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5. feedback control[1]


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  • 1. Chemical Process Dynamics and Control Feedback Control Eng. Misael González Macias
  • 2. Regulatory and Servo Control
    • Regulatory: s ystems designed to compensate for DISTURBANCES.
    • Servo: the controlled variable must follow the SET POINT.
  • 3. Basic Components of Control Systems
    • Sensors and Transmitters.
    • Controllers.
    • Final Elements Control: Control Valves
  • 4. Sensors and Transmitters
    • Range
    • 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.
    • Span
    • Is the difference between the high and low values of the range. For this case the span is 30 psig.
    • Zero
    • The low value of the range. The zero is 20 psig.
  • 5. Sensors and Transmitters ST(s ) Process Variable PV(s) Transmitter Output TO(s)
  • 6. Sensors and Transmitters
    • Gain of a sensor/transmitter
      • Consider an electronic pressure transmitter with a range of 0-200 psig with electronical signal output of 4 to 20 mA.
      • Consider an neumatic temperature transmitter with a range of 100-300 °F with neumatic signal output of 3 to 15 psig.
  • 7. Control Valves
    • A control valve acts as a variable restriction in a process pipe.
    • Safety
    • air-to-open or fail-closed valve.
    • air-to-close or fail-open.
  • 8. Control Valves
    • Example
    Control valve
  • 9. Control Valves
    • The Control Valve Actuator
    Transducer percent controller output
  • 10. Control Valves
    • Control Valve Capacity and Sizing
    • Liquid service.
      • F: liquid flow, GPM.
      • ΔP: pressure drop across the valve, psi.
      • G: specific gravity of liquid at flowing conditions.
    • Compressible Flow
      • Gas flow. - Steam flow
  • 11. Control Valves
    • Control Valve Capacity and Sizing
      • Q: gas flow, scfh (scfh = ft 3 /h at standard conditions of 14.7 psia and 60°F)
      • 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.
      • T: temperature at the valve inlet, R=°F + 460.
      • 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.)
      • P 1 : pressure at the valve inlet, psia.
      • W: gas flow, lb/h.
      • T SH : degrees of superheat, °F
  • 12. Control Valves
    • Control Valve Characteristics
  • 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. Typical Feedback Control Systems Flow Control
  • 15. Typical Feedback Control Systems Pressure Control Liquid-Level Control
  • 16. Typical Feedback Control Systems Temperature Control Liquid-Level Control
  • 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. 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. Types of Feedback Controller
    • Error
    • Proportional Controller (P)
    • Proportional Integral Controller (PI)
    • Proportional Integral Derivative (PID)
    percent controller output (50 %)
  • 20. Types of Feedback Controller
    • Proportional Controller (P)
  • 21. Types of Feedback Controller
    • Proportional Integral Controller (PI)
  • 22. Types of Feedback Controller
    • Proportional Integral Derivative Controller (PID)