Getting the most from your 4 gas monitor. Terri A. Pearce, PhD Emily R. T. Schmick, MSPH, CIH

Transcription

1 Getting the most from your 4 gas monitor Terri A. Pearce, PhD Emily R. T. Schmick, MSPH, CIH

2 NIOSH Disclaimer: The findings and conclusions in this report are those of the author(s) and do not necessarily represent the official position of the National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention. Mention of any company or product does not constitute endorsement by NIOSH.

3 What prompted us?

4 4 gas monitor Typically configured for confined space entry Oxygen deficiency Flammability Toxics Typically hydrogen sulfide and carbon monoxide May have sensors for other toxics or photoionization detector (PID)

5 Oxygen Concentration Optimized to normal Outdoor air = 20.9% O 2 OSHA range (29 CFR ) Oxygen deficient Less than 19.5% Oxygen enriched Greater than 23.5% Two common sensor types Electrochemical Fuel Cell Solid Polymer Electrolyte

6 Sensor technologies Capillary pore Solid Polymer Electrolyte (SPE)

7 Oxygen Depletion/Displacement Set air to 20% O 2 which means the other 80% of N 2 must be displaced too Using ratios, calculate the amount of N 2 that is also displaced: 20/80 = 1000/x x = = 5000 ppm displaced by another gas Every 0.1% drop in O 2 concentration means that there is 5000 ppm of something else in the air

8 Oxygen Sensing Dead band Changes in electronic signal around that for 20.9% oxygen, does not cause the output to change from reading 20.9 It can reduce the monitor s effectiveness as a broad band toxic sensor

9 Oxygen Sensing Dead band can reduce the perceived jumpiness of oxygen sensors but it can reduce their effectiveness as a broad band toxic sensor An oxygen shift will only become evident at 10,000 ppm because the first 0.1 ppm (5000 ppm of something else ) will be masked by the dead band It will take 10,000 ppm of something else to defeat the deadband (i.e., 20.9% drops to 20.7%) If oxygen drops AT ALL you have a LOT OF SOMETHING else in the air, so much so that you should expect response from most electrochemical sensors if only as a reading from crosssensitivity

10 Types of Sensors Wheatstone bridge catalytic bead Response, calibration & correction factors Poisons High range flammability Thermal conductivity Non dispersive infrared (NDIR) Photoionization detector (PID)

11 Thermal Conductivity Sensors The Thermal Conductivity (TC) sensor operates on the principle of the cooling effect caused by the gas as it passes over a heated coil Fixed Resistor Deactivated LEL Reference Bead; Used for TC/Vol% V 1 Flammable gases tend to conduct heat better than nitrogen which is a great insulator Fixed Resistor V OUT Active LEL Bead; Disconnected for Vol% Mode V 2 As the coil cools, the resistance decreases in proportion to the thermal conductivity of the gas Courtesy RAE Systems TN 153

12 Non Dispersive InfraRed (NDIR) Sensors for Combustible Gases NDIR sensors use the absorption of infrared light to make gas measurements Many molecules can absorb infrared light, causing them to bend, stretch or twist The amount of IR light absorbed is proportional to the concentration.

13 Non Dispersive InfraRed (NDIR) Sensors for Combustible Gases Light passes through the gas sample and is absorbed in proportion to the amount of C H bonds present The filter in front of the detector removes all the light except that at µm, corresponding to C H bonds Reference detector provides a real time signal to compensate the variation of light intensity due to ambient or sensor changes Concentration = Detector B Detector A (Light absorbed is proportional to concentration µm filters IR detector A IR detector B Measurement side CH 4 CH 4 CH 4 CH 4 CH 4 CH 4 Reference side

14 NDIR LEL sensors will miss some flammable gases Flammable gases and vapors that lack the C H bond will not be seen by the NDIR LEL sensors Some examples of flammable gases/vapors that NDIR LEL sensors miss: Hydrogen Carbon monoxide Ammonia

15 Electrochemical Toxic Gas Sensors Gas diffusing into sensor reacts at surface of the sensing electrode. Sensing electrode made to catalyze a specific reaction. EC sensors are often called 3 wire sensors as they have a sensing, reference and counter electrodes. Use of selective external filters further limits crosssensitivity for new sensors. Unlike fuel cell oxygen sensors EC sensors are not a one way trip. Similar to dry cell battery in construction.

16 Electrochemical Toxic Gas Sensors

17 EC Sensors Regenerative Process Unlike fuel cell oxygen sensors which have a one way trip from lead to lead oxide, electrochemical toxic gas sensors are more of a circular process Molecule comes in, reacts, generates electrical current, uses up water, current from the battery is returned to the sensor, regenerates water in the presence of oxygen Regenerative or circular process as long as you stay within the operating parameters (specs) of the sensor

18 Response Time: time for sensor to reach its final stable reading. Typically called T 90,or time to 90% of response and usually expressed in seconds.

19 Gas Concentration Response Carbon monoxide (CO) 300 ppm < 1.5 ppm Sulfur dioxide (SO 2 ) 5 ppm about 1 ppm Nitric oxide (NO) 35 ppm < 0.7 ppm Nitrogen dioxide (NO 2 ) 5 ppm about 1 ppm H 2 S sensor cross sensitivity* Hydrogen (H 2 ) 100 ppm 0 ppm Hydrogen cyanide (HCN) 10 ppm 0 ppm Ammonia (NH 3 ) 50 ppm 0 ppm Phosphine (PH 3 ) 5 ppm about 4 ppm Carbon disulfide (CS 2 ) 100 ppm 0 ppm Methyl Sulfide (C 2 H 6 S) 100 ppm 9 ppm Ethyl Sulfide (C 4 H 10 S) 100 ppm 10 ppm* Note: High levels of some chemicals including alcohols, ketones, and amines give a negative response. *Estimated from similar sensors. Methyl mercaptan (CH 4 S) 5 ppm about 2 ppm Ethylene (C 2 H 4 ) 100 ppm < 0.2 ppm Isobutylene (C 4 H 8 ) 100 ppm 0 ppm Toluene (C 7 H 8 ) ppm 0 ppm* Turpentine (C 10 H 16 ) 3000 ppm about 70 ppm*

20 Gas Concentration Response Hydrogen sulfide (H 2 S) 24 ppm 0 ppm Sulfur dioxide (SO 2 ) 5 ppm 0 Nitric oxide (NO) 25 ppm 0 ppm Nitrogen dioxide (NO 2 ) 5 ppm 0 ppm Hydrogen (H 2 ) 100 ppm 40 ppm Chlorine (Cl 2 ) 10 ppm 0 1 ppm Ammonia (NH 3 ) 50 ppm 0 ppm Phosphine (PH 3 ) 5 ppm 0 1 ppm Hexane (C 6 H 14 ) 100 ppm 0 ppm CO sensor cross sensitivity* Note: High levels of some chemicals including alcohols, ketones, and amines give a negative response. Used sensors show increasing response to VOCs Ethanol C 2 H 5 OH) 100 ppm 1 ppm Acetylene (C 2 H 2 ) 250 ppm 250 ppm Ethylene oxide (C 2 H 4 O) 125 ppm > 40 ppm Ethylene (C 2 H 4 ) 100 ppm 16 ppm Isobutylene (C 4 H 8 ) 100 ppm 1000 ppm 0 ppm 7 ppm Toluene (C 7 H 8 ) 400 ppm 0 ppm Nitrogen (N 2 ) 100% 0 4 ppm

21 What Does a PID Measure? Organic compounds: Aromatics e.g. benzene, toluene Ketones & Aldehydes e.g. acetone Amines & Amides e.g. diethyl amine Chlorinated hydrocarbons e.g. trichloroethylene Sulfur compounds e.g. carbon disulfide Unsaturated hydrocarbons - e.g. isobutylene Alcohols - e.g. ethanol Saturated hydrocarbons - e.g. butane Inorganic compounds: Hydride Gases: Ammonia, Phosphine Semiconductor gases: Arsine

22 Wheatstone Bridge Sensor Methane (CH 4 ) Compensating bead Heavier Hydrocarbons Rejected by the Flame Arrestor Flame arrestor Heavier Hydrocarbons Active bead

23 Wheatstone Bridge Catalytic Bead LEL Sensor Shortcomings Two mechanisms affect the performance of Wheatstone bridge LEL sensors and reduce their effectiveness when applied to all but methane: Gases burn with different heat outputs at their LEL Heavier hydrocarbon vapors have difficulty diffusing into the LEL sensor

24 Designed to Measure Methane Gas/Vapor LEL (% vol) Sensitivity (%)* Ignition Temp. F (C )** Acetone (465) Benzene (498) Diesel NA Gasoline (280) Hydrogen (500) Methane (537) MEK (404) n Pentane (260) Propane (450) Toluene (480) LEL sensor sensitivity varies with the gas/vapor * Relative sensitivities are for example only, please consult your detector manufacturer for sensitivities specific to your product ** NFPA 325 Guide to Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids, 1994 edition

25 Correction Factors Correction factor is the reciprocal of the relative response The relative response of LEL sensor (methane scale) to ethanol is 0.8 Multiplying the instrument reading by the correction factor for ethanol provides the true concentration Given a correction factor for ethanol of 1.25, and an instrument reading of 40 per cent LEL, the true concentration would be calculated as: 40 % LEL X 1.25 = 50 % LEL Instrument Reading Correction Factor True Concentration

26 Getting the most from your gas monitor: NOVEL USES FOR LEL METERS

27 The hazard of interest Upstream oil and gas industry workers have frequent contact with hydrocarbon gases (HCG) o o Manual tank gauging and oil sampling Fluid handling and loading Highly variable worker exposure to hydrocarbons o o o o Nine fatalities ( ) associated with acute hydrocarbon and oxygen displacement Potential for chronic effects Leukemia (Benzene) Peripheral Neuropathy (N Hexane) Photo Credit: Todd Jordan, OSHA Photo and Video Credit: John Snawder, NIOSH

28 Characterizing the hazard Graphic Credit: NIOSH OSHA Hazard Alert Methane Ethane Propane Butane Pentane N Hexane Benzene Toluene And more

29 Characterizing the hazard Graphic Credit: NIOSH OSHA Hazard Alert Methane Ethane Propane Butane Pentane N Hexane Benzene Toluene And more All detectable using your standard Catalytic Bead LEL Monitor! Can we use this monitor as an exposure assessment tool?

30 Example %LEL Datalog from Gas Monitor % LEL Time Catalytic combustion monitor Datalog provided by John Snawder, NIOSH

31 Example %LEL Datalog from Gas Monitor 16 Monitor in Alarm Mode >10% LEL, IDLH Condition % LEL Time Catalytic combustion monitor Datalog provided by John Snawder, NIOSH

32 Example %LEL Datalog from Gas Monitor Monitor NOT in Alarm Mode How do these peaks convert to a concentration? Is this a Safe Exposure? % LEL Time Catalytic combustion monitor Datalog provided by John Snawder, NIOSH

33 Example %LEL Datalog from Gas Monitor This monitor was calibrated to Methane, then exposed to a hydrocarbon mixture. So, what does 10% LEL really mean? % LEL Time Catalytic combustion monitor Datalog provided by John Snawder, NIOSH

34 Estimating Monitor Response: Correction Factors Monitors should be calibrated to the gas they are expected to sample, OR Apply a simple correction factor if sampled gas differs from calibration Generally published by the manufacturer: specific to the brand and type What about MIXTURES of gases? EQUATION 1: /

35 Testing this theory Flammable Gas Concentration in Blend (ppm) LEL of Gas (ppm) Methane Correction Factor (IS) Methane Correction Factor (Rae) Expected Monitor Response (IS) Expected Monitor Response (Rae) Denver Gas Blend (DGB) created with industry partners Methane , % 0.60% Ethane , % 1.32% Propane , % 1.85% Butane ,000 to 1.58 represent 1.7 ratios 1.27% of tank 1.18% Pentane , % 0.55% n Hexane 70 12,000 headspace % 0.28% Benzene 10 13, % 0.04% Toluene 9 12,000 Six 2.55Models 2.4 of Catalytic 0.03% 0.03% p Xylene 6 11, % 0.02% Combustion monitors SUM 2000 ppm 5.7% 5.9%

36 Monitor response to Denver Gas Blend (DGB) EQUATION 1: / Flammable Gas Concentration in Blend (ppm) LEL of Gas (ppm) Methane Correction Factor (IS) Methane Correction Factor (Rae) Expected Monitor Response (IS) Expected Monitor Response (Rae) Methane , % 0.60% Ethane , % 1.32% Propane , % 1.85% Butane , % 1.18% Pentane , % 0.55% n Hexane 70 12, % 0.28% Benzene 10 13, % 0.04% Toluene 9 12, % 0.03% p Xylene 6 11, % 0.02% SUM 2000 ppm 5.7% 5.9%

37 Monitor response to Denver Gas Blend (DGB) EQUATION 1: / Flammable Gas Concentration in Blend (ppm) LEL of Gas (ppm) Methane Correction Factor (IS) Methane Correction Factor (Rae) Expected Monitor Response (IS) Expected Monitor Response (Rae) Methane , % 0.60% Ethane , % 1.32% Propane , % 1.85% Butane , % 1.18% Pentane , % 0.55% n Hexane 70 12, % 0.28% Benzene 10 13, % 0.04% Toluene 9 12, % 0.03% p Xylene 6 11, % 0.02% SUM 2000 ppm 5.7% 5.9%

38 Monitor response to Denver Gas Blend (DGB) EQUATION 1: / Flammable Gas Concentration in Blend (ppm) LEL of Gas (ppm) Methane Correction Factor (IS) Methane Correction Factor (Rae) Expected Monitor Response (IS) Expected Monitor Response (Rae) Methane , % 0.60% Ethane , % 1.32% Propane , % 1.85% Butane , % 1.18% Pentane , % 0.55% n Hexane 70 12, % 0.28% Benzene 10 13, % 0.04% Toluene 9 12, % 0.03% p Xylene 6 11, % 0.02% SUM 2000 ppm 5.7% 5.9%

39 Monitor response to Denver Gas Blend (DGB) EQUATION 1: Flammable Gas Concentration in Blend (ppm) LEL of Gas (ppm) Methane Correction Factor (IS) Methane Correction Factor (Rae) / Expected Monitor Response (IS) Expected Monitor Response (Rae) Methane , % 0.60% Ethane , % 1.32% Based on EQN 1: Propane , % 1.85% Butane , % 1.18% Pentane , % 0.55% Denver Gas Blend (2000 ppm concentration) should produce response of % LEL on a Catalytic Combustion Monitor n Hexane 70 12, % 0.28% Benzene 10 13, % 0.04% Toluene 9 12, % 0.03% p Xylene 6 11, % 0.02% SUM 2000 ppm 5.7% 5.9%

40 Response to DGB (2000 ppm) AVERAGE MONITOR RESPONSE (% LEL) LEL 1 (CF=1) LEL 2 (CF=1) LEL 3 (CF=1) LEL 4 (CF=1) LEL 5 (CF=1) LEL 6 (CF=1)

41 Response to DGB (2000 ppm) 18 AVERAGE MONITOR RESPONSE (% LEL) Recall from EQN 1: Catalytic Combustion Sensors expected to produce a response of % LEL at 2000ppm DGB 2 0 LEL 1 (CF=1) LEL 2 (CF=1) LEL 3 (CF=1) LEL 4 (CF=1) LEL 5 (CF=1) LEL 6 (CF=1)

42 Gas Monitors for Protecting Lone Workers New technology allows for: GPS Worker Tracking No movement or Man Down response Fall Detection Example inet dashboard

43 Gas Monitors for Predictive Analytics Use Business Analytics to find trends and hazards N KEY: High Alarm Events Low Alarm Events Example only

44 What are the implications?

45 Monitor considerations Hazard assessment must happen before monitors are placed in use Limitations present risks that must be managed There must be awareness of the equipment limitations as much as there must be awareness of the hazards

46 Monitor considerations Must have the right detector for the application Configure monitor based upon specific hazards Understand cross sensitivities Cannot accurately detect combustible gases in a low oxygen environment most detectors require 10 15% oxygen Cannot detect combustible gases above the UEL UEL condition doesn t mean there is no immediate fire/explosion hazard Must know the detector actually works monitor must be bump tested and calibrated

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48 Managing monitor considerations Must develop a process that adequately evaluates the hazard while protecting the worker and accounting for the equipment limitations Training, Training, Training

49 THANK YOU! QUESTIONS?