22.05 What is important about tank emergency vents?

Tank emergency vents are designed to operate during a fire condition to open and relive pressure buildup in a tank. Standard aboveground tanks are atmospheric tanks rated at 2.5 PSI maximum pressure. The normal breathing tank vent maintains the tank at atmospheric pressure by allowing air flow in and out as the liquid level in the tank changes.

In a fire condition, fuel generates vapor as it increases in temperature, and the normal vent would be insufficient to allow discharge of these vapors, causing an increase in the pressure of the tank.

Emergency vents are sized on the “wetted area” of the tank, that surface area that would be transferring heat into the stored fluid in a fire condition. Based on this surface area a required minimum vent size is determined.

It is not necessary for the design engineer to size the emergency vent. The UL listing on the tank specifies the minimum emergency vent capacity required. The emergency vent device should be confirmed to provide that minimum capacity stamped on the tank by the UL requirement.

A special circumstance can occur where a local regulation requires emergency vents to be routed to the building exterior. If the discharge point is considerably higher than the top of the tank, then an overfill condition will cause fluid to rise in the vent pipe and increase the liquid head pressure in the tank. In this circumstance it is important to have an additional safety measure of a high level liquid sensor in the base of the vent pipe to act as a fuel system shutdown.