API-RP-576

The internal area of the bellows in a balanced-bellows spring-loaded pressure-relief valve is
referenced to atmospheric pressure in the valve bonnet. The bonnet of a balanced pressurerelief
valve shall be vented to the atmosphere at all times for the bellows to perform
properly. In the case of a bellows failure, the absence of the vent (i.e. if the bonnet were
closed), will allow the bonnet pressure to rise to the valve discharge pressure, defeating the
balanced design of the valve, and effectively converting it to a conventional valve. The
bonnet vent ensures nearly balanced performance even in the event of a bellows failure, and
therefore shall be kept open. If the valve is located where atmospheric venting would
present a hazard or is not permitted by environmental regulations, the vent should be piped
to a safe location that is free of backpressure that may affect the pressure-relief valve set
pressure.
Balanced safety-relief valves may have a backpressure limitation based on the mechanical
strength of the bellows. Consult the individual valve manufacturer to assure the allowable
backpressure is not exceeded.
Fion Zhnag/ Charlie Chong