A check valve (called also “non-return valve”) is used to prevent potentially damaging backflows and protect equipment like pumps and compressors. Indeed, a check valve allows the flow of the fluid only in one direction and blocks any flow in the opposite direction. The swing check design is the most common type, followed by the ball check, stop check and piston check configurations.
This type of valve does not require any type of manual or automated operation, as they open automatically when the fluid conveyed by the pipeline flows in the desired direction (the pressure of the fluid is sufficient to open the valve) and close when the flow changes direction, in an unwanted mode. The valve also closes automatically when the pressure in the pipeline drops at a low level.
A check valve is a unidirectional type of valve, i.e. it should be installed in a proper position to work as intended.
The key specifications for check valves are: API 6D (pipeline valves), API 603 (stainless steel stop check valves), BS 1868 (standard carbon and alloy steel body check valves), BS 1873, ASME B16.34 (pressure and temperature ratings), ASME B16.5/ASME B16.47 (flanged end connections), and, ASME B16.25 (butt weld connections).
Cast steel valves are available with flanged and butt weld ends. Forged, small size, valves are available with threaded and socket weld connections. To learn about the differences between the valve ends types, read this article.
CHECK VALVE SYMBOL IN P&ID DIAGRAMS
Check valves are represented by the following symbol in piping P&ID diagrams:
TYPES OF CHECK VALVE
The way the opening/closing and closing operation depends on the specific design of the check valve (the most common are the swing type, the lift type in its two variants ball and piston, butterfly, stop check and titling-disc). The basic features of each variant are described below.
SWING CHECK VALVE
A swing check valve features a simple design with a disc attached to a hinge at the top.
As the fluid passes through a swing check valve, the valve is open. When a reverse flow occurs, the changes in motion as well as gravity help to pull down the disc, closing the valve effectively.
Swing check valves are used for firefighting and flood prevention in sewage systems. They’re also designed to work with materials such as gas, liquids, and other types of media.
STOP CHECK VALVE
Stop-check can start, stop, and regulate the flow of fluids while preventing dangerous backflow that may damage other equipment like pumps and compressors. When the pressure in the system is below a certain value, stop check valves close automatically to impede a reverse flow. These types of check valves also feature an external override control to close the passage of the fluid manually (similarly to a gate valve).
Stop check valves are very common in power plants, boiler systems and oil & gas refining, hydrocarbon processing, and high-pressure safety services.
BALL CHECK VALVE
A ball check valve features a spherical ball positioned inside the body that opens and closes the passage of the fluid in only a wanted direction.
The ball rotates freely when the fluid passes through the pipeline in the desired direction. If the pipeline is subject to a pressure decrease or a reverse flow, the ball inside the valve moves towards the seat, sealing the passage.
Ball check valves are widely used available and recommended for viscous fluids.
The ball check valve belongs to the family of “lift valves”, and have a seat design similar to those of globe valves. A variant of the ball design is the so-called piston type. This type of check valves suits high-pressure services that generate a quick flow of the fluid (this due to the fact that the disc is precisely guided and fits perfectly into the seat).
Ball check and piston check valves can be installed both horizontally and vertically. The ball check valve design is adopted by smaller size check valves, due to the easier construction vs. a swing check valve design in small spaces.
DUAL PLATE CHECK VALVE
Dual plate check valves, designed according to API 594, are frequently used to protect pumps, compressors and other mechanical equipment installed onboard the pipeline.
PRESSURE SEAL CHECK VALVE
This type features a special design cover able to withstand high pressures.
SUMP PUMP AND AIR COMPRESSOR CHECK VALVE
SUMP PUMP CHECK VALVE
Anytime a new sump pump is installed, a new check valve shall also be installed. This is due to the fact that older check valves installed to protect the older sump pump may have been damaged by previous operations and the flow of corrosive materials.
A sump pump check valve prevents the fluid conveyed by the pipeline from flowing back into the sump pit as the pump is switched off by an operator or an automated control system. If no check valve is installed before the sump pump, the device would turn off automatically after a certain amount of fluid is pumped out of your sump pit or sump liner. The fluid remaining in the discharge pipe would fall back into the sump pit, raising the fluid level again so that pumping will need to resume.
If a sump pump is forced to move the same fluid twice, the device would burn out prematurely. The installation of a check valve before a sump pump would avoid this double pumping of the fluid and extend the pump life cycle. Therefore, it is always recommended to have the sump pump protected by a non-return valve.
AIR COMPRESSOR CHECK VALVE
Check valves are an important addition to any air compressor (reciprocating piston compressors, rotary screw compressor, and scroll compressors). Indeed, some air compressor features even more than one single check valve at once (it’s not uncommon to see up to five check valves installed on larger air compression systems).
The scope of an air compressor check valve is to allow the air flow in one direction only and to block any possible flow in the opposite direction. This action is particularly important as it helps the compressor to keep some parts pressurized and others de-pressurized. The non-return valve also prevents that the compressed air that leaves the compressor to the piping system or air-tank flows back into the compressor.
Air compressor check valves are rather simple devices, and share the same construction of any other non-return valve: inside the valve, there is a disc (or a ball) and a spring that keeps the disc or the ball in an open or closed position (pushed down/up) depending on the air flow direction.
If a wanted air flow enters the check valve (from the inlet), it keeps the disc or the ball open and the air flows freely through the valve and reaches the compressor. In case, instead, an unwanted flow is pushed back from the compressor to the check valve, the air pushes against the disc or the ball and closes the valve, preventing the backflow to the pipe installed at the check valve inlet. And this closing force is as strong and effective as the backflow pressure intensity.
CHECK VALVE MATERIALS
The body for check valves below 2 inches is generally made of forged steel (common grades are ASTM A105 for high-temperature service, ASTM A350 for low-temperature service and, ASTM A182 F304/F316 for corrosive service), whereas the bodies for larger bore sizes check valves are made of cast steel (ASTM A216 WCB for high-temperature service, ASTM A351 for low-temperature conditions and ASTM A351 CF8 and CF8M – i.e. SS304 and SS316).
CHECK VALVE PRICES
Looking for the price of check valves? Click on the image and check our online check valves price estimation tool!
The tool estimates the price for cast and forged check valves manufactured in Europe and Asia.
Even if our price estimates are reliable, prices shall always be double checked with check valves manufacturers and distributors.
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