Pneumatic, electric, and hydraulic actuators for control valves

Thread Content

The actuators used for control valves are mainly of three types: pneumatic, electric, and hydraulic (electro-hydraulic). Each type has its own advantages and disadvantages, which will be discussed below. 1. Pneumatic actuators: In most industrial control applications today, pneumatic actuators are used as the actuating elements. Since they are powered by air, they are more cost-effective compared to electric and hydraulic actuators. Moreover, they have a simple structure, making them easy to operate and maintain. From a maintenance perspective, pneumatic actuators are easier to operate and calibrate than other types of actuators, and it is also simple to swap them between forward, reverse, left, and right positions on site. Its greatest advantage is safety; when a locator is used, it is ideal for flammable and explosive environments, whereas telecommunication signals that are not explosion-proof or intrinsically safe pose a potential risk of starting a fire due to arcing. Therefore, although electric control valves are being used in an increasingly wide range of applications, in the chemical industry, pneumatic control valves still hold an absolute advantage. The main disadvantages of pneumatic actuators are slow response, poor control accuracy, and weak resistance to deviation, which is due to the compressibility of gas; especially when large pneumatic actuators are used, it takes time for the air to fill and empty the cylinder. But this shouldn’t be a problem, as in many operating conditions high control precision, extremely fast response, and the ability to resist deviations are not required. 2. Electric actuators: Electric actuators are primarily used in power plants or nuclear power plants, as high-pressure water systems require a smooth, stable, and gradual process. The main advantages of electric actuators are their high stability and the constant thrust they can provide to the user. The maximum thrust that can be generated by such actuators is up to 225,000 kgf; only hydraulic actuators are capable of producing such high levels of thrust, but hydraulic actuators are much more expensive than electric ones. Electric actuators have excellent resistance to deviation; the thrust or torque they generate remains essentially constant, which allows them to effectively counteract the unbalanced forces associated with the medium and thus achieve precise control over process parameters. As a result, their control accuracy is higher than that of pneumatic actuators. When equipped with a servo amplifier, it is easy to switch between forward and reverse operation, as well as to set the position of the shut-off valve (held/fully open/fully closed). In the event of a fault, it remains in its original position, which is something that pneumatic actuators cannot do; pneumatic actuators require a combination of protection systems to maintain their position. The main disadvantages of electric actuators are: their complex structure, which makes them more prone to failure; and due to this complexity, they require higher technical skills from the maintenance personnel on site ； The motor generates heat during operation; frequent adjustments can easily lead to overheating of the motor, triggering thermal protection mechanisms, and it also increases wear on the reduction gears ； Another issue is the slower operation speed; it takes a considerable amount of time for a signal to be sent from the regulator for the control valve to respond and move to the corresponding position. This is one of the reasons why it is inferior to pneumatic and hydraulic actuators. 3. Hydraulic actuators: When exceptional deviation resistance, high thrust, and a fast response time are required, hydraulic or electro-hydraulic actuators are often chosen. Due to the incompressibility of liquids, the advantage of using hydraulic actuators is their superior ability to resist deviation, which is important for regulating operating conditions. This is because the throttling condition becomes unstable when the regulating element approaches the valve seat, and this issue becomes more severe as the pressure difference increases. Furthermore, the hydraulic actuator operates very smoothly and responds quickly, enabling high-precision control. The electro-hydraulic actuator integrates a motor, an oil pump, and an electro-hydraulic servo valve into one unit; it can operate as long as it is connected to a power source and control signals. The hydraulic actuator is similar to a cylinder, but it can withstand higher pressures. Its operation requires an external hydraulic system, meaning that factories need to have hydraulic stations and oil delivery pipelines in place. In comparison, the electro-hydraulic actuator is more convenient to use. The main disadvantages of hydraulic actuators are their high cost, large and bulky size, complexity, and the need for specialized engineering; as a result, they are mostly used in special applications such as power plants and the petrochemical industry. Source: HuiCong.com, Wuxi Sheng Hans, Guo Kaijun. This post was last edited by guokaijun888 on 2009-2-5 at 14:21.]