Check valves are designed to prevent the reversal of flow in a piping system. These valves are activated by the flowing material in the pipeline. The pressure of the fluid passing through the system opens the valve, while any reversal of flow will close the valve, the check valve is self-actuating when the flow is reversed. Discs, wafers or membrane diaphragms are used in this type of valve.
They are also known as non-return valves. reflux valves, flap valves. retention valves and foot valves in different services.
Types of Check valves:
Swing Check valve:
Swing- or plate-type valves (swing/plate check valves), where the check mechanism is a hinged plate or flap, or disc-see chapter on Flap Valves. The butterfly check valve is a variant on this principle on Butterfly Valves. A swing check valve is normally recommended for use in systems employing gate valves because of Swing Check Valve of the low-pressure drop across the valve. Swing check valves are available in either Y-pattern or straight body design.
Straight body swing check valves contain a disk that is hinged at the top. The disk seals against the seat, which is integral with the body. This type of check valve usually has replaceable seat rings. The seating surface is placed at a slight angle to permit easier opening at lower pressures, more positive sealing, and less shock when closing under higher pressures.
Tilting Disk Check Valves:
The basis of the tilting disc check valve is a ‘lifting’ section disc. pivoted in front of its centre of pressure and counterweighted and/or spring-loaded to assume a normally closed position. With flow in one direction the disc lifts and ‘floats’ in the stream. offering minimum resistance to flow. The balance of the disc is such that as flow decreases the disc will pivot towards its closed position.
These valves are available with a soft seal ring, metal seat seal, or a metal-to-metal seal. The latter is recommended for high-temperature operation. The soft seal rings are replaceable, but the valve must be removed from the line to make the replacement.
Piston Check Valves:
The piston-type lift check valve incorporates a dashpot applied to the check mechanism, otherwise, it is basically similar to a lift-type disc valve. The advantage of the dashpot is that it provides a damping effect during operation.
Lift-type piston check valves are commonly used in conjunction with globe and angle valves on piping systems subject to surge pressures or frequent changes in flow direction.
Butterfly Check Valves:
Butterfly check valves have a seating arrangement similar to the seating arrangement of butterfly valves. Flow characteristics through these check valves are similar to the flow characteristics through butterfly valves.
The butterfly check valve design is based on a flexible sealing member against the bore of the valve body at an angle of 45°. The short distance the disk must move from full open to full closed inhibits the “slamming” action found in some other types of check valves