The ball valve is evolved from the plug valve. Its opening and closing part is a sphere, and the sphere is rotated 90° around the axis of the valve stem to achieve the purpose of opening and closing. The ball valve is mainly used to cut off, distribute and change the flow direction of the medium on the pipeline. The ball valve designed with V-shaped opening also has a good flow adjustment function. The advantages of the ball valve: it has the lowest flow resistance (actually 0); it can be reliably used in corrosive media and low-boiling liquids because it will not stick during operation (in the absence of lubricants); in larger Within the range of pressure and temperature, it can be completely sealed; it can realize rapid opening and closing, and the opening and closing time of some structures is only 0.05-0.1s, so as to ensure that it can be used in the automation system of the test bench. When the valve is opened and closed quickly, the operation has no impact; the spherical closing part can be automatically positioned at the boundary position; the working medium is reliably sealed on both sides; when the valve is fully opened and fully closed, the sealing surface of the sphere and the valve seat is isolated from the medium, so The medium passing through the valve at high speed will not cause erosion of the sealing surface; the structure is compact and light, and it can be considered as the most reasonable valve structure for low temperature medium systems; the valve body is symmetrical, especially the welded valve body structure, which can be well It can withstand the stress from the pipeline; the closing part can withstand the high pressure difference when it is closed; the ball valve of the fully welded valve body can be directly buried in the ground, so that the valve internal parts are not corroded, and the maximum service life can reach 30 years. The ideal valve for pipelines.
Disadvantages of ball valves: Because the most important valve seat sealing ring material of ball valves is PTFE, it is inert to almost all chemical substances, and has a small friction coefficient, stable performance, not easy to age, and a wide range of temperature applications. Comprehensive features of excellent sealing performance. But the physical properties of PTFE, including high coefficient of expansion, susceptibility to cold flow, and poor thermal conductivity, require that seat seal designs must be built around these properties. Therefore, when the sealing material becomes hard, the reliability of the sealing is damaged. Moreover, PTFE has a low temperature resistance grade and can only be used below 180°C. Above this temperature, the sealing material will degrade. In the case of long-term use, it is generally not used at 120 °C. Its adjustment performance is worse than that of globe valves, especially pneumatic valves (or electric valves).
