POTENTIAL FOR FLASHBACK THROUGH PRESSURE/VACUUM VALVES ON LOW-PRESSURE STORAGE TANKS SYNOPSIS

Low pressure or "atmospheric" storage tanks are commonly used for the storage of flammable hydrocarbons. In order to minimise losses of volatile organic compounds to the environment, these tanks are often fitted with pressure / vacuum valves (PV valves). These enable the tank to operate at pressures typically between +56mbar and -6mbar before the valve opens. The atmosphere within the tank is often in the flammable region and hence during outbreathing, the vapours vented from the PV valve are also flammable. These may be ignited by an external ignition source such as static discharge or hot work occurring near the vent. The common assumption has always been that the tank would be protected from internal explosion by the presence of the PV valve on the basis that the flame could not pass back through the valve. This paper presents the opinion that ignition can, in fact, flash back through a PV valve based on consideration of flame speed, minimum experimental safe gap (MESG) and flow rates. Experimental work done by Protego will also be presented demonstrating that flashback can occur, especially under conditions of continuous venting. It is therefore recommended that consideration should be given to installing a flame arrester on tanks protected by a PV valve.