The valve is available with either a rising stemor a non-rising stem. The rising stem valve usesthe same port body design as does the non-risingstem valve. The stem is threaded into the glandnut and screws outward as the valve is opened.This valve does not incorporate provisions fortightening the stem packing nor replacing thepacking while the valve is in service; therefore,complete valve disassembly is required formaintenance. Figure 6-10 illustrates a rising stemhydraulic and pneumatic globe valve. Additionalinformation on this valve is available in StandardNavy Valves, NAVSHIPS 0948-012-5000.PRESSURE CONTROL VALVESThe safe and efficient operation of fluidpower systems, system components, and relatedequipment requires a means of controllingpressure. There are many types of automaticpressure control valves. Some of them merelyprovide an escape for pressure that exceeds a setpressure; some only reduce the pressure to a lowerpressure system or subsystem; and some keep thepressure in a system within a required range.RELIEF VALVESSome fluid power systems, even when operat-ing normally, may temporarily develop excessivepressure; for example, when an unusually strongwork resistance is encountered. Relief valves areused to control this excess pressure.Relief valves are automatic valves used onsystem lines and equipment to prevent over-pressurization. Most relief valves simply lift (open)at a preset pressure and reset (shut) when thepressure drops slightly below the lifting pressure.They do not maintain flow or pressure at a givenamount, but prevent pressure from rising abovea specific level when the system is temporarilyoverloaded.Main system relief valves are generallyinstalled between the pump or pressure source andthe first system isolation valve. The valve mustbe large enough to allow the full output of thehydraulic pump to be delivered back to thereservoir. In a pneumatic system, the relief valvecontrols excess pressure by discharging the excessgas to the atmosphere.Figure 6-10.—Hydraulic and pneumatic globe valve (rising stem).6-6
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