Check valve, also called as clack valve, non-return valve or one-way valve is a valve normally allows media (liquid or gas) to flow through it in only one direction. Check valve is two-port valve, which means that there are two openings in the body of check valve, one for media to enter and the other for media to leave. There are various types of check valve used in a wide variety of applications. Accordingly, most of check valves do not have any valve handle or stem. Plastic or metal can be used for bodies (external shells) of the check valve. Typically, check valve is designed for and can therefore be specified for a specific cracking pressure.
Working principle of check valve
The check valve can also be used on the pipeline to supply the auxiliary system in which the pressure may rise above the system pressure. Check valves can be mainly divided into swing check valves (rotating according to the center of gravity) and lift check valves (moving along the axis). Check valve the function of this type of valve is to allow the medium to flow in only one direction and prevent the flow in the opposite direction. Usually, this valve works automatically. Under the action of fluid pressure flowing in one direction, the valve disc opens; When the fluid flows in the opposite direction, the fluid pressure and the self weight of the valve disc act on the valve seat, so as to cut off the flow. The check valve belongs to this type of valve, which includes swing check valve and lift check valve. The swing check valve has a hinge mechanism, and a valve disc like a door freely rests on the inclined seat surface. In order to ensure that the valve clack can reach the appropriate position of the valve seat surface every time, the valve clack is designed in the hinge mechanism, so that the valve clack has enough swing space and makes the valve clack really and comprehensively contact with the valve seat. The valve disc can be made of metal, or inlaid with leather, rubber or synthetic coverage, depending on the requirements of service performance. When the swing check valve is fully open, the fluid pressure is almost unimpeded, so the pressure drop through the valve is relatively small. The disc of the lift check valve is located on the sealing surface of the valve seat on the valve body. Except that the valve disc can rise and fall freely, the rest of the valve is like a stop valve. The fluid pressure lifts the valve disc from the sealing surface of the valve seat, and the medium backflow causes the valve disc to fall back to the valve seat and cut off the flow. According to the service conditions, the valve disc can be of all metal structure or inlaid with rubber pad or rubber ring on the valve disc frame. Like the stop valve, the passage of fluid through the lift check valve is also narrow, so the pressure drop through the lift check valve is larger than that of the swing check valve, and the flow of the swing check valve is rarely limited.
Classification of check valves
The valve whose opening and closing parts open or close by the force of medium flow to prevent medium backflow is called check valve. The check valve is an automatic valve, which is mainly used on the pipeline with one-way flow of medium. The medium is only allowed to flow in one direction to prevent accidents.
According to the structure, it can be divided into lifting check valve, swing check valve and butterfly check valve:
1. Lift check valves are divided into vertical and horizontal.
2. Swing check valves are divided into single valve, double valve and multi valve.
3. Butterfly check valve is straight through type.
The above check valves can be divided into threaded connection, flange connection, welding connection and clamp connection.
1. Cast iron check valve
2. Brass check valve
3. Stainless steel check valve
4. Carbon steel check valve
5. Forged steel check valve
1. DRVZ silent check valve
Stainless steel check valve
Stainless steel check valve
2. Drvg silent check valve
3. Nrvr silent check valve
4. SFCV rubber flap check valve
5. DDCV double disc check valve
Installation of check valve
Swing check valve: the disc of the swing check valve is in a disc shape and rotates around the rotating shaft of the valve seat channel. Because the channel in the valve is streamlined and the flow resistance is smaller than that of the lift check valve, it is suitable for large-diameter occasions with low flow rate and infrequent flow change, but it is not suitable for pulsating flow, and its sealing performance is not as good as that of the lift check valve. Swing check valve is divided into single disc type, double disc type and multi disc type. These three forms are mainly divided according to the valve diameter, in order to prevent the hydraulic impact from being weakened when the medium stops flowing or flows back.
Lift check valve: a check valve whose disc slides along the vertical centerline of the valve body. The lift check valve can only be installed on the horizontal pipeline. On the high-pressure small-diameter check valve, the disc can use a ball. The body shape of lift check valve is the same as that of stop valve (which can be used with stop valve), so its fluid resistance coefficient is large. Its structure is similar to the stop valve, and the valve body and disc are the same as the stop valve. The upper part of the valve disc and the lower part of the valve cover are processed with a guide sleeve. The valve disc guide sleeve can rise and fall freely in the valve disc guide sleeve. When the medium flows downstream, the valve disc opens by the thrust of the medium. When the medium stops flowing, the valve disc falls on the valve seat vertically to prevent the reverse flow of the medium. The direction of medium inlet and outlet channel of straight through lifting check valve is perpendicular to the direction of valve seat channel; For vertical lift check valve, the direction of medium inlet and outlet channel is the same as that of valve seat channel, and its flow resistance is smaller than that of straight through check valve.
Disc check valve: a check valve whose disc rotates around the pin shaft in the valve seat. Disc check valve has simple structure and can only be installed on horizontal pipeline with poor sealing.
Pipeline check valve: a valve whose disc slides along the center line of the valve body. Pipeline check valve is a new valve. It has small volume, light weight and good processing technology. It is one of the development directions of check valve. However, the fluid resistance coefficient is slightly larger than that of swing check valve.
Compression check valve: This valve is used as a boiler feed water and steam shut-off valve. It has the comprehensive function of lifting check valve, stop valve or angle valve.
In addition, there are some check valves that are not suitable for pump outlet installation, such as bottom valve, spring type, Y type, etc. Matters needing attention:
1. In the pipeline system, do not let the check valve bear the weight. The large check valve should be supported independently to prevent it from being affected by the pressure generated by the pipeline system.
2. During installation, pay attention that the direction of medium flow shall be consistent with the arrow marked on the valve body.
3. Lift type vertical disc check valves shall be installed on vertical pipes.
4. Lift type horizontal disc check valve shall be installed on horizontal pipeline.
Selection standard of check valve
The selection criteria of check valve are as follows:
1. In order to prevent the reverse flow of medium, check valves shall be installed on equipment, devices and pipelines;
2. Check valves are generally suitable for clean media, not for media containing solid particles and high viscosity;
3. Generally, horizontal lifting check valve shall be selected on horizontal pipeline with nominal diameter of 50mm;
4. Straight through lifting check valve can only be installed in horizontal pipeline;
5. For the pump inlet pipeline, the bottom valve should be selected. Generally, the bottom valve is only installed on the vertical pipeline at the pump inlet, and the medium flows from bottom to top;
6. The lifting type has better sealing performance and large fluid resistance than the swing type. The horizontal type should be installed on the horizontal pipeline and the vertical type on the vertical pipeline;
7. The installation position of swing check valve is not limited. It can be installed on horizontal, vertical or inclined pipelines. If installed on vertical pipelines, the medium flow direction shall be from bottom to top;
8. The swing check valve should not be made into a small-diameter valve, but can be made into a very high working pressure. The nominal pressure can reach 42MPa, and the nominal diameter can also be very large, up to more than 2000mm. According to the different materials of shell and seal, it can be applicable to any working medium and any working temperature range. The medium is water, steam, gas, corrosive medium, oil, medicine, etc. The working temperature range of medium is – 196-800 ℃;
9. Swing check valve is suitable for low pressure and large diameter, and the installation occasion is limited;
10. The installation position of butterfly check valve is not limited. It can be installed on horizontal pipeline or vertical or inclined pipeline;
11. The diaphragm check valve is suitable for pipelines prone to water hammer. The diaphragm can well eliminate the water hammer caused by the reverse flow of the medium. It is generally used in low-pressure and normal temperature pipelines, especially for tap water pipelines. Generally, the working temperature of the medium is between – 12-120 ℃ and the working pressure is < 1.6Mpa, but the diaphragm check valve can achieve a large diameter, DN can reach more than 2000mm at most;
12. Spherical check valve is suitable for medium and low pressure pipeline and can be made into large diameter;
13. The shell material of the spherical check valve can be made of stainless steel, and the hollow sphere of the seal can be wrapped with polytetrafluoroethylene engineering plastic, so it can also be used in the pipeline of general corrosive media. The working temperature is between – 101-150 ℃, the nominal pressure is ≤ 4.0Mpa, and the nominal diameter range is between dn200-dn1200;
14. When selecting the check valve for incompressible fluid, the first step is to evaluate the required closing speed, and the second step is to select the type of check valve that may meet the required closing speed;
15. When selecting the check valve for compressible fluid, it can be selected according to the similar method of the check valve for incompressible fluid. If the medium flow range is large, a speed reduction device can be used for the check valve for compressible fluid. If the medium flow stops and starts continuously and rapidly, such as the outlet of the compressor, a lifting check valve can be used;
16. The corresponding size of the check valve shall be determined, and the valve supplier must provide the data of the selected size, so that the valve size when the valve is fully open at a given flow rate can be found;
17. For high and medium pressure check valves below dn50mm, vertical lifting check valve and straight through lifting check valve should be selected;
18. For the low-pressure check valve below dn50mm, butterfly check valve, vertical lifting check valve and diaphragm check valve should be selected;
19. For high and medium pressure check valves with DN greater than 50mm and less than 600mm, swing check valves should be selected;
20. For medium and low pressure check valves with DN greater than 200mm and less than 1200mm, wear-free spherical check valves should be selected;
21. For low-pressure check valves with DN greater than 50mm and less than 2000mm, butterfly check valves and diaphragm check valves should be selected;
22. Slow closing swing check valve and slow closing butterfly check valve should be selected for the pipeline with small water hammer impact or no water hammer when it is required to be closed.