Fire or explosion can occur when flammable vapours are mixed with oxygen or air in proportions within critical values known as the Lower and Upper Explosive Limits (LEL and UEL, respectively). For most solvents the LEL lies within the range 1-5% in air and therefore good ventilation is essential in order to eliminate the risk of forming a flammable or explosive atmosphere (flashpoint) then such substances are used. However, it is significant that the LEL is usually considerably greater than the occupational (workplace) exposure limit for the concentration of vapour in the workroom air.
Vapours from flammable liquids are denser than air and thus tend to sink to ground level where they can spread over a large area. Care should be taken to minimise the production of such vapours and the associated risk of ignition by flashback from a remote source.
Storage and Handling of Flammable Substances
Wherever practical highly flammable (or hazardous) substances should be substituted with less flammable substances. Where this is not possible, quantities should be minimised and the following precautions taken.
- Flammable solvents, including waste solvent bottles must be stored in metal solvent cabinets and returned to the storage cabinet immediately after use.
- Vessels containing more than 2.5 litres of flammable solvent may not be stored in laboratories.
- Bottles containing flammable solvents on open shelves should be kept to a minimum; the container volume must not exceed 500 ml. Spill trays must be used.
- Ventilated wash bottles must be used for flammable solvents and must be labelled for contents.
- Empty bottles of flammable solvents must be stored safely as they may contain explosive vapour.
The total volume of highly flammable solvents (those with a flash point of less than 32⁰C) stored within the laboratory must not exceed 50 litres. Where these volumes are likely to be exceeded, a 'Dangerous Substances and Explosive Atmosphere Regs' (DSEAR) assessment must be completed with support of Health and Safety Services.