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5. feedback control[1]


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5. feedback control[1]

  1. 1. Chemical Process Dynamics and Control Feedback Control Eng. Misael González Macias
  2. 2. Regulatory and Servo Control <ul><li>Regulatory: s ystems designed to compensate for DISTURBANCES. </li></ul><ul><li>Servo: the controlled variable must follow the SET POINT. </li></ul>
  3. 3. Basic Components of Control Systems <ul><li>Sensors and Transmitters. </li></ul><ul><li>Controllers. </li></ul><ul><li>Final Elements Control: Control Valves </li></ul>
  4. 4. Sensors and Transmitters <ul><li>Range </li></ul><ul><li>Consider a pressure sensor/transmitter that has been calibrated to measure a process pressure between the values of 20 psig and 50 psig. Then, the Range is 20-50 psig. </li></ul><ul><li>Span </li></ul><ul><li>Is the difference between the high and low values of the range. For this case the span is 30 psig. </li></ul><ul><li>Zero </li></ul><ul><li>The low value of the range. The zero is 20 psig. </li></ul>
  5. 5. Sensors and Transmitters ST(s ) Process Variable PV(s) Transmitter Output TO(s)
  6. 6. Sensors and Transmitters <ul><li>Gain of a sensor/transmitter </li></ul><ul><ul><li>Consider an electronic pressure transmitter with a range of 0-200 psig with electronical signal output of 4 to 20 mA. </li></ul></ul><ul><ul><li>Consider an neumatic temperature transmitter with a range of 100-300 °F with neumatic signal output of 3 to 15 psig. </li></ul></ul>
  7. 7. Control Valves <ul><li>A control valve acts as a variable restriction in a process pipe. </li></ul><ul><li>Safety </li></ul><ul><li>air-to-open or fail-closed valve. </li></ul><ul><li>air-to-close or fail-open. </li></ul>
  8. 8. Control Valves <ul><li>Example </li></ul>Control valve
  9. 9. Control Valves <ul><li>The Control Valve Actuator </li></ul>Transducer percent controller output
  10. 10. Control Valves <ul><li>Control Valve Capacity and Sizing </li></ul><ul><li>Liquid service. </li></ul><ul><ul><li>F: liquid flow, GPM. </li></ul></ul><ul><ul><li>ΔP: pressure drop across the valve, psi. </li></ul></ul><ul><ul><li>G: specific gravity of liquid at flowing conditions. </li></ul></ul><ul><li>Compressible Flow </li></ul><ul><ul><li>Gas flow. - Steam flow </li></ul></ul>
  11. 11. Control Valves <ul><li>Control Valve Capacity and Sizing </li></ul><ul><ul><li>Q: gas flow, scfh (scfh = ft 3 /h at standard conditions of 14.7 psia and 60°F) </li></ul></ul><ul><ul><li>G: gas specific gravity with respect to air, calculated by dividing the molecular weight of the gas by 29, the average molecular weight of air. </li></ul></ul><ul><ul><li>T: temperature at the valve inlet, R=°F + 460. </li></ul></ul><ul><ul><li>C f : critical flow factor. The numerical value for this factor ranges between 0.6 and 0.95. (Figure C-44 C. A. Smith and A. B. Corripio, Control Automático de Procesos, Limusa.) </li></ul></ul><ul><ul><li>P 1 : pressure at the valve inlet, psia. </li></ul></ul><ul><ul><li>W: gas flow, lb/h. </li></ul></ul><ul><ul><li>T SH : degrees of superheat, °F </li></ul></ul>
  12. 12. Control Valves <ul><li>Control Valve Characteristics </li></ul>
  13. 13. Concept of Feedback Control Process m d y Process m d y Final Control Element Sensor/ transmitter Controller y sp y m c e Open loop Closed loop Controller Mechanism - +
  14. 14. Typical Feedback Control Systems Flow Control
  15. 15. Typical Feedback Control Systems Pressure Control Liquid-Level Control
  16. 16. Typical Feedback Control Systems Temperature Control Liquid-Level Control
  17. 17. Representation of Feedback Loops FC for flow control PC for pressure control LC for liquid-level control TC for temperature control CC for composition control FT for flow transmiser PC for pressure transmiser LC for liquid-level transmiser TC for temperature transmiser CC for composition transmiser
  18. 18. Representation of Feedback Loops Example Control valves on a flash drum See also Apendix A of A. Smith and A. B. Corripio, Control Automático de Procesos, Limusa
  19. 19. Types of Feedback Controller <ul><li>Error </li></ul><ul><li>Proportional Controller (P) </li></ul><ul><li>Proportional Integral Controller (PI) </li></ul><ul><li>Proportional Integral Derivative (PID) </li></ul>percent controller output (50 %)
  20. 20. Types of Feedback Controller <ul><li>Proportional Controller (P) </li></ul>
  21. 21. Types of Feedback Controller <ul><li>Proportional Integral Controller (PI) </li></ul>
  22. 22. Types of Feedback Controller <ul><li>Proportional Integral Derivative Controller (PID) </li></ul>