manner. Another reason for not leaving globevalves in the fully open position is that it issometimes difficult to determine if the valve isopen or closed. If the valve is jammed in the openposition, the stem may be damaged or broken bysomeone who thinks the valve is closed, andattempts to open it.It is important that globe valves be installedwith the pressure against the face of the disk tokeep the system pressure away from the stempacking when the valve is shut.NEEDLE VALVESNeedle valves are similar in design andoperation to the globe valve. Instead of a disk,a needle valve has a long tapered point at the endof the valve stem. A cross-sectional view of aneedle valve is illustrated in figure 6-8.The long taper of the valve element permitsa much smaller seating surface area than that ofthe globe valve; therefore, the needle valve is moresuitable as a throttle valve. Needle valves are usedto control flow into delicate gauges, whichmight be damaged by sudden surges of fluid underpressure. Needle valves are also used to controlthe end of a work cycle, where it is desirable formotion to be brought slowly to a halt, and at otherpoints where precise adjustments of flow arenecessary and where a small rate of flow isdesired.Although many of the needle valves used influid power systems are the manually operatedtype (fig. 6-8), modifications of this type of valveare often used as variable restrictors. This valve isconstructed without a handwheel and is adjustedto provide a specific rate of flow. This rate of flowwill provide a desired time of operation for aparticular subsystem. Since this type of valve canbe adjusted to conform to the requirements of aparticular system, it can be used in a variety ofsystems. Figure 6-9 illustrates a needle valve thatwas modified as a variable restrictor.HYDRAULIC AND PNEUMATICGLOBE VALVESThe valve consists of a valve body and a stemcartridge assembly. The stem cartridge assemblyincludes the bonnet, gland nut, packing, packingretainer, handle, stem, and seat. On small valves(1/8 and 1/4 inch) the stem is made in one piece,but on larger sizes it is made of a stem, guide,and stem retainer. The valve disk is made of nylonand is swaged into either the stem, for 1/8- and1/4-inch valves, or the guide, for larger valves.The bonnet screws into the valve body withleft-hand threads and is sealed by an O-ring(including a back-up ring).Figure 6-8.—Cross-sectional view of a needle valve.Figure 6-9.—Variable restrictor.6-5
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