Variable superimposed backpressure

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. 

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). 

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. 

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. 

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

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. 

For bellows valves with superimposed or variable backpressure see Appendix B. For pilot-operated valves see note below. 

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