Flash-point
The flash-point is a
measure of the ease of ignition of the liquid. It is the lowest temperature at
which the material will ignite from an open flame. The flash-point is a
function of the vapour pressure and the flammability limits of the material. It
is measured in standard apparatus, following standard procedures (BS 2000).
Both open- and closed-cup apparatus is used. Closed-cup flash-points are lower
than open cup, and the type of apparatus used should be stated clearly when
reporting measurements. Flash-points are given in Sax’s handbook, Lewis (2004).
The flash-points of many volatile materials are below normal ambient
temperature; for example, ether 45ŽC, petrol (gasoline) 43ŽC (open cup).
Autoignition temperature
The autoignition
temperature of a substance is the temperature at which it will ignite
spontaneously in air, without any external source of ignition. It is an
indication of the maximum temperature to which a material can be heated in air;
for example, in drying operations.
Flammability limits
The flammability limits
of a material are the lowest and highest concentrations in air, at normal
pressure and temperature, at which a flame will propagate through the mixture.
They show the range of concentration over which the material will burn in air,
if ignited. Flammability limits are characteristic of the particular material,
and differ widely for different materials. For example, hydrogen has a lower
limit of 4.1 and an upper limit of 74.2 per cent by volume, whereas for petrol
(gasoline) the range is only from 1.3 to 7.0 per cent.
A flammable
mixture may exist in the space above the liquid surface in a storage tank. The
vapour space above highly flammable liquids is usually purged with inert gas
(nitrogen) or floating-head tanks are used. In a floating-head tank a “piston”
floats on top of the liquid, eliminating the vapour space.
Flame traps
Flame
arresters are fitted in the vent lines of equipment that contains flammable
material to prevent the propagation of flame through the vents. Various types
of proprietary flame arresters are used. In general, they work on the principle
of providing a heat sink, usually expanded metal grids or plates, to dissipate
the heat of the flame. Flame arrestors and their applications are discussed by
Rogowski (1980), Howard (1992) and Mendoza et al. (1988).
Traps
should also be installed in plant ditches to prevent the spread of flame. These
are normally liquid U-legs, which block the spread of flammable liquid along
ditches.
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