In the field of industrial automation, pneumatic actuators play a crucial role, especially in the chemical and petrochemical industries. Choosing the right pneumatic actuator not only enhances production efficiency but also ensures the stable operation and safety of equipment. This article will provide you with a comprehensive guide on how to select the appropriate pneumatic actuator.
Before selecting a pneumatic actuator, it is essential to understand the characteristics of the valves that require automatic control. The three most commonly used types of rotary valves in the chemical and petrochemical industries are ball valves, plug valves, and butterfly valves. Each type of valve has its unique torque curve, making it crucial to choose an accurate and safe actuator.
When fluid flows through a rotary valve, the static pressure on the surface of the closing part (ball, butterfly plate, or plug) is not uniform. This uneven pressure distribution generates dynamic torque. Dynamic torque is formed by the flowing fluid and acts on the valve shaft, potentially aiding or resisting the operation of the actuator.
Ball Valves: Traditional ball valves typically generate a small amount of dynamic torque because their frictional torque is often greater than the dynamic torque. When determining the actuator model, the influence of dynamic torque can usually be ignored.
Plug Valves: Similar to ball valves, plug valves also have a frictional torque that is usually greater than the dynamic torque.
Butterfly Valves: Butterfly valves often have a low frictional torque. Dynamic torque only has a significant impact when the valve disc is fully closed against the seat. For symmetrical industrial butterfly valves with the valve stem at the center of the valve plate, dynamic torque can close the valve. However, for high-performance butterfly valves with an offset valve plate and an asymmetric valve plate surface, the influence of dynamic torque is more complex.
The operation of a pneumatic actuator is influenced by two external factors: signal and power source.
The signal is typically a discrete voltage of 120/240 VAC or 12/24 VDC, which can power solenoid valves. If a positioner is used to control the actuator's rotation, the signal is usually analog (4~20 mA) or digital. Solenoid valves or digital positioners control the supply and exhaust of air to the actuator's cylinder, thereby alternately controlling the valve's position.
Solenoid Valves: Used to switch the valve between open and closed positions.
Positioners: In addition to the function of solenoid valves, positioners are mainly used for control and can support more advanced applications, such as partial stroke testing in safety instrumented systems. With the decreasing cost of digital positioners, their application is continuously growing.
The common power source for pneumatic actuators is compressed air at approximately 60~100 psig. Each actuator has a fail position when the air supply is cut off.
Double-Acting Actuators: When the air supply is cut off, the valve will remain in its last position. However, if the torque generated by the pipeline fluid pressure is greater than the valve's frictional torque, the valve may rotate.
Spring-Return Actuators: When the valve loses air supply, it will return to its initial position. This design is typically chosen for critical applications with fail-safe requirements.
Once the torque requirements of the valve have been determined, the actuator can be correctly selected. Accurate information is essential before selection. Customers should provide the minimum air supply pressure and the mode of operation for the actuator (e.g., double-acting or spring-return). If a spring-return actuator is required, the fail mode (i.e., fail-closed or fail-open) must also be determined.
The output torque of the selected actuator at the minimum air supply pressure should be greater than the calculated valve torque.
Fail-Closed: The output torque of the selected actuator at the end of the spring stroke under the minimum air supply pressure should be greater than the torque required to close the valve.
Fail-Open: The torque output of the selected actuator at the end of the pneumatic stroke under the minimum air supply pressure should be greater than the torque required to open the valve.
When selecting gear-rack and scotch-yoke actuators for valves or dampers, engineers should ensure a safety margin between the estimated operating torque of the valve and the actual output torque of the actuator. If a safety factor is needed to specify and adjust the valve torque, the following guidelines can be followed:
Safety Factor: It should be added to the valve torque, not the actuator torque.
End-User Recommendation: If the end-user has a recommendation, or if the torque data in the valve catalog specifies "sizing torque" or "actuator sizing torque," no additional safety factor is needed.
No End-User Recommendation: If the end-user does not have a recommendation, or if the torque data in the valve catalog indicates "valve torque," the following safety factors should be added to the valve torque:
Double-Acting Actuator: Add 10% to the "valve torque."
Spring-Return Actuator: Add 15% to the "valve torque."
When selecting actuators for safety shutoff valves (SSOV) and emergency shutoff valves (ESD), there are usually special performance requirements.
If the frequency of valve operation is less than once a month, it is recommended to use a safety factor of twice the calculated valve torque.
Most SSOVs have a specific closure time limit (usually very fast). It is not uncommon for a customer to require a 4" automatic SSOV to close within one second. To ensure that this requirement is met, the actuator manufacturer should perform a calculation of the operating time. Calculations with supporting documentation are usually required to confirm the consistency and appropriateness of the actuator selection.
Most automated valve specifications require the inclusion of manual operation devices. To meet this requirement, it is crucial to specify advanced actuator control accessories. To ensure safe and proper operation, pneumatic actuators must be equipped with pneumatic locking and venting (LOV) devices. The LOV can vent residual air in the cylinder that may prevent piston movement and cause equipment damage. More importantly, the device can prevent improper operation, especially in remote operations.
Choosing the appropriate actuator is usually based on torque requirements, valve applications, and cost.
These are typically used for controlling valves of 6 inches or smaller. Designed with two pistons, double rack-and-pinion actuators deliver unmatched performance and reliability. For long-life and high-performance applications, advanced designs include internal support guide rods, which eliminate piston misalignment and the resulting lateral forces, thereby extending service life.
For valves larger than 6 inches, scotch yoke actuators are mainly used, as this design is better suited for driving high torque and thrust loads of up to millions of inch-pounds. Advanced designs also feature internal guide rods supporting the yoke and improved piston sealing technology, which cleans the sliding surface and enhances sealing performance. High-quality designs improve corrosion resistance by surface-treating critical internal components (such as cylinder walls and guide rods). At the same time, surface treatments on the exterior allow the product to withstand harsh environments.
For all actuators that are properly selected and installed according to specifications, the primary cause of failure is the use of air that does not meet the air quality defined by the ANSI/ISA-7.0.01 standard. Almost all valve actuators require permanent lubrication and should be equipped with the best high-quality instrument air (also applicable to air that may be invaded by the environment). The actuator's cylinder exhausts air during piston movement. When the piston moves inward, it draws in surrounding environmental air through the exhaust port. If the local environment is harsh, special control methods (such as breathing valves) must be employed to prevent contamination and premature equipment failure. If the air remains clean, the actuator can last longer.
Selecting the right pneumatic actuator is a complex but crucial process. By understanding the characteristics of valves, the basic requirements of actuators, the correct selection methods, considerations for special applications, the inclusion of manual operation devices and safety devices, and the selection of the appropriate actuator type, you can ensure the stable operation and safety of your equipment. Additionally, regular maintenance and service are key to extending the service life of the equipment. It is hoped that this article has provided you with a comprehensive guide to help you make wise decisions when selecting pneumatic actuators.
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