Page 330 - Offshore Electrical Engineering Manual

Ventilation 317

FIGURE 5.4.4
Typical sheet from hazardous source schedule.

The number of air changes per hour required to meet the natural ventilation
equivalent condition in the third case will vary to some extent, depending on the
degree of hazard, but a figure of 40 changes per hour is quoted by underwriters.
The positioning of fan inlets and exhaust outlets must be such that no unventilated
pockets are left. The adequate safeguard is usually an alarm followed, after a suitable
time delay, by the shutdown and venting to the flarestack of all process equipment
within the module.
Process modules are usually ventilated such that the internal air pressure
is slightly negative with respect to the outside, in order to retain any minor
gas leaks. Safe area modules are kept at a slightly positive pressure in order to
prevent the ingress of gas. The ventilation systems of different modules must
be well segregated to prevent cross-contamination of leaks. Fire and explosion
dampers are installed to automatically seal ventilation systems if fire or gas is
detected. Ventilation intakes for both safe and hazardous areas should be located
well within safe areas to minimise the risk of drawing gas into the ventilation
system.
A common problem on older installations is the location of secondary pres-
surised control rooms in the middle of a hazardous module. A typical example of
this is a gas compression module with its own control room. If the control room
pressurisation fails and remains failed for a timed period, the compression module