The Risk Management Tool Box Blog

What is the Lower Explosive Limit?

Graham Marshall - Monday, September 24, 2012

Chemical storage vessels and drums that have contained flammable liquids, such as fuels, solvents, and paints can ignite or explode when exposed to “hot work” such as sparks from angle grinders, plasma cutters, welding, oxyacetylene burners and electric metal cutting saws.

This is because flammable liquids give off vapour which has an upper and lower explosive limit that works like a range.  That means that vapour can ignite and/or explode when it is below the upper explosive limit (UEL), but above the lower explosive limit (LEL).  When a flammable or combustible liquid is exposed to heat (from direct or indirect flame), the vapours given off can reach into the range between the LEL and the UEL and ignite.  This reaction can cause a fire or an explosion. Such explosions have resulted in many fatalities occurring or severe burns from the resulting fire.

As recently as July 2012, a West Australian worker was killed when a chemical drum that he was cutting with an angle grinder exploded. The drum had previously contained a flammable liquid.

In another recent case, a Queensland high school student was killed when he was using a plasma-arc torch to convert a drum into a feed bin as part of his agricultural studies. The drum had previously contained diesel oil and the oil vapours that remained in the drum exploded.

The first step towards managing the risks associated with the UEL and LEL of any potentially explosive chemicals is to obtain important safety information.

The manufacturer or supplier of the chemicals must provide this information with the chemicals they provide. The information should be provided by a Material Safety Data Sheet (MSDS) that contains information about the hazards associated with the chemical and precautions for the safe use, transport, storage and disposal.

The MSDS should include information such as the temperature that will allow vapours to rise above the LEL and subsequently ignite, and the levels of exposure that are hazardous to humans through The chemicals should also be labeled in a way that can be easily read and understood.

If a drum of chemicals arrives at the workplace without a safety label and an MSDS attached, it should be returned to the supplier.

Once emptied, the storage containers should be stored in a safe location, away from ignition sources, with their labels still attached.

The best way to prevent an injury or fatality is to ban any form of hot work on or near a vessel or drum that contained (or still contains) flammable or toxic materials.

If hot work must be undertaken, then a Permit to Work should be issued following a thorough assessment of the task to be undertaken, and ensuring all appropriate controls are in place.

It is essential that competent, experienced persons who are familiar with the hazards associated with hot works review all hot work prior to issuing approval for the work to proceed.

It is also critical that workers performing any hot-work are provided with adequate training about the hazards associated with conducting hot work on or near drums that contained flammable or toxic substances.

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