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