TYPES OF VALVESDOE-HDBK-1018/2-93ValvesButterfly check valves may beFigure 27 Stop Check Valveinstalled horizontally orvertically with the vertical floweither upward or downward.Care should be taken to ensurethat the valve is installed sothat the entering flow comesfrom the hinge post end of thevalve; otherwise, all flow willbe stopped.StopCheckValvesA stop check valve, illustratedin Figure 27, is a combinationof a lift check valve and aglobe valve. It has a stemwhich, when closed, preventsthe disk from coming off theseat and provides a tight seal(similar to a globe valve).When the stem is operated tothe open position, the valveoperates as a lift check. Thestem is not connected to thedisk and functions to close thevalve tightly or to limit thetravel of the valve disk in the open direction.ReliefandSafetyValvesRelief and safety valves prevent equipment damage by relieving accidental over-pressurizationof fluid systems. The main difference between a relief valve and a safety valve is the extent ofopening at the setpoint pressure.A relief valve, illustrated in Figure 28, gradually opens as the inlet pressure increases above thesetpoint. A relief valve opens only as necessary to relieve the over-pressure condition. A safetyvalve, illustrated in Figure 29, rapidly pops fully open as soon as the pressure setting is reached.A safety valve will stay fully open until the pressure drops below a reset pressure. The resetpressure is lower than the actuating pressure setpoint. The difference between the actuatingpressure setpoint and the pressure at which the safety valve resets is called blowdown.Blowdown is expressed as a percentage of the actuating pressure setpoint.ME-04Rev.0Page 40
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