Electric and pneumatic actuators for pipeline valves: It seems that the two types of actuators are very different, and the choice needs to be made based on the power source available at the installation site. But in reality, this viewpoint is biased. In addition to the main and obvious differences, they also have several less obvious uniqueness.

As an electric actuator used for pipeline valves, its design feature is that all control components are located inside the actuator housing. The wiring terminal block, limit switch and torque switch, contactor and logic board (if any) are all located inside the electric actuator housing (Figure 4). If switching from shutdown mode to control mode, it will often lead to serious technical issues. Therefore, it is necessary to ensure that the mechanical design of the actuator is suitable for different functions. In addition, if it is necessary to install a locator to fix the base plate, it is not recommended for operators to directly carry out construction on site, but to transport the equipment back to the manufacturing plant.

Even the simplest electric actuator must be equipped with an electronic control system, which is responsible for driving the actuator to start, reverse, and stop according to limit switches, operator instructions, or instructions from the higher-level automatic process control system. In other words, the electric starting device is either integrated into the actuator housing or made into an independent component, such as the electric actuator control module. In either case, the high technological content will lead to an increase in product costs, and regular maintenance is also required. As an essential component of the entire technical system, the control system not only requires direct item costs, but also affects the reliability of the technical system. Pneumatic actuators do not require a control cabinet, as long as they are equipped with solenoid valves and solenoid valve control circuits. Compared with the control system of electric actuators, the control system of pneumatic actuators is simpler and cheaper. In addition, due to its simple structure and small number of parts, the reliability of pneumatic actuator control systems is much higher than that of electric actuator control systems.

During the use of pipeline valves, it may be necessary to extend the displacement period of the valve closing element (reducing the speed of the actuator output end). The reason for such adjustments is often due to design errors or the need to adjust after analyzing operating parameters. If it involves an electric actuator, the original actuator can be replaced with an actuator with a longer cycle period, or the RPM of the actuator motor rotor can be adjusted. Both methods can extend the cycle period. The control of frequency involves a series of technical difficulties, so it has not become a common method for pipeline valves in automation control systems. But if it is a pneumatic actuator, simply adjusting the exhaust port of the actuator (reducing the cross-sectional area of the flow channel, recommended practice) or the supply port (acceptable practice) can extend the cycle of the pneumatic actuator. An easier approach is to adjust the flow rate of compressed air. On this basis, different speeds can be used for the "on-off" stroke and the "on-off" stroke. In addition, the extension of cycle time is very significant. But as an electric actuator, specifically the motor of the electric actuator, its disadvantage is that it has a limit on the running time, that is, the starting frequency and running time per hour (expressed as a percentage). In other words, after starting the electric actuator and moving the valve shut-off component, it is necessary to pause the motor for a period of time in order to allow the temperature of the motor coil to return to normal. This' run pause 'mode allows the motor stator coil to dynamically maintain normal operating temperature. It is precisely because of this restrictive mode that electric actuators cannot operate continuously. The pneumatic actuator has no limit on the number of cycles or starts, and Figure 3 can run continuously.

Some process systems place special emphasis on fault safety functions: in the event of an emergency, the valve shut-off component will automatically reach a pre-set valve position. The usual practice is to equip a normally open or normally closed valve with an actuator with a fail safe function. When an emergency situation occurs, the actuator will automatically open or close the valve. As for electric actuators, the power source for the fail safe function is usually an external battery, which has enough electricity to support one or several cycles. Pneumatic actuators, on the other hand, install springs inside a cylinder and modify double acting actuators into spring reset actuators. Apart from the relatively simple modification task, it is worth mentioning that the reset spring can be easily installed at any stage during operation. In addition, "air springs" are widely used in pneumatic automation systems. Store enough gas in the compressed air tank to support several operating cycles. When the gas supply pipeline loses pressure, the gas in the tank will be automatically released and enter the actuator gas supply pipeline.

This article briefly analyzes the main technical characteristics of electric and pneumatic actuators, and compares them from the perspectives of operation and running. I hope it will help to comprehensively and systematically understand these two types of pipeline valve actuators, and better select and operate them.