possible to insure that any resisdual combustibles in the chamber are swept out, prior to light-off.
If it is a light-up than the purge has been completed already I would say. Therefore a 30% air flow is a minimum air necessary for a reliable and proper combustion process.
"Therefore a 30% air flow is a minimum
air necessary for a reliable and proper combustion process"
Minimum Combustion air is determined by fuel or fuels being burned. For natural gas, minimum air for complete combustion is roughly 20%. If you are using excess O2 or other data to control the combustion process, it will vary.
With 30% air flow & 300 Sec time delay we can ensure that complete volume of boiler is purge. hence it is ensured that there is no unburnt left inside the boiler.
Also, in boilers supplied by SEC china, purge will interrupt if air flow is increased above 40%. I think this upper limit in air flow is put, because during light when boiler is in hot condition more than required air flow will forced cool the boiler which can lead to distortion of panels.
About purge time and purge amount one should consult NFPA® 85
Boiler and Combustion Systems Hazards Code.
I don't have one copy with me and don't have an exact number on the top of my mind. But here's the deal. "In order to safely purge one must purge x times the volume of the flue gas volume from GT (or Fresh Air Fan, depending which component purges) or an x amount of volume times x amount of time. Which of the two is the shortest in time is the minimum allowed."
So. That being said about purging. Purging is a condition in order to do a light off.
Perhaps topic starter should give more detail about what is he lighting up and if he finished purging already.
And as my experience, either a boiler purge by GT (already on running on load) or FAF is done at a higher flow output than 30%.
For instance of FAF purge, the IGV will go to 100%. Once the purge is finished the IGV will go to a minimum position.
In case of GT on load it will be a calculated amount of time based upon the exhaust gas flow. If the GT's IGV at that moment is 30% than it is a minimum or optimal combustion air necessary for the GT combustion process at that moment. It is not set at that position for purging.
If the GT is starting one would purge with the crank engine. Again. This is pre-light off.