Five factors for the selection of valve electric devices
The valve electric device is an indispensable driving device to realize valve program control, automatic control and remote control. Its movement process can be controlled by stroke, torque or axial thrust. 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.
The electric device generally consists of the following parts:
Dedicated motor: It is characterized by strong overload capacity, large starting torque, small moment of inertia, and short-term intermittent work.
Deceleration mechanism: Used to reduce the output speed of the motor.
Stroke control mechanism: It is used to adjust and accurately control the opening and closing position of the valve.
Torque limiting mechanism: It is used to adjust the torque (or thrust) and make it not exceed the predetermined value.
Manual / electric switching mechanism: Interlocks for manual or electric operation.
Opening indicator: It is used to display the position of the valve during opening and closing.
Select the electric actuator according to the valve type
1. Quarter-stroke electric actuator (rotation angle <360 degrees) is suitable for butterfly valves, ball valves, plug valves, etc.
(1) The rotation of the output shaft of the electric actuator is less than one cycle, that is, less than 360 degrees, usually 90 degrees to realize the opening and closing process control of the valve. This type of electric actuator is divided into two types: direct connection type and base crank type according to the different installation interface methods.
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, globe valves, etc. The rotation of the output shaft of the electric actuator is more than one turn, that is, more than 360 degrees. Generally, it takes multiple turns to realize the opening and closing process control of the valve.
(3) Straight stroke (linear motion) is suitable for single-seat control valves, double-seat control valves, etc. The motion of the output shaft of the electric actuator is linear motion, not rotary.
2. Determine the control mode of the electric actuator according to the production process control requirements
(1) On-off type (open-loop control) :
On-off 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 a model, otherwise, mismatches such as conflicts with the control system during on-site installation often occur.
A) Split structure (commonly referred to as common type):
The control unit is separated from the electric actuator, and the electric actuator cannot control the valve alone. It must be controlled by an external control unit. Generally, it is implemented in the form of a controller or a control cabinet externally. matching. The disadvantage of this structure is that it is not convenient for the overall installation of the system, increases 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 into one body, and can be operated on the spot without an external control unit, and can be operated remotely only by outputting relevant control information. The advantage of this structure is that it facilitates the overall installation of the system, reduces wiring and installation costs, and is easy to diagnose and troubleshoot. However, the traditional integrated structure products also have many imperfections, so intelligent electric actuators have been produced.
(2) Regulating type (closed-loop control) :
The regulating type electric actuator not only has the function of an on-off integrated structure, but also can precisely 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), which needs to be specified when selecting the type Control signal type and parameters.
B) Working form (electrically open type, electric close type ) :
the working mode of the adjustable electric actuator is generally electric open type (take the control of 4 ~ 20MA as an example, the electric open type means that the 4MA signal corresponds to the valve close, 20MA Corresponding to the valve opening), 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 faults such as lines, the electric actuator will control the valve to open and close to the set protection value. The common protection values are fully open, fully closed, and maintain the original position.
3. According to the use environment and explosion-proof grade
Electric devices classified according to the use environment and explosion-proof grade According to the use environment and explosion-proof grade 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 explosion-proof level. The torque required for valve opening and closing determines the output torque of the electric actuator. Generally, it is proposed by the user or selected by the valve manufacturer. As an actuator manufacturer, only The output torque of the actuator is responsible. 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 the processing accuracy and assembly process of valve manufacturers, the torque required for valves of the same specification produced by different manufacturers There are also differences, even the torque of the same specification valve produced by the same valve manufacturer is also different. When the torque of the actuator is too small when selecting the model, it will cause the valve to fail to open and close normally. Therefore, the electric actuator must choose a reasonable torque range. .
5. The basis for the correct selection of valve electric devices
Operating torque: The operating torque is the most important parameter for selecting the valve electric device, and 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 the valve electric device: one is not equipped with a thrust disc, and directly outputs torque; the other is equipped with a thrust disc, and the output torque is converted into output thrust through the valve stem nut in the thrust disc.
The number of rotations of the output shaft: the number of rotations of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the valve stem, and the number of thread heads. It should be calculated according to M=H/ZS (M is the total rotation that the electric device should meet. The number of turns, H is the opening height of the valve, S is the pitch of the valve stem transmission thread, and Z is the number of threads of the valve stem).
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 part-turn valves and concealed stem valves in multi-turn valves, although it is not necessary to consider the diameter of the valve stem, the diameter of the valve stem and the size of the keyway should be fully considered when selecting and matching, so that it can work normally after assembly. 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 conditions of use.
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