How to correctly select and use electric actuators and electric devices for valves
1. Select the electric actuator based on the type of valve
1. Angular travel electric actuators (rotation Angle <360 degrees) are suitable for butterfly valves, ball valves, plug valves, etc.
The rotation of the output shaft of the electric actuator is less than one full circle, that is, less than 360 degrees, usually 90 degrees, to achieve the process control of opening and closing the valve. This type of electric actuator is further classified into two types based on the different installation interface methods: direct connection type and base crank type.
a) Direct connection type: It refers to the form in which the output shaft of the electric actuator is directly connected to the valve stem for installation.
b) Base crank type: It 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.
When the output shaft of the electric actuator rotates more than one full circle, that is, more than 360 degrees, it generally requires multiple turns to achieve the process control of opening and closing the valve.
3. Linear stroke (linear motion) is applicable to single-seat control valves, double-seat control valves, etc.
The movement of the output shaft of the electric actuator is linear motion, not rotational.
Second, determine the control mode of the electric actuator based on the requirements of the production process control
1. Switching type (open-loop control
On-off electric actuators generally achieve the opening or closing control of valves. The valves are either in the fully open position or the fully closed position. Such valves do not require precise control of the medium flow. It is particularly worth mentioning that switch-type electric actuators can be further classified into split structure and integrated structure due to their different structural forms. When selecting the type, this must be explained; otherwise, mismatches such as conflicts with the control system during on-site installation often occur.
a) Split structure (commonly known as the common type) : The control unit and the electric actuator are separated. The electric actuator cannot control the valve independently. An additional control unit is required to achieve control. Generally, an 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 wiring and installation costs, and is prone to faults. When faults occur, it is not easy to diagnose and repair, and the cost performance is not ideal.
b) Integrated structure (commonly referred to as integral type) : The control unit and the electric actuator are encapsulated into one unit, allowing for on-site operation without the need for an external control unit. Remotely, only relevant control information needs to be output to operate it. 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 eliminate faults. However, traditional integrated structure products also have many imperfections, so intelligent electric actuators have emerged.
2. Regulating type (closed-loop control
The regulating electric actuator not only has the function of the on-off integrated structure, but also can precisely control the valve and regulate the flow of the medium.
a) Control signal type (current, voltage) : The control signals of regulating electric actuators generally include current signals (4 ~ 20mA, 0 ~ 10mA) or voltage signals (0 ~ 5V, 1 ~ 5V). When selecting, it is necessary to clearly define the type and parameters of the control signal.
b) Working forms (electrically open type, electrically closed type) : The working mode of the regulating type electric actuator is generally the electrically open type (taking the control of 4-20mA as an example, the electrically open type means that the 4mA signal corresponds to valve closing and the 20mA signal corresponds to valve opening), and the other is the electrically closed type (taking the control of 4-20mA as an example, the electrically open type means that the 4mA signal corresponds to valve opening 20mA corresponds to valve closure.
c) Signal loss protection: Signal loss protection refers to the situation where the electric actuator opens or closes the control valve to the set protection value when the control signal is lost due to line faults or other reasons. Common protection values include fully open, fully closed, and remain in position.
Iii. Electric devices classified according to the usage environment and explosion-proof grade
According to the usage environment and explosion-proof grade requirements, the electric actuators of valves can be classified into ordinary type, outdoor type, flameproof type, outdoor flameproof type, etc.
4. Determine the output torque of the electric actuator based on the torque required by the valve
The torque required for the opening and closing of the valve 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, we are 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 differences in processing accuracy and assembly techniques among valve manufacturers, Therefore, the torque required for valves of the same specification produced by different manufacturers also varies. Even for valves of the same specification produced by the same valve manufacturer, the torque may differ. If the torque of the actuator is too small when selecting the type, it will result in the inability to open and close the valve normally. Therefore, an electric actuator must be selected within a reasonable torque range.
V. Basis for the Correct Selection of Valve Electric Actuators:
Operating torque: Operating torque is the most crucial parameter for selecting an electric actuator for valves. The output torque of the electric actuator should be 1.2 to 1.5 times the maximum operating torque of the valve.
Operating thrust: There are two main structures for the valve electric actuator: One is not equipped with a thrust plate and directly outputs torque; Another type is to configure a thrust plate, and the output torque is converted into output thrust through the valve stem nut on the thrust plate.
The number of rotations of the output shaft: The number of rotations of the output shaft of the valve electric actuator 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 as M=H/ZS (M is the total number of rotations that the electric actuator should meet, H is the opening height of the valve, S is the pitch of the valve stem transmission thread, and Z is the number of thread heads of the valve stem).
Valve stem diameter: For multi-turn type exposed stem valves, if the maximum valve stem diameter that the electric actuator is allowed to pass through cannot pass through the valve stem of the valve it is equipped with, an electric valve cannot be assembled. 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 the rising stem valve. For partial turn valves and concealed stem valves in multi-turn valves, although the issue of the diameter of the valve stem passing through does not need to be considered, the size of the valve stem diameter and keyway should also be fully taken into account when selecting and matching to ensure normal operation after assembly.
Output speed: If the opening and closing speed of the valve is too fast, water hammer is likely to occur. Therefore, the appropriate opening and closing speed should be selected according to different usage conditions.