The valve electric device is an indispensable driving device for realizing valve program control, automatic control and remote control. Its movement process can be controlled by the size of stroke, torque or axial thrust. Since the working characteristics and utilization rate of the valve electric 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 electric device is generally composed of the following parts:
Special motor, characterized by strong overload capacity, large starting torque, small moment of inertia, and short-term intermittent operation.
Speed reduction mechanism, used to reduce the output speed of the motor.
Stroke control mechanism, used to adjust and accurately control the opening and closing position of the valve.
Torque limiting mechanism, used to adjust the torque (or thrust) and make it not exceed the predetermined value.
Manual electric switching mechanism, an interlocking mechanism for manual or electric operation.
Opening indicator, used to display the position of the valve during the opening and closing process.
1. Select electric actuator according to valve type
1. Angular stroke electric actuator (angle <360 degrees) is suitable for butterfly valves, ball valves, plug valves, etc.
The rotation of the output shaft of the electric actuator is less than one circle, that is, less than 360 degrees, and usually 90 degrees is enough to control the opening and closing process of the valve. This type of electric actuator is divided into two types according to the different installation interface methods: direct connection type and base crank type.
A) Direct connection type: refers to the form in which the output shaft of the electric actuator is directly connected to the valve stem.
B) Base crank type: refers to the form in which the output shaft is connected to the valve stem through a crank.
2. Multi-turn electric actuators (rotation angle > 360 degrees) are suitable for gate valves, stop valves, etc. The rotation of the output shaft of the electric actuator is greater than one circle, that is, greater than 360 degrees, and generally multiple circles are required to control the opening and closing process of the valve.
3. Straight stroke (linear motion) is suitable for single-seat regulating valves, double-seat regulating valves, etc. The movement of the output shaft of the electric actuator is linear motion, not rotation.
2. Determine the control mode of the electric actuator according to the production process control requirements
1. Switch type (open loop control) Switch type electric actuators generally realize the opening or closing control of the valve. The valve is either in the fully open position or in the fully closed position. This type of valve does not require precise control of the medium flow.
It is particularly worth mentioning that switch type electric actuators can also be divided into split structure and integrated structure due to different structural forms. This must be explained when selecting, otherwise it will often conflict with the control system during on-site installation.
A) Split structure (usually called ordinary type): The control unit is separated from the electric actuator. The electric actuator cannot control the valve alone. An external control unit must be added to achieve control. Generally, the external controller or control cabinet is used for matching. The disadvantage of this structure is that it is not convenient for the overall installation of the system, increases the wiring and installation costs, and is prone to failure. When a failure occurs, it is not convenient for diagnosis and maintenance, and the cost performance is not ideal.
B) Integrated structure (usually called integral type): The control unit and the electric actuator are packaged as one, and can be operated on-site without an external control unit. Remote operation can be performed by simply outputting relevant control information. The advantages of this structure are that it is convenient for the overall installation of the system, reduces wiring and installation costs, and is easy to diagnose and troubleshoot. However, traditional integrated structure products also have many imperfections, so intelligent electric actuators have emerged.
2. Adjustable (closed-loop control) The adjustable electric actuator not only has the function of the switch type integrated structure, but also can accurately control the valve and adjust the medium flow.
A) Control signal type (current, voltage) The control signal of the adjustable electric actuator generally has a current signal (4~20MA, 0~10MA) or a voltage signal (0~5V, 1~5V). When selecting, it is necessary to clarify its control signal type and parameters.
B) Working form (electric opening type, electric closing type) The working mode of the adjustable electric actuator is generally electric opening type (taking 4~20MA control as an example, the electric opening type means that the 4MA signal corresponds to the valve closing, and the 20MA corresponds to the valve opening), and the other is the electric closing type (taking 4-20MA control as an example, the electric opening type means that the 4MA signal corresponds to the valve opening, and the 20MA corresponds to the valve closing).
C) Loss of signal protection Loss of signal protection means that when the control signal is lost due to line faults, the electric actuator will control the valve to open and close to the set protection value. Common protection values are fully open, fully closed, and keep in place.
3. According to the use environment and pressure level
According to the use environment and pressure level requirements, the electric device of the valve can be divided into ordinary type, outdoor type, explosion-proof type, outdoor explosion-proof type, etc.
4. According to the torque required by the valve
Determine the output torque of the electric actuator according to the use environment and pressure level. The torque required for the valve to open and close determines the output torque of the electric actuator. It is generally proposed by the user or selected by the valve manufacturer. As an actuator manufacturer, it is only responsible for the output torque of the actuator. The torque required for the normal opening and closing of the valve is determined by factors such as the valve diameter and working pressure. However, due to the differences in processing accuracy and assembly process of valve manufacturers, the torque required for valves of the same specification produced by different manufacturers is also different. Even the torque of valves of the same specification produced by the same valve manufacturer is different. When selecting the actuator, if the torque is too small, it will cause the valve to be unable to open and close normally. Therefore, the electric actuator must select a reasonable torque range.
5. The basis for correctly selecting valve electric devices
Operating torque: The operating torque is the most important parameter for selecting valve electric devices. The output torque of the electric device should be 1.2 to 1.5 times the maximum operating torque of the valve.
Operating thrust: There are two main structures of valve electric devices: one is not equipped with a thrust plate and directly outputs torque; 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.
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 valve stem pitch, and the number of thread heads. 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 valve stem transmission thread pitch, and Z is the number of valve stem thread heads).
Stem diameter: For multi-turn rising stem valves, if the maximum stem diameter allowed by the electric device cannot pass through the valve stem of the 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 valve stem of the rising stem valve. For partial-turn valves and concealed stem valves in multi-turn valves, although the problem of the passage of the valve stem diameter does not need to be considered, the valve stem diameter and the size of the keyway should also be fully considered when selecting, so that it can work normally after assembly. Output speed: If the valve opening and closing speed is too fast, water hammer will occur easily. Therefore, the appropriate opening and closing speed should be selected according to different use conditions.