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Date: Sun, 13 Jun 1999 14:23:14 -0500
To: techdiver@aquanaut.com
From: s_lindblom@co*.co* (Steve Lindblom)
Subject: Re: Advise on PP blending
Interesting how divergent and contradictory the replies to this have been.
I think this brings out an important point, that adiabatic compression is
not totally predictable or very well understood, and least not on the level
most of us are dealing with it, and it is very hard to say for sure what is
safe and what isn't, which is why entities like Global try to reduce it
into simple rules, which can often be less than totally accurate.

Adiabatic heating is caused when a surge of expanding gas comes to an
abrupt stop, which causes it to recompress and hence heat up. The problem
with a long run is is allows a pressure surge to accelerate and build
momentum, just the way the longer barrel of a rifle allows a bullet to
reach higher velocities. The higher the velocity, the greater the adiabatic
heating when it hits a dead end/restriction.

The worst case is a long run leading into an abrupt restriction, or a
series of increasingly smaller bores, and the best case a series of
increasingly larger bores. If the long run is followed by a larger space -
say a decent sized tank - it's fairly safe, since the tank will act as a
buffer, absorb the surge, and allow the gas to slow down gradually. If the
long run is followed by a series of restrictions or increasingly smaller
bores - say valves, or whip lines, it's bad since the unrushing surge of
gas gets funnelled into smaller and smaller spaces, which will further
increase its velocity, before it crashes to a stop at the end.

That said, there's no reason you can't get away with a 30' run - but it
will require a little more thought and care than for a shorter run. I'd
want to take all precautions possible to see that the system could not be
pressurized too quickly, and maybe get some expert help from someone who
really understands HP gas systems in figuring out the bore sizes, etc.




>Can anyone offer advise on the distance the O2 bank needs to be from the
>mixing panel? Because of my shop layout, the O2 bank as to be approx. 30ft
>away from the panel. But GMC state that this is very dangerous because of
>ADIABATIC COMPRESSION?
>I look forward to your replies.
>
>Regards
>
>Kurt
>
>
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