Is pressure good or bad in terms of cryogenic liquid cylinders?A) Good
B) Bad
C) It Depends? (The Millennium Answer)
Right- It depends. Pressure is necessary for liquid and gas withdrawal from a liquid cylinder. Too much pressure, though, can cause excessive venting or even severe damage if safety devices are absent or malfunction.
As a first simple step it is important to understand that when liquid is filled into a cryogenic liquid cylinder it goes in through a dip tube that goes down to near the bottom of the liquid cylinder. When liquid is withdrawn it comes back up from the bottom of the tank through the same dip tube. Since the valves and connections for a liquid cylinder are at the top it is necessary to have some head pressure on the liquid in order to push liquid oxygen, nitrogen or argon back up and out the liquid valve.
The simplest liquid cylinders then contain only two valves (Liquid and Vent) and are often referred to as “LS” vessels. They are used for liquid service only and typically fitted with 22psi reliefs to avoid spraying liquid out with too much force when withdrawn. They really on normal product evaporation to create head pressure for liquid withdrawal.
This is all pretty simple, right?. Next we’ll discuss “VGLs” and Gas Withdrawal.
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