API RP 576:2000 pdf download.Inspection of Pressure-Relieving Devices
4.1 PRESSURE RELIEF VALVE
A pressure relief valve is a pressure relief device designed to open and relieve excess pressure and to reclose and prevent further flow of fluid after normal conditions have been restored. A pressure relief valve opens when its upstream pressure reaches the opening pressure. It then allows fluid to flow until its upstream pressure falls to the closing pressure. It then closes, preventing further flow. Examples of specific types of pressure relief valves include; safety valve, relief valve, conventional safety relief valve, balanced safety relief valve, and pilot-operated pressure relief valve.
4.2 SAFETY VALVE
A safety valve is a direct spring loaded pressure relief valve that is actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. When the static inlet pressure reaches the set pressure, it will increase the pressure in the huddling chamber and over- come the spring force on the disk. This will cause the disk to lift and provide full opening at minimal overpressure. The closing pressure will be less than the set pressure and will be reached after the blowdown phase is completed. The spring of a safety valve is usually fully exposed, out- side of the valve bonnet to protect it from degradation due to the temperature of the relieving medium. A typical safety valve has a lifting lever for manual opening to ensure the free- dom of the working parts. Open bonnet safety valves are not pressure tight on the downstream side. Figure 1 illustrates a full-nozzle, top-guided safety valve.
Depending on the design, the pilot valve (control unit) and the main valve may be mounted on either the same connec- tion or separately. The pilot is a spring loaded valve that oper- ates when its inlet static pressure exceeds its set pressure. This causes the main valve to open and close according to the pressure. Process pressure is either vented off by the pilot valve to open the main valve or applied to the top of the unbalanced piston, diaphragm, or bellows of the main valve to close it. Figure 7 shows a low-pressure diaphragm-type pilot-operated valve. Figures 8 and 9 shows a high-pressure pilot-operated valve that uses an unbalanced piston and has an integrally mounted pilot. Figure 9 also illustrates optional remote pressure sensing from the vessel and optional dual outlets to equalize thrust.
API RP 576:2000 pdf download
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