Gas Detector Sensor Technology

Transcription

1 Gas Detector Sensor Technology Joel Myerson President Safety Inc ESafetyInc.com ETA Process Instrumentation etapii.com Martech Controls MartechControls.com ifacilityservices ifacilityservices.com

2 Standard technology and new developments Electrochemical (EC) Sensors Catalytic Bead (LEL) Sensors Infrared (IR) Sensors Photo Ionization Detectors (PID) Ceramic Metal Oxide (CMOS) Photoacoustic Infrared

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4 SERVICES WE PROVIDE: - On-site or in our state of the art calibration lab - Calibration and repair of portable and fixed instruments - Respirator maintenance - SCBA, PAPR, airline, breather boxes - Level A suit testing - to ASTM F 1052 Pressure Test Method - Fall protection equipment inspection - Competent Person Fall Protection - Equipment Inspections per ANSI Z requirements PRODUCTS CATEGORIES: On-Line Analyzers - For gases and liquids. Technologies: electrochemical, catalytic, infrared, spectroscopy, spectrometry, chromatography, fourier transform infrared. Process Control and Measurement - Flow, pressure, temperature, humidity, moisture, prowe, level. Instrumentation - Gas detection, sound, noise, dust, relative humidity, combustion efficiency, Indoor air quality, calibration gases and regulators. Safety Equipment - Personal protective equipment, fall protection, first aid, spill control Safety equipment and portable instruments - Safety Inc wws,esafetyinc.com Process Instrumentation : - New England - ETA Process Instrumentation - Upstate NY - Martech Controls www. martechcontrols.com Calibration and Repair services - ifacility Services

5 Standard 2 Electrode Electrochemical Sensor working electrode and anode

6 3 Electrode EC Sensor Temperature Sensor 3. Counter Electrode Electrolyte. Typically Sulph. acid in water or a salt solution, or an organic electrolyte without water Connection Header EEPROM Porous Wick Pressure Release 2. Reference Electrode (anode) Porous Teflon Housing 1. Working Electrode w/noble metal catalyst Replaceable Dust or Selective Filter

7 2 Electrode vs 3 Electrode EC Sensor Oxygen example Consumptive 2 Electrode Sensor Two electrode standard electrochemical sensor month life Oxygen is reduced to hydroxyl ions at the cathode: Cathode reaction O2 + 2H 2 O + 4e- 4OHHydroxyl Ions oxidize the (lead) anode: 2Pb + 4OH 2PbO + 2H2 O + 4e Overall cell reaction: 2Pb + O2 2PbO Non-consumptive 3 Electrode Sensor 3-electrode technology month life O2 + 4H+ + 4e 2H2O at the working electrode, and 2H2O O2 + 4H+ + 4e at the counter electrode

8 EC issues Reasons EC sensors fail or don t perform to spec: All of Pb anode converted to PbO (in 2 electrode) Electrolyte poisoned by exposure to contaminants: High concentrations of acid gases - H2S and CO2 Solvents cross contamination Electrolyte leakage Desiccation electrolyte dries out Excessive heat and humidity pay attention to specifications Blockage of capillary pore Calibration isn t done or isn t done correctly

9 Typical degradation curve for an EC sensor

10 EC sensors what to look for Sensor Size larger size = more electrolyte = longer life. Many fixed sensors are portable sensors in a housing (i.e. same sensor used in both portable and fixed instruments) Accuracy and Stability Cross Sensitivity data on interference from other gases Detection limits Temperature range Diagnostics. How do you know when to replace? Calibration Interval Warranty length Warranty provisions is it pro-rated, or a true replacement warranty?

11 Smart sensors predictive maint. With the addition of a micro chip in the sensor, a series of self tests are possible such as: Electrolyte Level Logging of Gas Concentrations over time Temperature Real Time for compensation as well as over time Sensitivity Resistance, etc.. Any parameter out of check will signal a calibration, warning or fault True predictive maintenance sensors have the ability to start sending a warning signal days before a sensor fails: Lower cost of ownership Sensors are replaced when they are nearing end of life, not proactively Costs associated with down time are avoided Sensor replacement can be scheduled, not done as an emergency

12 Sensor specifications details matter! Measuring range limit 100 ppm Detection limit 1 ppm Alarm response / on gas exposure with 5x alarm threshold t sec / on gas exposure with 1.6x alarm threshold t sec Calibration interval - default 6 months, range min./max. 1 day/12 months Warm up time Ready for use after max. 30 minutes, Ready for calibration after max. 120 minutes Measurement accuracy Measurement uncertainly (of meas. value) < ±5% Minimum < ±1.5 ppm Loss of sensitivity, per year <-15% Expected service life, in ambient air >24 months Ambient conditions Temperature, min./max. -40/65 C Rel. humidity, min./max. 15/95% Ambient pressure ±3% Cross sensitivities exist. Contact manufacturer

13 Ammonia monitoring Personnel protection, workplace monitoring For protecting workers in the plant and or visitors in buildings portable and fixed instrumentation can be used. The measuring range should match the TLV requirements of the applied regulation. Compressor and machinery room and ammonia tanks leak detection The machinery room houses the refrigeration system. Containing compressor, valves, pumps surge tanks and many joints it is prone to leaks. Small leaks might cause a constant background concentration. The instrumentation should monitor the range beyond the TLV levels to alert the control room and trigger ventilation and switch counter measures. Cold storage room leak detection and tainting protection A tiny Ammonia leak can have a damaging effect on stored goods. Small concentrations can taint food or other refrigerants or causes corrosion. Concentrations to be monitored are in the range up to TLV for getting early warnings. De-icing is an important process, where fast and reliable monitoring is requested to alarm emerging leaks. Local requirements regarding performance, alarm-thresholds, approvals result from different standards for Ammonia handling, cooling and refrigeration, transportation of dangerous goods, pressurized gases, workplace monitoring.

14 Electrochemical - ppm Gas Sensors Two methods of measurement have firmly established themselves over many years Electrochemical sensors are particularly suited to the detection of toxic gases in the lower ppm range Catalytic & IR - % LEL IR and catalytic methods are used to detect combustible substances in concentrations below the lower explosive limit

15 Ammonia exposure limits TWA (Time Weighted Avg) Niosh 25 ppm 8 hr exposure level OSHA 50 ppm 8 hr exposure level35 ppm STEL (Short Term Expsosure Limit)15 minute exposure limit) NIOSH 35 ppm IDLH (Immediately Dangerous to Life and Health)300 ppm (NIOSH) 500 ppm IDLH (OSHA) 1720 ppm Convulsive Cough Fatal <30min ppm Respiratory Spasm Rapidly fatal.

16 Installation Considerations In cooling areas, locate sensors near control / piping end of evaporators and valve stations. Do not mount in front, back or on top of evaporators. When installing sensors near evaporators, keep sensor out of direct airflow from and to the evaporator and away from any moisture created during defrost. Make drip loops for cables on the transmitter. Do not mount the transmitter over door in refrigerated area, because the sensor will become a chunk of ice. Seal all conduit connections In compressor rooms the transmitter should be installed at the ceiling. Ammonia is lighter than air and will rise first. So even if you don t smell Ammonia at ground levels the concentration at the ceiling can be high.

17 Catalytic Sensors measuring for combustion or flammability Lower explosive limit (lel) - Minimum concentration of a combustible gas or vapor in air which will ignite if a source of ignition is present Upper explosive limi (uel) - most but not all combustible gases have an upper explosive limit Maximum concentration in air which will support combustion Concentrations which are above the uel are too rich to burn Flammability range between LEL and UEL

18 Catalytic Bead Sensor

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21 Catalytic Bead rules of thumb

22 Catalytic Bead rules of thumb Rule 2: The more molecules react the higher the reaction rate, the higher the temperature increase Rule 2a: Anything forcing the molecules to move slower will reduce the reaction rate sensor sensitivity decreases. Example: diffusion rates through sintered discs, filters, contaminated sinter, etc. Rule 3: Oxygen concentration must be higher than %/volume Need enough oxygen for catalyzing of combustible gas can t support combustion on the active pellistor without enough O2. Rule 4: Catalyst poisoning reduces the reaction rate - Activated oxygen cannot form on poisoned surfaces. - Too few reactions produces too little heat Less reactions per time = less power = less temperature increase.

23 Issues with single bead catalytic sensors Using two different pellistors creates a zero drift/noise issue. Each pellistor degrades at a different rate, changing difference between the two. Require frequent calibration due to above (drift) Sintered disk is susceptible to poison and reactive gases. Sintered disk delays the response time since molecules have to travel a longer distance to reach the pellistor. Sensors typically cannot be used in applications where the physical orientation of the sensor may be variable (i.e. airplanes, ships, etc.)

24 New two active bead catalytic sensor

25 Two active bead advantages Optimum compensation both of the sensor s pellistors, and measuring and compensating elements, are part of a Wheatstone bridge and are completely uniform Both are active, both degrade at about the same rate Faster response times - using a wire mesh disc as the gas entrance and flame arrestor will make the diffusion paths noticeably shorter so that the response times increase Independent of orientation Improved poison resistance new ceramics and catalyst

26 Catalytic advantages and disadvantages Advantages: Low up front cost Can measure %LEL H2 Small physical footprint Disadvantages: Long term costs Finite lifetime High frequency of calibration (for standard single active pellistor sensors) Should calibrate with measurement gas No smarts built into sensor (no calibration curves) What to look for: Long term stability (zero drift) Calibration interval requirement Position requirement Speed of response

27 Infrared (IR) sensors for combustibles Infrared technology is based on the scientific principle that specific gases absorb infrared light at specific wavelengths. Identify and quantify gas volume by measuring infrared light absorption. Specific gases can be selectively targeted by narrowly filtering the wavelength of the infrared light introduced, thereby avoiding any infrared absorption by other non-targeted gases that may be present in the sample. Higher concentrations of gas will absorb more IR light energy.

28 Infrared (IR) sensors

29 Single source 2 beam IR sensor

30 Dual Source 4 beam IR Sensor

31 4 Beam Advantages

32 IR vs. Catalytic

33 Infrared advantages and disadvantages Advantages: Extremely stable measurement Non consumptive Calibration usually not necessary or can be extended. Low long term costs Can measure %LEL, PPM, %Vol depending on instrument used Gas Library depending on instrument used No false zero vs catalytic i.e. zero reading is all ir light getting to the receiver (100% of light) Disadvantages: Higher initial cost than a catalytic sensor Cannot measure H2 What to look for: Gas Library, Measure one gas, calibrate with another. Water proof Ease of configuring Accuracy Temp. coefficients Dust effects

34 Toxic vs Explosive protection vs combustion 100 LEL % = 1,000,000 ppm PPM = LEL % by Vol x % LEL. Toluene 1.1% LEL x 100% LEL = 11,000 ppm GAS LEL % by vol % LEL* PPM** OSHA exposure limits*** Toluene 1.1% 100% LEL = 11,000 ppm IDLH 500 ppm 10% LEL = 1,100 ppm TWA 100 ppm Xylene 0.9% 100% LEL = 9,000 ppm IDLH 900 ppm 10% LEL = 900 ppm TWA 100 ppm MEK 1.4% 100% LEL = 14,000 ppm IDLH % LEL = 1400 ppm TWA 300 ppm Ethylene Glycol 1.8% 100% LEL = 18,000 ppm IDLH not specified 10% LEL = 1,800 ppm TWA 50 ppm

35 NIOSH pocket guide - Toluene Synonyms & Trade Names Methyl benzene, Methyl benzol, Phenyl methane, Toluol CAS No RTECS No. XS DOT ID & Guide Formula C6H5CH3 Conversion 1 ppm = 3.77 mg/m3 IDLH 500 ppm NIOSH REL TWA 100 ppm (375 mg/m3) ST 150 ppm (560 mg/m3) OSHA PEL TWA 200 ppm C 300 ppm 500 ppm (10-minute maximum peak) See Appendix G Physical Description - Colorless liquid with a sweet, pungent, benzene-like odor. Molecular Weight 92.1 Boiling Point 232 F Freezing Point -139 F Solubility (74 F): 0.07% Vapor Pressure :21 mmhg Ionization Potential ev Specific Gravity Flash Point - 40 F Upper Exposive Limit - 7.1% Lower Explosive Limit- 1.1%

36 Combustible curves vs multipliers

37 Carbon Dioxide CO2 Ambient background in air ppm IDLH - 40,000 ppm (4%) Exposure Limits NIOSH TWA 5000 ppm, ST 30,000 ppm

38 CO2 detection with IR Measuring ranges 0 to 10 vol. % (Standard) 0 to 2,000 ppm 30 vol. % (configurable) Display Backlit graphic LCD; 3 Status LEDs (green/yellow/red) Normal operation 4 to 20 ma Maintenance Constant 3.4 ma or 4 ma ±1 ma 1 Hz modulation; (adjustable) Signal output analog Fault < 1.2 ma Power supply 10 to 30 V DC, 3-wire Power consumption (max.) w/o relay, non-remote 300 ma at 24 V w/ relay, remote 350 ma at 24 V Electrical data Relay specification (option) 2 alarm relays and 1 fault relay, single-pole two-way contact VAC, 5 30 VDC, resistance-bound Temperature -40 to 77 C (-40 to 170 F) without relay -40 to 70 C (-40 to 158 F) with relay Pressure 20.7 to 38.4 inch Hg / 700 to 1,300 mbar Environmental conditions Humidity 0 to 100% r. h., non-condensing Transmitter housing Epoxy coated copper-free aluminum or stainless steel SS316

39 Photo Ionization Detector (PID) PIDs use ultraviolet light as source of energy to remove an electron from neutrally charged target molecules creating electrically charged fragments (ions) This produces a flow of electrical current proportional to the concentration of contaminant The amount of energy needed (in electron volts ev) to remove an electron from a particular molecule is the ionization potential (or IP) The energy in ev must be greater than the IP in order for an ionization detector to be able to detect a particular substance

40 PID IP determines if the lamp can detect the gas If the IP of the gas is less than the ev output of the lamp the PID can detect the gas IP is a measure of the bond strength of a gas and does not correlate with the Correction Factor Ionization Potentials are found in the NIOSH Pocket Guide PIDs cannot distinguish between different contaminants they are able to detect anything present with an IP below the ev rating of the lamp will be ionized. Single total reading for all detectable substances present

41 What PIDs can and can t measure Common Gases Detectable by a PID 1. All hydrocarbons, whose chemical names end in ane, -ene or yne except: Methane & Ethane 2. All alcohols, whose chemical names end in ol 3. All aldehydes, whose names end in aldehyde 4. All ketones, whose names end in one 5. All esters, whose names end in -ate What PID s do not measure 1) Radiation 2) Air (N2, O2, CO2 & H20) 3) Common Toxics (CO, HCN & SO2) 4) Natural Gas (Methane and Ethane) 5) Acid Gases (HCL, HF & HNO3) 6) Others: Freons, Ozone (O3), Hydrogen Peroxide 7) Non-Volatiles: PCBs & Greases

42 PID sensor

43 Ionization potentials Quantified in electron volts (ev) IP is the amount of uv light required to ionize a chemical IP in ev Acetaldehyde Acetamide 9.77 Acetic acid Acetic anhydride 10 Acetone 9.67 Acetonitrile Acetophenone 9.27 Acetyl bromide Acetyl chloride Acetylene Acrolein 10.1

44 Correction factors lamps are 11.7, 10.6, & 9.8 ev Typically calibrated to Isobutylene response 1:1

45 PID advantages and disadvantages Advantages: - Flexible - can measure hundreds of gases - Detection limits of ppm, and sometimes ppb, levels - Stable, easy, inexpensive calibration with isobutylene - Multiple lamps can limit detection of some gases by choice of lamp Disadvantages: - Non-specific any gas present that has an ionzation potential below the power of the lamp will be ionized, reading is cumulative - Multiple lamps need to choose correct lamp - Correction factors different for every chemical, and for each type of lamp

46 Halocarbon refrigerants Halocarbon refrigerants are colorless, odorless and, in general, boil at or below room temperature. They are high molecular weight compounds with vapor densities three to five times that of air. When refrigerants escape from closed systems, they can be expected to settle into low lying areas (in the absence of air movement).

47 Halocarbon refrigerants In the past, notification of refrigerant loss has been by pressure, temperature, or liquid level sensor alarms caused either by inability of the system to maintain temperature or by mechanical malfunction of the equipment. Perhaps 20 to 40% of the total refrigerant in a system can be lost before temperature, pressure, or liquid level sensors alarm. This can amount to hundreds and sometimes thousands of pounds of refrigerant being lost.

48 Halocarbon refrigerants Detection through leak monitoring has the ability to detect the presence of minute concentrations (partsper-million) in the air of enclosed spaces thereby providing early and rapid notification that a leak exists. In most circumstances, refrigerant loss can be reduced significantly by monitoring. For example, in a 10,000 cu. ft. 90 F machine room, the loss of less than one pound of R 12 would trip a 300 ppm alarm. If the leak rate was 1/2 lb/min., an alarm would be given in less than 2 minutes.

49 Sensor technology options Sensor technology selection criteria include the requirements for gas selectivity, sensitivity, and the speed of response. There are tradeoffs between all three of these issues and instrument costeffectiveness. The two most widely applied technologies for refrigerant leak monitoring are IR (infrared) and CMOS (ceramic metal oxide semiconductor). Infrared technology is based on the absorption of a specific wavelength of IR light by the refrigerant molecule. CMOS technology is based on the change in conductivity of sensitized metal oxides upon exposure of refrigerant gases.

50 Ceramic Metal-Oxide Semiconductor (CMOS) CMOS is an older but still useful technology for refrigerant leak detection. CMOS technology is based on the change in conductivity of sensitized metal oxides upon exposure to refrigerant gases. While CMOS systems perform adequately to satisfy the safety requirements of all refrigerants, IR technology is recommended for toxic applications, refrigerant conservation efforts, or in areas where cross contamination by extraneous chemicals is a factor.

51 IR options NDIR & Photoacoustic IR Non-dispersive (NDIR) or absorptive infrared systems determine gas concentration by comparing an air sample to a sample of inert gas (usually nitrogen) stored in the monitor. The room sample and reference gases are individually irradiated with infrared light. The light absorption measured in each gas sample is compared, and the difference between the two measurements determines the concentration. Inaccuracies with this technology can occur when the assumed zero baseline measurement of the reference gas drifts due to changes in room temperature and atmospheric pressure, the aging of the infrared light source, or contamination of the reference cell. Systems must be pulled of/line to recalibrate the instruments, and such downtime can be costly.

52 Photoacoustic IR Photoacoustic infrared avoids the problems associated with zero drift by eliminating the need to compare results to a reference sample, and the associated downtime required for recalibration. PIR devices operate on the principle of sound. IR absorption causes the molecules of the gas to be excited to a higher vibration energy state and increases the pressure inside the instrument. This pressure creates an acoustic wave, which can be detected by an extremely sensitive microphone. Photoacoustic infrared devices direct measure the changes in pressure that occur when infrared light is absorbed by the refrigerant present in the sample. The greater the pressure, the greater the concentration. Conversely, when no pressure pulse occurs, then no gas is present.

53 Photoacoustic IR

54 CMOS vs Photoacoustic IR Application Factors To Consider for Refrigerant Monitors: Sensitivity - Different Refrigerant Safety Groups - Different Purpose of Installation Selectivity - Different Refrigerant Safety Groups - Different Refrigerants - Different Types of Equipment Present Speed of Response - High Pressure/low Pressure - Big Rooms/small Rooms

55 CMOS vs Photoacoustic IR P Infrared CMOS Sensitivity 1-100ppm 20-30ppm HCFC s 30-40ppm HFC s ppm CFC s Selectivity Selective Non-selective Cost Higher ($3-10K) Lower (<$3K) Calibration reference only every 3-6 months Sensor life long short

56 QUESTIONS?? Joel Myerson President Safety Inc ESafetyInc.com ETA Process Instrumentation etapii.com Martech Controls MartechControls.com ifacilityservices ifacilityservices.com