Pressure Safety Valves (PSV)


Conventional Relief Valve

The conventional relief valve is a differential pressure device. Back pressure on the outlet of the relief valve effects it. They are really only any use if they discharge to atmosphere. Discharging to a relief/flare header is not practical as the back pressure is always variable and will effect when they conventional relief valve pops.

Conventional   SRVs are normally used in any services where the super-imposed backpressure is constant and/or the built-up backpressure does not exceed 10% of the set pressure.

Balanced Bellows Relief Valve

The basic design of a balanced bellows spring valve is the same as a conventional valve, but a bellows is added to compensate for variable backpressures and the bonnet is vented to atmosphere. A leak at the bonnet vent indicates a failed bellows. Because balanced bellows valves must have their bonnets vented to the atmosphere, a safe location for piping the vent must be determined. The balanced bellows valve, with or without a supplementary balancing piston, only discharges process medium from the bonnet vent in the case of failure of the bellows.

Piloted Operated Relief Valves

The pilot-operated relief valve is not normally affected by back pressure (as to its ability to pop at set). The effect of back pressure on the performance of the relief valve is also minimal. They can have up to 80% back pressure with it only derating the capacity by 20%. There is one interesting effect: if the back pressure exceeds the upstream pressure of the relief valve the relief valve pops even if it is not at set. This can be prevented by purchasing a back flow preventer. PORVs also can handle high inlet losses by using a remote pilot. The sensing line for the remote pilot must use a special pickup to allow it to capture the dynamic pressure part of the gas.