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
• Repeatability
• Process measurement dynamics
• Calibration
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.
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.
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.
Pressure Measurements
• Usually based on mechanical balance bars
• Very fast measurement dynamics
• Repeatability less than ±0.1%
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.
Orifice Plate/DP Cell Flow Indicator in a Flow Control Loop
FC
FT
Flow Setpoint
Paddle Type Orifice Plate
Sizing an Orifice for a Differential Pressure Flow Indicator
• is the ratio of the orifice diameter to the pipe diameter.
• 0.2 < < 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 that satisfies each
of the above specifications.
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.
Example of a Vortex Shedding Meter
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.
Example of a Magnetic Flow Meter
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.
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.
Typical Differential Pressure Level Measurement
DPT
VaporDiaphragm
Lower Tap
Upper Tap
Liquid
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.
Bio-Sensors
Off Gas
Air
TurbidityMeter
RPM,Power
FT
AT
AT AT
TT
Redox
PTpHT
Dissolved O2
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
Common Bio-Sensors
• Ion-specific electrodes– pH sensor– DO sensor– Redox sensors
Schematic of an Ion-Specific Electrode
Silver wire (Cathode)Measuring electrode(Anode)
Membrane
AgCl(s) electrode
Saturated AgClsolution
Glass outertube
Fill solution
Porous plug
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
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