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Area Classification-Temperature Code

Area Classification-Temperature Code

Area Classification-Temperature Code

(OP)
Estimating an AIT for a mixture is a challenging problem
due to the complex nature of the autoignition-combustion
process. I haven't found any established standards or documents to calculate this.
If lowest AIT of any pure component
in the flammable mixture is used as the AIT. This typically
results in a very conservative AIT value and may not reflect
the true nature of the hazard.
Any idea of rule of thumb which has been accepted in industry? Theretically the autoignition
temperature of a mixture will be influenced more by the
properties of the major components than by the minor
components in the mixture.  Say, the AIT of any gas component over 5% in the mixture should be considered Maximum allowable temperature codes.

Any commments would be appreciated.  

 

RE: Area Classification-Temperature Code

Hamid,

My preference would be to stick with the conservative approach, choose the lowest AIT of any pure component of the mixture.  Estimating or calculating is extremely complex even for ideal gases.  The only way to satisfy me would be to determine such a mixture's AIT experimentally and document it with peer review.  Even then, it may not be allowed by a listing agency (UL, FM) since the mixture isn't listed in a reference published by one of the authorities such as NFPA.

That's my take, anyway...

Anyone else?

Good on ya,

Goober Dave

RE: Area Classification-Temperature Code

Oh, and I forgot to assert the first thought that popped into my head on this issue --

I'm sure you're not in this boat, Hamid, but others have been caught up in lots of debate when there was uncertainty as to which hazardous gas might be present in an installation. Days of debate and argument, finally agreeing to rate the place for hydrogen (1000F) or whatever.  Turns out that the fellow was only installing intrinsically safe temperature transmitters that were rated something like T4 (275 F). All the debate was for naught.  Could have picked the absolute most conservative AIT and been done with it, the outcome would not have changed.

Still get a bit of a giggle from it...

Goober Dave

RE: Area Classification-Temperature Code

it (the auto-ignition temp) is a function of pressure, concentration, etc, so you either have access to the test data for your vapor, or you have some one develop a best estimate, or you choose the conservative worse case

cetainly if the vapor is in the explosive range you have to observe the auto ignition temperature, but out side that range, then you have to be specific about the concentrations, disappation, etc.


 

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