Purging involved passing a specified quantity of the safe gas through the enclosure in order to remove any flammable gases which may have entered. The standards specify that the minimum volume of the safe gas required to achieved adequate purging is equivalent to 5 times the internal volume of the enclosure and associated ducting. Manufacturers may, however, recommend a greater number of air changes.
Depending on the pressurisation type the purge duration may be controlled manually, or automatically via a flow-rate sensor and timer. If there is a purge failure or loss of pressure the control system must automatically reset and the purge cycle be started again. Very large systems require on site tests to establish the purge duration necessary.
