Explosion Risks

Apart from choking lungs, irritating eyes and blocking pores, some seemingly
innocuous dusts can ignite to cause fires. Many materials, in a dust cloud, can ignite
and cause an explosion which could be capable of demolishing a factory. A
corn starch powder explosion at General Foods, Banbury in the UK did just this in
1981 and nine men were burned. The company pneumatically conveyed corn
starch, used in custard powder production, from a transfer hopper to feed bins, via
a diverter valve.
An accumulation of corn starch on the operating cylinder caused a malfunction
of this diverter valve. When one hopper was full, the flow should have been
diverted to the next hopper. An already full hopper, therefore, was over-filled
causing powder to be dispersed into the atmosphere. The actual explosion occurred
outside the processing plant where the dust cloud was ignited by electrical
arcing from nearby electrical switchgear, burning nine men and blowing out
brickwork and windows on all four walls [8].
When an explosive dust cloud is ignited in the open air there is a flash fire
but little hazardous pressure develops. If the dust cloud is in a confined situation,
however, such as a conveyor or storage vessel, then ignition of the cloud will lead
to a build-up of pressure. The magnitude of this pressure depends upon the volume
of the suspension, the nature of the material, and the rate of relief to atmosphere.
Research has shown that the particle size must be below about 200 urn for a hazard
to exist.
At some point in a pneumatic conveying system, or time in the conveying
cycle, whether dilute or dense phase, positive or negative pressure, the material
will be dispersed as a suspension. A typical point is at discharge into a receiving
vessel and a common time is during a transient operation such as start-up or shutdown.
Consideration, therefore, must be given to the possibility of an explosion
and its effects on the plant, should a source of ignition be present.
Because of legal and Health and Safety Executive requirements it is advisable
for specialist advice to be sought on dust explosion risks. Authoritative literature
on the subject is widely available and there are many tests that can be carried
out to determine the seriousness of the problem. It is strongly recommended