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Sensor Systems for CPI

Sensor Systems for CPI. Sensor temperature sensors flow sensors level sensors pressure sensors composition analyzers Transmitter. The Control Relevant Aspects of Sensors. The time constant/deadtime of the sensor The repeatability of the sensor. Sensor Terminology. Span Zero Accuracy

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Sensor Systems for CPI

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  1. Sensor Systems for CPI • Sensor • temperature sensors • flow sensors • level sensors • pressure sensors • composition analyzers • Transmitter

  2. The Control Relevant Aspects of Sensors • The time constant/deadtime of the sensor • The repeatability of the sensor

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

  4. Span and Zero Example • Consider a case in which the maximum temperature that is to be measured is 350ºF and the minimum temperature is 100ºF. • Then the zero is 100ºF and the span is 250ºF • In addition, if the measured temperature is known at two different sensor output levels (i.e., ma’s), the span and zero can be calculated directly.

  5. Smart Sensors • Sensors with onboard microprocesssors that offer a number of diagnostic capabilities. • Smart pH sensors determine when it is necessary to trigger a wash cycle due to buildup on the electrode surface. • Smart flow meters use statistical techniques to check for plugging of the lines to the DP cell. • Smart temperature sensors use redundant sensors to identify drift and estimate expected life before failure.

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

  7. Pressure Measurements • Usually based on mechanical balance bars • Very fast measurement dynamics • Repeatability less than ±0.1%

  8. Flow Measurements • Orifice plate/DP cell most common approach. Good repeatability and fast dynamic response. • Magnetic 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.

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

  10. Paddle Type Orifice Plate

  11. Sizing an Orifice for a Differential Pressure Flow Indicator • b is the ratio of the orifice diameter to the pipe diameter. • 0.2 < b < 0.7 • Pressure drop at minimum flow should be greater than 0.5 psi. • Pressure drop across the orifice should be less than 4% of the line pressure. • Choose the maximum value of b that satisfies each of the above specifications.

  12. Vortex Shedding Meters • A blunt object is placed in the flow path and the frequency of turbulent oscillations correlates with the flow rate. • Useful for clean low viscosity liquids and gases. • Ensure that cavitation does not occur in the measuring zone.

  13. Example of a Vortex Shedding Meter

  14. Magnetic Flow Meters • Based on measuring the current generated by the flow a conducting fluid through a magnetic field. • Have low pressure drop associated with them. • Are applied to conductive fluid (tap water is conductive enough) • Deposition on the electrodes is a limitation. • See picture in text.

  15. Example of a Magnetic Flow Meter

  16. Bottom Line on Flow Meters • Magnetic flow meters and vortex shedding flow meters require less maintenance and are generally more reliable than orifice plate flow meters BUT they are much more expensive.

  17. 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.

  18. Typical Differential Pressure Level Measurement

  19. 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.

  20. Bio-Sensors

  21. Common Bio-Sensors • Flow measurements: Coriolis meters and rotameters. • Off-gas analyzers: mass spectrometers (one mass spec can provide online measurements for up to 32 bio-reactors), O2 electrode for O2 concentration and infrared spectrometer for CO2 concentration. • Fermentation product analysis: HPLC and FIA

  22. Common Bio-Sensors • Ion-specific electrodes • pH sensor • DO sensor • Redox sensors

  23. Schematic of an Ion-Specific Electrode

  24. Table 2.3 • Lists the control-relevant aspects of actuators and sensors in the CPI and bio-tech industries: • Time constant • Valve deadband or repeatability • Turndown ratio, rangeability, or range

  25. 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

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