Ball valve and plug valve belong to the same type of valve. Only its closing part is a ball. The ball rotates around the center line of the valve body to open and close. Ball valve is mainly used to cut off, distribute and change the flow direction of medium in the pipeline. Ball valve is a new type of valve widely used in recent years. It has the following advantages: 1 The fluid resistance is small, and its resistance coefficient is equal to that of the pipe section with the same length. 2. Simple structure, small volume and light weight. 3. Tight and reliable. At present, plastic is widely used as the sealing surface material of ball valve, with good sealing performance, and has been widely used in vacuum system. 4. It is easy to operate and open and close quickly. It only needs to rotate 90 ° from full open to full close, which is convenient for remote control. 5. The maintenance is convenient, the structure of the ball valve is simple, the sealing ring is generally movable, and the disassembly and replacement are relatively convenient. 6. When fully open or fully closed, the sealing surface of the ball and valve seat shall be isolated from the medium. When the medium passes through, it will not cause the erosion of the valve sealing surface. 7. It has a wide range of applications, ranging from small to several millimeters and large to several meters. It can be applied from high vacuum to high pressure. Ball valve has been widely used in petroleum, chemical industry, power generation, papermaking, atomic energy, aviation, rocket and other departments, as well as people's daily life. Ball valve can be divided according to the structural form: first, the ball of floating ball valve is floating. Under the action of medium pressure, the ball can produce a certain displacement and press tightly on the sealing surface of the outlet end to ensure the sealing of the outlet end. The floating ball valve has simple structure and good sealing performance, but the load of the working medium borne by the ball is transmitted to the outlet sealing ring. Therefore, it is necessary to consider whether the sealing ring material can withstand the working load of the ball medium. This structure is widely used in medium and low pressure ball valves. 2、 The ball of the fixed ball valve is fixed and does not move after being pressurized. Fixed ball valves are equipped with floating valve seats. After being subjected to medium pressure, the valve seats move, so that the sealing ring is tightly pressed on the ball to ensure sealing. Usually, bearings are installed on the upper and lower shafts of the ball, with small operating torque. The rotor pump is suitable for high-pressure and large-diameter valves. In order to reduce the operating torque of the ball valve and increase the reliability of sealing, oil sealed ball valves have appeared in recent years. Special lubricating oil is injected between the sealing surfaces to form an oil film, which not only enhances the sealing performance, but also reduces the operating torque. It is more suitable for high-pressure and large-diameter ball valves. 3、 The ball of an elastic ball valve is elastic. The ball and valve seat sealing ring are made of metal materials, and the sealing specific pressure is very large. The pressure of the medium itself can not meet the sealing requirements, so external force must be applied. This valve is suitable for high temperature and high pressure media. The elastic sphere is formed by opening an elastic groove at the lower end of the inner wall of the sphere to obtain elasticity. When the channel is closed, the wedge head of the valve rod is used to open the ball and compress it with the valve seat to achieve sealing. Loosen the wedge head before rotating the ball, and the ball will return to its original shape, resulting in a small gap between the ball and the valve seat, which can reduce the friction and operating torque of the sealing surface. Ball valves can be divided into straight through type, three-way type and right angle type according to their channel position.