ValvesDOE-HDBK-1018/2-93TYPES OF VALVESFigure 28 Relief ValveRelief valves are typically used for incompressible fluids such as water or oil. Safety valves aretypically used for compressible fluids such as steam or other gases. Safety valves can often bedistinguished by the presence of an external lever at the top of the valve body, which is used asan operational check.As indicated in Figure 29, system pressure provides a force that is attempting to push the diskof the safety valve off its seat. Spring pressure on the stem is forcing the disk onto the seat.At the pressure determined by spring compression, system pressure overcomes spring pressureand the relief valve opens. As system pressure is relieved, the valve closes when springpressure again overcomes system pressure. Most relief and safety valves open against the forceof a compression spring. The pressure setpoint is adjusted by turning the adjusting nuts on topof the yoke to increase or decrease the spring compression.Rev. 0ME-04Page 41
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