The working principle of the check valve is that under the effect of fluid pressure running in a single direction, the valve disc opens, and when the fluid runs in the opposite direction, because the fluid pressure and the self-weight of the valve disc combine to affect the valve seat, the flow is cut off. The check valve uses the flow of the medium itself to control the valve disc of the switch, which can prevent the medium from flowing backward, so it can also be called a check valve. Common check valves are of swing-up type and lift-up type. The swing-up type has a hinge structure and a valve disc similar to a door, which can rest on the inclined seat surface at will.
Type:
（1）Lift check valve: vertical and horizontal. The valve body shape of the lift check valve is the same as that of the globe valve, so its fluid resistance is relatively large. The disc slides along the vertical centerline of the valve body. When the medium flows downstream, the valve disc is opened by the thrust of the medium, and when the medium stops flowing, the valve disc falls on the valve seat by hanging.
1. Vertical lift check valve. The direction of the medium inlet and outlet channels is the same as that of the valve seat channel, and its flow resistance is smaller than that of the straight-through type. The vertical lift check valve is installed in the vertical pipeline.
2. Straight-through lift check valves can only be installed in horizontal pipelines. Restricted by installation requirements, it is often used in small diameter occasions DN<50.
（2）Swing check valve: The disc of the swing check valve is disc-shaped and rotates around the shaft of the valve seat channel. Because the channel in the valve is streamlined, the flow resistance is lower than that of the lift check valve. Small, suitable for small and medium caliber, low-pressure large-diameter pipelines (large-diameter occasions with low flow rate and infrequently changing flow). The sealing performance is not as good as the lifting type.
1. Swing check valves are divided into three types: single-disc type, double-disc type and multi-disc type. These three types are mainly divided according to the valve caliber. The purpose is to prevent the hydraulic impact when the medium stops flowing or reverses. Single disc swing check valves are generally suitable for medium-caliber occasions. When a single disc swing check valve is selected for a large-diameter pipeline, in order to reduce the water hammer pressure, it is best to use a slow-closing check valve that can reduce the water hammer pressure. Double disc swing check valve is suitable for large and medium diameter pipelines. The double-disc swing check valve has a small structure and light weight, and is a rapidly developing check valve; the multi-disc swing check valve is suitable for large-diameter pipelines.
2. The installation position of the swing check valve is not limited, and it can be installed on horizontal, vertical or inclined pipelines.
（3）Butterfly check valve: straight-through type. The structure of the butterfly check valve is similar to that of a butterfly valve. The structure is simple, the flow resistance is small, and the water hammer pressure is also small. A check valve in which the disc rotates around a pin in the seat. The disc check valve has a simple structure and can only be installed on horizontal pipelines with poor sealing performance.
（4）Diaphragm check valve: There are many structural forms, all of which use diaphragm as the opening and closing part. Due to its good anti-hammer performance, simple structure and low cost, it has developed rapidly in recent years. However, the operating temperature and pressure of the diaphragm check valve are limited by the diaphragm material.
Diaphragm check valves are suitable for pipelines that are prone to water hammer. The diaphragm can well eliminate the water hammer caused by the reverse flow of the medium. It is generally used on low-pressure and normal temperature pipelines, especially for tap water pipelines.
