Check valves are valves that allow for the flow of a fluid in only one direction within a piping system. Their assembly typically consists of an inlet and an outlet and may also be referred to as non-return valves. This article covers the different types of check valves, valve construction and components, typical applications, and maintenance and repair.
Check valves utilize the pressure gradient across its assembly to operate, thus allowing flow in only one direction. On the inlet side, high-pressure fluid enters, and on the outlet side, it is released by the inner mechanism at a lower pressure. Once the pressure on the outlet side has become greater than that of the inlet, the valve will close off flow. Because of this operating principle, it is imperative that a check valve be oriented the right way in a system in order to prevent failure in a system. Similarly, the system must operate at a specified pressure in order for the valve to work. This minimum pressure is called cracking pressure.
There are many different types of mechanisms that may be used to create a check valve. This section covers some of the main types of check valves used in the industry.
Spring Loaded Inline
Spring loaded in line check valves are the most common type of check valve. It operates by allowing flow once the cracking pressure is attained. The high pressure inlet side must overcome the force of the inline spring to allow flow through the valve, once the pressure has decreased on the inlet side, the spring closes off the valve.
Spring Loaded Y
Spring-loaded Y check valves operate off the same principals as the spring-loaded inline check valve. The only difference is the orientation of the spring which is seated in a “Y” shape. The Y shape design allows for slightly easier maintenance that its inline counterpart.
Ball Check Valve
Ball check valves utilize the spring pressure actuation as the spring check valves. The difference is the incorporation of a ball into the design of the valve whose seating location allows control of the flow within the valve.
Diaphragm Check Valve
Diaphragm Check Valves use a rubber diaphragm to seal flow within the valve assembly. The pressure of the inlet portion must be large enough to deform the diaphragm to allow flow through the valve. They are ideal for low pressure applications as the rubber may wear out easier for higher pressure applications.
Lift Check Valve
Lift check valves operate by the sealing of a disk. The disk is seated so that it moves only up and down and seals properly when closed, the inlet flow must have enough pressure to lift the disk and sometimes a spring that is used to create a higher cracking pressure.
Swing Check Valve
Swing check valves are also known as tilting disk valves. The inner assemble consists of a hinged disk that swings shut and seals by the force of gravity. Once inlet pressure is high enough, the disk swings open allowing flow. Once the pressure decreases the valve shuts off.
Stop Check Valve
Stop check valves are spring loaded y check valves that have a manually operated stem. The stem allows for users to keep the flow shut or open, and to even throttle it manually, giving some control over the functions of the valve.
Most check valves are constructed from either brass or steel. Depending on the application of the check valve, the material must be chosen. While brass is ideal for most applications, it is often susceptible to saltwater due to its chemical properties. Brass is also not ideal for high temperature applications as they are not very heat resistant. Steel on the other hand is better suited for larger, high temperature and pressure applications. For smaller low pressure water systems, PVC may be used as it is inexpensive and easy to use.
The actuation of a check valve is totally dependent on pressure differences. As check valves are made to prevent any sort of backflow, the actuation of the valve depends on the high-pressure inlet to overcome the cracking pressure designated by the system. Once the pressure of the inlet has reached the cracking pressure, the valve opens allowing flow to continue through the valve and closes off once the inlet subsides under the cracking pressure.
Check valves are prevalent when backflow is a major concern. As the operation of a check valve automatically occurs with pressure changes, any backflow immediately shuts off flow. Any operations where backflow may cause system contamination makes check valve specification ideal. Due to their automatic actuation, check valves are used heavily on every scale, from major industrial processes, to residential appliances and water lines.
Maintenance and Repair
As the operation of a check valve relies solely on internal pressure gradients, check valve maintenance focuses on the seat, material, and actuation methods. Generally, routine maintenance intervals are performed and ensure the closing device properly seals flow when closed and opens properly at the desired pressure. Any flaws in the seals or sealing devices may cause detrimental backflow. For smaller applications, maintaining a check valve is relatively simple as low-pressure applications require cheaper valves that may are easily replaced. However, for larger systems, scheduled maintenance is crucial in ensuring the longevity and proper operation of the valve within the system.