Usually, the correct basis for selecting valve electric actuators is as follows:
1.Operating torque:
Operating torque is the most important parameter for the selection of electric valve devices, and the output torque of the electric actuator should be 1.2 to 1.5 times the maximum operating torque of the valve.
2.Structures for operating the electric device of the thrust valve:
There are two main structures for operating the electric device of the thrust valve: one is to output torque directly without configuring a thrust disc; another option is to configure a thrust disc, where the output torque is converted into output thrust through the valve stem nut in the thrust disc.
3.Number of output shaft rotation circle:
The number of output shaft rotations of the valve electric device is related to the nominal diameter, valve stem pitch, and number of thread heads of the valve, and should be calculated according to M=H/ZS (M is the total number of rotations that the electric device should meet, H is the valve opening height, S is the valve stem transmission thread pitch, and Z is the number of valve stem thread heads).
4.Stem diameter
For multi-rotary class of open-stem valves with stem diameter, if the maximum 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 greater than the outer diameter of the stem of the rising stem valve. For non rising stem valves in partial rotation valves and multi-rotary valves, although the diameter of the valve stem does not need to be considered, the size of the valve stem diameter and key way should also be fully considered during selection to ensure normal operation after assembly.
5.Output speed:
If the opening and closing speed of the valve is too fast, it is easy to cause water hammer phenomenon. Therefore, the appropriate opening and closing speed should be selected based on different usage conditions.
6. Special requirements
Valve electric actuators have a special requirement, that is, they must be able to limit torque or axial force. Usually, valve electric devices use torque limiting couplings. Once the specification of the electric device has been determined, its control torque is also determined. Generally, it runs within a predetermined time and the motor will not overload.
However, overloading may occur under the following conditions:
One is that the power supply voltage is low and the required torque cannot be obtained, causing the motor to stop rotating;
The second is to mistakenly set the torque limiting mechanism to exceed the stopping torque, causing continuous excessive torque generation and the motor to stop rotating;
The third is intermittent use, which generates heat accumulation that exceeds the allowable temperature rise of the motor;
Fourthly, due to some reason, the circuit of the torque limiting mechanism malfunctions, resulting in excessive torque;
The fifth reason is that the operating environment temperature is too high, which relatively reduces the thermal capacity of the motor.
In the past, the methods of protecting motors were to use fuses, over current relays, thermal relays, thermostats, etc., but each of these methods had its own advantages and disadvantages. There is no absolutely reliable protection method for variable load equipment such as electric devices.
Therefore, various combination methods must be adopted, which can be summarized into two types: one is to judge the increase or decrease of the motor input current; the second is to judge the heating condition of the motor itself. Both of these methods need to consider the time margin given by the motor’s thermal capacity.
Usually, the basic protection method for overload is as follows:
(1). Use a thermostat for overload protection during continuous operation or jog operation of the motor;
(2). Thermal relays are used to protect the motor from stalling;
(3). For short circuit accidents, use fuses or over current relays.
Valve electric actuators are indispensable equipment for achieving valve programming, automatic control, and remote control, and their movement process can be controlled by the magnitude of stroke, torque, or axial thrust.
The correct selection of valve electric devices is crucial to prevent overloading (working torque higher than control torque).
BCST Group is committed to the research development and manufacture of various types of valve electric actuators, welcome to consult and inquire, we will be happy to serve you. Please go to this URL (www.bcstgroup.com) to browse.
