Basis for Selecting a Valve Electric Actuator
1. Operating Torque: Operating torque is the most important parameter for selecting a valve actuator. The output torque of the electric actuator should be 1.2 to 1.5 times the maximum operating torque of the valve.
2. Operating Thrust: There are two main body structures for valve actuators: one without a thrust plate and directly outputting torque; the other with a thrust plate, where the output torque is converted into output thrust via the valve stem nut in the thrust plate.
3. Output Shaft Rotation: The number of output shaft rotations of a valve actuator depends on the nominal diameter of the valve, the valve stem pitch, and the number of thread starts. It should be calculated as M = H / ZS (M is the total number of rotations required by the actuator, H is the valve opening height, S is the valve stem drive thread pitch, and Z is the number of valve stem thread starts).
4. Valve Stem Diameter: For multi-turn rising-stem valves, if the maximum valve stem diameter allowed by the actuator cannot pass through the valve stem of the valve to which it is attached, the 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 valve stem of a rising-stem valve. For part-turn valves and non-rising-stem valves in multi-turn valves, while valve stem diameter fit is not a concern, the valve stem diameter and keyway dimensions should be carefully considered during selection to ensure proper operation after assembly.
5. Output Speed: Excessively fast valve opening and closing speeds can easily cause water hammer. Therefore, the appropriate opening and closing speed should be selected based on different operating conditions.
6. Electric valve actuators have special requirements, requiring the ability to limit torque or axial force. Typically, valve actuators use torque-limiting couplings. Once the actuator specifications are determined, its control torque is also determined. Generally, operating within a predetermined timeframe will prevent overloading of the motor.
Conditions that lead to overload:
- First, low power supply voltage prevents the required torque from being achieved, causing the motor to stall.
- Second, the torque limiter is incorrectly set, exceeding the stall torque, resulting in continuous excessive torque and stalling the motor.
- Third, intermittent operation generates heat that exceeds the motor's allowable temperature rise.
- Fourth, a malfunction in the torque limiter circuit causes excessive torque.
- Fifth, excessively high ambient operating temperatures reduce the motor's thermal capacity.
Previously, motor protection methods used fuses, overcurrent relays, thermal relays, and thermostats, but each of these methods has its pros and cons. There is no absolutely reliable protection method for variable-load devices like electric motors.
Therefore, various combinations of methods must be employed. These can be summarized into two types: one is to assess the increase or decrease in the motor's input current; the other is to assess the motor's internal heating. Both methods must take into account the time margin given by the motor's thermal capacity.
Basic Overload Protection Methods
Generally, basic overload protection methods are:
1. For overload protection during continuous or inching operation of the motor, use a thermostat;
2. For motor stall protection, use a thermal relay;
3. For short-circuit protection, use a fuse or overcurrent relay.
Electric valve actuators are indispensable for programmable, automatic, and remote control of valves. Their movement can be controlled by stroke, torque, or axial thrust.
Proper selection of valve actuators is crucial to preventing overload (operating torque exceeding control torque).
