Cryogenic Liquid Cylinder Hot Fill

A recent call (summarized):

"Our client purchased some new cryogenic liquid cylinders, filled them and put them into service. They seem to be venting excessively and they are concerned. Can you crack the inner vessel of a new cryogenic liquid cylinder if you improperly fill it? Is there a Hot Fill Procedure that should be followed to avoid excessive venting?"

I've never heard of a cryogenic liquid cylinder with a stainless inner vessel actually cracking..... even after sitting out in Houston or Phoenix sun for awhile. It would be interesting to hear from an engineering or physics standpoint just how much temperature differential a vessel can tolerate, but it is not something that is a cause for concern in everyday applications.

As for Hot Fill Procedures- Yes. It is important to cool liquid cylinders that are new or have been sitting around "hot."

During one memorable problem solving activity a client commented that his cryogenic liquid cylinders "are not hot. They are <0 degrees F inside." True or False?

Think about a hot (400 degree) skillet used for blackening or stir frying. Throw a little water into it and see what happens. The water will flash vaporize because the skillet is nearly 200 degrees above the boiling point of water.

Go back to a cryogenic vessel that is 0 to 80 degrees F inside. Run some liquid nitrogen, oxygen or argon into it that is about -300 degrees F and you can be sure that you will get violent flashing since the vessel is 300 to 400 degrees above the boiling point of the cryogenic liquid.

There are a number of ways to cool a hot cryogenic liquid cylinder and thereby decrease the normal vaporization and consequent venting of your liquid nitrogen, oxygen or argon. A very simple one is to:

a) Open the vent;
b) Connect your source line to the vessel; Open the liquid fill line and the open the source just enough to slowly run liquid into the cryogenic cylinder;
c) Very heavy venting will occur. Keep running liquid slowly into the cryogenic cylinder until it is about 1/4 full. Shut off the fill and leave the vent open.
d) Allow the vessel to vent until the venting becomes very moderate. This can take 1-2 hours.
e) Slowly fill the vessel the rest of the way until the full weight is reached; Some more heavy venting can occur;
f) Let the vessel vent until venting is almost completely stopped; Top off the vessel if more product is needed.

Remember- Fully venting cryogenic vessels is important when filling whether the vessel is hot or not. For more notes on this you can see http://cryonews.blogspot.com/2009/09/cryogenic-liquid-cylinder-filling.html