Generally, the sealing form of floating-structure ball valve is exit seal. After closing the valve ball, the valve ball under the effect of medium pressure moves a little toward the valve seat, making the valve seat produce elastic deformation and exert compaction effect on valve ball, thereby ensuring the sealing of valve. Greater medium pressure equals better valve sealing performance. (Provide that: the actual seating pressure is within the scope of the tolerance pressure of seat material). The material of seat usually are PTFE, PTFE, nylon and polymer materials. For floating ball valve, the sealing force of valve seat and the sealing pressure are greatly influenced by changing medium pressure. Too high pressure may damage the valve seat. Trunnion Ball Valve There are two kinds of sealing structure of trunnion ball valve: entrance sealing and exit sealing. The fixed shaft is installed in upper and lower end of the valve ball and valve ball rotates around the axis of the fixed shaft. Unlike floating structure, when the ball rotates to the closed position, the radial force produced by medium pressure acting on the ball won’t lead to radial displacement. The radial force of ball produced by medium pressure is transmitted to the upper and lower end of the fixed axis, so as to reduce damage for valve seat which produced by the radial force from medium pressure acting on the ball, thus ensuring a sealing pressure in the appropriate range between valve ball and valve seat. If there is no medium pressure, the sealing force relies on elastic element that installed on the back of the seat. When the medium flow into valve, the sealing of seat will change differently because of the structure of entrance seal and exit seal. According to the material of seat, It is very important to choose appropriate-stiffness elastic elements and proper seat materials. PTFE, PTFE, Nylon, high polymer material (soft), as well as Ni60/Ni55,stellite, tungsten carbide can be selected as metal to metal seat sealing materials.