Figure 8-6.-Crow section of a gate valve.
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 posi-
tion,
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
Figure 8-7.-Cross section of a swing check valve.
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).
8-7
Figure 8-8.-Lift check valve.
