Effects of Back Pressure
Back-pressure can affect either the set pressure or the capacity of a relief valve. The set pressure is the pressure at which the relief valve begins to open. Capacity is the maximum flow rate that the relief valve will relieve. The set pressure for a conventional relief valve increases directly with back-pressure. Conventional valves can be compensated for constant back-pressure by lowering the set pressure. For self-imposed back-pressure—back-pressure due to the valve itself relieving—there is no way to compensate. In production facility design, the back-pressure is usually not constant. It is due to the relief valve or other relief valves relieving into the header. Conventional relief valves should be limited to 10% back-pressure due to the effect of back-pressure on the set point.
The set points for pilot-operated and balanced-bellows relief valves are unaffected by back-pressure, so they are able to tolerate higher backpressure than conventional valves. For pilot-operated and balanced-bellows relief valves, the capacity is reduced as the back-pressure goes above a certain limit.
For balanced-bellows relief valves, above about 35% back-pressure, the back-pressure affects the stiffness of the bellows and decreases the relief valve’s capacity. Relief valves can be designed for higher back-pressure by increasing the size so that when the capacity is reduced the resulting size is adequate. The manufacturer’s suggested correction for back-pressure should be used when available. API RP 520 offers a generic backpressure correction factor for balanced-bellows relief valves shown in Figure 13-8. The back-pressure correction factor is calculated using gauge pressure. Balanced-bellows valves may be limited by the manufacturer to a back-pressure lower than 35% due to the design strength of the bellows.
All relief valves are affected by reaching critical flow, which corresponds to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pressures above 50%, API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow.
In summary, the back-pressure for relief valves should be limited to the following values unless the valve is compensated. We do not recommend using a relief valve with higher back-pressure than shown below without consulting a person knowledgeable in relief valve sizing and relief system design.
The relief piping design pressure is an additional limit to back-pressure. Relief piping is usually designed as ANSI 150 piping with a MAWP of 285 psig. Relief valves with ANSI 600 inlets usually have outlet flanges rated ANSI 150. A pilot-operated relief valve set at 1,480 psig could have a back-pressure of 740 psig without affecting the valve’s capacity, but that would overpressure the relief piping so the allowable back-pressure is limited to 285 psig. For this reason, ANSI 900 and above relief valves often have ANSI 300 outlet flanges to allow for higher back-pressure in the relief piping.
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