Moisture Sensors DP & Rh The basic low cost solution With a wide range

Transcription

1 Moisture Sensors DP & Rh The basic low cost solution With a wide range

2 Moisture technologies and their ranges DEW-POINT SENSOR TRAINING No single technology can cover the entire range All have limitations Impedance and capacitive sensors have certain key advantages TDL Quartz Sensors Chilled Mirror Hygrometer Impedance Dew-Point Sensors RH Sensors Dew Point o C 10ppb V 10ppm V 1000pp m V 23000ppm V 20 % V/V>50 % V/V

3 2 Technologies IMPEDANCE DEW-POINT SENSOR TRAINING Ceramic nanofarad Polymer Picofarad -100C -20C +60 C Impedance Capacitive

4 Dewpoint Technology Signal Levels IMPEDANCE DEW-POINT SENSOR TRAINING Polymer Picofarad Ceramic nanofarad Signal change 0 to Full Scale 0.1 nanofarads 115 nanofarads Clear challenges in electronic design to amplify and convert low capacitance values!

5 Impedance Technology Most widely used in industry Rugged-can work at high pressure Simple and compact Inexpensive Wide measurement range Less accurate than Cooled Mirror Can be affected by contaminants

6 Ceramic Impedance Sensor Construction DEW-POINT SENSOR TRAINING Plate 1 Plate 1 Dielectric Plate 2 Diaelectric Plate 2 Constructed using thin and thick film techniques. Adsorbs water vapor onto a porous hygroscopic layer between two conductive layers built on top of a base ceramic substrate. The active sensor layer is less than one micron (µm) and the porous top conductor that allows transmission of water vapor into the sensor is even thinner.

7 Moisture Adsorbtion DEW-POINT SENSOR TRAINING Moisture molecules Water changes dielectric constant (Ke) of the layer which causes shift in Impedance Water changes the physical properties of the sensor layer Therefore the sensor responds very rapidly to changes in applied moisture. Both when being dried on process start-up and when called into action if there is moisture ingress into a process.

8 Impedance Moisture Sensor raw output DEW-POINT SENSOR TRAINING Sensor Impedance, K ohms Y Dew point, C

9 Typical Impedance Sensor Response times WET TO DRY Reference Dew point Temperature T90 T95 ( C dp) ( C dp) h03 0h h04 0h h06 0h h30 2h h07 7h h15 9h27

10 More response times

11 Basics of Operation DEW-POINT SENSOR TRAINING Easidew Dewpoint Transmitter Customer Power Analogue Output Power Regulator Reg Micro Controller MSP430F155 Dew point Technology Customer Signal uc Customer Process RS485 Michell diagnostic interface Electronics Circuit

12 Ceramic Impedance Charge Curve Counts DEW-POINT SENSOR TRAINING Example at dry DP Example at wetter DP 100% % 90 Capacitor charge Capacitor charge Lower no. of counts = dry DP Higher no. of counts = wetter DP

13 Capacitive sensor technology DEW-POINT SENSOR TRAINING - Adsorption of humidity into a porous polymer film - Direct electrical response to changes in relative humidity. - Ambient temperature MUST be measured and recorded to give readings and validity. Fast response Higher dew points

14 Capacitive sensor technology IMPEDANCE DEW-POINT SENSOR TRAINING How does it work Actual water vapor pressure Max water vapor pressure directly proportional to RH (partial water vapor pressure) Calculated based on Ta When we know the above (in mbar), we can calculate the Td We can calculate Td when we have enough resolution (because of low RH levels there is a lot of noise in the signal). at 20 C (Ta) accuracy ±2 C Td Td -20 C -60 C Td -40 C -60 C RH 5.4% 0.08% RH ±0.16% ±0.018%

15 Sample Conditioning for Dew Point Determination

16 Moisture measurements are mostly measured indirectly, sample conditioning is essential to getting a correct result When getting an incorrect result the user usually blames the sensor Measure at pressure or at atmospheric? Different technologies require essentially the same sample system components but there are some differences

17 Direct insertion or use a sampling system? Direct insertion is low cost and simple but: X Does not allow flow control X Filtration is a problem X Maintenance requires total isolation and let down of whole system When looking at sampling systems you need to consider the following as well as the application: - Materials - Connections & leaks - Flow & pressure control - Filtration - Pressure regulation - Avoiding condensation

18 Materials It is important to ensure that the materials used are suitable for the application. Many materials commonly used are either porous or hydroscopic. The graph shows the level of dew point inside different tubing materials when flushed with very dry gas Stainless Steel (<-60 o C dew point) PTFE (Teflon) (-60 o C dew point) Brass (-40 o C dew point) There is no rubber on this list!

19 Careful Design Ensure smallest possible volume Avoid dead spaces they are hard to flush Avoid too many connections component leaks Leaks let the media out BUT LET WATER IN!

20 Flow Rate Flow Rate. The flow rate must be in the correct range. Flow rate too low:-ineffective purging -Accentuates absorption & desorption -Back migration of moisture Slow response Wetter Readings Flow Rate to high:- Problems with back pressure and slow response Flows in L/min Easidew MDM300 Chilled Mirror High Optimum Low

21 Avoid Contamination Use filters Particulate Coalescing Membrane All humidity measurement instruments are susceptible to errors caused by contamination. The hygrometric layer of the ceramic sensor is made of very small pores: Any contaminating particles carried along the process gas might eventually make contact with the active layer if the filtration failed to stop them. It generally causes permanent alteration to the electrical properties of the sensor resulting in a shift in calibration.

22 Standard type filters Particulate Coalescing

23 Special filters - Glysorb filters Contain a proprietary absorbent Removes glycol, lube oil, corrosion inhibitors Prevent electrolytic action which causes sensitivity loss in AlOx Glycol vapour contamination requires system cleaning and potentially expensive shutdowns Can be used before a particulate filter Copyright Caltech 2014

24 Special Filters-Membrane filters DEW-POINT SENSOR TRAINING Membrane filter for sampling system Membrane filtration offers better protection than conventional coalescing type Sample line still needs trace heating in case of sample at/near saturation Membrane does not effect composition of gaseous components A bypass flow is needed to ensure filter scrubbing Principle of operation Copyright Caltech 2014

25 Sample Pressure Regulation Joule-Thomson cooling must be counteracted C/bar in NG Such intermediate pressure samples would be more difficult to transport due to increased HCDP Heated pressure regulator Copyright Caltech 2014

26 Avoid Condensation At high dew points there is always a risk of condensation forming. This can be avoided by:- Close control of temperature (keep well above dew point) Avoid cold spots in system Trace heating if necessary Consider the winter/summer, daytime/nightime temperatures.

27 Sampling system designs for heating DEW-POINT SENSOR TRAINING Heated pressure regulator Analyzer Thermostat Heater Trace heated sample line Insulated enclosure

28 Diurnal temperature effects DEW-POINT SENSOR TRAINING

29 Actual case study

30 Simple sampling systems Standard range 10b (options to 200b) Filter-particulate Simple flow regulator but no flow meter Allows sampling at pressure or atmosphere

31 Easidew sampler

32 Drycheck: Complete package for compressed air Fully self contained Particulate filtration Flow meter Can be used at atmospheric or pressure up to 10 bar LCD Display

33 ES10:Build off plan Build off a data sheet Different components can be added Choice of enclosures or none at all Can use IS/IEC rated hygrometers Pressures up to 10 bar Pressure or atmospheric measurement

34 Instrument air Compressed air Medical gases Breathing air

35 DEW-POINT SENSOR TRAINING

36 Process: Natural and Process gas Water Dew Point DEW-POINT SENSOR TRAINING <0.1 ppm(v): alarm at 0.25 ppm(v) <3 Lb/mmscf (<50mgm3) Dependant of operator-user contract 20~30ppm(v)

37 Quartz Micro Balance Sample at lower pressures 1-3 bar-heated regulators required Needs to be temperature stabilised Dryers and permeation tubes need to be changed regularly

38 Moisture in hydrocarbon liquids DEW-POINT SENSOR TRAINING

39 Sample Extraction & Transportation DEW-POINT SENSOR TRAINING Sample from center 1/3 rd of pipe cross section to obtain representative sample Maintain sample line at pipeline pressure and temperature Any decrease in temperature (or pressure) may result in condensed liquid on route to analyzer, so false low readings Such condensed liquid will evaporate with rise in temperature (or pressure), so false high readings Keep sample lines short for best response to process changes

40 Dew-Point Transmitter Products DEW-POINT SENSOR TRAINING IMPEDANCE Transmitters, sensors and application

41 Dew-Point Transmitter Building Blocks IMPEDANCE A DEW-POINT SENSOR TRAINING Sensor packaging will decide suitability Mating Conn. Electrical Conn. Electronics Glass Metal Seal Tile Technology Guard Mini DIN PCB Mini DIN 3 wire digital 2 wire 4 to 20 ma 450 bar 20 bar Ceramic Imped. Polymer HDPE or Sintered

42 Dew Point Application: Compressed Air Process Plant Pneumatic tools Valve control Pneumatic press equipment Breathing air High pressure air systems Automotive (train, bus etc.)

43 The issues Poor quality compressed air leads to: Premature wear & tear Rust and corrosion in piping and systems Damage to instruments Spoiled paint surfaces in spray booths Increase in scrap rate Potential impact on safety and health Copyright Caltech 2014

44 What happens when air is compressed Compress air by 7 B = 800% increase in contamination Contaminants come from: Dirt, dusts and other particulates Water vapour Unburned hydrocarbons and oils Oxide and chlorine bases forming caustic gases Copyright Caltech 2014

45 Compressed air standard ISO Class micron Solid Particles micron micron Water Pressure DP Oil Mg/m3 Inc vapour ,000 1, Not specified Not specified Not specified 10, Not specified Not specified 1, ,00 0 Not specified Not specified Not specified 10 7 Not specified Not specified Copyright Caltech 2014

46 Types of dryer Refrigerant - Typically gives 2-4 C Dessicant - Typically gives -40 to -70 C Membrane Copyright Caltech 2014

47 Air or Gas Dryers Compressed Air Dryers Plastic Moulding Dryers Dryers for Ozone Generators Pure Gas Dryers Breathing Air Dryers Measurement Monitoring of dewpoint at outlet of dryer to ensure specification. Control of regenerative dryers switchover to increase efficiency and save energy costs Spot check of dryer outputs Copyright Caltech 2014

48 Adsorption Principle DEW-POINT SENSOR TRAINING Regenerated Tower Drying Tower

49 Rotary Adsorption Dryer Hot unsaturated air for regeneration Hot saturated air Dry air Cold saturated air

50 Refrigeration Dryer AIR/AIR HEAT EXCH

51 Other Types Combined Refrigeration / Adsorption Pre-cooling of intake air (capacity/power) Osmosis

52 Power Generation Hydrogen Coolant Moisture in hydrogen being used to cool electrical generator sets Measurement Techniques: Ensuring that the dewpoint of the hydrogen does not rise above ambient temperature causing condensation and thus the danger of damaging the generator. Upper limit is usually 0oC dewpoint. Checking outlet from H2 adsorption dryer Limit -20 C dp at operating pressure (typically 4 barg) Checking outlet from generator frame Limit 0 C dp at frame pressure (typically 2 to 4 barg)

53 Power Generation Application Moisture in Sulphur hexafluoride (SF6) arc quenching gas can cause rapid and severe deterioration of high-voltage switchgear. Measurement Techniques: Checking the moisture content of new SF6 in cylinders by industrial gas producers prior to use Dry gas purging of new switchgear enclosures In service field checks/ continuous monitoring

54 Power Generation DEW-POINT SENSOR TRAINING GAS TURBINES It is important to monitor the dewpoint of the inlet air of a gas turbine in order to ensure that ice does not form on the turbine blades.

55 Plastics Industry Polymer chip drying For PET

56 Industrial Gases Compressed gases are supplied to many industrial and research users by a relatively small number of large gas companies, for example: The gases are supplied either in cylinders (top picture) filled at a central filling station or in bulk (bottom) from an on-site generation plant, often managed by the gas supplier Gas Cylinder Checking Gas Generation Plant

57 Rh Products and Applications Swimming Pool Dehumidifier Details: Customer: Monitoring ambient humidity for control to prevent condensation Temperature range 20 to 35 C Humidity range % Sensors need to work well as they must have a good resistance to Chlorine Instruments:

58 Weather Station Details: Customer: Provide the %RH monitoring for Delta-T s weather station Humidity range %RH Temperature range -30 to 70 C Instruments:

59 Salt Spray Chamber Details: Customer: Humidity monitoring in corrosion chamber to speed up the ageing process of mechanical parts Humidity range 0-85%RH Temperature range 5 to 30 C Instruments:

60 Fish Smoker Details: Customer: Humidity monitoring of smoke circulation in fish smoker High humidity - up to 100%RH Temperature ranges from 50 C up to 150 C depending on application Instruments:

61 Details: Agricultural Food Storage Customer: Monitoring humidity for control in grain & potato stores Humidity needs to be controlled at 97%RH for potatoes, much less for grain Temperatures vary with the ambient condition Instruments:

62 Fume Cupboard Details: Customer: Monitoring ambient humidity within a fume cupboard Humidity range %RH Temperatures are only ambient room conditions Instruments:

63 Wood Kiln Drying Details: Customer: Monitoring of humidity for control and efficiency of the drying system Humidity range 20-75%RH Temperature range 25 to 100 C dependant on application Can also apply to ceramics and brick drying Instruments:

64 Engine Test Cell Details: Customer: Monitoring humidity of the air intake to the engine Humidity range 20-95%RH Temperature range 15 to 50 C Can apply to automotive and marine engines Instruments: