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From Wikipedia, the free encyclopedia

Tilting-disc inconel check valve
Tilting-disc inconel check valve
Check valve symbol on piping and instrumentation diagrams. The arrow shows the flow direction.
Check valve symbol on piping and instrumentation diagrams. The arrow shows the flow direction.
Vertical lift check valve
Vertical lift check valve

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

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, 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 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.

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Transcription

A check-valve is an amazing little device that lets a liquid or a gas flow one way only. We'll need some to make a low cost water pump, but buying them can be a little pricy. So in this project we'll be making our own, out of common materials, and for less than a dollar. For this project, I'm using a 3/4" male PVC slip adaptor, 3/4" tubing, a 3/4" plastic ball, and an O-ring. I chose this adaptor because there's a little ledge on the inside where the O-ring can sit. And it should look like this. The ball needs to be large enough that it won't push through the throat, but small enough that it doesn't touch the sides. Attaching the tube at this point won't really do any good because the ball will roll right out. We need a way to secure it in position, so let's make a mark on our tube 5/8" from the bottom and use a 1/16" bit to drill a small hole in the tube, continuing right on through the other side. Now you'll need a strong piece of metal, like a nail or a thick paperclip. I'm using one of these brad nails that go to my nail gun. Whatever you use, it needs to be strong, and long enough to push all the way through the tube. The ends of the nail will need to be flush with the sides of the pipe, so we can go ahead and clip the end with the nail head on it, and now we've got ourselves a retaining bar that's perfectly flush on both sides. The next step is to prime our connections and place the O-ring into place inside the adaptor. Now let's drop in the ball and double check that everything is seated properly. PVC cement is added to the pipe, making sure it covers the holes where the nail is set. Then the pipe is pushed down to the point where the holes dip just below the rim of the adaptor so that when the glue sets, this connection will be completely air-tight. You can build this valve into any length of PVC pipe you want, but I'm using this 1-1/2" piece because it's just long enough to accept another adaptor. When it's glued and pushed together, it makes a nice little compact unit, with only a slight gap separating the two ends. I took the opportunity to make 4 of these valves and a visual inspection on the inside shows that the ball and O-ring are still seated properly in place, and the retention bar keeps it all together. To finish up, I gave these a quick paint job, and wrapped one end with a bit of electrical tape to indicate the direction of flow, similar to the marking you'd find on an electrical diode. A fluid can pass freely in one direction, but if the valve is turned around, the pressure closes the valve and the fluid is blocked. The quality of the seal can be tested by dipping the pressurized valve in a bowl of water. This will show us if any air is escaping at all. The pressure can be released though, by pushing up on the inside of the valve to break the seal between the ball and the O-ring. I tried using a couple of these valves to make a hand pump, which you'll be able to find in another project. It'll compress air, and move water at over 5 gallons per minute. You can even create enough suction to form a makeshift vacuum pump. The threaded ends of the valves make them easy to connect, replace or rearrange whenever needed. Just for curiosity, let's pressure-test the valve by hooking it up to an air compressor using this pneumatic adaptor that we made in the Rocket Rifle project. The system's set at 20PSI, and you can see the valve close off and prevent any air from leaking. When I crank it up to 50PSI, the pressure difference is a little more obvious by the compression of the O-ring, and I'm really impressed at how well it works. As an alternative, I found that a rubber bouncy ball could replace the entire valve mechanism for low pressure applications. Since the ball is rubber, you don't need an O-ring to seal the port. To make it work, just grab a sharp knife and a cutting board, and begin cutting the top third of the ball very slowly, until you've sliced right through it. Now re-insert the ball with the round side facing down, and simply slide the tube in overtop. Amazingly it's that easy, and my kids use it for blowing up balloons. Well there's how to make some useful pressure rated check-valves for as little as 35¢. If you like this project, perhaps you'll like some of my others. Check them out at www.thekingofrandom.com

Contents

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

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.

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.
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.[11]

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.[12] 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.[13][14]

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.[15]

Domestic use

When a sanitary potable water supply is plumbed to an unsanitary system, for example lawn sprinklers, 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

References

  1. ^ Christopher., Dickenson, T. (1999). Valves, piping, and pipelines handbook (3rd ed.). Oxford, UK: Elsevier Advanced Technology. ISBN 9781856172523. OCLC 41137607.
  2. ^ "NEOPERL". www.neoperl.net.
  3. ^ "GF PVC Cone Check Valves Type 561 & 562 - U.S. Plastic Corp". www.usplastic.com.
  4. ^ "NIBCO® Chemtrol® TruUnion Ball Check Valves - U.S. Plastic Corp". 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. ^ "How Water Slides Work". howstuffworks.com. 31 August 2001.
  12. ^ "Nuclear Check Valves - Excess Flow Check Valves - Valcor Nuclear". valcor.com.
  13. ^ "Aircraft Check Valves in Hydraulic Systems - Valcor". valcor.com.
  14. ^ "Aerospace Check Valves - Valcor Aerospace Division". valcor.com.
  15. ^ "Simple Check Valves for Microfluidic Devices" (PDF). NASA's Jet Propulsion Laboratory, Pasadena, California. NASA Tech Briefs. 2010-05-01. Retrieved 2017-10-26.

External links

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