CO2 Sublimation

A client inquired why a CO2 vessel can be full by weight, but no product will come out. They mentioned that they'd been able to feed NF Nitrogen into the vessel and then get CO2 out.

"I think the principle issue here is one of pressure. CO2 is one of a few substances that go straight from the gas to solid phase and vice versa at atmospheric pressure. The only way to keep it in liquid form is to maintain adequate pressure as shown on this chart http://www.school-for-champions.com/science/dry_ice.htm.

We’ve found it necessary to use pressure to convert the sold back to a liquid once CO2 turns to dry ice (solid) due to a drop in pressure below the level needed to stay as a liquid. This can be done a number of ways and the addition of N2 as you have done is one. The key in your approach is to be certain that flow is tightly controlled and there are reliefs on all hoses and other trap points to prevent rupturing lines or tanks. Complete removal of all CO2 from the vessel can be pretty time consuming and take several days to accomplish. Once all CO2 is removed from the vessel, it is suggested that you use liquid nitrogen to test all functions of the vessel and perform a thorough leak check.

If you have a leak in a pressure building line or regulator, for instance, you get exactly the opposite result from the one desired when you open the PB isolation valve. The same can occur due to a leak in a valve stem or packing. When the valve is opened and a resultant leak occurs pressure will drop and then dry ice will form."

AWS Fabtech 2009

Bob Burg and John David Mann provide an interesting perspective in a passage of "The Go-Giver" where The Chairman explains, "Most of us have grown up seeing the world as a place of limitation rather than a place of inexhaustible treasures. A world of competition rather than one of co-creation."

The great joy of this year's Fabtech show was the incredible number of "Co-Creators" we had the pleasure of visiting with. People from all over the U.S. and the world had incredibly inspiring success stories to share... both past and in the making. All sorts of industries (both cryogenics related and not) were included. And- there was no regard for the actual location. People from Iowa, Michigan, Alabama, Texas, New York, Eastern Europe, Latin American, Asia and anywhere else you could imagine all had great experiences and enthusiasm to share. It was a cup we gladly drank from and are still savoring.

Most assuredly, people all over had confronted issues this year, but the prevailing attitude was that it was just another challenge like so many before.

All I can really say is THANK YOU to everyone who made this last week so enjoyable. It will be a big part of this year's Thanksgiving celebration remembrances.

4L DOT Standards- Size by Default

A client called today with an issue related to a 4L-DOT cryogenic liquid vessel. At question was the size of the vessel and the concern was that the vessel was over 119 gallons.

It will be interesting to see how their issue is resolved since according to the 49CFR Sec. 178.57 Specification 4L welded insulated cylinders. "(a) Type, size, service pressure, and design service temperature. A DOT 4L cylinder is a fusion welded insulated cylinder with a water capacity (nominal) not over 1,000 pounds water capacity and a service pressure of at least 40 but not greater than 500 psig......"

Since water weighs 8.3453 pounds per gallon this means that the absolute maximum capacity of a 4L vessel would be 119.83 gallons. Most vessels have some head space allowance so nominal capacities for the largest 4L cryogenic liquid cylinders tend to be under 119 gallons. But- depending on the specific vessel, the water capacity could technically be 119 point something, but under 120 gallons.

So- like I said, it will be interesting to see what comes of their question. As always- any comments on this are welcomed.

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