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Chapter 14

Chapter 14. PID Implementation Issues. Overall Course Objectives. Develop the skills necessary to function as an industrial process control engineer. Skills Tuning loops Control loop design Control loop troubleshooting Command of the terminology Fundamental understanding Process dynamics

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Chapter 14

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  1. Chapter 14 PID Implementation Issues

  2. Overall Course Objectives • Develop the skills necessary to function as an industrial process control engineer. • Skills • Tuning loops • Control loop design • Control loop troubleshooting • Command of the terminology • Fundamental understanding • Process dynamics • Feedback control

  3. Reset Windup for PID Controllers • Windup results when the manipulated variable is not able to control to the setpoint resulting in sustained offset causing the integral of the error from setpoint to accumulate. • When control returns, accumulated error causes an upset. • Windup can occur when a control valve saturates or when a control loop is not being used (e.g., select control).

  4. Reset Windup • Note that controller output saturates causing area “A” to accumulate by the integral action. • After the disturbance returns to its normal level, the controller output remains saturated for a period of time causing an upset in y.

  5. Anti-Reset Windup • When the manipulated variable saturates, the integral is not allowed to accumulate. • When control returns, the controller takes immediate action and the process returns smoothly to the setpoint.

  6. Methods for Anti-Reset Windup • Turn off the integral when a valve saturates or a control loop is not in use. • Clamp the controller output to be greater than 0% and less than 100%. • Apply internal reset feedback • Apply external reset feedback

  7. Industrial Approach • External reset feedback • Controller output clamping • Digitally turn-off integral calculation

  8. Internal Reset Feedback

  9. Conventional PI Controller • Therefore, internal reset feedback is equivalent to a conventional PI controller. • It still has windup, but controller output can be clamped.

  10. External Reset Feedback • An extension of internal reset feedback, therefore, it is equivalent to a conventional PI controller. • When u saturates, windup will cease preventing windup. • Less windup than clamping, but requires umeas.

  11. Bumpless Transfer • When a control loop is turned on without bumpless transfer, the process can become unduly upset. • With bumpless transfer, an internal setpoint is used for the controller and the internal setpoint is ramped at a slow rate from the initial conditions to the actual desired setpoint to order to provide a smooth startup of a control loop.

  12. Comparison of True and Internal Setpoints

  13. Control Performance With and Without Bumpless Transfer

  14. Split Range Flow Control • In certain applications, a single flow control loop cannot provide accurate flow metering over the full range of operation. • Split range flow control uses two flow controllers (one with a small control valve and one with a large control valve) in parallel. • At low flow rates, the large valve is closed and the small valve provides accurate flow control. • At large flow rates, both valve are open.

  15. Split Range Flow Controller

  16. Coordination of Control Valves for Split Range Flow Control

  17. Example for Split Range Flow Control

  18. Titration Curve for a Strong Acid-Strong Base System • Therefore, for accurate pH control for a wide range of flow rates for acid wastewater, a split range flow controller for the NaOH is required.

  19. Other Split-Range Flow Control Examples • When the controlled flow rate has a turn down ratio greater than 9 • See value sizing examples in Chapter 2

  20. Split Range Temperature Control

  21. Split Range Temperature Control

  22. Overview • All controllers that employ integral action should have anti-reset windup applied. • Bumpless transfer provides a means for smooth startup of a control loop. • When accurate metering of a flow over a very wide flow rate range is called for, use split range flow control.

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