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Measurement of Lower Explosive Limits (LEL) and Upper Explosive Limits(UEL)
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Interesting discussion going on amongst my HSE colleagues. The question is around measuring the atmosphere for volatile organic solvent vapours which under certain conditions can become explosive. An expert suggests that the lowest LEL of the vapours present should be taken . My view is that different organic solvents have different LEL's and therefore the LEL's of each organic solvent needs to be measured. Any thoughts ?
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Rank: Super forum user
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De
If I understand what you have written then I agree with you. If you have a number of products present if you just monitor the one that has the lowest LEV how does this ensure that one of the others does not reach its lev?
Unless of course there is some way that you can show that the build up of these solvents will be equal?
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Rank: Super forum user
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If you are measuring the LEL for the purposes of establishing your hazardous areas and for setting up/installing gas detectors, then I would work to your 'worst case' i.e. the solvent with the lowest LEL.
e.g. if you have 2 solvents, one with LEL of say 10% and the other 15%, then there is not much point in having gas detectors set at 15% - you may already have an explosive atmosphere present, before getting any alarm.
You also need to consider the boiling points of the solvents, as the BP is a good indicator of their volatility and how easily gases/vapours will be released.
Good reference documents for classification of hazardous areas are BS60079-10 and the Energy Institute document 'IP15'
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Rank: Super forum user
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JJ
for my own education I would be grateful if you could explain the following. Are you able to monitor for solvent levels in general? In which case I would agree with you.
My question is if you have two products one with and lel (apologies for the lev error) of 10% and a second with 15% if you are just monitoring for the 1st what is to stop the second building above 15% before the 1st reaches 10%?
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Rank: Super forum user
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It depends on why you are measuring/monitoring, which is more important e.g. for purposes of Atex/DSEAR/Hazardous areas or to satisfy CoSHH/EH40 WEL requirements.
Arguably of course, both limits need to be complied with.
Just discussing % LEL isn't that helpful, unless the context is known and the solvent properties known.
Indeed the 15% LEL solvent may be more important than the 10% LEL solvent, if the 15% LEL solvent is the more volatile solvent i.e. has a low boiling point
Hence why, if the quantities justify it, hazardous area classification should be done following one of the guidance documents I mentioned in my first post.
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Rank: Super forum user 
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Do all of them. The cost is in having the specialist come in in the first place, each additional test is then just peanuts, so get the full analysis done while they are present and that should answer any questions. A specialist will also be able to give you a combined reading of all VOCs present as well so you can ascertain the total effect.
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Rank: Forum user
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The combustible gas sensor on your basic monitoring device will not distinguish between different combustible gases. The sensor is calibrated with a single gas, normally methane. If you use a sensor calibrated with methane in an environment with say methane and propane gas, then the %LEL reading will not be accurate, however both gases will contribute to the sensor's "response".
Generally the response of the sensor to other combustible gases is less than methane. This means that the sensor (when placed in an atmosphere with a combustible gas other than methane or mixture of combustible gases) will report a lower than actual value for the %LEL. If you know which gas is present then you can correct the %LEL reading to obtain an accurate concentration.
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Rank: Forum user
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DeMemezes. I largely agree with AndrewO. However Measuring organic solvent vapours is different to flammable gases.
Flammable liquids will only give off flammable vapours in sufficient quantities to reach their LEL, IF they are above their "Flashpoint" FP. Industrial LEL meters such as Draeger or Crowcon etc are usually calibrated on Pentane. (can be an other if there is special need) As such they are a general indicator of solvent vapour as % of LEL. Normally the alarms are set at 10% of LEL -something is present and you need to know what, and 50% LEL Action is needed now to prevent LEL being reached!
As for Coshh, as a crude rule of thumb, the OES /WEL for solvent vapours are 100 to 1000x less than the LEL for that solvent. So if you are compliant with Coshh you are doing it right.
To find out all solvent components present and their levels then you will have to take an air sample for gas chromatography analysis.
If you are playing with LELs then DSEAR applies.
In practice on most industrial sites it is impossible to differentiate between different solvent ppm or LELs on a real time basis.
BW Peter
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