GATE VALVE..— The gate valve (fig. 8-6) has a wedge-shaped, movable plug, called a gate, that fits tightly against the seat when the valve is closed. When the gate is opened, an unrestricted flow passage is provided. It allows fluid to flow through in a straight line with little resistance and less friction and pressure drop, provided the valve gate or disk is kept fully opened. The gate valve releases a variable amount with each turn of the gate.

Gate valves must always be operated in either their fully opened or fully closed position, never in any position to adjust the rate of flow. A partly closed gate will cause vibration and chattering, damaging the seating surfaces.

CHECK VALVE.— The check valve is used principally to prevent backflow in pipelines automatically. The valves are entirely automatic and are used where flow of liquids, vapors, or

gases in one direction only is required. Check valves fall into two main groups: swing check valves and lift check valves. A SWING CHECK VALVE, shown in figure 8-7, is used where an unrestricted flow is desired. A LIFT CHECK VALVE is usually used for air or gases or when operation of the check valve is frequent (fig. 8-8).

GLOBE VALVE.— The globe valve (fig. 8-9), so-called because of its globular-shaped body, is used for regulating liquids, gases, and vapor flow by means of throttling (adjusting rate of flow). They are well suited for services requiring regulated flow and/or frequent valve settings (throttling).

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