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Industrial Instrumentation

Industrial Instrumentation. Dr. –Ing. Naveed Ramzan. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 1.

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Industrial Instrumentation

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  1. Industrial Instrumentation Dr. –Ing. Naveed Ramzan PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 1

  2. “In any given plant, the number of pressure gauges used is probably larger than all other instruments put together” Pressure Sensors

  3. Pressure = Force / Area Pressure can be used inferentially to measure other variables such as Flow and Level Pressure plays a major role in determining the Boiling Point of Liquids Fluids exerts pressure on the containing vessel equally and in all directions Pressure Measurement

  4. Pressure Measurement

  5. Mechanical Methods Electrical Methods Pressure Measurement Methods

  6. Elastic pressure transducers Manometer method Vacuum Pressure Measurement Electric pressure transducers Pressure measurement by balancing forces produced on a known area by a measured force Pressure Measurement Methods

  7. Bourdon tube pressure gauge Diaphragm pressure transducers Bellows Uses flexible element as sensor. As pressure changed ,the flexible element moved, and this motion was used to rotate a pointer in front of dail. Elastic Pressure Transducers

  8. Bourdon tubes are generally are of three types; C-type Helical type Spiral type Bourdon Tube Pressure Gauge 8

  9. Bourdon Tube Pressure Gauge 9

  10. Diaphragm are popular because they required less space and the motion they produce is sufficient for operating electronic transducers Diaphragm and Bellows Pressure Gauge

  11. They are used to measure gauge pressures over very low ranges. Two types of diaphragm pressure gauges are: Metallic diaphragms gauge (brass or bronze) 2. Slack diaphragms gauge (Rubber) Diaphragm Pressure Gauge

  12. More sensitive than bourdon type gauge. Used to measure low pressures Brass, phosphor bronze, stainless steel, beryllium copper etc. Bellows

  13. Mechanical Pressure Transducers to Pneumatic Pressure Transmitters to Electric Pressure Transmitters Electric Pressure Transducers PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 13

  14. Why Electrical Pressure Transducers? Transmission requirements for remote display as electric signal transmission can be through cable or cordless. Electric signals give quicker responses and high accuracy in digital measurements. The linearity property of the electric signal produced to pressure applied favors simplicity. They can be used for extreme pressure applications, i.e. high vacuum and pressure measurements. EPTs are immune to hysteresis, shock and mechanical vibrations. Electric Pressure Transducers 8/28/2014 14 Group Se7en

  15. Pressure sensing element such as a bellow , a diaphragm or a bourdon tube Primary conversion element e.g. resistance or voltage Secondary conversion element Electric Pressure Transducers PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 15

  16. Strain gauge pressure transducers Capacitive pressure transducers Potentiometer pressure transducers Resonant Wire pressure transducers Piezeoelectric pressure transducers Types of Electric Pressure Transducers PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 16

  17. Strain Gauge Pressure Transducer A strain gauge is a passive type resistance pressure transducer whose electrical resistance changes when it is stretched or compressed The wire filament is attached to a structure under strain and the resistance in the strained wire is measured PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 17

  18. Strain Gauge Pressure Transducer Construction and Working

  19. Strain Gauge Pressure Transducer -- Strain gauge pressure transducers are used for narrow pressure span and for differential pressure measurments -- Available for pressure ranges as low as 3 inches of water to as high as 200,000 psig -- Inaccuracy ranges from 0.1 % of span to 0.25 % of full scale PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 19

  20. Capacitive Pressure Transducer C=ε0 εr A/d Where, C = the capacitance of a capacitor in farad A = area of each plate in m2 d = distance between two plates in m εr= dielectric constant ε0 = 8.854*10^-12 farad/m2 Thus, capacitance can be varied by changing distance between the plates, area of the plate or value of the dielectric medium between the plates. Any change in these factors cause change in capacitance. In capacitive transducers, pressure is utilized to vary any of the above mentioned factors which will cause change in capacitance and that is a measureable by any suitable electric bridge circuit and is proportional to the pressure. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 20

  21. The sensing diaphragm and capacitor form a differential variable separation capacitor. When the two input pressures are equal the diaphragm is positioned centrally and the capacitance are equal. A difference in the two input pressure causes displacement of the sensing diaphragm and is sensed as a difference between the two capacitances Capacitive Pressure Transducer PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 21

  22. -- Originally developed for use in low vacuum research -- Wide rangeability from high vacuum in the micron range to 10,000 psig -- Differential pressure as low as 0.01 inch can be readable -- Accurate within 0.1 % of reading or 0.01 % of full scale -- More Corrosion resistant Capacitive Pressure Transducer PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 22

  23. Potentiometer Pressure Transducer The device consists of a precision potentiometer whose wiper alarm is mechanically linked to bourdon tube or bellow. The movement of wiper alarm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement using a Wheatstone bridge circuit. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 23

  24. Potentiometer Pressure Transducer -- Extremely small and installed in very tight quarters such inside the housing of 4.5 in dial pressure gauge -- Provide strong output so no need of additional amplifier -- Range 5 to 10,000 psig -- Accurate within 0.5 % and 1 % of full scale PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 24

  25. Resonant Wire Pressure Transducer A wire is griped by a static member at one end and by the sensing diaphragm at the other. An oscillatory circuit causes the wire to oscillate at its resonant frequency. A change in process pressure changes the wire tension which in turn changes the resonant frequency of the wire. A digital counter circuit detects the shift. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 25

  26. Resonant Wire Pressure Transducer -- Used for low differential pressure applications -- Generates inherently digital signal -- Sensitive to shock and variation -- Range : From Absolute pressure 10 mm Hg Up to Differential pressure 750 in Water or Gauge pressure 6000 psig -- Accuracy 0.1 % of Calibrated Spam PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 26

  27. Piezoelectric Pressure Transducer When a pressure , force or acceleration is applied to a quartz crystal , a charge is developed across the crystal that is proportional to the force applied PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 27

  28. Piezoelectric Pressure Transducer -- Signals generated by crystals decays rapidly so unsuitable for static force or pressure measurements -- measure rapidly changing pressure resulting from blasts, explosions or pulsation pressures -- Range : 5,000 to 10,000 psi -- Rugged construction, small size and high speed PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 28

  29. Where and How have EPTs failed? EPTs require a constant supply of electricity for them to function. They do not come with built-in power supply. High performance comes at a cost. Installation of auxiliary display modules and electrical circuitry increases capital cost. Physical properties, like temperature, which can affect electrical constants may affect the consistency of EPTs. For this reason, temperature compensation is always required with EPTs. Some electrical phenomena, like piezolectric energy, have limited applicability. This limits their use in industry. Electricity exposes personnel to potential hazards. 8/28/2014 29 Group Se7en

  30. Home Work INDUCTIVE/RELUCTIVE PRESSURE TRANSDUCERS PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 30

  31. High Pressure and Vacuum Measurement -- Synthetic Diamond Manufacturing industry: Normal reaction pressure = 100,000 psig Some fiber and Plastic extruders operates at 10,000 psig -- But in average plant pressure exceeding 1000 psig are considered high INDUCTIVE/RELUCTIVE TRANSDUCERS: PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 31

  32. High Pressure and Vacuum Measurement High pressure designs -- Can detect pressure up to 10,000 psig and operate up to 8000 degree F -- The pressure of the output air signal follows the process pressure in inverse ratio to the areas of the two diaphragms. If the diaphragm area ratio is 200:1, a 1,000-psig increase in process pressure will raise the air output signal by 5 psig. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 32

  33. High Pressure and Vacuum Measurement High pressure designs -- May include as many as twenty coils -- can measure pressures well in excess of 10,000 psig -- standard element material is heavy-duty stainless steel -- measurement error is around 1% of span -- Suitable for fluctuating pressure service PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 33

  34. High Pressure and Vacuum Measurement High pressure designs -- primary standards in calibrating high-pressure sensors -- Typical inaccuracy of an industrial deadweight tester is 1 part in 1,000 or 0.1%. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 34

  35. High Pressure and Vacuum Measurement Very High pressure The bulk modulus cell consists of a hollow cylindrical steel probe closed at the inner end with a projecting stem on the outer end . When exposed to a process pressure, the probe is compressed, the probe tip is moved to the right by the isotropic contraction, and the stem moves further outward. This stem motion is then converted into a pressure reading. detect pressures up to 200,000 psig with 1% to 2% full span error PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 35

  36. High Pressure and Vacuum Measurement PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 36

  37. High Pressure and Vacuum Measurement In general, for high vacuum services , either cold cathode or Bayard-Alpert hot cathode gauges are suitable. Neither is particularly accurate or stable, and both require frequent calibration. -- For vacuums in the millitorr range the capacitance manometer is a good choice. For intermediate vacuum Applications capacitance manometers are the best in terms of performance, but are also the most expensive. The lowest priced gauge is the thermocouple type, but its error is the greatest. Digital Pirani gauges can represent a good compromise solution, with accuracy between that of capacitance and thermocouple sensors. For low vacuums (higher pressures) between atmospheric and 10-2 torr, Bourdon tubes, bellows, active strain gages, and capacitance sensors are all suitable PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 37

  38. High Pressure and Vacuum Measurement PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 38

  39. High Pressure and Vacuum Measurement -- A basic manometer can consist of a reservoir filled with a liquid and a vertical tube . -When detecting vacuums, the top of the column is sealed evacuated. -- A manometer without a reservoir is simply a U-shaped tube, with one leg sealed and evacuated and the other connected to the unknown process pressure -- The difference in the two column heights indicates the process vacuum. -- An inclined manometer can consist of a well and transparent tube mounted at an angle. A small change in vacuum pressure will cause a relatively large movement of the liquid. --Manometers are simple, low cost, and can detect vacuums down to 1 millitorr. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 39

  40. High Pressure and Vacuum Measurement A capacitance sensor operates by measuring the change in electrical capacitance that results from the movement of a sensing diaphragm relative to some fixed capacitance electrodes Accuracy is typically 0.25 to 0.5% of reading. Thin diaphragms can measure down to 10-5 torr, while thicker diaphragms can measure in the low vacuum to atmospheric range. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 40

  41. High Pressure and Vacuum Measurement McLeod Gauge: (Originally invented in 1878), -- The McLeod gauge measures the pressure of gases by compressing a known volume with a fixed pressure. The new volume is then a measure of the initial absolute pressure. -- The McLeod gauge has been used until recently for calibrating other gauges. -- It covers the vacuum range between 1 and 10-6 torr. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 41

  42. High Pressure and Vacuum Measurement Molecular Momentum: -- This vacuum gauge is operated with a rotor that spins at a constant speed. Gas molecules in the process sample come in contact with the rotor and are propelled into the restrained cylinder. The force of impact drives the cylinder to a distance proportional to the energy transferred, which is a measure of the number of gas molecules in that space. -- The full scale of the instrument depends on the gas being measured. -- The detector has to be calibrated for each application. PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 42

  43. High Pressure and Vacuum Measurement Thermal Designs: The thermal conductivity of a gas changes with its pressure in the vacuum range. If an element heated by a constant power source is placed in a gas, the resulting surface temperature of the element will be a function of the surrounding vacuum. Because the sensor is an electrically heated wire, thermal vacuum sensors are often called hot wire gauges. Typically, hot wire gauges can be used to measure down to 10-3 mm Hg. Pirani: In this design, a sensor wire is heated electrically and the pressure of the gas is determined by measuring the current needed to keep the wire at a constant temperature PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 43

  44. High Pressure and Vacuum Measurement Ionization Gauges PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 44

  45. High Pressure and Vacuum Measurement Ionization Gauges PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 45

  46. High Pressure and Vacuum Measurement PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 46

  47. Summary of Pressure Sensor Characteristics PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 47

  48. Discussion & Questions? Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 48

  49. Discussion Examples Example 1 PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 49

  50. Discussion Examples Example 2 PEC – UET Lahore Dr. Shahid Naveed 28. August 2014 / Dr. –Ing Naveed Ramzan 50

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