Pressure-relieving devices Balanced, valves

Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid.) Safety relief valves are classified as "Conventional" or "Balanced", depending upon the effect of back pressure on their performance. 

Pressure-relieving devices, 399, 435-455 Balanced, valves, 400, 405, 407, 441 Calculadon of relief areas, 436, 440,... 

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.)...

The eventual supplementary balancing piston is actually fitted as a backup device if the bellows fail in service this device will ensure that the valve still relieves at the correct set pressure. Balancing piston valves are uncommon and expensive, but without the balancing piston fitted, the variable backpressure has an adverse effect on the set point of the SRV. This may result in the valve not relieving at its hill capacity or not achieving hill lift within 10% overpressure... 

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