Pressure-relieving devices Conventional 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. 

Figure 5-5. Constant back pressure sizing factor, for conventional safety 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.)...

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. 

To evaluate the use of pressure-relief devices, a number of tests were conducted by King (3-6J. Water was forced into NaK through a i-inch pipe under a pressure differential of 200 pounds per square inch. The pressure generated was vented through a 1-inch relief valve set at 200 pounds per square inch. These reaction studies are notable, because they demonstrated that the pressure resulting from the reaction can be relieved by conventional relief valves. 

Conventional relief valves should only be used where the discharge is routed independently to atmosphere, or if installed in a header system, the back-pressure build-up when the device is relieving must be kept below 10% of the set pressure so the set point is not significantly affected. The set point increases directly with back-pressure. 

Should the rupture disk rupture, the relief valve instantly pops fully open, and after tank pressure is relieved and returns to a safe working pressure, the relief valve will reseat and function as a conventional spring-loaded relief valve until it is convenient to replace the rupture disk portion of the device. 

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