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Check Valve

» Check Valve

Check Valve

Check valves are also called non return valve, reflux valve, retention valve or one-way valve. This type of industrial valve is automatically opened and closed by the force generated by the flow of the medium itself in the pipeline, which can categorize as an automatic valve. Check valves are used in pipeline systems, their main functions are to prevent backflow of the medium, to prevent the pump and its drive motor from reversing, and to backflow of the medium from container. The check valve can also be used to replenish the auxiliary system where the pressure may rise above the pressure of the main system.


 
Table of Content

1. Check Valve Types

2. The structure and working principle of check valve

3. Features of Check Valves

4. Design Standard

5. Application Of Check Valve

6. General Maintenance

7. Troubleshooting Guideline

 
1. Check Valve Types

Based on Body, internal components, seat configuration

The flapper, the movable part to block the flow, swings on a hinge, either onto the seat to block reverse flow or off the seat to allow forward flow. The following figure shows the most common single flap check valve. The nominal size is generally DN50~DN500. The swing check valve is one of the most commonly used check valves

swing check valve

Swing Check Valve

  • Lift Check Valve

The valve disc of the lift check valve works automatically. Generally, a conical valve disc is pressed against the metal valve seat. Spring can be preloaded to the flapper. When the fluid is flowing in the forward direction, under the action of the fluid pressure, the conical valve flapper disengages from the wide seat and the valve opens. During reverse flow, the valve flapper will fall back to the valve seat under the combined action of its own weight, spring force (if spring preload is provided) and fluid return pressure, and the valve will be closed.

lift check valve

Lift Check Valve

The main advantage of the lift check valve is its simple structure. Since the conical flapper is the only moving part, the valve is strong and durable, requiring minimum maintenance. Because it is a metal seat, it wears very little. The limitation of the lift check valve is that it must be installed on a horizontal pipeline.

The dual plate check valve has two spring-loaded disc flaps that are placed on a ribbed shaft that spans the valve through hole. The distance of the centre of gravity of the valve flapper is short. Compared with the same size single flapper swing check valve, the double disc check valve is 50% lighter. Because of the loaded spring, the response to reverse flow is very fast.

dual plate check valve

Dual Plate Check Valve

  • Slow Closing Check Valve

In some special occasion, such as the pipeline pressure often changes, or when there are special requirements, the double plate check valve, swing check valve, and lift check valve are provided with buffer devices to form a slow-closing type Check valve. This valve is suitable for systems with very strong backflow, it can reduce the water hammer pressure.

slow closing check valve

Slow Closing Check Valve

The movement of the valve flapper of the no slam check valve is back and forth along the centre line of the valve, the flapper size and stroke distance are small. The sealing configuration: Standard rubber ring and O-ring are embedded in the flapper seat. When the valve flapper is closed, the valve seat cone surface will contact with the rubber ring and the rubber ring is compressed to form an elastic seal. Then the valve flap cone surface contacts the valve seat cone surface, and the taper forms a secondary metal seal. When the valve flap opens, the rubber ring restores to its original shape. This type of configuration prevents the rubber ring from being damaged due to excessive compression, which will be resulting leakage. Even if the rubber ring is worn and scratched by impurities, the elastic seal fails, the metal seal can ensure that the valve does not leak, and does not affect the normal operation of the system. The combination of these two seals increases the service life of the valve and reduces the maintenance costs.

Non Slam Check Valve

Non Slam Check Valve

The pressure seal check valve is a top-entry design, under high pressure and large diameter conditions it reduces the bolting connection of the valve body, which enhances the reliability and overcome the weight influences of the system on the normal operation of the valve. The higher the medium pressure in the valve body, the better the sealing performance
 
2. The structure and working principle of check valve

  • Lift Check Valve

The structure of the lift check valve is generally similar to globe valve, and the valve flapper is lifted along the centre line of the flow path. The sealing principle is it depends on its own weight when it is closed. Once the valve flapper is closed, the pressure of the medium when the flow is reversed and is applied to the valve flap to form a sealing pressure. The higher the pressure, the tighter the seal. Conversely, the lower the medium pressure, the lower the sealing pressure ratio between the valve flapper and the seat sealing surface, and the lesser the sealing.

  • Swing check valve

The valve flapper of the swing check valve rotates around the rotating shaft in the valve body. The axis of the rotating shaft must be on the same plane as the sealing surface of the valve body to ensure that the seal is achieved when closed. Swing check valves can only be installed on horizontal pipelines.

  • Dual Plate Check Valve

The structure of the double flap check valve is similar to a butterfly valve, and its sealing principle is similar to a butterfly valve. It is opened by the pressure when the medium flows forward, and it is closed by the torsional moment generated by the torsion spring. Then it is sealed by the specific pressure of the seal generated by the pressure when the medium flows backward.
 
3. Features of Check Valves

  • The valve flapper of the dual plate check valve rotates around the pin in the valve seat. The structure is simple but poor sealing performance and can only be installed on horizontal pipelines.
  • The disc of the pipeline check valve slides along the centre line of the valve body. Pipeline check valve is small in size.
  • The valve flapper of the lift check valve slides along the centre line of the valve body and can only be installed on horizontal pipelines, has a large fluid resistance coefficient.
  • The disc of the swing check valve rotates around the pin outside the valve seat.

 
4. Design Standard

No Type Design Standards
a Lift Check Valve BS5352/ BS1868
b Dual Plate Check Valve API594
c Swing Check Valve API6D
d Non Slam Check Valve ASME B16.34
e Stop Check Valve BS1873

 
5. Application Of Check Valve

1) The dual plate check valve is suitable for the water supply system of high-rise buildings, chemical corrosive medium system, pipeline with limited installation space, and also for the sewage system.
2) Non slam check valve is suitable for water supply and drainage system, high-rise building piping, can be installed at the outlet of the pump. With slight modification, it can be used as a suction valve, but not suitable for sewage piping.
3) Rubber lined check valve is suitable for domestic water pipe network; but not for sewage with a lot of sediment
4) Single disc swing check valve is suitable for water supply system, petroleum, chemical industry, metallurgy and other industrial sectors. In case of limited installation space, it is a good choice.
 
6. General Maintenance

1)Check valve should be stored in dry and ventilated room, outdoor storage is not allowed. Both ends should be covered with end caps to prevent debris from entering the valve.

2)Check valves for long-term storage should be regularly inspected to remove dirt from the internal cavity and machined surfaces should be greased.

3)The in-service check valve installed and operated on the pipeline shall be regularly checked for normal operation. If small faults are found, they should be eliminated in time. If there are major faults, they should be removed for maintenance. Check valve after repair and assembly should be re-tested. The records for troubleshooting and maintenance shall be maintained.

4)Welding and repair under pressure is prohibited, and do not dismantle and replace valve parts without referring to check valve manufacturer.
 
7. Troubleshooting Guideline

1)Broken Disc or flapper and seat

The equilibrium of pipeline pressure may cause the flapper to continuously flapping on the valve seat and caused the disc/flapper/seat to be broken or chip off if the flapper is made of fragile materials (such as cast iron, brass, etc.)

A tougher material shall be selected during purchase.

2)Medium Backflow

Medium backflow/passing is caused by sealing surface damaged or debris stuck between flapper and seat. Repair/replace the sealing surface or clean the sealing surface.

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    NTGD Valve (China) Co. LTD
    North Avenue Zhuqu Town ,Weishi County, Kaifeng, Henan,P.R.China
    Email: sales@ntgdvalve.com
    Website:www.ntgdvalve.com

    NTGD Valve (UK) Co. LTD
    ADD.: 19,King Street, Gillingham, England ME7 1EO
    Email: sales@ntgd.co.uk