Pressure and vacuum relief valves are used on liquid storage tanks and other process vessels or

systems to prevent structural damage due to excess internal pressure or vacuum.

Storage tanks are pressurized when liquid is pumped in, compressing the existing vapor or when rising

temperatures cause increased evaporation or expansion of existing vapor.

Conversely, a vacuum condition may be created when pumping out or due to falling temperature.

To prevent tank damage, vapor must be allowed into or cout of the tank at specified pressure/vacuum

condition. The volume rate of venting depends upon the tank size, volatility of the tank contents, the

pumping rates and the temperature. Refer to API Standard 2000 for the procedures to determine venting requirements.

A relief valve must be carefully maintained by a qualified valve technician. It should only be assembled

under clean conditions, preferably in a service shop environment.