The influencing factors of ball valve torque mainly include pressure, temperature, medium type, property and quantity of suspended substance in fluid and operation frequency, as well as the structure and processing quality of ball valve. Therefore, when selecting the pneumatic actuator of the ball valve, the maximum output torque of the pneumatic actuator should conform to and include the safety factors necessary for the above conditions. Therefore, in the selection of pneumatic actuator, the valve torque should be determined first, and then multiplied by the appropriate safety factor according to the medium conditions. Generally, the safety value should be increased by 25% for steam or non lubricating medium, 60% for non lubricating dry gas medium, 100% for non lubricating pneumatic conveying granular powder medium, and 20% for clean and friction free lubricating medium. Then, according to the working pressure of the air source and the action mode of the actuator and the output torque table specified by the operating conditions, the appropriate type of pneumatic actuator is selected.
In order to make the pneumatic actuator work normally, it is necessary to provide more than the starting torque of the ball valve and have the same or similar torque curve characteristics as the ball valve driven by it. This is the basic principle for selecting the pneumatic actuator. Therefore, the best method is to fit the torque curve of the ball valve and the output torque line of the pneumatic actuator into the same coordinate system, and require the output torque line of the pneumatic actuator to be higher than the opening and closing torque line of the ball valve, and ensure that there is no intersection between the two curves, so as to avoid the phenomenon of insufficient power in the opening and closing process of the ball valve. The distance between the two curves is the safety factor（ As shown in the figure).