Sensor Systems for CPI Sensor temperature sensors flow

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 p. H 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 620 seconds for RTDs, thermistors and TC’s. • Additional thermal resistance on inside or on the outside of 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

Paddle Type Orifice Plate

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.

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

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

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), O 2 electrode for O 2 concentration and infrared spectrometer for CO 2 concentration. • Fermentation product analysis: HPLC and FIA

Common Bio-Sensors • Ion-specific electrodes – p. H sensor – DO sensor – Redox sensors

Schematic of an Ion-Specific Electrode

Table 2. 3 • Lists the control-relevant aspects of actuators and sensors in the CPI and biotech 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