Re: Why does Oxy Acet crack sometimes?

On Fri, 26 May 2006 18:11:25 GMT, Deb <d.lu***@xxxxxxxxxxxx> wrote:

I often get a laud CRACK and this occasionally blows out the flame,

Don't do that, it's a bad thing to have happening. In particular the
more you do it, the more often it will happen.

Technically it's caused by the flame propagating backwards past the
nozzle orifice, because you're no longer keeping the gas velocity
forwards high enough to "blow the flame forward". When the gas burns
faster than it's all travelling, the flame front migrates inside.

Lots of things cause it, some are technique, some are kit. Generally
it's some variant of "too small a flame on too big a hole" which you can
cause by:

- Setting the flame size too small by adjusting the gas down (switch to
a smaller nozzle)

- Holding the nozzle too close and disturbing the gas flow (move back).

- Using a worn nozzle, particularly one that's oval or bell-mouthed (go
easy with those prickers and change worn-out nozzles)

- Sticky regulators with creeping pressures and varying gas mixtures.

- Dirty kit, particularly a dirty mixer.


If you have such a backfire, then shut the gases off and let it all cool
down before even thinking about re-lighting. A small pop should make you
count to 10 first. A "screamer" where it actually burns inside the
nozzle (you'll know it when it happens!) is an excuse for a cup of tea.

OK, so you're learning to weld so it'll be about positioning. No big
deal, we all do it. Maybe try a smaller nozzle and faster gas flow
though - it's easier to control.

REALLY don't re-light a hot nozzle or dirty nozzle. If the rig is making
a habit of popping back, such that it's actually getting warm or sooty
somewhere it shouldn't, then fix the problem before carrying on welding.
This is the situation that leads to a flashback into the torch body or
hoses, and you really don't want one of those.
.

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