Overview

Certain chemicals pose fire and explosion risks because:

• They ignite easily. Vapours often travel a considerable distance to an ignition source remote from the point of chemical escape.

• Considerable heat is generated. Many volatile substances liberate heat at a rate some ten times faster than burning wood.

• The fire spreads easily by, e.g., running liquid fire, a pool fire, a fire ball, heat radiation or thermal lift (convection).

• Explosion: a confined vapour cloud explosion (CVCE) can result from ignition of vapour within a building or equipment; a boiling liquid expanding vapour explosion (BLEVE) can result when unvented containers of flammable chemicals burst with explosive violence as a result of the build-up of internal pressure; unconfined vapour cloud explosion (UVCE) can result from ignition of a very large vapour or gas/air cloud.

Clearly, flammable chemicals also pose a health risk if the substance or its thermal degradation or combustion products are toxic, (e.g. carbon monoxide) or result in oxygen deficiency because oxygen is consumed. Hot smoke and other respiratory irritants, e.g. aldehydes, are also produced.

Ignition and Propagation of a Flame Front

Normally flame propagation requires

1. fuel, gas or vapour (or combustible dust) within certain concentration limits,

2. oxygen supply (generally from air) above a certain minimum concentration, and

3. ignition source of minimum temperature, energy and duration.

All three, represented by the three corners of a triangle (Figure 6.1), must generally be present. But no ignition source is needed if a material is above a specific temperature (see p. 214), and...