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A Check valve or Clack valve is a mechanical
device, a valve, that normally allows fluid (liquid
or gas) to flow through it in only one direction.
Check valves are two-port valves, meaning they have
two openings in the body, one for fluid to enter and
the other for fluid to leave. There are various
types of check valves used in a wide variety of
applications. Check valves are often part of common
household items. Although they are available in a
wide range of sizes and costs, many check valves are
very small, simple, and/or cheap. Check valves work
automatically and most are not controlled by a
person or any external control; accordingly, most do
not have any valve handle or stem. The bodies
(external shells) of most check valves are made of
plastic or metal.
An important concept in check valves is the cracking
pressure which is the minimum upstream pressure at
which the valve will operate. Typically the check
valve is designed for and can therefore be specified
for a specific cracking pressure.
Heart valves are essentially inlet and outlet check
valves for the heart ventricles, since the
ventricles act as a pump.
Types of check valves
An open ball check valve.
A ball check valve is a check valve in which the
disc, the movable part to block the flow, is a
spherical ball. In many ball check valves, the ball
is spring-loaded to stay shut, but also many do not
have a spring inside. For those designs without a
spring, reverse flow is required to move the ball
toward the seat and create a seal. The interior
surface of the main seats of ball check valves are
more or less conically-tapered to guide the ball
into the seat and/or form a positive seal when
stopping reverse flow.
Ball check valves are often very small, simple, and
cheap. They are commonly used in liquid or gel
mini-pump dispenser spigots, spray devices, some
rubber bulbs for pumping air, etc., manual air pumps
and some other pumps, and refillable dispensing
syringes. Although the balls are most often made of
metal, they can be made of other materials, or in
some specialized cases out of artificial ruby. High
pressure HPLC pumps and similar applications
commonly use small inlet and outlet ball check
valves with balls made of artificial ruby and seats
made of artificial sapphire, both for hardness and
chemical resistance. After prolonged use, such check
valves can eventually wear out or the seat can
develop a crack, requiring replacement. Therefore,
such valves are made to be replaceable, sometimes
placed in a small plastic body tightly-fitted inside
a metal fitting which can withstand high pressure
and which is screwed into the pump head.
There are similar check valves where the disc is not
a ball, but some other shape, such as a poppet
energized by a spring. Ball check valves should not
be confused with ball valves, which is a different
type of valve in which a ball acts as a controllable
rotor to stop or direct flow.
A diaphragm check valve uses a flexing rubber
diaphragm positioned to create a normally-closed
valve. Pressure on the upstream side must be greater
than the pressure on the downstream side by a
certain amount, known as the pressure differential,
for the check valve to open allowing flow. Once
positive pressure stops, the diaphragm automatically
flexes back to its original closed position.
A swing check valve is a butterfly-style check valve
in which the disc, the movable part to block the
flow, swings on a hinge or trunnion, either onto the
seat to block reverse flow or off the seat to allow
forward flow. The seat opening cross-section may be
perpendicular to the centerline between the two
ports or at an angle. Although swing check valves
can come in various sizes, large check valves are
often swing check valves.
This Siamese clappered inlet allows one or two
inputs into a deluge gun.
A clapper valve is a type of check valve used in
firefighting, and has a hinged gate (often with a
spring urging it shut) that will only remain open in
the outflowing direction.
A stop-check valve is a check valve with override
control to stop flow regardless of flow direction or
pressure. When the valve is open, it acts as a check
valve, but the valve can be deliberately shut to
stop flow.
A lift-check valve is a check valve in which the
disc, sometimes called a lift, can be lifted up off
its seat by higher pressure of inlet or upstream
fluid to allow flow to the outlet or downstream
side. A guide keeps motion of the disc on a vertical
line, so the valve can later reseat properly. When
the pressure is no longer higher, gravity or higher
downstream pressure will cause the disc to lower
onto its seat, shutting the valve to stop reverse
flow.
A double check valve is often used as a backflow
prevention device to keep potentially contaminated
water from siphoning back into municipal water
supply lines.
There are also double ball check valves in which
there are two ball/seat combinations sequentially in
the same body to ensure positive leak-tight shutoff
when blocking reverse flow; and piston check valves,
wafer check valves, and ball-and-cone check valves.
[Applications
Check valves are often used with some types of
pumps. Piston-driven and diaphragm pumps such as
metering pumps and pumps for chromatography commonly
use inlet and outlet ball check valves. These valves
often look like small cylinders attached to the pump
head on the inlet and outlet lines. Many similar
pump-like mechanisms for moving volumes of fluids
around use check valves such as ball check valves.
Check valves are used in many fluid systems such as
those in chemical, and power plants, and in many
other industrial processes.
Check valves are also often used when multiple gases
are mixed into one gas stream. A check valve is
installed on each of the individual gas streams to
prevent mixing of the gases in the original source.
For example, if a fuel and an oxidizer are to be
mixed, then check valves will normally be used on
both the fuel and oxidizer sources to ensure that
the original gas cylinders remain pure and therefore
nonflammable.
Some types of irrigation sprinklers and drip
irrigation emitters have small check valves built
into them to keep the lines from draining when the
system is shut off.
Also used with most home made snowmakers.
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