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Check valves
Check valves prevent reversal of flow, controlling the direction
of flow rather than stopping or starting flow. Some basic types
include swing check, ball check, wafer check, silent check, and
stop-check valves. Most check valves are available in screwed
and flanged body styles.
Swing check valves have hinge-mounted disks that open and close
with flow (Figure 28). The seats are generally made of metal,
while the disks may be of metallic or nonmetallic composition
materials. Nonmetallic disks are recommended for fluids containing
dirt particles or where tighter shutoff is required. The Y-pattern
check valve has an access opening to allow cleaning and regrinding
in place. Pressure drop through swing check valves is lower than
that through lift check valves due to the straight-through design.
Weight- or spring-loaded lever arm check valves are available
to limit objectionable slamming or chattering when pulsating flows
are encountered.
Lift check valves have a body similar in design to a globe or
angle valve body with a similar disk seating. The guided valve
disk is forced open by the flow and closes when flow reverses.
Due to the body design, the pressure drop is higher than that
of a swing check valve. Lift check valves are recommended for
gas or compressed air or in fluid systems not having critical
pressure drops.
Ball check valves are similar to lift checks, except that they
use a ball rather than a disk to accomplish closure. Some ball
checks are specifically designed for horizontal flow or vertical
upflow installation.
Wafer check valves
Wafer check valves are designed to fit between pipe flanges
similar to butterfly valves and are used in larger piping (4 in.
diameter and larger). Wafer check valves have two basic designs:
(1) dual
spring-loaded flapper, which operates on a hinged center post,
and
(2) single flapper, which is similar to the swing check valve.
In silent or spring-loaded check valves, a spring positively and
rapidly closes a guided, floating disk. This valve greatly reduces
water hammer, which may occur with slow-closing check valves
like the swing check. Silent check valves are recommended for
use
in pump discharge lines.
STOP-CHECK VALVES
Stop-check valves can operate as both a check valve and a stop
valve. The valve stem does not connect to the guided seat plug,
allowing the plug to operate as a conventional lift check valve
when
the stem is in the raised position. Screwing the stem down can
limit
the valve opening or close the valve. Stop-check valves are used
for
shutoff service on multiple steam boiler installations, in accordance
with the ASME Boiler and Pressure Vessel Code, to prevent back
flow of steam or condensate from an operating boiler to a shutdown
boiler. They are mandatory in some jurisdictions. Local codes
should be consulted.
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