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Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control

Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control ( Strumentazione e Controllo dei Processi Chimici ) Measuring devices of the main process variables Pressure measurements Rev. 2.4 – April 3, 2019. ABSOLUTE PRESSURE :

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Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control

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  1. Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control (Strumentazione e Controllo dei Processi Chimici) Measuring devices of the main process variables Pressure measurements Rev. 2.4 – April 3, 2019

  2. ABSOLUTE PRESSURE: Pressure referenced to the perfect vacuum. BAROMETRIC PRESSURE: Pressure exerted from the atmosphere on the Earth’ssurface (it changes with the altitude and the weatherconditions). DIFFERENTIAL PRESSURE: Pressure difference between two different points. GAUGE (GAGE in USA) PRESSURE: Difference between the absolute and the barometric pressure. RESIDUAL PRESSURE: Absolute pressure that is referenced to the perfect vacuum and is below the barometric pressure. VACUUM: Pressure measured below the barometric pressure after taking this latter as a reference. PRELIMINARY DEFINITIONS From “Fundamentals of Instrumentation & Process Control”, 2006 Control Station absolute pressure = gauge pressure + barometric pressure Process Instrumentation and Control - Prof M. Miccio

  3. The standard atmospheric pressure, referred to as P atm (n), corresponds to the value the barometric pressure exerted by the atmosphere at the sea level at 0 ºC with average weather conditions. It is defined as being equal to: P atm (n)= 101325 Pa = 1.01325 bar = 1 atm = 1.03323 kg/cm2 = 14.6959 psi = 760 mm (Hg) = 10.3323 m (H2O) STANDARD ATMOSPHERIC PRESSURE Process Instrumentation and Control - Prof M. Miccio

  4. TYPES OF PRESSURE MEASUREMENTS pressure under measurement Perfectvacuum • Absolute pressure • Gage pressure • Differential pressure with a variable reference • Differential pressure with a constant reference pressure under measurement Atmosphericpressure Manometer Variablereferencepressure pressure under measurement pressure under measurement Constantreferencepressure Process Instrumentation and Control - Prof M. Miccio

  5. Quite often, all pressure sensors have been improperly called manometers even though it is actually necessary to distinguish between pressure sensors: Manometeris the instrument which measures positive gage pressures and with pressures zero-referenced to the barometric pressure. Deprimometer is the instrument that measures absolute pressures lower than the barometric pressure and has zero at barometric pressure. Measure the vacuum. Vacuum gauge is the instrument that measures the absolute pressures between the absolute vacuum and the barometric pressure, the vacuum meter has therefore the zero of the scale that corresponds to the absolute vacuum. Measure the residual pressure. Barometer is the instrument that measures only the absolute value of the atmospheric pressure (barometricpressure). DEFINITIONSPressure sensors  Process Instrumentation and Control - Prof M. Miccio

  6. CLASSIFICATIONPressure Sensors • In the most common sensors, the pressure measurement can be performed by: • 1. a variation of the liquid level; • 2. a deformation of an elastic element; • 3. a change of an electromagnetic quantity. • 1. In the first category there are instruments based on the measurement of the level of a liquid column: • Ex.: “U” manometers • 2. In the second category, there are instruments based in the deformation of an elastic element because of the action of pressure forces. They are generally based on macro-deformations. e.g.: • Bourdon pressure gauges; • Diaphragm manometers • Bellows manometers • 3. In the third category there are the transducers in which a variation in pressure, which may involve a micro-deformationof an elastic element (diaphragm, wire, etc.) that produces an output signal generated by the change of a electrical property (capacity, resistance, inductance, frequency). Ex.: strain-gauge  Process Instrumentation and Control - Prof M. Miccio

  7. PRESSURE MEASUREMENT SENSORS 1. a variation in height of liquid2. a deformation of an elastic element3. a variation of an electromagnetic quantity Process Instrumentation and Control - Prof M. Miccio

  8. LIQUID COLUMN MANOMETERor “U” MANOMETER • STEVIN’S LAW • Instruments based on the measurement of a level of a liquid column • Measuring ranges: 10 -1500 mm Hg • 0.01 - 2 bar • 10 - 2500 mm H2O • NOTE: they are used: • in the range of low differential pressures • Almost exclusively for gases and vapors • as local indicator and not in automatic control  WaterColumn.swf PI Process Instrumentation and Control - Prof M. Miccio

  9. PRESSURE MEASUREMENT SENSORS 1. a variation in height of liquid2. a deformation of an elastic element3. a variation of an electromagnetic quantity Process Instrumentation and Control - Prof M. Miccio

  10. BOURDON PRESSURE GAUGE • it consists of a closed-end elastic tube with an elliptic section, shaped as an arc, with a fixed terminal; • the free terminal changes its position because of the deformation stress exerted by the pressure of internal fluid; • the measurement of the terminal displacement provides the pressure measure. • NOTE: • They are used as local indicators in Piping & Instrumentation Diagram PI Process Instrumentation and Control - Prof M. Miccio

  11. Standard manometer with quadrant Ø 40 mm Quadrant with double graduation: in psi and kg/cm2 Manometer with quadrant Ø 50 mm Quadrant with double graduation: in bar and in psi Connection Central threaded male 1/8" BSP BOURDON PRESSURE GAUGEand ASSEMBLING ACCESSORIES Process Instrumentation and Control - Prof M. Miccio

  12. PRESSURE MEASUREMENT SENSORS 1. a variation in height of liquid2. a deformation of an elastic element3. a variation of an electromagnetic quantity Process Instrumentation and Control - Prof M. Miccio

  13. CAPACITIVE PRESSURE TRANSDUCERS BEFORE the deformation F = (P1 - P2)* Area with P1>P2 AFTER the deformation Measurement principle: change of the CAPACITY C errelative dielectric constant e0dielectric constant of the vacuum Process Instrumentation and Control - Prof M. Miccio

  14. pressione CAPACITIVE PRESSURE TRANSDUCERS CAPACITIVE CELLS with CERAMIC MEMBRANES CELL with METALLIC DIAPHRAGM ATMOSPHERIC PRESSURE VACUUM PRESSURE PRESSURE PRESSURE CELL with DOUPLE COMPENSATOR for DIFFERENTIAL PRESSURE NOTE: The dielectric fluid can move from a side to the other one.In this way, it automatically compensates the change of the dielectric fluid density. Process Instrumentation and Control - Prof M. Miccio

  15. Transduction deformation - capacity An a.c.Wheatstonebridge is used as rectifier in order to produce a direct signal. The sensitivity is redoubled if there are two capacities and they have opposite variations, inserting them on two different legs of the bridge. TRANSDUCTION OF A CAPACITY VARIATION IN AN ELECTRIC SIGNAL Process Instrumentation and Control - Prof M. Miccio

  16. Strain-gauge It operates on the variation of an electrical resistance due to the change of the length and the cross sectional diameter (2°Ohm’s law) when a conductor is submitted to a tensile stress. thin (few microns) metallic plate; very high resistance (hundreds ohms) which changes with the deformation; extension 0.2-5% bonded with insulating material NOTE: - Cheaper sensors Termination Wires Strain gauge Adhesive Substrate ELECTRICAL RESISTANCE-BASED PRESSURE TRANDUCERS PROTECTION Tratto da Dr. Chi-fu Wu, ME 4903-Special Problem in ME, GTREP, Savannah (USA), Oct. 21, 2004 Piezoelectric transducers They can be alternative to the strain-gauges. In this case the electrical resistance and the voltage across the sensible element, which is a crystal of a certain solid material (silicon or another semiconductor), changes directly because of the deformation. Process Instrumentation and Control - Prof M. Miccio

  17. Valim +e - e + e -e E + e -e +e - e TRANSDUCTION OF A RESISTANCEVARIATION IN AN ELECTRIC SIGNAL Transduction deformation - resistance (with Wheatstone bridge) 4 extensometers (2 for tensile and 2 for compression) on 4 legs of a bridge ADVANTAGES Accuracy 0.1% on measured value Rangeability (75 100):1 Measuring range: “The pressure range is 0 – 14000 bar” [see the University of Michigan Chemical Engineering Process Dynamics and Controls Open Textbook at http://controls.engin.umich.edu/wiki/index.php ] Process Instrumentation and Control - Prof M. Miccio

  18. INDUCTIVE DIFFERENTIALPRESSURE TRANSDUCER Measure principle: Variation of the INDUCTANCE L Transduction deformation - inductance The deformation of the metallic diaphragm produces a translation of two units of ferrite respect to 2 fixed coils placed on the two sides of diaphragm. The deformation occurring as a consequence of the pressure difference is transduced in a inductance variation (of opposed direction in the 2 coils). This is further transduced in an electric signal by an a.c. Wheatstone bridge with rectifier or including inductances as tuning elements of oscillators (LC circuits). P2 P1 Process Instrumentation and Control - Prof M. Miccio

  19. RESONATORS ON SILICON Measurement principle: Variation of the RESONANT FREQUENCY • double diapason of a “resonant” silicon mono-crystal immersed in a permanent magnetic field • resonant frequency depends on the transverse deformation produced by the measuring pressure • MEMS = Micro ElectroMechanical Systems • ADVANTAGES • No attrition • No AC/DC converter • Better performances and greater stability with time (little calibration requirement) Process Instrumentation and Control - Prof M. Miccio

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