

“The experimental results are improving our understanding of mist hazards,” HSE comments. “This will assist validation of mist dispersion and ignitability models, helping industry improve their guidance and standards to prevent fires and explosions of ignited mists such as diesel.”

The research is continuing but HSE considers that the results obtained so far indicate a need for revision of existing guidance and standards, such as that included in the Energy Institute’s (EI) EI15 guide, the part of its Model Code of Safe Practice that deals with hazardous area classification for installations handling flammable fluids. In particular, HSE concludes, there seems to be a need for a “significant increase in size of zone” that might be required below a vertically oriented mist release, rather than a larger radius around it.

There also appears to be a difference in behaviour depending on the location and energy of the ignition source; this consideration will be a particular focus of continuing work. www.hse.gov.uk