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GASES AND COMBUSTION

ESTIMATION OF A FLAMMABLE ENVELOPE
This calculation estimates the total Flammable envelope for a flammable Gas (or mixture) based on the room temperature values of LEL and UEL and shows the effects of inclusion of inert components.  This is particularly useful when dealing with lean mixtures or checking the efficacy of flare purge gas.  The curve uses the condition of LEL as the cold flame temperature which will just inhibit flame propagation and apportions inert content as equivalent nitrogen on the basis of a generalized ratio of the volumetric heat content of the various diluents.

How to read the charts.
The left hand chart shows the complete flammable envelope.
The horizontal baseline is inert content resolved as nitrogen.
The vertical scale is flammable gas in the mixture.
Oxygen is the balance (100 - Flammable - Inert)%.
All possible mixtures lie within the main boundary triangle.
The (imaginary) grid lines all slope parallel to the boundaries.
The vertical line is a constant ratio of N2 to O2 (79:21= air)
The Stoichiometric line is a constant ratio of  O2 to flammable.
SPEC GAS shows the true inert/flammable ratio of your gas.
Flammable condition exist between the UEL and LEL lines
"Safe" conditions exist outside the NFPA86 lines.

 

 The right hand chart shows an enlargement of the envelope for values of Nitrogen greater than 79%, effectively setting Air as a datum.
The horizontal axis is shown in total nitrogen including the air like the main chart and does not directly show the ratio of inert to flammable.

How to use the chart:

YOUR GAS is the mixture in the flare line and should normally be primarily flammable material.   In the yellow boxes, enter the amounts of flammable, inert and oxygen which are in the main stream.  The number need not add up to 100 but the amounts MUST be in volumes or moles.
For the flammable portion only, enter the LEL, UEL and stoichiometric ratio (air/gas) from published data.   US Bureau of Mines Publications 503 and 627 are useful for this.
If you have a flammable mixture use the formula   100% / L  = Σ [ flammn% / Ln ] where L is the limit in question for the mixture or nth component.   (I will have a page up soon to help with this but it's not ready yet !).
Common Inert gases which could impact flammability are listed in the third section.  Apportion those which apply in your case.
If you have hardly any inert and the flammable portion is nearly 100%, the graphs may not work properly.  But then you don't need them because you already know the LEL and UEL.
The charts should update automatically as you enter the data. Good luck with this.  If you have any questions drop me an email.

 

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My disclaimer:

This calculation is ONLY an estimation for general assistance and information and DOES NOT guarantee flammability or lack thereof for any particular gas or mixture.  For  critical calculations, a more detailed study may be needed.  The author of this calculation makes no warranties of suitability for any purpose.  See "Terms and Conditions of Site Use".

Copyright © 2003 David Shore. All rights reserved.
Last Revised: 7 July 2003