A final control element is defined as a mechanical device that physically changes a process in response to a change in the control system setpoint. Final control elements relevant to actuators include valves, dampers, fluid couplings, gates, and burner tilts to name a few. Final control elements are an essential part of process control systems, allowing an operator to achieve a desired process variable output by manipulating a process variable setpoint.
Current trends in industry focus on improving quality and efficiency in an effort to reduce process costs. Many industrial facilities are recognizing the bottom line savings found from improved process control performance through precise and consistent control of final control elements.
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Traditional methods of positioning final control elements include electric actuators with squirrel-cage motors that suffer from duty-cycle limitations, poor resolution, and poor reliability. Commonly used pneumatic actuators suffer from stick/slip overshoot and high maintenance requirements. While it may seem like a given that your actuator should move when and where the controller tells it to, in many cases dead time and stick/slip overshoot cause delays in reaching the setpoint. The inefficiencies of these technologies can subject the process to relatively poor control and unplanned outages causing process variability and increased process costs. Many times these costs go unnoticed because they are not direct and immediately apparent. When selecting an actuator for a final control element, the actuator should have performance characteristics that will enable a control system to perform as designed.
The key final control element actuator performance characteristics are as follows:
- Precise, repeatable positioning typically better than 0.15% of span.
- The ability to start and stop instantaneously without dead time or position overshoot.
- Continuous duty rating without limitations on the number of starts per minute.
- Perform consistently and unaffected by load.
- Rugged industrial design capable of operating in difficult environments without an effect on performance.
- Minimal periodic maintenance required.
A final control element actuator designed with these characteristics provides two extremely important advantages:
- An ability to follow the demand signal from the controller precisely and instantly. This ensures that the actuator responds exactly as directed by the controller. Thus, the actuator is not the limiting factor in the control loop and the controller can function to its optimal levels.
- A high degree of maintenance-free reliability. An actuator designed to function as outlined above by default is more rugged than typical actuators. By design, then, it is capable of a much higher degree of reliability.
Process Control for Valves
When it is necessary to improve process control performance, the first step is to improve final control element performance. Beck electric actuators provide the necessary control and reliability that is required for many final control elements. The Group 11 actuator product line is often found on low to medium torque applications (20 lb-ft to 5,200 lb-ft). Group 22 actuators are utilized for high torque applications (3,000 lb-ft to 8,000 lb-ft), and Group 31 compact rotary actuators are utilized on low torque applications (15 lb-ft to 30 lb-ft).