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Personal experiences aside, O2 remains a serious fire hazard, and there are more things to be aware of in an O2 system than just compression heating (technically "adiabatic heating" is a contradictory phrase). The safest method of preventing fire is to separate the the 3 elements required: an oxidizer, the fuel, and the ignition source. In an O2 fill station these 3 elements are in constant contact and cannot be separated, the best we can do is to choose the best materials and methods that will reduce the likely-hood of fire. When a gas flows from a high pressure (supply tank) to a lower pressure (empty fill whip) it can often reach extremely high velocities (the o2 is also cooling due to expansion), sometimes even approaching sonic velocity through small restrictions (like the seat of a valve). When the high velocity O2 suddenly stops at a closed valve, it compresses the downstream o2 to the original pressure and the o2 heats up. The result is almost adiabatic compression, and the higher the pressure differential, the higher the final temperature. It is this "compression heating" that causes the problems. This is probably why most people recommend needle valves as opposed to ball valves. A ball valve is very course to open, and are used principally as on/off devices. With a needle valve, the flow of o2 can be metered down to a more reasonable level resulting is lower o2 velocity. It is also a good practice to open the valve on the empty cylinder first, and the valve with the supply pressure last. With the o2 flowing into the large volume of the SCUBA tank compression heating is dramatically reduced. Now all we need to be concerned with is organic contamination and small metal burrs in the gas stream. -----Original Message----- From: Steve Lindblom <s_lindblom@co*.co*> To: techdiver@aquanaut.com <techdiver@aquanaut.com> Date: Sunday, June 13, 1999 2:20 PM 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 >> >> >>______________________________________________________ >>Get Your Private, Free Email at http://www.hotmail.com >>-- >>Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >>Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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