Check Valves - How They Work
Figure 1: Check valve
A check valve, also called a one-way valve, is a device that allows the flow of fluids to move only in one direction. The primary purpose of a check valve is to prevent backflow in the system. Check valves are cheap, effective, and easy solutions to a potential issue. Backflow can cause a problem if the flow is contaminated and, therefore, contaminates the media upstream. For example, a sewer line will have a non-return valve to ensure that waste can leave but not re-enter the system it came from. Figure 1 shows an example of a check valve.
There are various sizes, designs, and materials to ensure there is a check valve for every application. Two-way check valves are commonly used. Check valves with vents are used as three-way check valves to protect the incoming water supply from contamination caused by backflow. The atmospheric vent allows the disposal of purged water into an existing drain. An arrow on its bonnet indicates the check valve flow direction.
Table of contents
- How does a check valve work?
- Check valve types
- Double check valves and dual check valves
- Check valve materials
- Selection criteria
- Check valve symbols
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How does a check valve work?
A check valve requires a minimum upstream pressure (pressure differential between inlet and outlet) to open the valve and pass the media through it. This minimum upstream pressure at which the valve opens is called the check valve's ‘cracking pressure.’ The specific cracking pressure changes based on the valve design and size, so ensure that the system’s pressure can generate the cracking pressure of the chosen check valve(s).
If the upstream pressure falls below the cracking pressure or there is back pressure (flow attempting to move from the outlet to the inlet), the check valve will close. Typically, check valves have a gate, ball, diaphragm, or disc that is pressed against a seal to close the check valve. Gravity or spring can assist the closing process. As the inlet pressure decreases below the cracking pressure or there is backpressure, the valve closes by gravity, spring, and/or by using the backpressure.
As a one-way valve only allows flow in one direction, it is crucial to know the correct installation orientation. Typically, an arrow on the valve’s housing indicates the flow direction. If there isn’t an arrow, examine the valve to ensure it is installed in the intended flow direction. If the valve is installed backward, media will not be able to move through the system, and the resulting pressure build-up can cause damage.
Normally open and normally closed check valves
A normally open check valve allows the medium to flow freely but shuts off the flow in case of backflow. A normally closed check valve prevents the flow of media through it until the cracking pressure builds up, at which point the valve opens.
Check valve types
Check valves operate differently depending on their design. The most common check valve is a spring-loaded in-line check valve; however, we will discuss multiple types below.
Spring-loaded in-line check valve
In-line spring-loaded check valves are common, easy to understand and have a simple design. Figure 2 shows a spring-loaded in-line check valve in the open and closed positions. The arrows indicate flow direction. When flow enters the input port of the valve, it should have enough pressure (force) to overcome the cracking pressure and the spring force. The pressure pushes the disc, opening the orifice and allowing flow to move through the valve. When the input pressure is no longer high enough or there is enough back pressure, the back pressure and spring push the disc against the orifice and seal the valve shut. The spring, along with the short travel distance for the disc, allows for a quick reaction time for closing. This valve design also prevents pressure surges in the line, preventing water hammer.
Common types of spring-loaded in-line check valves are also called ‘nozzle check valves,’ ‘poppet check valves, ‘axial check valves,’ ‘excess flow check valves,’ ‘silent’ or ‘quiet’ check valves, or ‘antiwater hammer check valves.’ They can be installed in a vertical or horizontal orientation. However, as they are in line with the system, they must be fully removed from the line to be inspected and/or maintained. A dual plate check valve has two spring-loaded plates mounted on a central pin. This design prevents slamming and water hammer effectively.
Figure 2: Spring-loaded in-line check valve open (left) and closed (right). The working components are the valve body (A), disc (B), spring (C), and guide (D).
Spring-loaded Y-check valve
Spring-loaded y-check valves operate similarly to in-line spring-loaded check valves. The difference is that the spring and movable disc are positioned at an angle. This creates a ‘y’ shape, hence the name of the valve. It works the same way as an in-line valve, but since the moveable components are at an angle, they can be inspected and serviced while the valve is connected to the system. These valves are larger and take up more room within the system.
Figure 3: Y-check valve
Ball check valve
A ball check valve uses a free-floating or spring-loaded ball that rests on the sealing seat to close the orifice. The sealing seat is normally conically tapered to guide the ball into the seat and create a positive seal, thereby stopping reverse flow. When the pressure of the fluid in the inlet side exceeds the cracking pressure, the ball is dislodged from its seat, which permits flow. When the inlet pressure doesn’t exceed the cracking pressure, or there is back pressure, the ball will close with the back pressure or via the spring, effectively closing the orifice. True union ball check valves allow for easy removal and replacement of balls, eliminating the need to buy a new valve. Read our article on ball check valves for more information.
Figure 4: Spring loaded ball check valve in the open position allowing flow (A), and in the closed position preventing backflow (B)
Diaphragm check valve
Diaphragm check valves consist of a rubber diaphragm that flexes open when the inlet pressure increases. Figure 5 (left) shows a normally open diaphragm check valve with ‘minimal’ inlet pressure, which allows media to flow through. As the inlet pressure increases, the diaphragm will flex open, allowing more flow, as seen in Figure 5 (middle). If backpressure occurs (or it is a normally closed diaphragm check valve), the diaphragm will be forced against the opening and will seal it to prevent any backflow, as seen in Figure 5 (right). Diaphragm check valves are ideal for low-pressure or vacuum applications.
Figure 5: Diaphragm check valve normally open (left), open with inlet pressure (middle), and closed due to backflow pressure (right).
Lift check valve
A lift check valve consists of a guided disc that raises (lifts) off the valve seat to allow media flow. It requires a cracking pressure to overcome gravity and/or spring resistance. The guide keeps the disc in a vertical line to be re-seated with the correct alignment and seal.
Most commonly, lift check valves force the media to make a 90-degree turn, as seen in Figure 6. If there is no spring to assist in closing, mounting orientation is important to consider to ensure that the disc swings shut with gravity.
Figure 6: Lift check valve in an open position (left) and closed position (right)
Swing check valve
Swing check valves are called ‘tilting-disc’ or ‘flapper’ check valves. They have a disc on a hinge (or trunnion) that swings open with an inlet pressure. The disc swings shut as the inlet pressure decreases or if there is backflow. If there is no spring to assist in closing, mounting orientation is important to consider to ensure that the disc swings shut with gravity. A dual disk or double door check valve has a central disk that is split into two semicircular doors that function independently hinged on a central pivot point. Figure 7 shows an example of a swing check valve. Read our article on swing check valves for more information.
Figure 7: Swing check valve. Bolted bonnet (A), hinge or trunnion (B), valve body (C), disc (D), seal (E)
Stop check valve
A stop check valve is typically a spring-loaded y-check valve or a lift check valve with a manual override feature. This allows the valve to function as a normal check valve and prevent backflow. An external mechanism can be used to maintain the valve in an open or closed state. Therefore, a stop check valve can function as two valves in one: a flow regulating valve and a backflow preventing valve. They are commonly used in power plants, boiler circulation, steam generators, turbine cooling, and safety systems.
Figure 8: Check Valve: Figure 1 shows the valve closed by the spring. In figure 2, the pressure overcomes the spring force causing the valve to open. In figure 3, the valve is opened by the actuator, keeping the valve open. The parts of a valve include an actuator (A), actuator shaft and thread (B), spring (C), and disc (D).
Butterfly or wafer check valves
The terms butterfly check valve and wafer check valve are often used interchangeably. They consist of a butterfly, or wafer, style disc on a hinge and a spring. The two sides open when inlet pressure overcomes the cracking pressure, as seen in Figure 9. When the inlet pressure decreases, or there is backflow, the spring on the hinge (or backpressure) will close the disc, effectively sealing it. This valve type allows a straight media flow with minimal obstruction. Wafer swing check valves are sleek in design and can be fitted into tight flange spaces.
Figure 9: Butterfly, or wafer, check valve
Duckbill check valve
Duckbill valves allow flow to proceed through a soft tube of which the end has a natural flattened shape, as seen in Figure 10. This flattened shape resembles a duck beak, hence the name of the valve. The flow opens the flattened end of the duckbill, permitting media to pass. When pressure is removed from the inlet side, the duckbill end returns to its flattened state, cutting off the flow.
Figure 10: Duckbill check valve
A foot valve is a check valve combined with a strainer on the inlet side. The strainer prevents debris that could clog or damage something downstream from entering the check valve. This valve is installed at the end of a section of piping as its input doesn’t have a connection point. Common check valve types included in a foot valve are in-line spring assisted or an in-line ball check valve. They are typically installed at the end of a pump suction line of a water well, fuel tank, or any other application where the suction line is situated below the pump. They can be used to keep pumps primed, prevent liquid from siphoning back, and keep debris out of the line. Figure 11 shows an example of a foot valve.
Figure 11: Foot valve
Dual plate check valve
A dual plate check valve has two spring-loaded plates mounted on a central pin. This design prevents slamming and water hammer effectively.
Double check valves and dual check valves
Double check valve
A double check valve assembly consists of two general check valves assembled in series. A double-check valve has two features:
- If one check valve is jammed open, the other valve takes over the necessary function.
- Closing one check valve reduces the pressure differential across the other valve enabling a tighter seal and avoiding even minor leakage.
A double-check valve backflow preventer is used to prevent back pressure and back siphonage in applications such as fire sprinklers and lawn irrigation. A single check valve is commonly used for fluid category 2 protection where there may be changes in the fluid's temperature, taste, or odor. A double check valve is commonly used for fluid category 3 protection, which involves substances of low toxic levels (like common disinfectants).
Dual check valve
A dual backflow preventer check valve is similar to a double check valve. The configuration has two spring-loaded check valves in series and usually does not include shutoff valves. A dual check valve is powerful against back-siphonage and backpressure. However, it is approved only for residential connections.
Check valve materials
Brass check valve
Brass check valves are excellent for air, water, oil, or fuel applications. However, they are not resistant to seawater, purified water, or chlorinated water. They are less resistant to heat and corrosion compared to stainless steel and are typically used for applications with low pressure.
Stainless steel check valve
Stainless steel check valves have superior resistance against corrosion, heat, and low temperature and have excellent mechanical properties. For applications that don’t require high durability or resistance, stainless steel is typically not a cost-effective solution compared to PVC or brass check valves. Stainless steel wafer check valves are typically high-quality check valves used for high-temperature and high-pressure applications.
PVC (Polyvinyl Chloride) check valve
Plastic check valves like PVC or CPVC are frequently used in irrigation and water management systems. They are corrosion resistant to most corrosive media like seawater, acids, bases, chloride solutions, and organic solvents. However, they are not immune to aromatic and chlorinated hydrocarbons and typically have a max temperature resistance of around 60°C.
Polypropylene (PP) check valve
Polypropylene check valves are used for water, aggressive media, and liquid food products. They are resistant to most corrosive media like inorganic acids, bases, and aqueous solutions that rapidly corrode metals. However, they are not resistant to concentrated acids and oxidizing agents and typically have a max temperature resistance of around 80°C.
Cast iron check valves
Cast iron check valves are typically used as high-temperature check valves. Cast iron is extremely strong and is immune to vibrations. The material has excellent resistance to wear and tear and temperature tolerance. But cast iron is not ductile in nature. Hence, any bending can cause the cast iron material to crack and become useless. Cast iron can work at higher temperatures when compared to PVC but gets corroded over time. These valves find applications in sugar industries, paper industries, and oil lubrication systems.
Check valves have the following criteria to consider when selecting one for an application:
- Material compatibility with the medium
- Line size for connection points
- Max pressure and cracking pressure requirement
- Installation orientation horizontal or vertical
- Envelope dimensions
- Accessibility needs for inspections and repairs
- Temperature (external and media)
Due to how check valves function, they are typically used for one of four different reasons in a variety of applications:
- To protect equipment downstream from backflow damage
- To prevent contamination due to reverse flow
- To prevent siphoning
- To keep a vacuum seal
Due to their function, they are used in almost every industry. They are used on common household appliances, like dishwashers, washing machines, and wastewater lines. For industrial purposes, they are used on boilers, furnaces, gas systems, pumping applications, or vacuum systems. They are also frequently used as aquarium check valves on water and CO2 lines. Also, a miniature check valve is a popular choice where the space is limited, yet a reliable operation is essential. Two of the most common check valve applications are for water and air, which are discussed in more depth below.
Check valves for water
Check valves are used in numerous water applications, like drinking water and wastewater, and are simply called one-way water valves. For drinking water applications, they ensure that no media from the environment (outlet side of the valve) can enter the system with the safe, clean drinking water and contaminate it. For wastewater applications, they ensure that the waste water cannot re-enter the system and cause an overflow or additional contamination. For water pumping applications, a foot valve is often used to ensure no debris will enter the line and to keep internal pressure for priming purposes. Duckbill valves can also be used for discharges on water lines. Sump pump check valves ensure that the discharged water does not come back into the sump pump with gravity when the pump is turned off.
Pneumatic check valve
A pneumatic check valve, or air check valve, allows airflow and prevents it from going out. They are often simply called one-way air valves. The most common application is for an air compressor. A pneumatic check valve allows the compressor to keep certain parts pressurized and other parts de-pressurized. They can be located on an air receiver, discharge pipe, or as a piston check valve on the piston compressor's inlet and outlet sides.
Check valve symbols
The P& ID symbols for check valves are shown in Figures 12-13.
Figure 12: Different P&ID symbols used for check valves. It points in the orientation that it allows the flow with a vertical line showing it doesn’t allow backflow.
Figure 13: Pilot-operated check valve symbol. The dashed line is the pilot line that is used to lift and open the check valve.
What is a check valve?
A check valve is a unidirectional valve that passes fluid in one direction but prevents any flow in the opposite direction.
What is the purpose of a check valve?
The main purpose of a check valve in a system is to prevent backflow, which could damage equipment or contaminate media upstream.
What are common check valve problems?
Common check valve problems are noise, water hammer, vibration, reverse flow, sticking, leakage, and component wear/damage.
Will a check valve stop water hammer?
A check valve can prevent water hammer if it is fast-acting like a spring-actuated check valve. This prevents pressure surges, which create shock waves throughout the media.