Check valve

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Tilting-disc inconel check valve Inconel-Tilting disc check valve--The-Alloy-Valve-Stockist.JPG
Tilting-disc inconel check valve
Check valve symbol on piping and instrumentation diagrams. The arrow shows the flow direction. Check Valve.svg
Check valve symbol on piping and instrumentation diagrams. The arrow shows the flow direction.
Vertical lift check valve Vertical check valve.jpg
Vertical lift check valve

A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid (liquid or gas) to flow through it in only one direction. [1]

Contents

Check valves are two-port valves, meaning they have two openings in the body, one for fluid to enter and the other for fluid to leave. There are various types of check valves 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 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 valves are made of plastic or metal.

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

Heart valves are essentially inlet and outlet check valves for the heart ventricles, since the ventricles act as pumps.

Technical terminology

Cracking pressure – Refers to the minimum pressure differential needed between the inlet and outlet of the valve at which the first indication of flow occurs (steady stream of bubbles). Cracking pressure is also known as unseating head (pressure) or opening pressure. [2]

Reseal pressure – Refers to the pressure differential between the inlet and outlet of the valve during the closing process of the check valve, at which there is no visible leak rate. Reseal pressure is also known as sealing pressure, [3] seating head [4] (pressure) or closing pressure. [5]

Back pressure – a pressure higher at the outlet of a fitting than that at the inlet or a point upstream

Types of check valves

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

A ball check valve is a check valve in which the closing member, the movable part to block the flow, is a 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 other pumps, and refillable dispensing syringes. Although the balls are most often made of metal, they can be made of other materials; in some specialized cases out of highly durable or inert materials, such as sapphire. High pressure HPLC pumps and similar applications commonly use small inlet and outlet ball check valves with balls of (artificial) ruby and seats made of sapphire [6] or both ball and seat of ruby, [7] 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 metal fitting which 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 poppet energized by a spring. Ball check valves should not be confused with ball 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. [8]

Swing check valve opening and closing

A swing check valve or tilting disc check valve is a check valve in which the disc, the movable part to block the flow, swings on a hinge or trunnion, 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. A common issue caused by swing check valves is known as water hammer. This can occur when the swing check closes and the flow abruptly stops, causing a surge of pressure resulting in high velocity shock waves that act against the piping and valves, placing large stress on the metals and vibrations in the system. Undetected, water hammer can rupture pumps, valves, and pipes within the system. [9]

The flapper valve in a flush-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 such firefighting and 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. Another example is the backwater valve (for sanitary drainage system) that protects 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.

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 the 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.

An in-line check valve is a check valve similar to the lift check valve. However, this valve generally has a spring that will 'lift' when there is pressure on the upstream side of the valve. The pressure needed on the upstream side of the valve to overcome the spring tension is called the 'cracking pressure'. When the pressure going through the valve goes below the cracking pressure, the spring will close the valve to prevent back-flow in the process. [10]

A duckbill 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.

A pneumatic non-return valve .

A reed valve is a check valve formed by a flexible flat sheet that seals an orifice plate. The cracking pressure is very low, the moving part has low mass allowing rapid operation, the flow resistance is moderate, and the seal improves with back pressure. These are commonly found in two stroke internal combustion engines as the air intake valve for the crankcase volume and in air compressors as both intake and exhaust valves for the cylinder(s). Although reed valves are typically used for gasses rather than liquids, the Autotrol brand of water treatment control valves are designed as a set of reed valves taking advantage of the sealing characteristic, selectively forcing open some of the reeds to establish a flow path. [11]

A flow check is a check valve used in hydronic heating and cooling systems to prevent unwanted passive gravity flow. A flow check is a simple flow lifted gravity closed heavy metal stopper designed for low flow resistance, many decades of continuous service, and to self-clean the fine particulates commonly found in hydroninc systems from the sealing surfaces. To accomplish self cleaning, the stopper is typically not conical. A circular recess in a weight that fits over a matching narrow ridge at the rim of an orifice is a common design. The application inherently tolerates a modest reverse leakage rate, a perfect seal is not required. A flow check has an operating screw to allow the valve to be held open, the opposite of the control on a stop-check valve, as an aide for filling the system and for purging air from the system. [12]

Multiple check valves can be connected in series. For example, a double check valve is often used as a backflow prevention device to keep potentially contaminated water from siphoning back into municipal water supply 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 shutoff when blocking reverse flow; and piston check valves, wafer check valves, and ball-and-cone check valves.

Applications

Pumps

The check valves on this steam locomotive are located under the small cover between the chimney and the main dome. Cricklewood 2 railway statio geograph-2206898-by-Ben-Brooksbank.jpg
The check valves on this steam locomotive are located under the small cover between the chimney and the main dome.

Check valves are often used with some types of pumps. Piston-driven and diaphragm pumps such as metering pumps and pumps for chromatography commonly use inlet and outlet ball check valves. These valves often look like small cylinders attached to the pump head on the inlet and outlet lines. Many similar pump-like mechanisms for moving volumes of fluids around use check valves such as ball check valves. The feed pumps or injectors which supply water to steam boilers are fitted with check valves to prevent back-flow.

Check valves are also used in the pumps that supply water to water slides. The water to the slide flows through a pipe which doubles as the tower holding the steps to the slide. When the facility with the slide closes for the night, the check valve stops the flow of water through the pipe; when the facility reopens for the next day, the valve is opened and the flow restarts, making the slide ready for use again. [13]

Industrial processes

Check valves are used in many fluid systems such as those in chemical and power plants, and in many other industrial processes.

Typical applications in the nuclear industry are feed water control systems, dump lines, make-up water, miscellaneous process systems, N2 systems, and monitoring and sampling systems. [14] In aircraft and aerospace, check valves are used where high vibration, large temperature extremes and corrosive fluids are present. For example, spacecraft and launch vehicle propulsion propellant control for reaction control systems (RCS) and Attitude Control Systems (ACS) and aircraft hydraulic systems. [15] [16]

Check valves are also often used when multiple gases are mixed into one gas stream. A check valve is installed on each of the individual gas streams to prevent mixing of the gases in the original source. For example, if a fuel and an oxidizer are to be mixed, then check valves will normally be used on both the fuel and oxidizer sources to ensure that the original gas cylinders remain pure and therefore nonflammable.

In 2010, NASA's Jet Propulsion Laboratory slightly modified a simple check valve design with the intention to store liquid samples indicative to life on Mars in separate reservoirs of the device without fear of cross contamination. [17]

Domestic use

When a sanitary potable water supply is plumbed to an unsanitary system, for example lawn sprinklers, a dish washer or a washing machine, a check valve called a backflow preventer is used to prevent contaminated water from re-entering the domestic water supply.

Some types of irrigation sprinklers and drip irrigation emitters have small check valves built into them to keep the lines from draining when the system is shut off.

Check valves used in domestic heating systems to prevent vertical convection, especially in combination with solar thermal installations, also are called gravity brakes.

Rainwater harvesting systems that are plumbed into the main water supply of a utility provider may be required to have one or more check valves fitted to prevent contamination of the primary supply by rainwater.

Hydraulic jacks use ball check valves to build pressure on the lifting side of the jack.

Check valves are commonly used in inflatables, such as toys, mattresses and boats. This allows the object to be inflated without continuous or uninterrupted air pressure.

History

Frank P. Cotter developed a "simple self sealing check valve, adapted to be connected in the pipe connections without requiring special fittings and which may be readily opened for inspection or repair" 1907 (U.S. patent 865,631).

Nikola Tesla invented a deceptively simple one-way valve for fluids in 1916, called a Tesla valve. It was patented in 1920 (U.S. patent 1,329,559).

Images

See also

Related Research Articles

Pump Device that imparts energy to the fluids by mechanical action

A pump is a device that moves fluids, or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they use to move the fluid: direct lift, displacement, and gravity pumps.

Valve Device that controls the flow of a fluid

A valve is a device or natural object that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.

Diaphragm pump

A diaphragm pump is a positive displacement pump that uses a combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either side of the diaphragm (check valve, butterfly valves, flap valves, or any other form of shut-off valves) to pump a fluid.

Fuel pump Pump

A fuel pump is a component in motor vehicles that transfers liquid from the fuel tank to the carburetor of the internal combustion engine.

Ball valve

A ball valve is a form of quarter-turn valve which uses a hollow, perforated and pivoting ball to control flow through it. It is open when the ball's hole is in line with the flow and closed when it is pivoted 90-degrees by the valve handle. The handle lies flat in alignment with the flow when open, and is perpendicular to it when closed, making for easy visual confirmation of the valve's status. The shut position 1/4 turn could be in either CW or CCW direction.

A flow control valve regulates the flow or pressure of a fluid. Control valves normally respond to signals generated by independent devices such as flow meters or temperature gauges.

Butterfly valve

A butterfly valve is a valve that isolates or regulates the flow of a fluid. The closing mechanism is a disk that rotates.

Relief valve

A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. The pressure is relieved by allowing the pressurized fluid to flow from an auxiliary passage out of the system. The relief valve is designed or set to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the set pressure is exceeded, the relief valve becomes the "path of least resistance" as the valve is forced open and a portion of the fluid is diverted through the auxiliary route. In systems containing flammable fluids, the diverted fluid is usually routed through a piping system known as a flare header or relief header to a central, elevated gas flare where it is usually burned and the resulting combustion gases are released to the atmosphere. In non-hazardous systems, the fluid is often discharged to the atmosphere by a suitable discharge pipework designed to prevent rainwater ingress which can affect the set lift pressure, and positioned not to cause a hazard to personnel. As the fluid is diverted, the pressure inside the vessel will stop rising. Once it reaches the valve's reseating pressure, the valve will close. The blowdown is usually stated as a percentage of set pressure and refers to how much the pressure needs to drop before the valve reseats. The blowdown can vary from roughly 2–20%, and some valves have adjustable blowdowns.

Globe valve Type of device for blocking or regulating the flow of fluids

A globe valve, different from ball valve, is a type of valve used for regulating flow in a pipeline, consisting of a movable plug or disc element and a stationary ring seat in a generally spherical body.

Solenoid valve

A solenoid valve is an electromechanically-operated valve.

Piston pump

A piston pump is a type of positive displacement pump where the high-pressure seal reciprocates with the piston. Piston pumps can be used to move liquids or compress gases. They can operate over a wide range of pressures. High pressure operation can be achieved without a strong effect on flow rate. Piston pumps can also deal with viscous media and media containing solid particles. This pump type functions through a piston cup, oscillation mechanism where down-strokes cause pressure differentials, filling of pump chambers, where up-stroke forces the pump fluid out for use. Piston pumps are often used in scenarios requiring high, consistent pressure and in water irrigation or delivery systems.

Hydropneumatic devices such as hydropneumatic accumulators or pulsation dampeners are devices which prevent, but do not absorb, alleviate, arrest, attenuate, or suppress a shock that already exists, meaning that these devices prevent the creation of a shock wave at an otherwise earlier stage. These can include pulsation dampeners, hydropneumatic accumulators, water hammer preventers, water hammer arrestors, and other things.

Metering pump

A metering pump moves a precise volume of liquid in a specified time period providing an accurate volumetric flow rate. Delivery of fluids in precise adjustable flow rates is sometimes called metering. The term "metering pump" is based on the application or use rather than the exact kind of pump used, although a couple types of pumps are far more suitable than most other types of pumps.

Artificial lift refers to the use of artificial means to increase the flow of liquids, such as crude oil or water, from a production well. Generally this is achieved by the use of a mechanical device inside the well or by decreasing the weight of the hydrostatic column by injecting gas into the liquid some distance down the well. A newer method called Continuous Belt Transportation (CBT) uses an oil absorbing belt to extract from marginal and idle wells. Artificial lift is needed in wells when there is insufficient pressure in the reservoir to lift the produced fluids to the surface, but often used in naturally flowing wells to increase the flow rate above what would flow naturally. The produced fluid can be oil, water or a mix of oil and water, typically mixed with some amount of gas.

A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.

Pressure regulator Control valve that maintains the pressure of a fluid or gas

A pressure regulator is a valve that controls the pressure of a fluid or gas to a desired value. Regulators are used for gases and liquids, and can be an integral device with a pressure setting, a restrictor and a sensor all in the one body, or consist of a separate pressure sensor, controller and flow valve.

Instrumentation is used to monitor and control the process plant in the oil, gas and petrochemical industries. Instrumentation comprises sensor elements, signal transmitters, controllers, indicators and alarms, actuated valves, logic circuits and operator interfaces.

A microvalve is microscale valve, i.e. a microfluidic two-port component that regulates the flow between two fluidic ports. Microvalves are basic components in microfluidic devices, such as Labs-on-a-chip, where they are used to control the fluidic transport. During the period 1995-2005, a lot of Microelectromechanical systems-based microvalves were developed.

The mechanism of diving regulators is the arrangement of components and function of gas pressure regulators used in the systems which supply breathing gases for underwater diving. Both free-flow and demand regulators use mechanical feedback of the downstream pressure to control the opening of a valve which controls gas flow from the upstream, high-pressure side, to the downstream, low-pressure side of each stage. Flow capacity must be sufficient to allow the downstream pressure to be maintained at maximum demand, and sensitivity must be appropriate to deliver maximum required flow rate with a small variation in downstream pressure, and for a large variation in supply pressure, without instability of flow. Open circuit scuba regulators must also deliver against a variable ambient pressure. They must be robust and reliable, as they are life-support equipment which must function in the relatively hostile seawater environment, and the human interface must be comfortable over periods of several hours.

References

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  2. "NEOPERL". www.neoperl.net.
  3. U.S. Plastic Corp. "GF PVC Cone Check Valves Type 561 & 562". www.usplastic.com.
  4. U.S. Plastic Corp. "NIBCO® Chemtrol® TruUnion Ball Check Valves". www.usplastic.com.
  5. "NEOPERL". www.neoperl.net.
  6. "Chromatography on line, from J.W. Dolan, LCGC North Am. 26(6), 532–538 (2008)".
  7. "Industrial valve store>>Valves>>Check Valves, paragraph on ball check valves".
  8. Wright, Stephen. "Norval valve performance". Northvale Korting. Archived from the original on 2009-04-27. Retrieved 2009-05-19.
  9. "Check Valves". DFT Valves. Retrieved 2017-10-26.
  10. Fleming, Jennifer. "ValveMan Blog". ValveMan Brand VM6800 In-Line Check Valve- Product Overview. ValveMan LLC. Archived from the original on 2012-06-10. Retrieved 1 August 2012.
  11. "Autotrol". pentairaquaeurope.com.
  12. "Flow Check Valves". www.watts.com.
  13. "How Water Slides Work". howstuffworks.com. 31 August 2001.
  14. Valcor Nuclear. "Nuclear Check Valves – Excess Flow Check Valves". valcor.com.
  15. Valcor. "Aircraft Check Valves in Hydraulic Systems". valcor.com.
  16. Valcor Aerospace Division. "Aerospace Check Valves". valcor.com.
  17. "Simple Check Valves for Microfluidic Devices" (PDF). NASA's Jet Propulsion Laboratory, Pasadena, California. NASA Tech Briefs. 2010-05-01. Retrieved 2017-10-26.