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Chapter 2

Chapter 2. Control Loop Hardware. Disturbance. Setpoint. e. c. u. +. Process. Controller. Actuator. -. Controlled. Variable. Sensor. Schematic of Feedback Loop. Control Diagram of a Typical Control Loop. Components and Signals of a Typical Control Loop.

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Chapter 2

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  1. Chapter 2 Control Loop Hardware

  2. Disturbance Setpoint e c u + Process Controller Actuator - Controlled Variable Sensor Schematic of Feedback Loop

  3. Control Diagram of a Typical Control Loop

  4. Components and Signals of a Typical Control Loop

  5. Controllers/Control Computers • Pneumatic controllers • Electronic analog controllers • Supervisory control computers • Distributed Control Systems (DCS) • Fieldbus technology

  6. Distributed Control System- DCS • Introduced in the late 1970’s. • Based upon redundant microprocessors for performing control functions for a part of the plant. SUPERIOR RELIABILITY • Less expensive per loop for large plants. • Less expensive to expand. • Facilitates use of advanced control.

  7. DCS Architecture

  8. Control Relevant Aspects of a DCS • The most important control aspect of a DCS is the cycle time for controller calls. The shortest cycles time are typically around 0.2 seconds while most loops can be executed every 0.5 to 1.0 seconds. These cycle times affect flow control loops and other fast control loops.

  9. Fieldbus Technology • Based upon smart valves, smart sensors and controllers installed in the field. • Uses data highway to replace wires from sensor to DCS and to the control valves. • Less expensive installations and better reliability. • Can mix different sources of sensors, transmitters, and control valves. • Now commercially available and should begin to replace DCS’s.

  10. Fieldbus Architecture

  11. Actuator System • Control Valve • Valve body • Valve actuator • I/P converter • Instrument air system

  12. Typical Globe Control Valve

  13. Cross-section of a Globe Valve

  14. Valve Deadband • It is the maximum change in instrument air pressure to a valve that does not cause a change in the flow rate through the valve. • Deadband determines the degree of precision that a control valve or flow controller can provide. • Deadband is affected by the pressure drop across the valve, the wear on the valve, and the design of the valve plug and seat.

  15. Sensor Systems • Sensor • temperature sensors • flow sensors • level sensors • pressure sensors • composition analyzers • Transmitter

  16. Sensor Terminology • Span • Zero • Accuracy • Repeatability • Process measurement dynamics • Calibration

  17. Temperature Sensing Systems • RTD’s are an order of magnitude more precise but are less rugged and cost more than thermocouples. • Typical dynamic response time constant is 6-20 seconds. • Additional thermal resistance on inside or on the outside of the thermal well can result in an excessively slow responding temperature measurement.

  18. Pressure Measurements • Usually based on strain gauges • Very fast measurement dynamics • Repeatability less than ±0.1%

  19. Flow Measurements • Orifice plate/DP cell most common approach. Good repeatability and fast dynamic response. • Magnet flow meters and vortex shedding flow meters are also used in certain situations. They are more expensive but more reliable and require less maintenance. • A straight run of pipe required for good accuracy for all flow meters.

  20. Orifice Plate/DP Cell Flow Indicator in a Flow Control Loop

  21. Level Sensors • Usually based on the hydrostatic head in a vessel measured by the differential pressure. • Has a repeatability of about ±1% with a time constant less than 1 second. • Level measurements based upon a float or x-rays are also used in special situations.

  22. Typical Differential Pressure Level Measurement

  23. Analyzer Sensor Systems • GC- most common composition analyzer. Based on plug flow of a volatile sample through a packed bed--behaves as deadtime. Deadtime and repeatability depend on the particular components being measured. • Radiation absorption- infrared, ultraviolet, and visible. Can be effective for certain components. • Sample system can affect dynamics and reliability of composition measurement.

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