Open the Gas Use valve on a filled cryogenic liquid cylinder and gas will come out. Interestingly enough, the same thing happens if you open the Vent valve. So……. Does the gas coming out of each valve come from the same place inside the vessel?If you answered “yes” or “no” you are sometimes right. As they say, though, even a broken clock has the right time twice each day. It is more accurate to say “It depends.” To understand the “why” of this, it is helpful to first look at the Vent connection.
We previously affirmed that liquid withdrawn from a 4L-DOT liquid cylinder comes from the bottom of the vessel. This is easy since the liquid is heavier. Gas that is formed by the normal evaporation of liquid nitrogen, argon or oxygen collects in the top of the liquid cylinder inner vessel “head space”. As more and more molecules crowd into the head space the inner vessel contents pressure increases. This pressure is useful to push liquid back out through the dip tube when the liquid valve is opened.
When the Vent valve is opened, free gas molecules are allowed to escape and the pressure goes down. If the Vent valve and Gas Use valves were both connected to the head space only then there would be no reason for both. There would also be a big operational problem.
Let’s consider a case where Mike the Scrapper is using oxygen from a cryogenic liquid cylinder to feed a cutting torch or oxygen lance. If he were using a simple LS (Liquid Service Only) vessel then his work cycle would look very much like the profile of a wedge or the topography of the White Cliffs of Dover. He’d have a long wait while pressure built in the liquid cylinder. Then, once the pressure was high enough he’d fire up his torch and start pulling off gas. The pressure would drop precipitously until it got too low and the torch would go out. Mike would then have to wait a long time again for pressure to build and then fire up for a little while, lose pressure and go out. This would be unlikely to occur more than a couple of times before Mike would give up on the liquid cylinder and go back to high pressure cylinders that would give him a constant supply of gas until empty.
If gas doesn’t just pull right off the top, then where does it come from?
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