Hi. I was taking a qualification for FGAs - CPA1 - and something happened I'm thinking through... The assessor caught me checking the burner-pressure of an Open Flued boiler while the boiler was running and, even though I'd been mega-quick, he told me that since its a CF boiler I could've caused the gas to 'light back' by doing this. Did he mean that the gas could light all the way back from the burner into the gas valve? How come this would be more likely to happen on an open-flued appliance than one where the burner is sealed to the air around the test nipple I took off? Maybe the assessor meant that the danger was in gas from the valve being drawn by convection into the burner and light-back happening outside the pipe-line - is this possible? I read an article in Gas Installer about whether its safe to purge to an open flame and it said that gas rarely lights back down any line since it has a very slow rate of catching on. Doesn't this apply to the above scenario then... any comments?
I assume, from your "mega quick" comment that you removed / opened the pressure nipple while the burner was firing? If this is the case, the instructor was obviously commenting on the fact that any escaping gas could ignite of the burner. On a room sealed appliance, this can't happen because - hey - the flame is room sealed! Your further comments, I think, are referring to the theoretical possibility of "flashback", where the gas inside a pipe ignites, and can "flash" all the way back to the meter! It is an unlikely scenario, because of the flammability range of 5 - 15% gas in air, but is based on flame speed. I don't know if this has happened in real life but was a standard demonstration at college (the good old days) and the reason why a meter should be disconnected before soldering. Being OF or RS would make no difference.
Your assessor was correct before attaching the u gauge to the working pressure isolate the gas cock then turn back on once connected. Before attaching the ugauge to the burner pressure make sure you have no call for the burner. And the same goes for removing the ugauge. Will it light back to the test nipple ? with nat gas i doubt it but it is possible ive never had it happed yet but im always "mega quick" Will it light back with LPG ? OH YES trust me i once found out when i had a six inch flame coming from the BP test nipple on a baxi BB that was some years ago now and i always test in the correct manor even on RS appliances just as its a good habit to get into. SS
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