Figure 8-6.-Crow section of a gate valve.GATE VALVE..— The gate valve (fig. 8-6) hasa wedge-shaped, movable plug, called a gate, thatfits tightly against the seat when the valve isclosed. When the gate is opened, an unrestrictedflow passage is provided. It allows fluid to flowthrough in a straight line with little resistance andless friction and pressure drop, provided the valvegate or disk is kept fully opened. The gate valvereleases a variable amount with each turn of thegate.Gate valves must always be operated ineither their fully opened or fully closed posi-tion,never in any position to adjust therate of flow. A partly closed gate will causevibration and chattering, damaging the seatingsurfaces.CHECK VALVE.— The check valve is usedprincipally to prevent backflow in pipelinesautomatically. The valves are entirely automaticand are used where flow of liquids, vapors, orFigure 8-7.-Cross section of a swing check valve.gases in one direction only is required. Checkvalves fall into two main groups: swing checkvalves and lift check valves. A SWING CHECKVALVE, shown in figure 8-7, is used where anunrestricted flow is desired. A LIFT CHECKVALVE is usually used for air or gases orwhen operation of the check valve is frequent(fig. 8-8).8-7Figure 8-8.-Lift check valve.
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