Balanced bellows pressure

In general, the total back pressure on a balanced bellows pressure relief valve (superimposed plus built-up) should be limited to 50% of set pressure, because of the marked effect of higher back pressures on valve capacity, even when appropriate correction factors are used in sizing. In exceptional cases, such as a balanced bellows PR valve discharging into another vessel, total pressure up to 70% of set pressure may be used. 

For balanced bellows pressure relief valves, the maximum superimposed baek pressure should be specified. (For balanced bellows valves discharging to a safety valve header which is at atmospheric pressure under non-flowing conditions, the maximum back pressure is zero.)... 

In the case of a balanced bellows pressure relief valve, to the maximum pressure permitted by considerations of bellows and bellows bonnet flange mechanical strength. This maximum pressure may be obtained by applying the following correction factor to the maximum back pressure listed for 38 °C. 

Figure 13-8 Bock-pressure sizing factor K, for balanced-bellows pressure relief valves—vapors ono gases.

Figure 5-6. Back pressure sizing factor, K, for balanced-bellows pressure relief valves in vapor or gas service. (API Recommended Practice 520, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1, 5th ed., 1990. Reprinted courtesy of the American Petroleum Institute.)...

Figure 4.3 Balanced-bellows pressure relief valve. (Courtesy American Petroleum Institute, 1220 L Street, NW, Washington, D.C.)...

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