Knowledge of check valves

What is a gate valve?

Check Valve, also known as a one-way valve or Check Valves. Pipeline used to prevent reflux of a fluid valve. So pipeline fluid can only make one-way flow of fluid when the clock back, the flap valve can be shut down automatically to prevent backflow. According structure can be divided into two categories : elevating and spin-Kai (or a shake-flap). Former perpendicular to the valve flap valve passageway movements with Valve are generally designed as a cut-off valve and valve common. The latter is a flap valve around the sealing surface for rotating movement, resistance than elevating small, poor sealing performance.

Check valves are probably the most misunderstood valves ever invented. If you mention check valves to most plant personnel, the typical response is “they don’t work.” In fact, those personnel may well have taken out the internals or re-piped the system to avoid check valves. In other words, these valves are the least popular valve in use today.

This article will explore the basics of check valves, how they work, what types there are, how to select and install them, how to solve their problems, and, finally, why they are not always the cause of the problem.

Simply put, a check valve allows flow in one direction and automatically prevents back flow (reverse flow) when fluid in the line reverses direction. They are one of the few self-automated valves that do not require assistance to open and close. Unlike other valves, they continue to work even if the plant facility loses air, electricity, or the human being that might manually cycle them.

Check valves are found everywhere, including the home. If you have a sump pump in the basement, a check valve is probably in the discharge line of the pump. Outside the home, they are found in virtually every industry where a pump is located.

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Like other valves, check valves are used with a variety of media: liquids, air, other gases, steam, condensate, and in some cases, liquids with fines or slurries. Applications include pump and compressor discharge, header lines, vacuum breakers, steam lines, condensate lines, chemical feed pumps, cooling towers, loading racks, nitrogen purge lines, boilers, HVAC systems, utilities, pressure pumps, sump pumps, wash-down stations, and injection lines.

Operation principle of check valve

Check valve allows flow in one direction and prevents flow in the reverse direction. Check valves are available with different spring rates to give particular cracking pressures.

The simplest type of directional control valve is the non-return or check valve which allows flow in one direction and prevents flow in the reverse direction. Such a valve, its symbols and characteristic curve is shown in Figure 1.

Check valves are available with different spring rates to give particular cracking pressures. The cracking pressure is that at which the check valve just opens. If a specific cracking pressure is essential to the functioning of a circuit, it is usual to show a spring on the check valve symbol. The pressure drop over the check valve depends upon the flow rate; the higher the flow rate, the further the ball or poppet has to move off its seat and so the higher the spring force.

Ball-type check valves have the least expensive form of construction, but as the ball is not guided there is a tendency for leakage to occur. Although manufacturers claim their check valves are leak-free in one direction of flow and allow free flow in the reverse direction, a tiny scratch, wear mark ,or imperfection on the poppet or scat will permit some leakage. Soft-seated check valves use Delrin or similar polymer material for the waling and 100% scaling is possible but at the expense of valve life. However, they are no generally suitable for pressures above 200 bar or temperatures above 35°C. Valves which seal satisfactorily at high pressure may leak at lower pressures. At high pressure, the poppet is forced onto the seat hydraulically giving a good seal; at low pressures, the sealing force is less and the valve may leak.

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Figure 1: Poppet-type check valve with symbols and curves.

Pilot-operated check valves

These are normally closed check valves which may be opened by a pilot signal or less commonly held closed by a pilot signal. The pilot pressure needed to open the check valve against a load pressure depends upon the ratio of the areas of the pilot piston and check valve. A pilot-operated check valve is shown in Figure 2 (a). Most manufacturers offer I a range of pilot ratios, i.e. if the pilot ratio is 4:1, the pilot pressure required to open the valve is 25% of the load pressure. A typical application is shown in the circuit in Figure 2 (b) where a pilot-operated check valve is used to lock in pressure to prevent a load from falling. With a long stroke cylinder the lowering motion of the load may be jerky. If the load overruns, the pressure in the full bore end of the cylinder drops, the check valve closes and the cylinder jerks to a stop. The pressure at the full bore end increases, the check opens, the cylinder lowers the load, the load overruns and so on. This problem can be overcome by using:

1. A meter-out flow control valve to limit the cylinder speed;

2. A counterbalance valve to prevent overrun; or

3. An over-center valve.

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Figure 2: Pilot-operated check valve. (a) Section. (b) Application.

With the directional control valve in the mid-position and the load raised, there will be a tendency for the pilot-operated check valve to leak at low loads, since the hydraulic sealing force on the check valve poppet is also reduced. Zero leakage is possible by using soft-seated versions of the valve.

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Figure 3: Vented pilot-operated check valve.

Pressure on the pilot port X of the pilot-operated check valve shown in Figure 2 has not only to overcome the closing pressure which is present at cylinder port C but is also sensitive to any back-pressure at the valve port V. This can be overcome as shown in Figure 3 by incorporating a seal on the pilot stem and a separate vent or drain connection D for the spring chamber. Any back-pressure at port V will assist the pilot to open the valve.

Prefill valves

Prefill valves are basically large pilot-operated check valves. They are used in hydraulic press circuits to prefill the main cylinder with fluid whilst the press dies are being closed. The valve shown diagrammatically in Figure 4 is similar to a large pilot-operated check valve both in construction and operation but incorporates a decompression feature.

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Figure 4: Prefill valve with decompression feature.

Hydraulic fluids are compressible to varying degrees and the volume of free fluid compressed into a cylinder is greater than its internal capacity. For example, in a cylinder having an internal volume of 0.3 m3 , approximately 0.31 m3 of a typical mineral oil hydraulic fluid at atmospheric pressure will compress into the cylinder at 400 bar. (This quantity will be much greater if the oil is aerated). Special valves have to be employed to control the decompression of large cylinders because the additional fluid (10 liters in this case) will attempt to discharge instantaneously resulting in extremely high shock forces.

The decompression feature incorporated in the prefill valve (Figure 4) is composed of small poppet built within the main poppet. When the valve is piloted open by a pat port X, the main poppet is initially held firmly on its seat by pressure within the cylinder .The first part of the movement operates the pilot poppet opening up a small flow path facilitating a controlled decompression. Further movement opens up the main poppet and the valve functions as a normal pilot-operated check.

Figure 5 is a press circuit which utilizes a prefill valve. Operating directional control valve. A to the crossover condition initiates the closing of the dies. The side cylinder B drives down the main ram C and fluid from the reservoir which is mounted above the press,is sucked in through the prefill valve D to charge the full bore end of C. As the dies close onto the work piece, pressure builds up opening the sequence valve E and flow from the pump pressurizes the full bore end of the main cylinder. During the pressing operation the prefill valve isolates the cylinder from the reservoir. On the retract stroke, (valve A in tramline condition) the prefill valve is piloted open and as the side cylinder pulls back the main ram, fluid from the full bore end is pushed into the reservoir. Using a prefill valve in this manner enables rapid movement of a large bore cylinder from a small delivery circuit.

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Figure 5: Press circuit utilizing a prefill valve.

Pilot to close check valves

In the valve shown in Figure 6(a), application of sufficient pilot pressure at port X prevents flow through the check in either direction. At other times the valve performs as a normal check, valve with free flow one way (B to A) and blocked flow in the opposite direction (A to B). A representative application could be as a safety valve. In Figure 6(b) if pressure is lost in circuit number 1, circuit number 2 exhausts immediately.

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Figure 6: Pilot to close check valve. (a) Section. (b) Application.

Sandwich plates

Both check valves and pilot-operated check valves are manufactured as single or double units in sandwich plates for mounting in a valve stack between the directional control valve and base plate.

Restrictor checks

Check valve are available with small holes through or bypassing the poppet to give a controlled leakage rate in the normally blocked direction. Such a valve may be pilot as a safety feature in tome circuits, or to give u pilot supply through the closed check to the downstream circuit.

Shuttle valve

The shuttle valve is a single-ball check valve with two alternative inputs A and B and one output C. It is used for load-sensing and will accept a signal from the higher of two pressure inputs.

A typical application is in a reversible brake motor circuit (Figure 7) where used to release the brake when the motor is driven in either direction.

A double-ball shuttle valve or back-to-back check valve is able to sense signals from different inputs but prevents pressure feedback or interaction from other circuits . Care has to be taken in its use as it is possible to ‘lock in’ a pressure signal on the output side.

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Figure 7: Shuttle valve in reversible brake motor circuir.

So what is the ideal check valve?

Regardless of type or style of valve, the longest trouble-free service will come from valves sized for the application, not the line size, whereby the disc is stable against the internal stop in the open position or fully closed. When these conditions are met, no fluttering of the disc will occur. Unfortunately, most check valves are selected in the same way on/off valves are selected: based on line size and the desire for the largest Cv available. This ignores the fact that, unlike on/off valves, the flow conditions determine the internal performance of the check valve since its disc is always in the flow stream.

As mentioned earlier, unlike on/off valves, check valve internals are flow sensitive. If there is not enough flow, disc movement occurs inside the valve since the disc is always in the flow path. This results in wear, potential for failure, and a higher pressure drop than calculated.

Whenever a metal part rubs against another metal part, wear is a result, which leads to eventual failure of the component. A component failure can result in the valve not performing its function, which in the case of a check valve is to prevent reverse flow. In extreme cases, failure could result in the component or components escaping into the line, causing failure or nonperformance of other valves or equipment in the line.

Typically, pressure drop is calculated based on the check valve being 100% open, as with on/off valves. However, if the flow is not sufficient to achieve full opening and the check valve is only partially open, the pressure drop will be greater than calculated since the flow passage is restricted by the disc being in the flow path. In this situation, a large-rated Cv actually becomes detrimental to the check valve (unlike with on/off valves), resulting in fluttering of the disc and eventual failure. Such is not the case with some other valves. With a gate valve, for example, if the valve is fully open, the wedge is out of the flow path and the flow through the valve does not affect the performance of the wedge whether that flow is low, medium, or high.

Types of check valves

There are many types of check valves in use today. Some of the more popular types include: ball; dual plate check valve or double-door; spring assisted in-line or nozzle or silent; piston or lift; and swing checks. As with other types of valves, specialty check valves can be found for special applications. While no one type of valve is good for all applications, each has its advantages. Take time to contact the manufacturer to assist in selection of the best check valve, especially if you are incurring problems with whatever type of check valve that is currently installed.

Parts of a check valve

Check valves are generally used in pipes and other similar devices or equipment to enable the materials or fluids to flow in only one direction while preventing a reverse flow from happening. Although this control valve comes in different forms, sizes, and functions, they typically operate on the same principle. Likewise, most of these valves have 3 common parts or components:

1. The inward port that allows entry to the material (gas or liquid).

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2. The body through which the material passes.

3. The outward port which allows the material to exit the device.

Various technologies are employed to regulate the inward and outward flow of gases and liquids. Although some parts may be similar, these are not interchangeable between types of valve controls as they differ in function, size and materials. Here are various types of technologies used and the components or parts that make the valves work:

  • Single Disc: This is designed with the closure disc hinged at the top of the cap and can be pushed open by the flow. It automatically closes once the flow reverses. This type of valve is not recommended for applications where flow reversals are frequent because the disc will wear out easily with the constant pounding it will get. The single disc check valve may be set up either in a vertical or horizontal position. However, when it is vertically mounted, there is a tendency for the valve to stay open when there are gradual velocity changes. A lever or a counter balance must be used to neutralize this tendency.
  • Double Disc or Wafer: This uses 2 half circular discs that are attached to each other. They fold together with forward flow and closes to full circle when there is reverse flow. This is usually installed between two flanges and most commonly used in liquid dispensers. It is popular because it is inexpensive and compact.
  • Lift Type: Lift type non-return valves have a guided disc. It may be spring loaded that can work whether it is horizontally or vertically installed. If the device has no spring, mounting should be done in such a way that gravity will enable the disc to close. This is commonly used for granular liquid dispensers.
  • Ball Type: This uses either a spring loaded or a free floating ball nestled on a seat that serves as the closure component. When the flow reverses, the ball settles back to its seat to prevent the back flow. This is one of the most popular types and mentioned in any check valve selection guide. It can be used in most applications and will work even for liquids that leave sticky deposits. Because the ball moves freely, there is even distribution of pressure and the valve seat is kept clean.
  • Silent or Center Guide: These is similar to a lift check valve and has a center guide that extends from the inward port to the outward port. This type features a stopper made of spring and bushing that is specifically designed to minimize noise created by its movements.

How to select the right check valve

A Check valve is normally used in piping systems to decrease the flow reversal and allow the material, which may be gas or liquid, to flow freely in just one direction. These devices are automatically activated when liquid flows in the pipeline. They can also be installed in some systems that tend to overheat or get contaminated as a result of reverse flows; these contraptions are then used to prevent such occurrences. When selecting the most appropriate check valve type to use in your pipe lines, there are a few things that need to be considered.

Depending on the purpose, clack valves come in a variety of types such as swing, ball, double, clapper, disk, solenoid and dual disc types, and many others. When selecting the most appropriate type to use in your pipe lines, there are a few things that need to be considered. Here are a few guides to help you select the right valve for your particular need:

1. Know the available check valve types: Before buying, it is advisable to know first the various types and the common applications they are used for. This will give you a clearer picture of what you really need and what type will be most appropriate to use.

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  • Swing Check Valve: This is a type of pressure-reducing valve that is normally used for systems with gate valves. It has a movable disc that regulates the flow of liquid.
  • Double Check Valve: Commonly used in water supply lines, this type of control valve prevents the reverse flow of liquid and prevents possible contaminants from getting into the lines.
  • Ball Type: This valve type uses a ball-like component to block the liquid flow. Usually small and inexpensive, this can be seen in spray devices, refillable dispensing syringes, manual air pumps, and other similar equipment or devices. As this type is more prone to the rigors of wear and tear, it is designed for convenient installation and replacement.
  • Diaphragm: This type utilizes a flexible rubber diaphragm that is strategically located to allow for a usually-closed valve. The valve works when the upstream pressure is greater than that on the downstream. This pressure differential will force the valve to open to allow the outflow. The diaphragm will automatically close when the pressure stops.
  • Clapper: This type is similar to the diaphragm type. It features a hinged gate that is open when the material is flowing in an outward direction and is most commonly used in dry pipe, and fire safety systems.

2. Know the manufacturer: As there are so many types of non-return valves available, it can sometimes be difficult to select and decide on which one to go for. Sometimes, valve control manufacturers produce only one or two specific types with various models, sizes and specific functions. These manufacturers who specialize only in specific models usually produce the best brands available in the market. Of course, there are exceptions but to simplify the selection process, it may be advisable to identify certain check valve types with specific manufacturers. Initially, you can ask the store attendant for the best brands and manufacturers or you can do some research on your own through the internet and look for check valve reviews.

Problem Solving

When replacing a check valve it helps to ask the following simple questions:

  • “Why am I replacing this valve?”
  • “What is the problem?”
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Check valve at a compressor station.

Sometimes we get so busy or absorbed in other things, we forget that the cause can help with the solution. Common check valve problems include noise (water hammer), vibration, reverse flow, sticking, leakage, missing internals, component wear, or damage. However, it is worth mentioning that normally the real cause is the wrong style check valve for the application. In such cases, the problem is the application, not the check valve.

Two of the most common problems with check valves are reverse flow and water hammer. In both situations, a fast-closing valve is desired. Reverse flow can be costly, especially if it occurs at the discharge of a pump and the pump spins backwards. The cost to repair or replace the pump, plus the plant downtime, far exceeds the cost of installing the right check valve in the first place. With water hammer, you need a faster-closing check valve to prevent pressure surges and the resulting shock waves that occur when the disc slams into the seat, sending noise, vibration, and hammering sounds that can rupture pipelines and damage equipment and pipe supports.

If the internals are missing or exhibiting wear, two factors may be occurring. First, if the check valve selected does not have enough flow passing through to keep it against its stop, a valve with a lower Cv is needed to prevent the moving/fluttering of the internals. Second, if the check valve is used at the discharge of a reciprocating air or gas compressor, a valve with a damped design or dashpot to handle high-frequency cycling is needed.

Sticking can occur when scale or dirt is trapped between the disc and body bore. Leakage can happen from damage to the seat or disc or simple trash in the line. An elastomer is needed to provide zero leakage.

Installation of check valves

This sounds simple, but when installing check valves, point the “flow arrow” in the direction of the flow to allow the valve to perform its function. The flow arrow can be found on the body or tag. Make sure the valve type will work in the installed position. For example, not all check valves will work in a vertical line with flow down, nor will conventional or 90-degree piston check valves perform in a vertical line without a spring to push the disc back into the flow path. The disc in some check valves extends into the pipeline when the valves are fully open. This could interfere with the performance of another valve bolted directly to the check valve. If possible, install the check valve a minimum of five pipe diameters downstream of any fitting that could cause turbulence. Notice the phrase “if possible.” After all, how many check valves have you seen bolted to the discharge of a pump? Many! A good source of reference for installing check and other styles of valves is MSS SP-92 “Valve Users Guide,” published by the Manufacturers Standardization Society.

How to maintain a check valve

The best way to maintain a check valve properly is follow the recommendations of the manufacturer especially where repairs or replacements are involved.

For the proper maintenance of your installed check valve, it is always best to follow the recommendations of the manufacturer especially where repairs or replacements are involved. This is because different products are made of different materials and parts; and therefore, must be maintained differently. However, there are some common maintenance steps that you can undertake on your own to compliment the proper procedures prescribed by the manufacturer.

The control valves installed in your basement or any part of your home that are installed as protection from sewage backflows must be regularly inspected and tested just to make sure that they are working properly. As in any other mechanical devices, the valves are also subject to normal wear and tear. Regular maintenance checks will provide the opportunity to identify possible problems so they can immediately be addressed before they get worse.

Refer to the manual that came with the product for the inspection and testing procedures, intervals, and replacement of parts if necessary. If there are any procedures which you are not comfortable in doing yourself, you may seek the help of a contractor to do the job for you. This is much better than taking the risk of doing more damage to your one way valve assemblies.

  • The Check valve must be inspected ideally every six months (or as often as the manufacturer may recommend); or when heavy rains are forthcoming; or if you are leaving your home for a long period of time either for a vacation, a work engagement, or for any other reason. Visual inspection must be performed and the valve assemblies must be cleaned.
  • Remove excessive debris and dirt that may affect the seal quality. Clean the affected parts. If required for proper washing, remove then reinstall. Make sure you wear protective gloves before exposing yourself to the potentially harmful dirt and debris.
  • Try to see if any component or part is broken. Replace worn out parts of a check valve if necessary. Do not attempt to replace worn out parts if you think you cannot do it on your own or if other adjacent components are affected. Call a professional if you think you will need help.
  • If you see that the flapper of the valve is dirty, take the flapper off and rinse with clean water. If you have to, scrub it to remove the dirt. Put the valve control component back to its original place. Test it to make sure that it will be forced open with outflow from the drain. Run water through it for proper testing. When pushed towards the direction of the drain, the flapper must properly close. If this procedure is not possible probably because of time constraints, a quicker alternative would be to flush each of the check valve with clean water. Flushing the nearest toilet bowl that uses the drain several times; or letting the sink run for 3 to 5 minutes; or flushing the floor drain with a couple of pails o water will also help clean the assemblies up.

Source: China Check Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@metallicsteel.com

Please notice that you might be interested in the other technical articles we’ve published:

  • How To Choose A Valve

  • Knowledge of a filter and a gate valve

  • What Are the Needle Valves

  • What are Thread Valves

  • What is a Butterfly Valve

  • What are Pipe Valves

  • Knowledge of globe valves

  • Knowledge of ball valves

  • Knowledge of Gate valves

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