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Flashback


#1

Was: Meco midget

As I recall this story the problem was a result of work being done
with a platinum casting pressure set up.

Having cast platinum many times I can understand what happened. The
oxygen gage is set up at over 45 psi.

With the higher oxygen pressures required for casting, a flashback
can travel back through the fuel line if there is no flashback
arrester on the torch or tank or built unto the torch.

This means that the flame burns back through the torch and into the
fuel hose to the source. The higher oxygen pressure pushed this flame
backwards.

In this case to a propane tank. It is like a fuse to a stick of
dynamite.

If you have ever popped your torch flame out, you have done the same
thing.

Only your torch, check valve or flashback arrestor has stopped the
traveling of the flame.

It took this high pressure to have this happen in Chicago with no
flashback arrestor. I have never heard of this any other time. In my
life time of jewelry work which is going on 43 years, this was the
only incident I’ve heard about. If there are other specific
incidents, not rumors let me know.

Teaching in Minneapolis many years ago the same thing happened with
the natural gas and oxygen torch we cast with. A flashback happened
casting. I felt the heat of the flame travel back through the natural
gas hose line. I also smelled the rubber of the hose burning. I shut
off the oxygen control on the torch and it stopped the flame
traveling. The flashback stopped. I had to replace the hose and it
did smell burnt. Because the gas was not in a propane container right
next to me there was no explosion. Also, using natural gas, the
distance in the line was to my advantage. Nothing got past the hose
as far as I could determine. There are no flashback arrestors for low
pressure natural gas at this point. We had under 1/2 pound of natural
gas pressure on this system.

I have never used acetylene so I cannot comment on this fuel.
Propane/oxygen and natural gas/oxygen are part of my jewelry making
torch set up.

My comments are because I have had experience with this. I’m not
making anything up.

Regards,
Todd Hawkinson


#2

Hello Todd,

My understanding is that when people started increasing the psi to
cast Pt, the flashback potential became critical and municipalities
began requiring the arresters to be installed. As you pointed out,
the natural gas (NG) line pressure is quite low (only about a quarter
pound here), and unless a pressure booster is installed, there are no
flashback arresters available. The scientific glass blowing lab on
campus has a NG pressure booster and the necessary flashback
arrester. According to the NG company representative with whom I
spoke, your comments about the lengthy gas line, etc. are correct. I
don’t cast and I like NG with an air compressor for the torch. If I
did cast, it would be a different story.

Judy in Kansas, who is puzzled by birds eating snow when unfrozen
water is available nearby.