The most extensive definition of actuator is: a kind of driving device which can provide linear or rotary motion. It uses some driving energy and works under some control signal. The actuator uses liquid, gas, electricity or other energy sources and converts them into driving force through motors, cylinders or other devices.
The basic actuator is used to drive the valve to the fully open or fully closed position. With the control valve actuator can accurately make the valve go to any position. Although most of the actuators are used to open and close valves, the design of today's actuators is far beyond the simple switching function. They include position sensing device, torque sensing device, electrode protection device, logic control device, digital communication module and PID control module, etc., all of which are installed in a compact shell.
1. Select electric actuator according to valve type
There are many kinds of valves, and the working principle is not the same. Generally, the opening and closing control is realized by turning the angle of the valve plate and lifting the valve plate. When matching with the electric actuator, the electric actuator should be selected according to the type of the valve.
1.1. Angular stroke electric actuator (angle < 360 degrees)
When the rotation of the output shaft of the electric actuator is less than one circle, that is, less than 360 degrees, usually 90 degrees, the valve opening and closing process can be controlled. This kind of electric actuator can be divided into direct connection type and base crank type according to the different installation interface mode.
a) Direct connection type: it refers to the direct connection between the output shaft of the electric actuator and the valve stem.
b) Base crank type: it refers to the form that the output shaft is connected with the valve stem through the crank. This kind of electric actuator is suitable for butterfly valve, ball valve, plug valve, etc.
1.2. Multi turn electric actuator (angle > 360 degrees)
The rotation of the output shaft of the electric actuator is more than one circle, that is, more than 360 degrees. Generally, it takes more than one circle to realize the opening and closing process control of the valve. This kind of electric actuator is suitable for gate valve, globe valve, etc.
1.3. Straight stroke (linear motion)
The movement of the output shaft of the electric actuator is linear, not rotational. This kind of electric actuator is suitable for single seat control valve, double seat control valve, etc.
2. Determine the control mode of electric actuator according to the production process control requirements
The control modes of electric actuators are generally divided into switch type (open-loop control) and regulation type (closed-loop control).
2.1. Switch type (open loop control)
The switch type electric actuator generally realizes the open or close control of the valve. The valve is either in the fully open position or in the fully closed position. This kind of valve does not need to accurately control the medium flow. It is particularly worth mentioning that the switch type electric actuator can also be divided into split structure and integrated structure due to different structural forms. This must be explained when selecting the model, otherwise it will often happen in the field installation conflict with the control system and other mismatches.
a) Split structure (commonly referred to as common type): the control unit is separated from the electric actuator. The electric actuator can not control the valve independently, and the control unit must be added to realize the 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 whole system installation, increases the wiring and installation costs, and is prone to failure. When the failure occurs, it is not convenient for diagnosis and maintenance, and the cost performance is not ideal.
b) Integrated structure (commonly known as integrated): the control unit and the electric actuator are packaged as a whole, which can be operated on the spot without the external control unit, and the remote control unit can be operated only by outputting the relevant control information. The advantage of this structure is that it is convenient to install the whole system, reduces the cost of wiring and installation, and is easy to diagnose and remove faults. But the traditional integrated structure products also have many imperfections, so the intelligent electric actuator is produced, which will be explained later.
2.2. Regulated (closed loop control)
The adjustable electric actuator not only has the function of switch type integrated structure, but also can accurately control the valve, so as to accurately adjust the medium flow.
a) Control signal type (current, voltage)
The control signals of adjustable electric actuators generally include current signals (4 ~ 20mA, 0 ~ 10mA) or voltage signals (0 ~ 5V, 1 ~ 5V). The type and parameters of control signals should be determined when selecting the type.
b) Working mode (electric on type, electric off type)
The working mode of regulation type electric actuator is generally electric open type (taking 4-20mA control as an example, electric open type refers to 4mA signal corresponding to valve closing and 20mA corresponding to valve opening), and the other type is electric close type (taking 4-20mA control as an example, electric open type refers to 4mA signal corresponding to valve opening and 20mA corresponding to valve closing).
c) Loss of signal protection
Loss of signal protection means that when the control signal is lost due to the line fault, the electric actuator will open and close the control valve to the set protection value. The common protection values are full open, full close and in situ, and it is not easy to modify after leaving the factory.
3. Determine the output torque of the electric actuator according to the torque required by the valve
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 the 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 the valve diameter, working pressure and other factors. However, due to the differences in processing accuracy and assembly process of valve manufacturers, the torque required for the same specification of valves produced by different manufacturers is different The required torque is also different, even the torque of the same specification valve produced by the same valve manufacturer is also different. If the torque of the selected actuator is too small, the valve cannot be opened and closed normally. Therefore, the electric actuator must select a reasonable torque range.
4. Determine the electrical parameters according to the selected electric actuator
Because the electrical parameters of different actuator manufacturers are different, it is generally necessary to determine the electrical parameters in the design and selection, mainly including motor power, rated current, secondary control circuit voltage, etc. As a result, the parameters of the control system and the electric actuator do not match, resulting in open tripping, fuse fusing, thermal overload relay protection tripping and other faults.
5. Choose the protection grade and explosion-proof grade according to the application situation
5.1. Enclosure protection level
The enclosure protection grade refers to the external object and waterproof grade of the electric actuator. It is indicated by the letter IP followed by two digits. The first digit represents the external object protection grade from 1 to 6, and the second digit represents the waterproof grade from 1 to 8.
5.2. Explosion proof grade
In places where explosive gas, steam, liquid and combustible dust may cause fire or explosion hazard, it is necessary to put forward explosion-proof requirements for electric actuators, and select explosion-proof forms and types according to different application areas. Explosion proof grade can be indicated by ex and explosion-proof content (refer to explosion proof electrical equipment for explosive environment GB3836-2000). The contents of explosion-proof signs include: explosion-proof type + equipment category + (gas group) + temperature group.