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Nitrogen Purge Purity Requirements 2

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chemEcaleb

chemEcaleb

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Nitrogen gas is used to displace hydrogen off gas from pipes generated in an electrolytic process to prevent explosions during maintenance, and cell line shutdowns when oxygen generation is increased. How do you determine the minimum purity of nitrogen required for this activity?
 
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It doesn't require pure nitrogen. I'm trying to spec out a nitrogen generator that will actually have a return on investment. One idea is to set the purity of the nitrogen below the LEL of hydrogen and oxygen by some safety margin. 4% oxygen in hydrogen is generally accepted as an explosive mixture, so I should be able to safely set the purity at 98%.
 
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basically says that <4% hydrogen in air is the lowest possible flammability limit, which means that if you generate less than 4% hydrogen by volume in air, you have no flammability hazard, and you don't even need a nitrogen generator.

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I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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Did you read my original post? Here's a question for you, would you cut in to a 20" fiberglass pipe with a hacksaw that has hydrogen in it?
 
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If it were mixed down to 4% volume fraction with air, the risk is relatively minor. I asked whether you should consider using air instead of nitrogen; I never suggested leaving it unmixed.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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This electrolysis process has a long history, and up until the 80s air was used to purge the lines. After enough explosions, plants stopped being designed with air purges, and at this point internal corporate groups mandate that nitrogen be used as a purge gas.
 
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Anyway, I'm asking the question because I'm looking at getting a nitrogen generator instead of using liquid nitrogen delivered by bulk tank trucks, and I kind of already got the answer because I'm just having the vendor quote a skid with 99.9% purity and 99% purity because I know that the company politics would make it a stretch to ask to use 99% nitrogen, and 99.9% is definitely acceptable.
 
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Each end user should determine the nitrogen purity which best suits their application. You have not presented enough information to understand your application.

This link outlines some applications:

Application Link
 
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The published LOC (lower oxidant concentration) required for H2 is 5% v/v, while LFL for H2 at ambient P/T = 4%v/v. Keeping a safe margin on LOC to say 3% as required by NFPA 69 (since 0.6*0.05=0.03), that will require an inert gas purity of at least 97% v/v from your PSA nitrogen generator( since (1-0.04)*(1-0.97)=.029), so 97-98% v/v N2 purity should be okay. Remember to install a inert gas purity trip sequence at the corresponding 2-3% v/v of O2 at the PSA unit exit just in case the PSA unit were to malfunction. One of the chief causes of malfunction is poor feed air quality to the PSA unit, which is attributed to lube oil carryover from non oil free air compressors into the mole sieves adsorber bed. So to get you off on the right foot, check that you have a oil free air compressor. Typically online residual O2 analysers are low reliability, and given the attendant risks involved, we installed a 2oo3 voting system for this on a past project (at the least, go for 1oo2 on the analysers if you are not too fussed about high round the clock availability of this PSA unit).
 
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Hey thanks for the good information Georgeverghse!
 
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The hot ceramic sensor oxygen sensors are much more reliable than the fuel cell versions.
Georges suggestion of going 2oo3 is very well founded.
I have also seen people put dew point sensors in the output line, if you are passing too much oxygen then it is also likely that moisture is passing. Just another check.

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P.E. Metallurgy, Plymouth Tube
 
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Ed, thanks for the tip on the ceramic oxygen sensors. I was only aware of fuel cell, paramagnetic, and laser analyzers. I'll have to do some more research on that, do you have any recommended vendors for these sensor?
 
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It has been a long time and I don't recall.
The ceramic ones are similar to the sensors used in autos, they must be hot in order to work.
I have also had good luck with paramagnetic sensors.

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P.E. Metallurgy, Plymouth Tube
 
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