Superimposed backpressure

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

Balanced safety relief valves may be used in systems where built-up and/or superimposed backpressure is high or variable. In general, the capacity of a b anced valve is not signincantly affected by backpressures below 30 percent of set pressure. Most manufacturers recommend keeping tne backpressure on balanced valves below 45 to 50 percent of the set pressure. 

Backpressure The pressure existing at the outlet of a relief device. The value under no-flow conditions is superimposed backpressure. The value under flowing conditions consists of both... 

Superimposed backpressure Superimposed backpressures acting on the outlet of an SRV can be either constant or variable. Superimposed backpressure occurs when the valve is closed and pressure already exists at the outlet of the valve. This is due to existing constant and/or variable pressures which exist in the discharge system. 

Constant superimposed backpressure Usually backpressures that occur when a safety valve outlet is connected to a static pressure source and doesn t change appreciably under any conditions of operation. In this case, conventional valves may be used if the valve spring setting is reduced by the amount of the constant backpressure (Figure 3.8). 

Variable superimposed backpressure Usually the result of one or more SRVs discharging into a common header. The backpressures may be different at each moment and at each relief cycle Bellows or pilot design is always required since no predetermined set pressure is possible when the oudet pressure is acting on the trim of the valve therefore direcdy influencing the set pressure, and the set point will vary with backpressure (Figure 3.9). 

Cold differential set pressure The inlet static pressure at which an SRV is adjusted to open on the test stand. This test pressure includes corrections for service conditions of superimposed backpressure and/or low or high temperature. 

Superimposed backpressure The static pressure existing at the outlet of a pressure relief device at the time the device is required to operate. It is the result... 

Variable backpressure A superimposed backpressure that will vary with time. Also see Backpressure. 

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

The superimposed backpressure varies, so the set pressure of the conventional valve will vary proportionally. This is acceptable if the valve set pressure increased by the maximum backpressure is equal to or bebw the maximum albwable pressure of the protected installation. 

Many valves and other equipment are connected to the same header system, so the pressure in this system can vary greatly, causing a permanent variable superimposed backpressure on the SRV, as described in Section 3.4. This variable backpressure will act directly on the top of the disc of a conventional SRV and add itself to the original set point (opening pressure) of the SRV. 

Superimposed backpressure is PRV s backpressure, when the PRV is not relieving. This pressure can be constant or variable, depending on the relieving system downstream of the PRV. 

Buildup backpressure is PRV s additional backpressure due to the relief load from the PRV. During relieving, the total backpressure of Che PRV equals its superimposed backpressure plus its buildup backpressure. 

L Conventional spring-loaded PRV is restricted to be used for PRV has constant or no superimposed backpressure, and very low buildup backpressure service. 

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