Explosion caused by explosion Suppression system

Accident summary

An explosion occurred within a grinder on a plastics manufacturing plant which blew off the grinder door. As it was not practicable to protect the system from the effects of a dust explosion with explosion vents, a chlorofluorocarbon explosion suppression system had been fitted. The system operated off a pressure sensor, which was designed to detect the onset of a dust explosion and rapidly release the CFC to quench the explosion before sufficient pressure was generated to damage the equipment or associated piping.

The grinder had operated without incident for many years, and the suppression system had never been put to the test. At the time of the incident in question, water had inadvertently got into the plastic grinding system and accumulated in the pipework below the grinder. Eventually the water pressure became sufficient to activate the suppression system. As the plastic powder was wet, it did not flow freely, and hence the suppressing agent was not able to flow freely through the system. The pressure of the CFC released into the grinder caused the over-pressurisation of the grinder, which failed at the grinder door.

Failings in technical measures

It was only through good fortune that an operator was not injured during this incident as the viewing port used to inspect the grinder system was in the grinder door. This was relocated following the incident.
Explosion Relief: separation distances
The failure mode of water entering the system and activating the suppression agent had never been considered during the design of the system. It was assumed during the design of the system that the grinder and pipework would be filled with dry, free flowing powder if the system was triggered.
Design Codes - Plant: suitability to withstand foreseeable temperature and pressure over whole life of plant
There was more suppressing agent in the system than required, which exacerbated the over-pressure effect.
Quench Systems: sizing
It was foreseeable that water could enter the system following process upsets elsewhere within the plant. No facility for draining the system was provided.
Design Codes - Plant: HAZOP

References

A.M. Dowell, D.C. Hendershot and G.L. Keeports, 'Explosion Caused By Explosion Suppression System', Loss Prevention Bulletin, Issue No. 146, April 1999, pp 3-4, Institution of Chemical Engineers, ISSN 0260-9576.