Balanced-bellows pressure relief valve

Besides reduce the backpressure influence on PRV s disk, bellows has another usage. It sometimes is used to isolate the guide, spring, and bonnet fiom the relieving fluid. It is important that balanced-bellows PRV s bonnet be vented to atmosphere, since it is part of valve s design basis. 

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Pg = superimposed back pressure in pounds per square inch gauge, 

The bellows isolate an area of the disk, approximately equal to the disk seat area, from the back pressure. Thus, the installation of bellows will allow the set pressure to remain constant in spite of variation in back pressure. 

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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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