TABLE OF CONTENTS The type of large enclosure which is envisaged here is exemplified by an offshore module. It is with such enclosures that the account given in this section is primarily concerned.
An offshore module tends to have a high aspect, or length/diameter ratio, and to contain a number of obstructions in the form of equipment and pipework.
The overpressure generated in the combustion of a vapour cloud is due to two effects. One contribution to the overpressure comes from the production of a large quantity of hot burned gas, the volume production. The other is due to the effect of the flame speed. The overpressure which might occur due to the volume production would be up to about 8 bar, where as the overpressure due to the flame speed effect might have any value up to that associated with a detonation.
Much work has been done on the development of explosion overpressures in vapour clouds. This work has shown that in the part of the cloud that is free of obstacles the flame speed is relatively low. If the flame passes first through a region which is unobstructed, then through one which is obstructed and finally through one which is unobstructed, the flame accelerates on entering the obstructed region and decelerates on leaving it. The flame speed in the obstructed space can reach quite high values.
Merc X (1992, 1998) conducted experiments in which two congested areas, each 2 m wide, are separated by an...
