The valve electric device is an indispensable device to realize valve program control, automatic control and remote control. Its movement process can be controlled by stroke, torque or axial thrust. Because the working characteristics and utilization of the valve electric device depend on the type of valve, the working specification of the device and the position of the valve on the pipeline or equipment. Therefore, the correct selection of valve electric devices is essential to prevent overload (working torque is higher than control torque).
Generally, the correct selection of valve electric device 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 device should be 1.2 to 1.5 times the maximum valve operating torque.
2. Operating Thrust
There are two main structures of the valve electric device: one is not equipped with a thrust plate, and the torque is directly output; the other is equipped with a thrust plate, and the output torque is converted into output thrust through the valve stem nut in the thrust plate.
3. Number of Turns of 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 stem, and the number of thread heads. It should be calculated as M=H/ZS (M is the total number of turns that the electric device should meet, and H is the valve Opening height, S is the thread pitch of the stem drive thread, Z is the number of stem threads).
4. Stem Diameter
For multi-turn rising stem valves, if the maximum stem diameter allowed by the electric device cannot pass through the stem of the matched 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 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 stem diameter, the stem diameter and the size of the keyway should also be fully considered when selecting, so that it can work normally after assembly.
5. Output Speed
If the opening and closing speed of the valve is too fast, water hammer will easily occur. Therefore, the appropriate opening and closing speed should be selected according to different usage conditions.
Valve electric devices have special requirements, that is, they must be able to limit torque or axial force. Usually valve electric devices use torque-limiting couplings. When the specifications of the electric device are determined, the control torque is also determined. Generally, it runs within a predetermined time and the motor will not be overloaded. However, overload may occur if the following conditions occur: First, the power supply voltage is low, and the required torque cannot be obtained, so that the motor stops rotating; Second, the torque limit mechanism is set incorrectly to make it greater than the stopped torque. Causes the continuous generation of excessive torque, causing the motor to stop rotating; third, intermittent use, the heat accumulation generated exceeds the allowable temperature rise of the motor; fourth, due to some reason, the torque limit mechanism circuit fails, causing the torque to be excessive Fifth, the use 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 summed up in two ways: one is to judge the increase or decrease of the input current of the motor; the other is to judge the heating of the motor itself. Regardless of these two methods, the time margin given by the motor thermal capacity must be considered.
Generally, the basic protection methods for overload are: the use of thermostats for the overload protection of the continuous operation or jog operation of the motor; the use of thermal relays for the protection of the motor stalls; the use of fuses or overcurrent relays for short-circuit accidents.
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