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    5. feedback control[1] 5. feedback control[1] Presentation Transcript

    • Chemical Process Dynamics and Control Feedback Control Eng. Misael González Macias
    • Regulatory and Servo Control
      • Regulatory: s ystems designed to compensate for DISTURBANCES.
      • Servo: the controlled variable must follow the SET POINT.
    • Basic Components of Control Systems
      • Sensors and Transmitters.
      • Controllers.
      • Final Elements Control: Control Valves
    • 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.
    • Sensors and Transmitters ST(s ) Process Variable PV(s) Transmitter Output TO(s)
    • 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.
    • 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.
    • Control Valves
      • Example
      Control valve
    • Control Valves
      • The Control Valve Actuator
      Transducer percent controller output
    • 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
    • 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
    • Control Valves
      • Control Valve Characteristics
    • 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 - +
    • Typical Feedback Control Systems Flow Control
    • Typical Feedback Control Systems Pressure Control Liquid-Level Control
    • Typical Feedback Control Systems Temperature Control Liquid-Level Control
    • 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
    • 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
    • Types of Feedback Controller
      • Error
      • Proportional Controller (P)
      • Proportional Integral Controller (PI)
      • Proportional Integral Derivative (PID)
      percent controller output (50 %)
    • Types of Feedback Controller
      • Proportional Controller (P)
    • Types of Feedback Controller
      • Proportional Integral Controller (PI)
    • Types of Feedback Controller
      • Proportional Integral Derivative Controller (PID)