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PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 1 Industrial Instrumentation Dr. –Ing. Naveed Ramzan.

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Presentation on theme: "PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 1 Industrial Instrumentation Dr. –Ing. Naveed Ramzan."— Presentation transcript:

1 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 1 Industrial Instrumentation Dr. –Ing. Naveed Ramzan

2 Pressure Sensors PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 2 In any given plant, the number of pressure gauges used is probably larger than all other instruments put together

3 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 3 Practical Considerations in Pressure Measurements In industrial applications, good repeatability often is more important then absolute accuracy. If process pressures vary over a wide range, transducers with good linearity and low hysteresis are the preferred choice. Ambient and process temperature variations also cause errors in pressure measurements, particularly in detecting low pressures and small differential pressures. In such applications, temperature compensators must be used

4 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 4 Practical Considerations in Pressure Measurements Selection Criteria Pressure transducers usually generate output signals in the millivolt range (spans of 100 mV to 250 mV). When used in transmitters, these are often amplified to the voltage level (1 to 5 V) and converted to current loops, usually 4-20 mA dc. The transducer housing should be selected to meet both the electrical area classification and the corrosion requirements of the particular installation. If the installation is in an area where explosive vapors may be present, the transducer or transmitter and its power supply must be suitable for these environments. This is usually achieved either by placing them inside purged or explosion proof housings, or by using intrinsically safe designs. The single most important decision in selecting a pressure transducer is the range When high process temperatures are present, one can consider the use of various methods of isolating the pressure instrument from the process. These include loop seals, siphons, chemical seals with capillary tubing for remote mounting, and purging.

5 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 5 Practical Considerations in Pressure Measurements Maintenance Without exception, pressure sensors require scheduled, periodic maintenance and/or recalibration.

6 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 6 Practical Considerations in Pressure Measurements Calibration Pressure transducers can be recalibrated 1. on-line or 2. in a calibration laboratory. Laboratory recalibration typically is preferred, but often is not possible or necessary. In the laboratory, there usually are two types of calibration devices: 1.deadweight testers that provide primary, base-line standards, and 2.laboratory or field standard calibration devices

7 Self Assessment Test PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 7

8 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 8 Question No. 1 The difference between gauge and absolute pressure is A.a Vacuum B.0.433 psia C.atmospheric pressure D.zero

9 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 9 Question No. 2 Pressure is defined as A.Force per unit area B. F. A C.A/F D.None of the above

10 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 10 Question No. 3 Metals used in the bourdon tubes should not undergo A. Fatigue B. hysteresis C. creep D.All of the above

11 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 11 Question No. 4 A capsule diaphragm is made by welding two diaphragms A. to a solid base B. together at the center C. together around the edges D.To two other diaphragms

12 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 12 Question No. 5 Dead weight tester is used for A. testing dead weights B. measuring process pressures accurately C. producing high pressure D. calibrating pressure instruments

13 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 13 Question No. 6 One torr is defined as A.One mm Hg B. One inch Hg C. One atmosphere D. One kilopascal

14 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 14 Question No. 7 Which gauge measures pressure by sensing changes in the thermal conductivity of the gas A.Pirani gauge B.Slack diaphragm gauge C.Mcleod gauge D. None of them

15 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 15 Question No. 8 A thermocouple gauge is one type of A.Ionization gauge B.Thermal conductivity gauge C.Mcleod gauge D. None of these

16 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 16 Question No. 9 Diaphragms used in pressure applications are A.light B.Small in size C.slack D. bimetallic

17 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 17 Question No. 10 A Mcleod gauge can measure pressure as low as A.0.05 torr B.0.005 torr C.0.0005 torr D. 0.00005 torr

18 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 18 Question No. 10 What type of manometer is best for measuring low pressures A.Well B.Inclined C.U-Tube D. Multiple tube

19 PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 19 Question No. 11 A capacitance pressure transducer indicates changes in pressure by changing the A.Voltage output of an ac circuit B. frequency C. capacitance D. alternating current

20 Home Work PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 20 Make similar 10 MCQs from Temperature and Pressure Sensors. Solve Them and attached the sheet with the test on coming Tuesday to get 5 marks. Additional 5 marks for the student who prepare the best MCQs.

21 Total Lectures held 8 Fundamentals of Electrical Technology and digital logic employed in the measurement Review of Scientific principles employed in instruments Parts of Instrument Performance Characteristics of Instruments Selection and Calibration of Instruments Instruments Identification and Line Symbols Principle measurements desired in industry (a) Temperature (b) Pressure, Load (c) Level (d) Flow (e) Others ( Weight, Composition, pH etc.) (f) Transducers Installation and Installation Costs Case Studies Quiz Held = 1 : Next Test due on = 3-10-2009 (B) 6-10-2009 (A) A Quick View! What we have covered? Assignment due on 10-10-2009

22 Level Sensors PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 22

23 Level is another common process variable that is measured in many industries. The method used will vary widely depending on the nature of the industry, the process, and the application. Inventory: -- a constant supply or storage of material Control: -- continuous, batch, blending, and mixing control -- stabilize flow to the next process Alarming: -- hi/lo limits, safety shut down Data Logging: -- material quantities for inventory and billing purposes and where regulatory requirements are necessary Level Measurement PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 23

24 What is measured? The measured medium can be liquid, gas or solid and stored in vessels (open/closed tanks), silos, bins and hoppers. Units of level can be expressed in: feet (meters) gallons (liters) pounds (kilograms) cubic volume (ft 3, m 3 )

25 Hydrostatic Head Float Load Cells Magnetic Level Gauge Capacitance Transmitters Magnetostrictive Ultrasonic Microwave Laser Radar Guided Wave Radar Dip Stick Vibration Methods ---- Direct or Indirect (inferential) PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 25

26 Direct Methods Direct methods sense the surface or interface of the liquid and is not affected by changes in material density (Specific Gravity) Examples: Dip Stick Resistance Tapes Sight Glass Floats Ultrasonic

27 Indirect Methods (Inferential) Indirect methods infer liquid level by measuring some other physical parameter such as pressure, weight, or temperature. Changing materials means a corrective factor must be used or recalibrating the instrument. Examples: Hydrostatic head methods Load Cells Capacitance Conductivity

28 When determining the type of level sensor that should be used for a given application, there are a series of questions that must be answered: Open tank or closed tank? Can the level sensor be inserted into the tank or should it be completely external? Contact or non- contact? Continuous measurement or point measurement? Direct or Indirect measurement? What type of material is being measured? Liquid or Solid? Clean or Slurry? Selection Criteria PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 28

29 For all liquids you will need: The system operating temperature with max. and min. excursions? two wide range – expensive the sensor The system operating pressure? Check that system T and P do not conflict with the materials of construction? Selection Criteria PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 29

30 For Solids: Bulk density Be careful with very large silos as compaction at the bottom can greatly change assume bulk densities Flow characteristics? Expected particle size distribution? Is solid abrasive and/or corrosive and what is the moisture/solvent content? Selection Criteria PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 30

31 Simple and cheap Can be used with any wet material and not affected by density. Can not be used with pressurized tanks Visual indication only (electronic versions are available) RodGauge - similar to a dipstick found in a car, it has weighted line markings to indicate depth or volume For Liquids PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 31 Dip Stick

32 Another simple direct method of measuring liquids. Can be used in pressurized tanks (as long as the glass or plastic tube can handle the pressure) Good for applications where non-contact measurement is needed (like beverages) Sight Glass PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 32 For Liquids

33 Float rides the surface level to provide the measurement. Many different styles are available. Usually used for pump control, high/low level alarms and emergency shut-off Liquid density does not affect measurement Floats PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 33 For Liquids

34 Point Level MeasurementContinuous Level Measurement Advantages and disadvantages Low Cost Conductive, non-coating liquids only Insulating coatings can cause problems Conductivity Level Measurement PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 34 For Liquids

35 The pressure of the fluid in the tank causes the tape to short-circuit, thus changing the total resistance of the measuring tape. An electronic circuit measures the resistance; it's directly related to the liquid level in the tank. Resistance Tape PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 35 For Liquids

36 Bubblers allow the indicator to be located anywhere. The air pressure in the tube varies with the head pressure of the height of the liquid. Bottom of tube determines reference point P Regulated purge system (air or nitrogen) Instrument input does not matter Cant be used in closed tanks or where purging a liquid is not allowed (soap). Very popular in the paper industry because the air purge keeps the tube from plugging. Bubblers PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 36 For Liquids

37 Advantages: -- Easy installation -- Continuous reading providing analogue or digital signal -- No moving parts -- Good accuracy and repeatability Bottom of tube determines reference point P Regulated purge system (air or nitrogen) Instrument input does not matter Bubblers PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 37 For Liquids

38 Limitations: -- Not suitable for pressurized tanks -- Sediments may block tube or probe -- Tanks must be freely vented Bottom of tube determines reference point P Regulated purge system (air or nitrogen) Instrument input does not matter Bubblers PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 38 For Liquids

39 These methods infer level by measuring the hydrostatic head produced by the liquid column. A pressure sensing element is installed at the bottom of the tank and pressure is converted to level. Different liquid densities or closed tank applications must be accounted for. Hydrostatic Head Level Sensors PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 39

40 General Theory for Head Measurement The Pressure exerted by the Height of the liquid is: P = H x Density* If the Density of the liquid is known then H = Pressure Density* Height (H) Pressure PSI Liquid Density (D) *Note: For liquids other than water, use the density of water 0.0361 lb/in3 as a reference and multiply by the SG of the liquid. Hydrostatic Head Level Sensors

41 Example Height (H) Tank 1 PSI Water Density (D) Height (H) Tank 2 PSI Oil Density (D) A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85) P = ? psi

42 Example Height (H) Tank 1 PSI Water Density (D) Height (H) Tank 2 PSI Oil Density (D) A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85) P = H x Density = 30 ft x 0.0361 lbs/in 3 = (30 x 12) x 0.0361 = 13 psi P = ? psi

43 Example Height (H) Tank 1 PSI Water Density (D) Height (H) Tank 2 PSI Oil Density (D) A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85) P = H x Density x SG = 30 ft x 0.0361 lbs/in 3 x 1 = (30 x 12) x 0.0361 = 13 psi P = H x Density x SG = 30 ft x 0.0361 lbs/in 3 x 0.85 = (30 x 12) x 0.0361 x 0.85 = 11 psi

44 Practical Considerations when using head type instruments The reference point of the tank vs instrument input must be considered. Height (H) Pressure PSI Liquid Density (D) Height (H) Pressure PSI Liquid Density (D) This may not be practical in some applications where the tank elevation is below grade or where a remote visual reading is required. Hydrostatic Head Level Sensors (Contd) PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 44

45 Tank Elevations Vertical rises and drops contribute to the overall height and therefore head pressure. Horizontal runs have no effect. Height (H) P Water Density (D) P Hydrostatic Head Level Sensors (Contd)

46 Closed Tank Applications Open tanks are vented to atmosphere so the pressure at the bottom of the tank is only due to the head pressure of the liquid. Closed tanks are not vented to atmosphere so the pressure at the bottom of the tank is due to the head pressure of the liquid + the vapour pressure above the surface. P Head = h x D P (atmospheric) P Head = (h x D) + P vapour P vapour Hydrostatic Head Level Sensors (Contd)

47 Used where the sight glass level gauge can not be used. Magneto-resistive types can provide an electrical output. Liquid/liquid interface (such as water and oil) can be measured by changing the buoyancy of the magnetic float Magnetic Level Sensor PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 47

48 Not the same as a float. The displacer is immersed in the tank and the buoyant force of the liquid produces a torque which is proportional the amount of liquid level. The output force can be converted to provide a proportional pneumatic or electrical continuous output of tank level. Displacers PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 48

49 Displacer – side mounted unit The displacer float size and diameter is selected according to the size of tank and the height of the level to be measured. The output force can be converted to provide a proportional pneumatic or electrical continuous output of level. Advantages and disadvantages: Very little movement of displacer float Affected by different liquid densities Limited to smaller tanks sizes Displacers

50 Non-Contact direct level sensor Level is a function of the time it takes an ultrasonic pulse to hit the surface and return Limitations include: Surface foam absorbs signal, agitation create reflections High Pressure & High Temperatures affect the signal speed Vapour and condensate create false echos UltraSonic Level Measurement PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 50

51 Similar to ultrasonic but at a much higher frequency (6.3 GHz) Various designs -- Frequency Modulated Continuous Wave -- Pulsed Wave -- Guided Wave These sensors have better performance in applications where vapour, dust or uneven surfaces exist. Radar Level Sensors (Microwave) PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 51

52 Tank level is determined by the weight of the quantity of material Load Cells (strain gauge transducers) placed at the bottom of the tank measure the weight and then convert it to an electrical signal. Load Cells PEC – UET Lahore Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 52

53 Capacitance Level Detection (RF Level Probes) RF Capacitance level sensors are similar in operation to conductivity sensors and are typically used when non- conductive liquids are measured. The only variable is the dielectric of the tank material that varies in proportion to the level. Conductive liquids like mineral water will short out the probe to the tank wall. Good for solids

54 Can be used for non-liquids RF capacitance probes are used in a wide range of applications and are subjected to certain limitations such as: Electrode Coating Low sensitivity Temperature Different materials Shielding & noise

55 Summary Level is measured by locating the boundary between two media, called the interface Level can be measured directly or indirectly Noninvasive devices are preferred when the material is corrosive, hazardous, sterile, or at a high temperature or pressure

56 Batch application using point level measurement Detect if the level is at a predetermined point Generally used to detect either a low-level condition or high- level condition The output of a point level sensor is on-off, or 1 or 0 state digital signal A batch process does NOT use a continuous level measurement device. Creative Assignment! HAZOP?????

57 Discussion & Questions? Dr. Shahid Naveed 14. Januar 2014 / Dr. –Ing Naveed Ramzan 57


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