Page 52 - Electrical Installation in Hazardous Area

Area classification 3 1

The jashpoint; the temperature at which sufficient vapour is released
from a liquid to allow the production of an explosive atmosphere above
its surface. Below this temperature the gas or vapour and air mixture is
not normally ignitable although a mist may still be capable of forming an
explosive atmosphere below this temperature.
The ignition temperature; the minimum temperature at atmospheric pres-
sure necessary to ignite an ideal mixture of the flammable gas or vapour
in air. This determines the limit of maximum temperature of equipment
permitted within the hazardous area.
The flammable range; the range of percentage mixtures of the flammable
gas or vapour with air between which ignition is possible at atmospheric
pressure. The lower flammable limit is normally the important one as it
determines the extent of the explosive atmosphere. Figure 2.1 shows typical
flammable range and sensitivity information.
The guslvupour density; normally quoted as relative to air which is
assumed as for the flashpoint. This parameter has an effect upon the
dispersion of the flammable gas or vapour insofar as it may be lighter
or heavier than air and may also be necessary if calculation is necessary.
The boiling point; the temperature at which a flammable liquid will boil
at atmospheric pressure. If it is below the actual temperature at the point
of release then the released liquid will vaporize quickly.
The molecular weight; this information is necessary in case calculation is
required.

I
I I I 1 I I I I I I
50 100
Percentage gas in air
Lower Upper
explosive explosive
limit ("/.) limit (Yo)
Hydrogen 4.1 74.2
Propane 2.1 9.5
Min. ignition energy a = 260 pJ, b = 20 pJ
Fig. 2.1 Typical ignition characteristics