Generally, the correct selection of valve electric actuators is based on the following:
1 Operating torque: The operating torque is the most important parameter for selecting the valve electric device. The output torque of the electric actuator should be 1.2 to 1.5 times the maximum operating torque of the valve.
2. There are two main structures for operating the thrust valve electric device: one is not equipped with a thrust plate, and the torque is directly output; the other is with a thrust plate, and the output torque is converted to output thrust through the valve stem nut in the thrust plate.
3. Number of turns of the output shaft: The number of turns of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the ceramic rod, and the number of thread heads. It should be calculated according to MH/si (M is the total rotation that the electric device should meet The number of turns, H is the opening height of the valve, S is the thread pitch of the steering rod driving thread, and z is the number of thread heads of the steering rod).
4. Ceramic stem diameter For multi-turn rising stem valves, if the maximum wide stem diameter allowed by the electric device cannot pass through the valve stem of the equipped valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric device must be larger than the outer diameter of the wire stem of the rising stem valve. For part-turn valves and dark-stem valves in multi-turn valves, although there is no need to consider the passage of the wide rod diameter, the valve stem diameter and the size of the keyway should be fully considered when selecting so that the assembly can work normally.
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5•Output speed: If the opening and closing speed of the valve is too fast, water hammer is easy to occur. Therefore, the appropriate opening and closing speed should be selected according to different conditions of use.
6, The valve electric actuator has its special requirements, that is, it must be able to limit the torque or axial force. Usually, the valve electric device adopts a torque-limiting coupling. When the specifications of the electric device are determined, the control torque is also determined. Generally, run at a predetermined time, the motor will not be overloaded.
The following conditions may cause the motor to be overloaded:
1) The power supply voltage is low, and the required torque cannot be obtained so that the motor stops rotating;
2) Wrongly set the torque limit mechanism to make it larger than the stopping torque, resulting in continuous generation of excessive torque and stopping the motor
3) High-frequency use, the heat accumulation generated exceeds the allowable temperature rise value of the motor;
4) For some reason, the torque limit mechanism circuit fails, which makes the torque too large;
5) The operating environment temperature is too high, which relatively reduces the thermal capacity of the motor.
In the past, the way to protect the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. There is no absolutely reliable protection method for variable-load equipment such as electric devices.
Therefore, various combinations must be adopted, which can be summarized in two ways:
1) Judge the increase or decrease of the input current of the motor;
2) Judge the heating of the motor itself.
Regardless of these two methods, the time margin given by the motor’s thermal capacity must be considered.
The basic protection methods of overload are as follows:
1) The use of a thermostat is used for overload protection of the motor for continuous operation or jog operation;
2) Thermal relay is used to protect the motor from stalling;
3) For short-circuit accidents, use fuses or overcurrent relays.
Valve electric actuator is an indispensable device for realizing valve program control, automatic control, and remote control. Its movement process can be controlled by stroke, torque, or axial thrust.
The correct selection of valve electric devices is essential to prevent overloading (working torque is higher than control torque).
Article source: Global Valve Network
