Misuse of Combustible Gas Meters Supervisor Training 1/11/08

Case History  An incident investigation raised concerns that users of combustible gas meters may be greatly underestimating fire and explosion hazards.  Errors result when meters calibrated on a specific gas are used to measure other explosive gases or vapors.  In one case a methane calibrated meter was used to measure gasoline vapors; the meter gave a reading of 14% LEL (Lower Explosive Limit) while the actual concentration in the tank was 73% of the LEL.

How Combustible Gas Meters Work  Most combustible gas detectors these days literally measure the contaminant by combustion at a catalytic detector. –Older style used the wheat-stone bridge method.  The heat produced is used as a measure of the “explosivity” of the contaminant in air.  Different compounds produce different amounts of heat when they are burned. So the meters respond differently to different chemical mixtures in air.

Meter Variation  Combustible gas meters can only be expected to respond accurately to the gas for which they were calibrated.  To measure other gases with the same meter, consideration must be given to the specific properties of the gas and of the detector.  Most manufacturers have responded to the need to estimate more accurately the concentration of other gases by providing correction factors which allow calculation of percent LEL from the measured level.

What is LEL & UEL? When vapors of a flammable or combustible liquid are mixed with air in the proper proportions in the presence of a source of ignition, rapid combustion or an explosion can occur. The proper proportion is called the flammable range and is also often referred to as the explosive range. The flammable range includes all concentrations of flammable vapor or gas in air, in which a flash will occur or a flame will travel if the mixture is ignited.

What is LEL & UEL? There is a minimum concentration of vapor or gas in air below which propagation of flame does not occur on contact with a source of ignition. There is also a maximum proportion of vapor in air above which propagation of flame does not occur. These boundary-line mixtures of vapor with air are known as the lower and upper flammable limits (LFL and UFL) respectively, and they are usually expressed in terms of percentage by volume of vapor in air. The LFL is also known as the lower explosive limit (LEL). The UFL is also known as the upper explosive limit (UEL).

Flammable Limits In popular jargon, a vapor/air mixture below the lower flammable limit is too "lean" to burn or explode, and a mixture above the upper flammable limit is too "rich" to burn or explode. No attempt is made to differentiate between the terms flammable and explosive as applied to the lower and upper limits of flammability.

Limitations of Meters  These instruments can not be relied upon to: –measure highly toxic gases such as hydrogen sulphide. –respond accurately in atmospheres which do not contain 20.9% oxygen in air. –provide a reliable indication of the degree of explosive hazard when meters are not calibrated before and after each use. –compensate for poor field sampling technique or an operators failure to consider the work environment.

Limitations of Meters  ALWAYS remember… 1% = 10,000 ppm  QUESTION… OSHA’s acceptable limit for O2 is 19.5%-23.5%. Your meter gives you an O2 reading of 19.9%. Would you enter this space based on this information?

Limitations of Meters  Oxygen concentrations in air, other than those normally occurring in the atmosphere (i.e. 20.9%), may result in underestimating the explosive hazard.  Meter response depends on its ability to burn the combustible gas.  If there is not enough oxygen to support combustion, the meter then would give a lesser LEL, even if high levels of combustible gas were present.

Limitations of Meters  Since it is quite common for the meter sensors to fail, bump-test before use is required.  Exposure of the sensor to moisture, lead compounds, silicon compounds, or chlorinated hydrocarbons may result in instrument failure during use.

How to use Correction Factors K-Factors & Correction Factors  Various means of correcting for different meter response have been employed by a number of manufacturers.  In each case, however, the correction factors given are specific to the model of meter and the gas/vapor being measured.  Individual suppliers must be contacted to obtain the appropriate correction factors for your meter.

How to use Correction Factors K-Factor Formula K-factor (calibration gas) x observed LEL K-factor (measured gas/vapor)

How to use Correction Factors Trick Question #1…  In a tank used to store anhydrous ammonia an operator records a reading of 18% LEL using a meter calibrated for methane gas (what you use).  What is the actual % LEL in the tank?  Given: –Observed reading: 18% LEL –ammonia K-factor = –Calibration gas was methane, K- factor = 112.0

How to use Correction Factors Answer..  Then: K-factor (calibration gas) x observed LEL K-factor (measured gas/vapor) x 18 = 14% 141.7

How to use Correction Factors Trick Question #2  The area around a leaking gas line gives a reading of 56% LEL on a meter calibrated with methane.  What is the actual % LEL in the area? Given: Observed reading = 56% LEL Correction factor (propane) = 1.1

How to use Correction Factors Answer: Corrected % LEL = correction factor x observed % LEL = 1.1 x 56 = 62%

Correction Factors for some of your chemicals Some flammables have a correction factor as high as 2.5!!! (xylene & Hexane)

Four Gas Meters  Your meters are 4-gas meters, meaning they measure ONLY four gases! –Oxygen –LEL –H2S –CO  A few meters have a NH3 sensor instead of a CO or H2S sensor

Four Gas Meters  What do you do when you want to measure a potential toxic atmosphere for Sulphuric Acid? –Will your four gas meter work?  Colormetric/Diffusion Detector Tubes are your answer.

Response Times  When sampling an area, one sample is NOT enough  Sampling a PRCS –Top - Middle - Bottom –Vapor Density  Last Trick Question of the day…

Response Times  You are sampling a tank for entry and the tank is 50 ft tall. You are on top of the tank and lower your 50 ft sample hose to the bottom of the tank. It takes ~5 minutes for the air sample to reach the instrument. You then sample the middle of the tank (25 ft down)…how long should you leave the sample tube at the 25 ft mark, since it is half the distance of your first sample?  A. 5 minutes  B. 2.5 minutes  C. I do not have to sample the middle?

Industrial Scientific M40  FREE On-Line Video for Operation of the M40 Meters can be viewed at: Click on “Services” tab and select “Training” from drop down On the “Training Page” Click on “FREE Online Video Training” link just below the image of your M40 meter. Once on that page you will see an image of your meter, CLICK on it and begin with the first 2 minute video.