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About Check Valves
Check valves, also referred to as “non-return” or “one-way
directional” valves, are very simple devices that allow fluid,
air or gas to flow in only one direction. When the fluid moves in the
pre-determined direction, the check valves open. Any backflow is prevented
by the moveable portion of the valves. A swinging disc, ball, plunger
or poppet moves out of the way of the original flow. Since these devices
are slightly larger than the through hole, the pressure of backflow will
cause them to tightly seal, preventing reversal of flow. Gravity or a
spring assists in the closing of the check valves.
Check valves are indispensable. Domestically, check valves are found in many
devices such as faucets, toilets and dishwashers. Without check valves, fresh
water would be impossible and common plastics would be unheard of. Industries
use check valves to control flows of all types—from the thinnest gas to
radio active materials, from molten metal to highly corrosive materials. Check
valves can range in size from less than an inch in diameter to 30 feet across.
The simplest check valves can be purchased at the local hardware store, but check
valves may also be precision-designed for highly sophisticated systems.
As aforementioned, check valves use different means of preventing backflow. Swing
disc check valves are typically used with liquids, such as slurries, that can
easily damage the valve seat. These check valves and custom check valves may
be installed either vertically or horizontally, but a lever and counter-balance
is recommended when vertically installing swing disc valves. Ball
check valves use a ball with a small hole in the middle, which can be either
free-floating or spring-loaded. Custom check valves featuring ball mechanisms
have many uses in a wide variety of applications. Custom check valves are more
resistant to plugging than other check valves, and thus can be used to handle
even fluids that deposit gummy residue. Check valves may also utilize a plunger,
which is spring-loaded and usually used to prevent backflow of pressurized gases,
or backpressure. A poppet in check valves is spring-loaded and typically installed
in systems that require prevention of backpressure.
The versatile custom check valve is often made of a wide variety of materials.
Some check valve manufacturers exclusively manufacture plastic check valves.
Other check valve manufacturers, check valve distributors, and check valve suppliers
provide brass check valves, electroless nickel plated brass check valves and
stainless steel check valves. The components of check valves can be made of different
materials, such as cast iron check valves or bronze check valves, depending upon
their application. Many leading check valve manufacturers may also be construct
their products out of Teflon. Elastomer products can also be used to make check
valves.
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http://www.engineering.usu.edu/cee/faculty/rahm/check.html
Types of Check Valves
- Ball Check Valves are valves where liquids can flow
only one way. The pressure from the liquid forces the ball to move,
when the pressure from the fluid stops the ball also stops and the
valve closes.
- Clapper Valves are a type of check valve used in firefighting. This
valve has a hinged gate (often with a spring pulling it shut) that
will only remain open in the outflowing direction.
- Double Check Valves consist of at least two independently
acting check-valves. They are typically used in liquid services.
- Foot Valves are often used in conjunction with a
pump. The valve aids in pumping water and helps prevent backflow.
- Hydraulic Check Valves prevent backflow in hydraulic
systems, typically using a ball or a poppet.
- Lift Check Valves has a disc, sometimes called a lift, that can be
lifted up off its seat by higher pressure of inlet or upstream flow.
A guide keeps motion of the disc on a vertical line, so the valve can
later reseat properly.
- Plastic Check Valves can be used for industrial
purposes or for lower grade home use, such as in a swimming pool or
aquarium. Typically, they are used when resistance to corrosion is
needed.
- Pneumatic Check Valves allow fluids to flow in one
or two ways. When the fluid is allowed to flow more than one way it
is often metered.
- Relief Valves relieve excess and potentially dangerous
pressure from piping systems or closed-top vessels and are normally
closed until the set pressure is reached.
- Sanitary Check Valves are used when the fluids contained
in the pipes need to remain germ free. These check valves are often
used for medical, food, or dairy purposes.
- Silent Check Valves have a spring-operated return mechanism. They eliminate
the shock of sudden reversal of flow, making them a favorite for use
in clear liquid systems.
- Stop Check Valves are a type of 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.
- Swing Check Valves use outside levers to slow down
the flow of materials through a pipe. The lever pulls the disk into
the flow which causes the flow to stop more quickly.
- Wafer Check
Valves are often used for liquid services. The check valve is made
of two half-circle disks hinged together. With positive flow, the two
disks fold together to allow materials to flow through the pipe. With
reverse flow, they retract to a full circle to restrict flow.
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