ValvesDOE-HDBK-1018/2-93TYPES OF VALVESStem seals are accomplished either with packing in a conventional stuffing box or by means ofO-ring seals. Some valve manufacturers, particularly those specializing in the handling ofcorrosive materials, place a stem seal on the inside of the valve so that no material beinghandled by the valve can come into contact with the valve stem. If a stuffing box or externalO-ring is employed, the fluid passing through the valve will come into contact with the valvestem.NeedleValvesFigure 20 Needle ValveA needle valve, as shown in Figure 20, isused to make relatively fine adjustmentsin the amount of fluid flow.The distinguishing characteristic of aneedle valve is the long, tapered, needle-like point on the end of the valve stem.This "needle" acts as a disk. The longerpart of the needle is smaller than theorifice in the valve seat and passesthrough the orifice before the needleseats. This arrangement permits a verygradual increase or decrease in the size ofthe opening. Needle valves are oftenused as component parts of other, morecomplicated valves. For example, theyare used in some types of reducingvalves.NeedleValveApplicationsMost constant pressure pump governorshave needle valves to minimize the effectsof fluctuations in pump dischargepressure. Needle valves are also used insome components of automaticcombustion control systems where veryprecise flow regulation is necessary.Rev. 0ME-04Page 33
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