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How does it work: Check Valves

Check valves, also known as non-return or one-way valves, are designed to allow fluid toflow 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 thevalve body. The basic design of a check valve inhibits backflow in a line.

Because of their simple design, check valves generally operate without automation orhuman 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 orlever. The minimum upstream pressure required to operate the valve is called the“cracking pressure”. Check valves are generally designed specifically with this number inmind 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 theflow rate, the more open the valve will be until it reaches its maximum, full openposition. 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 isconvex on the upstream inlet side and flat on the downstream outlet side. For API600 valves, this disc swings on a hinge or trunnion that is mounted to the bottom ofthe valve bonnet. For API 6D valves, the typical construction is a cast pocket in thevalve body with a drop in shaft/pin and bushing arrangement that the clapper willturn on. This traditional type of construction requires some sort of restraint to keepthe clapper in the pocket. Newer designs include shaft/pin bosses that are drilledthrough 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 thespherical design, these valves can easily wear from prolonged use and requirefrequent maintenance; therefore, they should be installed in places that are easy forrepair teams to access.

Diaphragm check valves – Consist of a rubber diaphragm clapper that flexes openwhen the pressure on the upstream side is greater than the pressure on thedownstream side, and closes when this pressure is equalized or lowered below a setpressure differential.

Stop-check valves – Usually constructed similar to a swing check valve, stop-checkvalves 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 tubethat feeds into the downstream side of the valve, wherein backpressure collapsesthe tube and cuts off the flow.

Features

The main advantage of a check valve is its simple design. Generally, check valves also aresmaller in size and easier to install than other valve types, making maintenance easierand more efficient.

Check valves can be designed with specific non-slam features to reduce the noise inside aplant and wear to the seal. Cameron’s ENTECH™ non-slam nozzle design is commonlyselected for this feature in LNG pump stations as well as in gas compressor stations.Additionally, because of their simple, streamlined design, Cameron’s line of TOMWHEATLEY® swing check and ENTECH non-slam, nozzle check valves have lowerpressure drop (less than 1 psi) than comparable piston check valves.

Applications

Because of their simple design and versatile material options, check valves are utilized ina 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 areused in the industrial market. These valves are usually reduced port, have special boltingand cover sealing, have angle seating, a two-piece clapper and arm bolted together,hard chrome trims and reference various wall thicknesses, and pressure and temperatureratings per ASME B16.34. API 600 valves, which are generally designed for use in the oiland gas industry, typically have removable seats and bonnet-to-body joint connectionsfor seals, bonnet gaskets and cover bolting.”

In many power plant operations, check valves are selected for piping and pump design tosave space and weight. Cameron’s TECHNO™ elastomer-hinged check valves includean unrestricted full-port seatless design, tight shutoff, and the ability to be mounted inalmost any position. This makes them ideal in industrial processes for controlling liquids,gases and air for pumps and blowers, and for process separation.

Specialty valves for subsea operation at very high pressures and low temperatures can bedesigned into a system. Cameron’s TOM WHEATLEY swing check valve features aunique, 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 openposition.

Cameron’s NEWCO® cast steel, pressure seal check valves are ideal for standardand critical power industry applications. The pressure seal bonnet joint eliminates thebody/bonnet flanges, reducing weight and simplifying the application of exteriorinsulation. Contrary to bolted bonnet valves, internal pressure applied to a pressure sealvalve 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-