A check valve is a valve that allows fluid to flow in a fixed direction and automatically prevents it from flowing in the opposite direction (backflow), also known as a non-return valve. It is an automatic closing valve with a valve disc or rocker plate in the valve body. When the medium flows in the same direction, the fluid automatically pushes the valve disc open; when the fluid flows back, the fluid (or spring force) automatically closes the valve disc.
The lift check valve disc moves vertically to the valve body channel and is generally used on horizontal or vertical pipes; the valve disc of the swing check valve is often called a rocker plate, one side of which is connected to the shaft and can rotate around the shaft. The swing check valve is generally installed on horizontal pipes. For small diameters, it can also be installed on vertical pipes, but it should be noted that the flow rate should not be too large.
Check valves are generally suitable for pipelines with clean media, and should not be used in pipelines containing solid particles and high viscosity media. The closing performance of the lift check valve is better than that of the swing check valve, but the fluid resistance of the swing check valve is smaller than that of the lift check valve. In general, swing check valves are suitable for large-diameter pipelines.
According to the different structures of check valves, they are divided into lift check valves, swing check valves, axial flow check valves, butterfly check valves and other special check valves.
1. Swing check valve
The valve disc of the swing check valve is disc-shaped and rotates around the rotating axis of the valve seat channel. Because the channel inside the valve is streamlined, 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 changes, but it is not suitable for pulsating flow. Its sealing performance is not as good as the lift check valve. Swing check valves are divided into single-flap type, double-flap type and multi-flap type. These three forms are mainly divided according to the valve diameter. The purpose is to prevent the medium from stopping flowing or backflowing and weakening the hydraulic impact.
2. Lift check valve
The lift check valve is a check valve whose valve disc slides along the vertical center line of the valve body. The valve disc on the high-pressure small-diameter check valve can be a ball. The valve body shape of the lifting check valve is the same as that of the stop valve (it can be used with the stop valve), so its fluid resistance coefficient is relatively large. Its structure is similar to that of the stop valve, and the valve body and valve disc are the same as those of the stop valve. The upper part of the valve disc and the lower part of the valve cover are processed with guide sleeves. The valve disc guide cylinder can be freely lifted and lowered in the valve cover guide cylinder. When the medium flows downstream, the valve disc is opened by the thrust of the medium. When the medium stops flowing, the valve disc falls on the valve seat by its own weight to prevent the medium from flowing back.
3. Axial flow check valve
With the construction and expansion of large-scale domestic refining and ethylene and other petrochemical projects, the long-distance oil and gas pipeline system and the compressors and large pumps in large ethylene units with more demanding working conditions require the use of check valves with fast closing, low pressure loss and noise. Especially for gas phase media, for ordinary swing and double-plate check valves, the valve disc often cannot be fully opened, resulting in unstable medium flow, large pressure loss, easy mechanical vibration, high noise, etc., which affects the normal operation of large compressors and pumps. At this time, a special type of axial flow check valve is needed to ensure the safe and effective operation of the above equipment pipeline system.
4. Butterfly check valve
The installation position of the butterfly check valve is not restricted. It can be installed on a horizontal pipeline or a vertical or inclined pipeline;
Micro-resistance slow-closing butterfly check valve: mainly composed of a valve body, two semicircular valve discs, a return spring, an oil storage cylinder, a slow-closing small cylinder group, and a needle valve (micro-regulating valve). It can be used in drainage pipes for media such as clean water, sewage, and seawater. It can prevent the backflow of the medium and effectively limit destructive water hammer to ensure the safety of the pipeline. It is not suitable for medium containing solid particles and granular media with high viscosity. It is small in size, light in weight, and the structural length is only about one-third of that of a swing check valve.