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(November 2008)

Tilting disc inconel check valve

Check valve symbol onP&IDs.The arrow shows the flow direction.

A check valve, clack valve, non-return valve orone-way valve is amechanical device, avalve, which normally allowsfluid(liquidorgas) to flow through it in only one direction.

Check valves are two-port valves, meaning they have two openings in the body, one for fluid t o enter and the other forfluidto leave. There are various types of check v alves used in a wide variety of applications. Check valves are often part of common household items. Although they are available in a

wide range of sizes and costs, check valves generally are very small, simple, and/or inexpensive. Check valves work automatically and most are not controlled by a person or any external control; accordingly, most do not have any valve handle or stem. The bodies (external shells) of most check v alves

are made of plastic or metal.

An important concept in check valves is the crackingpressurewhich is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.

Heart valvesare essentially inlet and outlet check valves for theheartventricles, since the ventricles act aspumps.

Contents

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1 Types of check

valves

2 Applications

o

2.1 Pumps

o

2.2 Industrial processes

o

2.3 Domestic use

3 Hist ory

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4 Image s

5 See also

6 References

7 External links

[edit]Types of check valves

A ball check valve in the open position to allow forward flow and closed position to block reverse flow

Backwater valve (for sanitary drainage system) protects lower located rooms against flooding caused by return flow of sewage waters. Such risk occurs most often in sanitary drainage systems connected to combined sewerage systems and in rainwater drainage systems. It may be caused by intense

rainfall, thaw or flood. Backwater valve prevents rats and other rodents entering the sanitary and rainwater drainage systems and, consequently, the building interiors. It protects also against unpleasant smells in case of longer breaks in system use.

A ball check valve is a check valve in which the closing member, the movable part to block the flow, is a spherical ball. In some ball check valves, the ball is spring-loaded to help keep it shut. For those designs without a spring, reverse flow is required to move the ball toward the seat and create a

seal. The interior surface of the main seats of ball check valves are more or less conically-tapered to guide the ball into the seat and form a positive seal when stopping reverse flow.

Ball check valves are often very small, simple, and cheap. They are commonly used in liquid or gel minipump dispenser spigots, spray devices, some rubber bulbs for pumping air, etc., manual air pumps and some otherpumps, and refillable dispensing syringes. Although the balls are most often made

of metal, they can be made of other materials, or in some specialized cases out of artificialruby. High pressureHPLCpumpsand similar applications commonly use small inlet and outlet ball c heck valves with both balls and seats made of artificial ruby, for both hardness and chemical resistance. After

prolonged use, such check valves can eventually wear out or the seat can develop a crack, requiring replacement. Therefore, such valves are made to be replaceable, sometimes placed in a small plastic body tightly-fitted inside a metalfittingwhich can withstand high pressure and which is screwed

into the pump head.

There are similar check valves where the disc is not a ball, but some other shape, such as a poppetenergized by a spring. Ball check valves should not be confused withball valves, which is a different type of valve in which a ball acts as a controllable rotor to stop or direct flow.

A diaphragm check valve uses a flexing rubber diaphragm positioned to create a normally-closed valve. Pressure on the upstream side must be greater than the pressure on the downstream side by a certain amount, known as the pressure differential, for the check valve to open allowing flow. Once

positive pressure stops, the diaphragm automatically flexes back to its original closed position.[1]

A swing check valve ortilting disc check valve is check valve in which the disc, the movable part to block the flow, swings on ahingeortrunnion , either onto the seat to block reverse flow or off the seat to allow forward flow. The seat opening cross-section may be perpendicular to the centerline

between the two ports or at an angle. Although swing check valves can come in various sizes, large check valves are often swing check valves. The flapper valve in aflush-toilet mechanism is an example of this type of valve. Tank pressure holding it closed is overcome by manual lift of the flapper. It

then remains open until the tank drains and the flapper falls due to gravity. Another variation of this mechanism is the clapper valve, used in applications suchfirefightingand fire life safety systems. A hinged gate only remains open in the inflowing direction. The clapper valve often also has a spring that

keeps the gate shut when there is no forward pressure.

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A stop-check valve is a check valve with override control to stop flow regardless of flow direction or pressure. In addition to closing in response to backflow or insufficient forward pressure (normal check-valve behavior), it can also be deliberately shut by an external mechanism, thereby preventing any

flow regardless of forward pressure.

A lift-check valve is a check valve in which the disc, sometimes called a lift, can be lifted up off its seat by higher pressure of inlet or upstream fluid to allow flow to t he outlet or downstream side. A guide keeps motion of the disc on a vertical line, so the valve can later reseat properly. When the

pressure is no longer higher, gravity or higher downstream pressure will cause the disc to lower onto its seat, shutting the valve to stop reverse flow.

Aduckbill valve is a check valve in which flow proceeds through a soft tube that protrudes into the downstream side. Back-pressure collapses this tube, cutting off flow.

Multiple check valves can be connected in series. For example, adouble check valveis often used as abackflow prevention deviceto keep potentially contaminated water fromsiphoning back intomunicipalwatersupply lines. There are also double ball check valves in which there are two ball/seat

combinations sequentially in the same body to ensure positive leak-tight s hutoff when blocking reverse flow; and piston check valves, wafer c heck valves, and ball-and-cone check valves.

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