Working principle of check valves:

Check valves are used in pipeline systems. Their main functions are to prevent backflow of the medium, prevent the pump and its driving motor from reversing, and release the medium in the container. Check valves can also be used on pipes that supply supplementary systems with pressure that may rise above the main system pressure. Check valves can be applied to various media pipelines depending on their material. Check valves are installed on pipelines and become one of the fluid components of this complete pipeline. The valve disc's opening and closing process is affected by the transient flow state of the system in which it is located; conversely, the closing characteristics of the valve disc have an effect on the fluid flow state.

Classification of check valves:

1. Swing check valve: The valve disc of the swing check valve is disc-shaped and rotates around the rotating shaft of the valve seat channel. Due to the streamlined channel inside the valve, the flow resistance is smaller than that of the lift check valve, suitable for large-diameter occasions with low flow rate and stable flow, but not suitable for pulsating flow. Its sealing performance is inferior to the lift check valve. Swing check valves are divided into single-leaf, double-leaf, and multi-leaf types. These three forms are mainly classified by the diameter of the valve opening to prevent hydraulic shock when the medium stops flowing or flows in reverse.

2. Lift check valve: The lift check valve has a valve disc that slides along the vertical center line of the valve body. The lift check valve can only be installed on horizontal pipelines. The valve disc can be a round ball on high-pressure small-diameter check valves. The valve body shape of the lift check valve is the same as that of the globe valve (can be used interchangeably with the globe valve), so its fluid resistance coefficient is relatively large. Its structure is similar to that of the globe valve, with the valve body and valve disc being the same as those of the globe valve.

3. Silent check valve: The upper part of the valve disc and the lower part of the valve cover are machined with guide sleeves. The valve disc guide sleeve can freely rise and fall within the valve cover guide sleeve. When the medium flows in the same direction, the valve disc opens due to the medium thrust. When the medium stops flowing, the valve disc falls down on the valve seat due to its own gravity, serving to prevent the medium from flowing in reverse. The inlet and outlet channels of the straight-through lift check valve are perpendicular to the valve seat channel direction; the vertical lift check valve has the inlet and outlet channels of the medium in the same direction as the valve seat channel direction, and its flow resistance is smaller than that of the straight-through type.

4. Butterfly check valve: The check valve with the valve disc rotating around the pin shaft inside the valve seat. The butterfly check valve has a simple structure and can only be installed on horizontal pipelines, with poor sealing performance. 

V. Pipeline Check Valve: A valve where the valve disc slides along the centerline of the valve body. The pipeline check valve is a newly emerged type of valve. It has a small size, is lightweight, and has good processing manufacturability. It is one of the development directions of check valves. However, its flow resistance coefficient is slightly larger than that of the swing check valve.

VI. Compression Check Valve: This valve is used as a shut-off valve for boiler feed water and steam. It has the combined functions of a lift check valve, a globe valve, or a angle valve.

In addition, there are some check valves that are not suitable for installation at the pump outlet, such as bottom valves, spring-type, Y-type, etc.

Precautions for installing check valves:

1. Do not allow the check valve to bear the weight in the piping system. Large-sized check valves should be independently supported to be unaffected by the pressure generated by the piping system.

2. Pay attention to that the direction of the medium flow should be consistent with the arrow direction marked on the valve body.

3. Vertical lift check valves with vertical flaps should be installed on vertical pipes. 4. The lift-type horizontal flap check valve should be installed on a horizontal pipeline.