① Because the main valve seat sealing ring material of the ball valve is polytetrafluoroethylene, which is inert to almost all chemicals, and has the comprehensive characteristics of small friction coefficient, stable performance, aging resistance, wide temperature application range and excellent sealing performance. However, the physical characteristics of PTFE, including high expansion coefficient, sensitivity to cold flow and poor thermal conductivity, require that the design of valve seat seal must focus on these characteristics. Therefore, when the sealing material becomes hard, the reliability of the seal will be damaged. Moreover, the temperature resistance grade of PTFE is low, and it can only be used at less than 180 ℃. Above this temperature, the sealing material will age. In case of long-term use, it is generally used at 120 ℃.
② Its regulating performance is worse than that of the stop valve, especially the pneumatic valve (or electric valve).
