I will try to confirm the question by restating it slightly.
"I am working with flammable gases. I need to know how to find when a mixture of non-flammable and flammable gases, when mixed with air, becomes flammable."

If this is the question you need to look up the Minimum Oxygen Concentration (MOC)for the flammable gas or gases in the non flammable gas or gases and air. For example the MOC for methane where the "purge gas" is nitrogen is about 12 mol%. This just an approximate value, I don't have the real value at hand. Another common purge gas is carbon dioxide. You can find the data in a book on explosions by Bodurtha. He also includes a guide on plotting the data on triangular graph paper. As long as the oxygen content of the mixture is below the MOC then the mixture will not burn. If the oxygen content is above the MOC and depending on the flammable gas (say methane) content, the mixture might burn. You really need to plot it out to get the whole story.

If this was not the question, sorry.

HAZOP at www.curryhydrocarbons.ca

Visit Thread798-33615.

I also just posted an additional clarification at Thread798-33615