Check valves are devices that only allow media to flow in one direction. Due to this, they are often called “one-way” or “non-return” valves. They are two-port valves, which mean they have one inlet and one outlet. They do not require a user or actuator to function, as they work automatically based on the pressure in the system. Check valves are mainly used to prevent contamination and to prevent equipment damage that could be caused if the media were to flow backwards.
How do check valves work?
Since a check valve operates with no user, it requires a pressure differential to operate. For flow to move through the valve from inlet to outlet, it requires a higher upstream pressure. The minimum difference between inlet and outlet for the valve to work is called “cracking pressure”, which changes based on the valve size and design. If there is higher pressure on the outlet side, the flow will attempt to move backwards through the check valve. It is designed to then close due to this “back pressure”.
Since a check valve allows flow in one direction, it is crucial that it is installed in the correct orientation. Often times, there is an arrow clearly depicting the orientation, which can be seen in Fig.1. If installed backwards, media will not be able to move through the valve in the desired flow direction.
Types of check valves
Depending on the design of the valve, they will operate slightly differently. Spring loaded inline check valves are the most common. However, other frequently used types are listed below. There are additional types available that have special features, like inlet strainers, flexible diaphragms, or mechanical override levers, but they are less common.
Spring Loaded In-Line Check Valve
In-line spring loaded check valves have a simple design and are the most commonly used. This valve type has a sealing disc that closes with the assistance of a spring. When media flows into the inlet of the valve, the pressure needs to be stronger than the spring force (cracking pressure). When the inlet pressure is less than the spring force (no inlet pressure or there is a back pressure), the disc will seal the opening preventing the flow of media. For inspection and maintenance, the valve would need to be fully removed from the line. Fig.1 and Fig.2 are examples of a spring loaded in-line check valve that can be screwed in-line to the pipe.
This same concept can be applied to spring loaded Y-check valves. However, the disc and spring are at an angle in relationship to the pipe, which gives the valve housing a Y-shape. This allows for inspection and maintenance to take place with the valve still installed.
Swing Check Valves
This variant simply has the disc swing open and closed based on media flow direction. The disc is attached to a hinge that is connected to the valve body. As media flows in, the inlet pressure is higher than the outlet pressure, so the disc swings open allowing flow to pass through. With no pressure or back pressure, it forces the swinging disc closed and seals the opening.
Lift Check Valves
A lift check valve requires the media to make a 90-degree turn. As the inlet pressure increases, it pushes up on a disc (overcoming gravity and/or a spring) to allow media to flow through. The disc is kept on a vertical line to ensure that when inlet pressure decreases the disc will re-seat properly and seal the opening.
Ball Check Valves
These use a ball that rests on the sealing seat to close the opening. The sealing seat is typically conically tapered to ensure the ball is guided to the seat and creates a proper seal. It is either free-floating or spring-loaded. When inlet pressure is higher than the cracking pressure, the ball will be dislodged from the seal and allow media flow. To close, the back pressure (reverse flow) is required for ball check valves with no spring, otherwise, the back pressure forces the ball to the seal.
Duckbill Valves
A duckbill valve is similar to a hose instead of a metal housing. The outlet of the valve is squeezed/flattened together, which is how the valve type gets its name. Media flow from the inlet opens the flattened end allowing the media to flow through. When inlet pressure is removed, the duckbill end flattens back together to prevent backflow.
Selection criteria
After deciding that your application requires a check valve, it is crucial to understand the varying selection criteria to ensure it meets the application requirements. The main selection criteria to choosing the right valve are:
- Connection and Pipe Size: It is important to know how the valve will be installed (most commonly it is threaded) and to ensure it fits within the system.
- Material: The media will come into contact with the valve, so it is important that it can withstand the media characteristics and that it won’t contaminate the media.
- Pressure: Ensure that the media can create the cracking pressure and, if needed, create a backflow for closing the valve.
- Flow Rate: Ensure the check valve can allow the needed flow rate through it for the application.
- Operating Conditions: The media temperature, external temperature, maintenance schedules, installation orientation, etc. can all affect the check valve type.
Applications
Check valves are used in numerous industries and even on common house hold water lines. They have five main purposes:
- To protect equipment from backflow damage
- To prevent contamination due to reverse flow
- For sealing and holding vacuum
- To prevent siphoning
- To keep liquid pumps primed
You will find check valves for pneumatic, hydraulic, and gas applications. A common example is on your house’s waste water line. They are used to allow waste water to exit the house, but not come back in. Other common household applications are the dishwashers and washing machines. In industry, you can find them in reverse osmosis systems, water treatment plants, boilers, engines, gas systems, etc.
Need a check valve? Visit Tameson’s online store for a wide range of sizes, material selections, and pressure ranges giving you a valve you can count on at the lowest price for any application. If you need further information or assistance, please visit Tameson’s technical information center.
