Thread Content
The electric valve devices manufactured by Shanghai Automation Instrument Co., Ltd. are essential equipment for achieving programmed, automatic, and remote control of valves, and their operation can be controlled by the magnitude of travel distance, torque, or axial thrust. Since the operating characteristics and efficiency of electric valve assemblies depend on the type of valve, the operating specifications of the assembly, and the position of the valve in the pipeline or equipment, selecting the appropriate electric valve assembly is crucial to prevent overload conditions (where the operating torque exceeds the control torque). Generally, the criteria for properly selecting an electric valve actuator are as follows: Operating torque – The operating torque is the most important parameter when choosing an electric valve actuator; the torque output by the electric actuator should be 1.2 to 1.5 times the torque required to operate the valve. There are two types of main structures for operating thrust electric valve devices: one type does not have a thrust disc and delivers torque directly; the other type has a thrust disc, with the torque being converted into thrust through the valve stem nut located in that thrust disc. The number of rotations of the output shaft in electric valve devices is determined by the valve’s nominal diameter, the pitch of the valve stem threads, and the number of thread turns. It should be calculated using the formula M = H/ZS, where M represents the total number of rotations required by the electric device, H is the opening height of the valve, S is the pitch of the valve stem threads, and Z is the number of thread turns on the valve stem. For multi-turn straight-stem valves, the diameter of the valve stem is important; if the **valve stem diameter that the electric actuator can accommodate is larger than that of the valve’s stem, it is not possible to assemble an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric actuator must be larger than the outer diameter of the valve stem of the straight-stem valve. For some rotary valves and stem-type valves among multi-rotary valves, although there is no need to consider the issue of the valve stem diameter fitting through, the dimensions of the valve stem diameter and the keyway should still be taken into full consideration during selection to ensure proper operation after assembly. If the opening and closing speed of the output speed control valve is too fast, water hammer can occur. Therefore, the appropriate opening and closing speed should be selected based on different operating conditions. Electric valve devices have specific requirements, namely the ability to limit torque or axial force. Typically, electric valve systems use couplings that limit torque. Once the specifications of the electric device are determined, its control torque is also determined. It generally operates within a predetermined time frame, so the motor will not be overloaded. However, overload can occur under the following circumstances: first, a low supply voltage results in insufficient torque, causing the motor to stop rotating; second, the torque limitation mechanism is set incorrectly to a value higher than the torque required for operation, leading to excessive torque and thus the motor stopping; third, intermittent use causes heat to accumulate, exceeding the motor’s allowable temperature rise; fourth, a fault occurs in the circuit of the torque limitation mechanism for some reason, resulting in excessive torque; fifth, the operating environment has an excessively high temperature, which reduces the motor’s heat capacity. In the past, fuses, overcurrent relays, thermal relays, thermostats, and similar devices were used to protect motors, but each of these methods has its advantages and disadvantages. For variable-load devices such as electric drives, there is no reliable method of protection. Therefore, various combinations must be adopted, which can be summarized into two types: one is to determine the increase or decrease in the current input to the motor; the other is to assess the heating condition of the motor itself. For either of these two methods, the time margin provided by the motor’s heat capacity must be taken into account. Generally, the basic protection methods against overload are as follows: a thermostat is used for protecting the motor against overload during continuous operation or intermittent operation; a thermal relay is used to protect the motor against stalling; and fuses or overcurrent relays are used to deal with short-circuit faults.