The basis for the correct selection of electric valve actuator

April 17, 2019
Industry News

The electric valve actuator is an indispensable device for realizing valve program control, automatic control and remote control. The movement process can be controlled by the stroke, torque or axial thrust. Since the working characteristics and utilization rate of the electric valve device depend on the type of valve, the working specifications of the device and the position of the valve on the pipeline or equipment, the correct selection of the valve electric device is crucial to prevent overloading (the working torque is higher than the control torque). In general, the basis for the correct selection of valve electric devices is as follows:

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

There are two types of mainframe structures for operating the thrust valve electric device: One is that the thrust disk is not arranged and the torque is directly output, and the other is to configure the thrust disc, and the output torque is converted into the output thrust through the stem nut in the thrust disc.

Output shaft rotation
The number of turns of the output shaft of the valve electric device is related to the nominal diameter of the valve, the screw pitch of the valve stem and the number of threads. It should be calculated according to M=H/ZS (M is the total number of turns that the electric device should meet, H is the valve opening height, S is the stem drive thread pitch, and Z is the stem thread number).

Valve stem diameter for multi-turn type rising stem valves
If the valve stem of the valve being fitted cannot pass through the maximum stem diameter that the electric device allows to pass, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow shaft of the electric device must be larger than the outer diameter of the stem of the rising stem valve. For some rotary valve and the non-rising stem valve in the multi-turn valve, although the problem of the diameter of the stem is not considered, the diameter of the stem and the size of the keyway should be fully considered in the selection, so that the electric actuator can work normally after assembly.

If the opening and closing speed of the output speed valve is too fast, water hammer is likely to occur. Therefore, the appropriate opening and closing speed should be selected according to different conditions of use.

Valve electric devices have special requirements that must be able to limit torque or axial forces. Usually, the valve electric device uses a torque limiting coupling. When the specifications of the electric device are determined, the control torque is also determined. In general, running in a predetermined time, the motor will not be overloaded. However, if the following conditions occur, the overload may be caused: first, the power supply voltage is low, the required torque is not obtained, and the motor stops rotating; second, the torque limiting mechanism is set incorrectly to be greater than the stopped torque, causing continuous excessive torque to stop the motor from rotating; thirdly, The accumulated heat generated by the intermittent use exceeds the allowable temperature rise of the motor; fourth, for some reason, the torque limiting mechanism circuit is faulty and the torque is too large. Fifth, the ambient temperature is too high, and the thermal capacity of the motor is relatively reduced.

In the past, the protection of the motor was to use fuses, overcurrent relays, thermal relays, thermostats, etc., but these methods have their own advantages and disadvantages. For the variable load device, like the electric device, there is no absolute and reliable protection method. Therefore, various combinations must be adopted, which can be summarized in two ways: one is to judge the increase and decrease of the input current of the motor; the other is to judge the heat of the motor itself. In either case, the time margin given by the thermal capacity of the motor must be considered.

Generally, the basic protection method of overload is: using a thermostat for overload protection of continuous or jog operation of the motor; using a thermal relay for the protection of the motor stall; and using a fuse or an overcurrent relay for a short circuit accident.