Pressure-relieving devices Pilot operated valves

Pressure Relief Valves Definitions for pressure relief valves, relief valves, pilot-operated pressure relief valves and safety v alves, are found in the ASVIE Boiler and Pressure Assel Code, Section Division 1, Rules for Construction of Pressure Assels, Paragraphs UC-125 and UC-126, The pressure-relief valve is an automatic pressure relieving device designed to open when normal conditions are exceeded and to close again when normal conditions are restored. Within this class there are relief valves, pilot operated pressure relief valves, and safety valves. 

When properly designed, this type of valve arrangement conforms to the ASME code. It is a pilot operated pressure relief valve in which the major relieving device is combined with and is controlled by a self-activating auxiliary pressure relief valve. See Figures 7-5A and B. 

Pressure relief device (PRD) is a safety device to protect equipment or piping from damage due to overpressure or vacuum, but in this chapter, we wiU discuss FRD to provcfit overpressure only. Commonly used pressure relief devices are spring-loaded and pilot-operated pressure relief valve (PRV), rupture disk, or pin-actuated pressure relief device. For equipment, it is installed cither directly on the equipment or at its inlet/outlet piping. When the equipment or piping pressure reaches the pressure relief de vice set pressure, it will open to relieve fluid inside the equipment or piping to avoid overpressure. The relief fluid can be vapor, liquid, or two phase mixture. The relieved fluid is either sent to floic system to be disposed or relieved to atmosphere. 

Among the relief devices mentioned above, spring-ioaded and pilot-operated pressure relief valve (PRV) are the ones used most fiequently, PRV is a reclosing pressure relief device, since after it is opened and relieved, it will be reclosed, when equipment or piping pressure is dropped below PRV set pressure. Once it is dosed, h will continue to protect the equipment or piping from overpressure- Rupture disk or pin-actuated relief device is the non-reclosing pressure relief device, since it cannot be reused after it is ruptured or actuated - its rupture disk or pin needs to be replaced. 

The set pressure tolerances of pressure relief valves are not to exceed +2 psi for pressures up to and including 70 psig and +3% for pressures above 70 psig. Indirect operation of safety valves, for example, by pilot valve, is not acceptable unless the primary unloading valve will automatically open at not over the set pressure and will operate fully in accordance with design relieving capacity conditions if some essential part of the pilot or auxiliary device should fail [1]. 

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