Check valves, also known as non-return or one-way valves, are designed to allow fluid to flow one way in a pipeline. They’re constructed of a clapper which hangs from a hinge, the clapper shaft or pin, which is mounted to the underside of the bonnet, inside the valve body. The basic design of a check valve inhibits backflow in a line.
Because of their simple design, check valves generally operate without automation or human interaction and instead rely on the flow velocity of the fluid to open and close. This means they generally do not have a method of outside operation, like a handle or lever. The minimum upstream pressure required to operate the valve is called the “cracking pressure”. Check valves are generally designed specifically with this number in mind and, depending on the size and style of check valve, this number is 1 psi to 5 psi.
The degree of opening on a check valve is determined by the flow rate. The higher the flow rate, the more open the valve will be until it reaches its maximum, full open position. On many check valves, the full open position is approximately 85 degrees.
There are a variety of types of check valves, including:
- Swing check or tilting disc check valves – Consist of a clapper with a disc that is convex on the upstream inlet side and flat on the downstream outlet side. For API 600 valves, this disc swings on a hinge or trunnion that is mounted to the bottom of the valve bonnet. For API 6D valves, the typical construction is a cast pocket in the valve body with a drop in shaft/pin and bushing arrangement that the clapper will turn on. This traditional type of construction requires some sort of restraint to keep the clapper in the pocket. Newer designs include shaft/pin bosses that are drilled through with the shaft/pin inserted into the bosses to keep the clapper in place. Generally, large check valves are swing check valves.
- Ball check valves – Consist of a spherical ball clapper which is sometimes spring-loaded to provide sealing at pressures below the cracking pressure. Because of the spherical design, these valves can easily wear from prolonged use and require frequent maintenance; therefore, they should be installed in places that are easy for repair teams to access.
- Diaphragm check valves – Consist of a rubber diaphragm clapper that flexes open when the pressure on the upstream side is greater than the pressure on the downstream side, and closes when this pressure is equalized or lowered below a set pressure differential.
- Stop-check valves – Usually constructed similar to a swing check valve, stop-check valves have an additional external control mechanism (e.g., an actuator, lever/handwheel, etc.) that allows the valve to be deliberately closed regardless of flow pressure.
- Duckbill valve – This type of check valve allows flow to proceed through a soft tube that feeds into the downstream side of the valve, wherein backpressure collapses the tube and cuts off the flow.
The main advantage of a check valve is its simple design. Generally, check valves also are smaller in size and easier to install than other valve types, making maintenance easier and more efficient.
Check valves can be designed with specific non-slam features to reduce the noise inside a plant and wear to the seal. The Cameron ENTECH non-slam nozzle design is commonly selected for this feature in LNG pump stations as well as in gas compressor stations. Additionally, because of their simple, streamlined design, our line of TOM WHEATLEY swing check and ENTECH non-slam, nozzle check valves have lower pressure drop (less than 1 psi) than comparable piston check valves.
Because of their simple design and versatile material options, check valves are utilized in a variety of markets and applications.
“Check valves can be designed to API 600 or API 6D standards,” explains David Atkinson, Cameron check valve product manager. “Typically, API 600 designed check valves are used in the industrial market. These valves are usually reduced port, have special bolting and cover sealing, have angle seating, a two-piece clapper and arm bolted together, hard chrome trims and reference various wall thicknesses, and pressure and temperature ratings per ASME B16.34. API 600 valves, which are generally designed for use in the oil and gas industry, typically have removable seats and bonnet-to-body joint connections for seals, bonnet gaskets and cover bolting.”
Specialty valves for subsea operation at very high pressures and low temperatures can be designed into a system. The Cameron TOM WHEATLEY swing check valve features a unique, free-swinging clapper. If reverse flow or critical pigging operation is required, the clapper can be manually, ROV or hydraulically actuated, and held in the full open position.
Our NEWCO cast steel, pressure seal check valves are ideal for standard and critical power industry applications. The pressure seal bonnet joint eliminates the body/bonnet flanges, reducing weight and simplifying the application of exterior insulation. Contrary to bolted bonnet valves, internal pressure applied to a pressure seal valve forces the sealing elements into tighter contact – the higher the internal pressure, the tighter the seal. The tilt disc check valve design yields minimal restriction to low-velocity environments and is ideal for preventing backflow in unidirectional flow applications in horizontal flow. Additionally, the tilting disc design offers closing that reduces slamming.