The check valve, also known as the check valve, is an automatic valve. It relies on the pressure of the flowing medium in the pipeline to push the valve disc to close and open the valve. When the medium stops flowing and the check valve disc closes, it can effectively prevent the backflow of the medium in the pipeline, which plays a great role in ensuring the safety of the pipeline.

止回阀按结构划分，可分为升降式止回阀、旋启式止回阀和蝶式止回阀三种。升降式止回阀可分为立式和直通式两种。旋启式止回阀分为单瓣式、双瓣式和多瓣式三种。蝶式止回阀分为蝶式双瓣、蝶式单瓣，以上几种止回阀在连接形式上可分为螺纹连接、法兰连接、焊接和对夹式连接四种。

According to the structure, check valves can be divided into lift check valve, swing check valve and butterfly check valve. Lift check valves can be divided into vertical and straight through. Swing check valves are divided into single valve, double valve and multi valve. Butterfly check valves are divided into butterfly double valve and butterfly single valve. The above check valves can be divided into threaded connection, flange connection, welding and wafer connection.

安装止回阀是为了保证水泵内部水流的流向，从而保障水泵的正常工作。所以关于止回阀的安装位置，到底是安装在水泵前还是水泵后，可以分两种情况：

The check valve is installed to ensure the flow direction of water inside the water pump, so as to ensure the normal operation of the water pump. Therefore, the installation position of the check valve can be divided into two situations: whether it is installed in front of or behind the water pump:

一种是安装在水泵前竖吸管端头，也叫底阀。目的是为了不用抽一次水向水泵充一次水，因为水泵内以及泵前的吸管内无水时水泵只能空转抽不上水，所以水泵内必须充满水才能抽出水。这种水泵的安装是高于液面时的一种方法，也叫负压法。

One is installed at the end of the vertical suction pipe in front of the water pump, also known as the bottom valve. The purpose is to fill the water pump with primary water without pumping primary water, because when there is no water in the water pump and the suction pipe in front of the pump, the water pump can only idle and can not pump water, so the water pump must be filled with water to pump water. The installation of this water pump is a method above the liquid level, also known as negative pressure method.

另一种是安装在水泵的后方。这种安装方法是液面高于水泵的情况下，方便启停水泵时不用关水泵出水阀，特别是对于母管制水泵系统，防止发生倒流的情况。

The other is installed behind the water pump. This installation method is convenient when the liquid level is higher than the water pump. When starting and stopping the water pump, there is no need to close the water outlet valve of the water pump, especially for the main pipe water pump system to prevent backflow.