single acting pneumatic actuators

There are only three types of actuators used for control valves: pneumatic, electric, and hydraulic (electro-hydraulic), and their performance has its advantages and disadvantages, as described below:

 

1. Pneumatic actuators: currently used in most industrial control occasions are pneumatic actuators. Due to the use of gas as the power source, it is more economical than electric and hydraulic actuators, simple in structure, and easy to grasp and maintain. From a maintenance point of view, pneumatic actuators are easier to operate and calibrate than other types of actuators and can also be easily interchanged in the field. It has the advantage of being safe. It is ideal for use in flammable and explosive atmospheres when using positioners. If the electrical signal is not explosion-proof or intrinsically safe, there is a potential risk of fire causing a fire. Therefore, although electric control valves are becoming more widely used, pneumatic control valves are still dominant in the chemical industry.

The main disadvantages of pneumatic actuators are slow response, poor control accuracy, and poor resistance to deviation. This is due to the compressibility of the gas, especially when using large pneumatic actuators where air needs to fill the cylinder and evacuate Time. However, this should not be a problem as many operating conditions do not require a high degree of control accuracy response and resistance to deviation.

 

2. Electric actuators: Electric actuators are mainly used in power or nuclear plants where smooth, stable, and slow processes are required for high-pressure water systems. The main advantages of electric actuators are the high stability and the constant thrust that can be applied by the user. Such large thrusts can only be achieved with hydraulic actuators, but hydraulic actuators are more expensive than electric actuators. The electric actuators have excellent deflection protection. Its output thrust or torque is essentially constant and it can well overcome the unbalanced forces of the medium to achieve control of process parameters with better control accuracy than pneumatic actuators. If equipped with a servo amplifier, the exchange of positive and negative effects can be easily achieved, and the state of the shut-off signal valve position can also be easily set (hold/fully open/fully closed), which must remain in the original position when a fault occurs. This is something that pneumatic actuators cannot do. Pneumatic actuators have to use a combined protection system to achieve position holding.

The disadvantages of electric actuators are: the structure is more complex, prone to failure, and, due to its complexity, the technical requirements for site maintenance personnel are relatively high; the motor needs to generate heat and, if adjusted too frequently, can easily cause the motor to overheat, creating thermal protection and also increasing the wear and tear on the reduction gear. In addition, it runs at a slow speed. It takes a long time to output a signal from the regulator to the corresponding position of the adjusting valve in response to the movement. It is not as good as pneumatic and hydraulic actuators.

 

3. Hydraulic actuators: We usually choose hydraulic or electro-hydraulic actuators when abnormal deflection resistance, high thrust, and fast formation speed are required. The advantage of using hydraulic actuators is better resistance to deviation due to the incompressibility of the liquid, which is important for regulating conditions, as the throttling conditions are unstable when the regulating element is close to the valve seat. The greater the pressure difference, the worse the situation. In addition, hydraulic actuators operate very smoothly and have a fast response time, allowing for high precision control. The electro-hydraulic actuator is an integrated motor, oil pump, and electro-hydraulic servo valve. It only needs to be connected to the power supply and the control signal to work. A hydraulic actuator is similar to a cylinder, but it can withstand higher pressures than a cylinder. An external hydraulic system is required for the work. The plant needs to be equipped with a hydraulic station and piping. In contrast, electro-hydraulic actuators are more convenient.

The main disadvantages of hydraulic actuators are high cost, large body inspection, and heavyweight, especially for complex and special projects, so they are mostly used in power plants, petrochemicals, and other special occasions.