Investigation into the Role of Electrostatics in a Vapour Ignition Incident during Refuelling at a Service Station

Abstract

During the delivery of unleaded petrol to a Snax service station on the Southend Arterial Road a small fire occurred in the manhole chamber containing the underground offset fill pipes. The manhole involved has four fill pipes and is constructed from GRP with a metal frame at the top. The pipes are plastic but have a metal transition fitting for connection to the tanker hose. When not in use the fittings have an aluminium cap with an o-ring seal in place. It is understood that the delivery driver had two hoses connected to the offset fill points and was delivering the product. One delivery had finished and he opened up the cap of the next fill point to commence delivery to that tank. He then went to place the cap on the pipe of the tank to which he had completed the first delivery when the ignition occurred. The fire was quickly extinguished and no one was hurt. Initially it was suspected that the probable cause of the ignition was static electricity igniting an uncommonly high concentration of petrol vapour. This is not an unreasonable assumption since the metal termination on the plastic pipe was ungrounded. In order for an ignition to occur at this point however there must be an electrostatic discharge between the cap and the termination fitting of sufficient energy to exceed the minimum ignition energy of the vapour mixture. Normally the caps made from aluminium are connected to the termination fitting by means of a short length of chain but in this case the chain was broken and consequently there was no electrical continuity between the fitting and the cap. It follows that electrostatic ignition would be reliant on either the fitting to be raised to high potential or the cap/delivery driver being raised to high potential.

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