The check valve is widely used in the water supply and drainage pumping station system. The main function of setting the check valve on the pump outlet pipe is to prevent a large amount of backflow of pressure water in the pipeline when the pump stops suddenly. At present, the most commonly used check valves in China are lift check valves, swing check valves, micro-resistance slow-closing check valves, etc. In recent years, on the basis of learning from foreign advanced experience and technology, new products such as energy-saving shuttle check valve, ball check valve, elastic film silencing check valve have been developed, providing a broader choice space for the majority of water supply and drainage engineering designers. In this paper, the structural characteristics, working principles and main performances of the above check valves are comprehensively reviewed and compared.

1. Conventional check valve

1.1 Lift check valve
A check valve that has been applied earlier in China, its working principle is that when the pump is opened, the valve disc will lift under the action of pressure water to make the water flow through. When the water pump is suddenly powered off, the pressure water will flow back, and the valve disc will close quickly under its own gravity to prevent the flow back, but at the same time, it will cause water hammer in the pipeline system. Because of its large flow resistance and high power consumption, the valve has been listed as a obsolete product.

1.2 Swing check valve
The valve also has a long history in China. There is a valve disc that can be turned on in the valve body. The valve disc is washed up by the water flow when the water flow stops. When the water flow stops, the valve disc closes by its own gravity to prevent the water from flowing back. The main disadvantages of the valve are high local resistance, the connecting rod, rocker, copper sleeve, hammer and other parts on the valve disc are easy to wear, poor water stop performance, short maintenance period, etc. At the same time, there is also the problem of pump stop water hammer, At present, large and medium diameter swing check valves have been explicitly eliminated by the state.

1.3 Slight-resistance slow-closing swing check valve
The first two types of check valves are developed and produced for their large flow resistance and easy to cause pump stop water hammer. During normal operation, the valve clack will open automatically by the water flow rate and head to make the water flow through, and the water in the valve body will push out the piston by means of the one-way valve; When the water pump stops working and the pressure water suddenly reverses, the piston will hold the valve disc, making the valve disc unable to close quickly, thus weakening the effect of water hammer. With the slow return of the piston, the valve disc can close slowly. This valve is applicable to large and medium-sized water supply and drainage pump stations. The slow closing time can be adjusted arbitrarily between 0 and 60 seconds. At the same time, the flow resistance is small, and the energy saving is significant, which is about 50% less than the swing check valve.

2. Several new check valves

2.1 The slow-closing silencing lifting check valve is improved on the basis of the lifting check valve. As shown in Figure 1, the valve body is composed of two water chambers. The intercepting port of the diaphragm lower chamber is a flow passage, and the upper water chamber of the diaphragm is a pressure regulating chamber. When the water pump stops working, the intercepting port is quickly closed by 90% due to the self-weight of the valve disc and the pressure of the upper water chamber, and the remaining 10 pipes are used to transmit the pressure behind the valve to the upper water chamber, With the increase of the water chamber pressure, the interceptors are slowly closed. The valve is applicable to the secondary pressurized water supply system of high-rise buildings, and the diameter is generally below DN300

2.2 Energy-saving shuttle check valve
It is developed based on the principle of differential pressure shuttle, as shown in Figure 2. The internal part is composed of check valve core, guide valve core and sealing gasket. When the pressure at the lower end of the check valve core is greater than the pressure at the upper end, it will move upward, and the valve will open, otherwise it will close. The current maximum diameter of the valve is DN400, which is mostly used in the water supply system of high-rise buildings, whether in terms of flow resistance, sealing, or cost Its service life is better than that of swing check valve.

2.3 Ball check valve
This valve is especially suitable for industrial and domestic sewage systems with high viscosity and suspended solids, as shown in Figure 3. The valve body adopts a full-channel structure, with the ball as the movable seal (the ball is hollow iron ball wrapped with rubber). When the water pump is working, the pressure in front of the valve is greater than the pressure behind the valve, the ball is pushed along the track to the side chamber of the valve body under the action of the water flow rate and head, thus forming a full channel. When the water pump stops working, the ball falls along the track, and under the return pressure, the channel is closed to prevent the water from flowing back. The valve has good sealing performance and small flow resistance, It is especially suitable for small and medium-sized sewage pumping stations, but generally not for water supply systems with high working pressure

2.4 Elastic film silencing check valve
A new type of check valve is developed to solve the water hammer phenomenon and noise hazard commonly existing in the water supply system of high-rise buildings. As shown in Figure 4, when the pressure in front of the valve is greater than the pressure behind the valve, the diaphragm is pushed away from the valve seat to make the water flow through. When the pump stops working, the diaphragm is tightly attached to the valve seat under the effect of the return pressure, so that the channel is fully closed. It is characterized by replacing the traditional metal moving parts with flexible natural rubber, and because the sealing surface of the valve seat adopts concave design, it has small opening pressure, good sealing performance and can eliminate destructive water hammer. The valve is suitable for small water supply system and fire control system, with low production cost, and is a new product with great development prospects