When the pressure in the equipment or pipeline exceeds the set pressure of the pressure relief valve, it automatically opens to relieve the pressure, to ensure that the equipment and pipeline medium pressure under the set pressure, to protect the equipment and pipeline, to prevent accidents. It is a kind of hydraulic control valve. The main valve is divided into two parts by the diaphragm, the lower chamber of the diaphragm is the water flow channel, the upper chamber is the control chamber, which controls the opening and closing of the main valve flap. Pilot valve itself is a pressure relief valve, it also has a control room and water flow channel, valve opening and closing by the control room control. Needle valve that throttle valve, it controls the flow of water in the connecting pipe.
Drainage system (sewersagesystem) refers to the collection of drainage, transportation, treatment and discharge of water quality and other facilities in a certain way combined into a total. Used to remove waterlogging, prevention of stains, salt at all levels of drainage ditches (pipes) and the general term of the building. It mainly consists of field drainage regulation network, all levels of drainage ditches, flood storage lakes, drainage gates, pumping stations and drainage capacity of the drainage area (see Figure). Drainage area of excess water first into the field drainage regulation network, and then by all levels of drainage ditch or by the lake storage and then by the drainage gate or pumping station to the discharge area.