Definition of actuator: a driving device that can provide linear or rotary motion. It uses a certain driving energy and works under the action of a certain control signal. The actuator uses liquid, gas, electricity or other energy and converts it into driving action through motor, cylinder or other devices. Its basic types are part turn, multi turn and linear.
The basic actuator is used to drive the valve to the fully open or fully closed position. The actuator used to control the valve can accurately move the valve to any position. Although most actuators are used to switch valves, the design of today's actuators goes 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, all of which are installed in a compact shell.
Because more and more factories adopt automatic control, and manual operation is replaced by machinery or automatic equipment, people require the actuator to play the role of the interface between the control system and the mechanical movement of the valve, and also require the actuator to enhance the performance of work safety and environmental protection. In some dangerous situations, automatic actuator devices can reduce personal injury.
Some special valves require emergency opening or closing under special circumstances. The valve actuator can prevent the further spread of danger and minimize plant losses. For some high-pressure and large-diameter valves, the required actuator output torque is very large. At this time, the required actuator must improve mechanical efficiency and use high output motor, so as to operate the large-diameter valves smoothly. For some small torque valves, fine and small electric valves are also used. Compared with ordinary valves, they have the advantages of light weight, compact structure and complete functions.
In order to successfully realize process automation, the most important thing is to ensure that the valve itself can meet the special requirements of the process and the medium in the pipeline. Usually, the production process and process medium can determine the type of valve, the type of valve core, the structure and material of valve internals and valves.
After the valve is selected, the requirements of automation and the selection of actuator should be considered. The actuator can be simply considered according to two basic valve operation types.
Rotary valves include plug valves, ball valves, butterfly valves and dampers or baffles. This type of valve requires an actuator that rotates 90 degrees with the required torque.
Multi turn valves can be non rotating lifting stem or rotating non lifting stem, or they need multi turn operation to drive the valve to the open or closed position. Such valves include: through valve (stop valve), gate valve, knife gate valve, etc. As an option, pneumatic or hydraulic cylinders or diaphragm actuators with linear output can also drive the above valves.
The main advantages of electric actuator are high stability and constant thrust that can be applied by users. The thrust generated by the maximum actuator can be as high as 225000kgf. Only hydraulic actuator can achieve such a large thrust, but the cost of hydraulic actuator is much higher than that of electric actuator.
The anti deviation ability of the electric actuator is very good, and the output thrust or torque is basically constant, which can well overcome the unbalanced force of the medium and achieve the accurate control of process parameters. Therefore, the control accuracy is higher than that of the pneumatic actuator. If the servo amplifier is used, the exchange of positive and negative effects can be easily realized, and the off signal valve position state (hold / fully open / fully closed) can be easily set. In case of fault, it must stay in the original position, which can not be done by the pneumatic actuator. The pneumatic actuator must realize position preservation with the help of a set of combined protection system.
At present, there are two types of electric actuators, which are generally divided into part turn electric valve actuator and multi turn electric valve actuator. The former mainly controls valves requiring partial rotation, such as ball valve and butterfly valve, while the latter requires valves requiring multiple turns of rotation, such as gate valve.
Electric driven multi rotary actuator is one of the most commonly used and reliable types of actuator. The single-phase or three-phase motor is used to drive the gear or worm gear, and finally drive the valve stem nut. The valve stem nut moves the valve stem to open or close the valve. Multi rotary electric actuator can quickly drive large valves. In order to protect the valve from damage, the limit switch installed at the end of the valve stroke will cut off the power supply of the motor. At the same time, when the safety torque is exceeded, the torque sensing device will also cut off the power supply of the motor. The position switch is used to indicate the switching state of the valve. The handwheel mechanism installed with the clutch device can manually operate the valve in case of power failure.