10% LEL ON HOT WORK PERMITS
Good morning - I would like to pose a question to the group. I am trying to understand if a chemical has an LEL of less than 10%, why do most hot work permits allow hot work up to 10%? For example, ethyl alcohol has an LEL of 3% but the industry standard is to have 10% on a hot work permit. My understanding is you would have an explosive atmosphere at 3% concentration. Is it because the permit is giving authorization for 10% of the 3% chemical LEL? In which case it would be 0.3%
Comments (5)
Yes to your question. The LEL is the lowest concentration that can support combustion. In your example, ethanol has an LEL of 3%. If the Combustible Gas Indicator is calibrated to ethanol, then a reading of 10% means that the 0.3% ethanol in the atmosphere.
If your CGI is not calibrated to ethanol (most aren't), then you need to take an extra step of looking out the conversion factor and multiplying the 'CGI reading by the conversion factor to get the actual percent LEL.
Correct. In addition to the correction factor issue Michael mentioned, the LEL is a standalone number and is based on STP (standard temperature and pressure, which is 68⁰F at 1 atmosphere/bar). With that being said, unless it's 68⁰F and 1 bar at all times, your flammability range will actually be different than what's on the SDS - typically a wider range than what is on the SDS since higher temps increase vapor pressure.
Make sure you have the proper meter, as well. If you have any chance of an oxygen-deficient atmosphere, you will need to ensure the LEL sensor is an NDIR (non-dispersive infrared) sensor, not the typical catalytic bead/wheatstone bridge sensors that are oxygen-dependent. Catalytic bead sensors will not function properly in low O2 atmospheres since they use oxidation to get a reading, whereas NDIRs do not rely on oxygen at all (they rely on infrared signatures).
As you mentioned, the requirement for hot work is 10% OF the LEL, not a 10% LEL in general. Your calculations are correct. If the LEL is 3%, then the most allowed during hot work operations is 0.3% concentration of ethyl alcohol.