How to Choose the Right Valve Electric Actuator

The valve electric actuator is an indispensable device for realizing the remote control, automatic control and remote control of valves. Its movement process can be controlled by the stroke, torque or axial thrust. Since the working characteristics and utilization rate of the valve electric actuator depend on the type of valve, the working specifications of the device and the position of the valve on the pipeline or equipment, correctly choosing the valve electric actuator is crucial to preventing overloading (where the operating torque is higher than the control torque).

Generally, the basis for correctly choosing a valve electric actuator is as follows:

1. Operating Torque:

The operating torque is the most important parameter for selecting a valve electric actuator. The output torque of the electric actuator should be 1.2 to 1.5 times the maximum torque of the valve operation.

2. Operating Thrust:

There are two main structures for the main body of the valve electric actuator: one is without a thrust disc and directly outputs the torque; the other is with a thrust disc, and the output torque is converted into output thrust through the valve stem nut.

3. Number of Rotations of the Output Shaft:

The number of rotations of the output shaft of the valve electric actuator is related to the nominal diameter of the valve, the valve stem pitch, and the number of threads of the valve stem. It can be calculated by M = H/ZS (M is the total number of rotations that the electric actuator should meet, H is the opening height of the valve, S is the pitch of the valve stem transmission thread, and Z is the number of threads of the valve stem).

4. Valve Stem Diameter:

For multi-turn type open-end valves, if the maximum valve stem diameter allowed by the electric actuator cannot pass through the valve stem of the matched valve, the electric actuator cannot be assembled into 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 open-end valve stem. For some multi-turn valves and the concealed stem valves in multi-turn valves, although the valve stem diameter passing issue does not need to be considered, the valve stem diameter and the key slot size should still be fully considered when selecting to ensure normal operation after assembly.

5. Output Speed:

If the opening and closing speed of the valve is too fast, water hammer phenomena may occur. Therefore, an appropriate opening and closing speed should be selected according to different usage conditions.

The valve electric actuator has its special requirements, that is, it must be able to limit torque or axial force. Usually, the valve electric actuator uses a torque-limiting coupling. After the specification of the electric actuator is determined, the control torque is also determined. Generally, it operates within a pre-determined time, and the motor will not overheat. However, the following situations may lead to overloading: first, the power supply voltage is low, and the required torque cannot be obtained, causing the motor to stop rotating; second, the torque limit mechanism is wrongly adjusted, making it greater than the stop torque, resulting in continuous generation of excessive torque and causing the motor to stop rotating; third, intermittent use, the accumulated heat exceeds the allowable temperature rise of the motor; fourth, the torque limit circuit of the torque limit mechanism fails, causing excessive torque; fifth, due to some reason, the torque limit mechanism circuit fails, causing excessive torque; sixth, the operating environment temperature is too high, which reduces the thermal capacity of the motor.

In the past, the methods for protecting the motor were using fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their advantages and disadvantages. For this variable-load equipment like the valve electric actuator, there is no absolutely reliable protection method. Therefore, various combinations must be adopted, and there are two ways: one is to judge the increase or decrease of the input current of the motor; the other is to judge the heating condition of the motor itself. Both of these methods must consider the time margin given by the motor's thermal capacity.

Generally, the basic protection method for overloading is: for the overloading protection of continuous operation or point operation of the motor, use a thermostat; for the protection of motor stall, use a thermal relay; for short-circuit accidents, use a fuse or overcurrent relay.