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

Quality- Half Way to the Wall

Motorola presented a National Quality Day segment several years ago where they discussed “Half Way to the Wall.” It is a pretty neat concept that doesn’t seem to have become cliché, but possibly should.

Summarizing- Initial targets for quality are established by determining current baselines and then cutting the defect rate in half. If you are currently 50% good at something the idea is to move this up to 75%. A 75% you go for 87.5% good. There are several neat points to this system:

1) Fact-Based Decision Making is required. It is cliché to say you can’t make good decisions with bad information. The challenge in this age of TMI (too much information) is deciding what you really need to know and then gathering the information accurately;
2) Initial Gains are relatively much easier than later gains. It is as easy (or easier) to move from 80% to 90% than it is to move from 90% to 95%. The next step to 97.5% is even more challenging. Once defect rates become very low the challenge in keeping them good is not only not slipping back to old ways, but also not allowing new defects to creep in.
3) Marginal Utility (aka Law of Diminishing Returns) also comes into play at a point. The question “at what price success?” has value. The cost of training, test equipment, inspectors or other cures necessary to make small incremental gains just might not be worth it.

These concepts can be and are actively applied to liquid cylinder repairs by West Cryogenics and our clients. One of the most valuable tools used is the ability West has to monitor incoming vacuum levels on previously repaired liquid cylinders. Active use of this information allows for identification of potential issues so we can work together with clients to find root causes and affect solutions. A large percentage of cylinders returned with good vacuums can indicate operational or training issues at the client side. A rise from 2% to 8% “bad vacs” could indicate an issue at the supplier or user side.

One case found that a client’s cylinders were being exposed to excessive levels of cleaning chemicals resulting in high corrosion. Another found that liquid cylinders were too close to a welding or cutting procedure. Sparks from the procedure were melting tiny holes in vacuum rupture disks.

SIMPLE NER Method for Checking Cryogenic Liquid Cylinder Vacuum Insulation

How do you know if a liquid cylinder marked "BAD" actually needs to be sent in for repair or if the “problem” is something that can be fixed at your shop? What is “BAD?” Is it venting too much or just doing what is normal? Or is the problem a leak or a stripped fitting?

Ideally, liquid cylinders with bad vacuums should be identified and separated from other “bad” vessels since it is clear that they will need to be sent in for rehab. Segregating those with bad vacuums also allows more focus time to trouble-shoot vessels that you might be able to repair yourself.

The SIMPLE NER test method was developed to provide an easy, clear-cut identifier for determining bad vacuums and procedures can be found in the West LCR Handbook at http://www.westcryogenics.com/Liquid_Cylinders.php. It allows for testing without instrumentation and in many cases shows there to be no problem with a vessel at all. Just a couple of weeks ago, a client called with a near urgent need to get a vessel shipped in and repaired. I walked him through hot-fill procedures in the morning and SIMPLE NER testing in the afternoon. The next morning we found the vessel was “fixed” and able to go back into service.

Cryogenic Liquid Cylinder NER (Normal Evaporation Rate) Testing

Full NER Steps include:

- Make sure your pressure builder and gas use valve are securely closed
- Open the Vent valve and connect a fill source to the liquid connection on the liquid cylinder
- Fill the Vessel with liquid nitrogen to “full” level by weight and leave the vent open allowing the liquid to fully stabilize (ie- until venting stops)
- If the vessel vents continuously for several (3-4) hours after filling and shows no sign of slowing down then check to make sure your pressure building line is not leaking past the valve. If it is then your pressure building valve needs to be rebuilt or replaced. If the PB is fine then stop the test and save whatever product you can as you already know that your vessel needs to be sent in for service.
- Now, top off liquid level to replace initially vented product
- Log the weight of the filled vessel and leave the vent open
- After 24 hours, reweigh the vessel and log this (minus vessel tare weight) as “Net Starting Weight”
- Measure the weight of the vessel in two successive 24 hour periods
- Subtract the final weight from the starting weight to determine the total weight of product lost. In all cases subtract the tare weight of the vessel so that you are just logging the weight of the product. (net weight)
- Divide the net weight of product lost by the net starting weight to get the percentage of loss over two days.
- Divide that figure by two to get the daily NER.

Cryogenic Liquid Cylinder Testing- NER Intro

NER is a TLA (Three Letter Acronym) for Normal Evaporation Rate. This is the expected boil off rate of cryogenic liquids due to heat incursion and associated temperature rise in liquid nitrogen, argon, or oxygen above their boiling points. As the liquids go above the boiling point they (by definition) turn to gas. As gas accumulates in the vessel’s head space pressure builds. If the gas is not used as product the pressure will build to the point that the vessel vents off. Excessive venting is an indication that a vessel is not performing properly and might have a weakened vacuum allowing greater heat incursion.

The release of molecules through venting reduces the weight of the vessel. So- the easiest way to measure NER is by weighing the vessel over a period of several days. Conducting a fully controlled NER is difficult to accomplish in most production environments as it requires fully stabilizing the liquid, not moving the vessel during testing, and taking readings at nearly the same time over a period of days along with having a scale capable of measuring nearly 1000 that is not needed for other purposes.

Tomorrow, we’ll detail steps for a full NER test. In the meantime, it is important to pressurize your vessel and check all plumbing and fitting connections for leaks with an approved solution prior to starting the NER. You should also use the leak test on the outlet of all connections to assure that none of the valves are leaking.

DOT 4L Authorized Repairs?

“There is no fact which I fear or do not care to know”
- Thomas Jefferson

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If asked, “Is your service company authorized by the DOT to repair 4L cryogenic liquid cylinders?” would you say-

a) “Well of course they are. They wouldn’t be working on them if they weren’t.”
or
b) “We, at XYOGZ Welding Supply, personally verified them for high pressure cylinder repair so they are OK’d for DOT 4L vessel repairs, too.”
or
c) “Yes. They are and we know because they install their date stamped DOT ‘K’ tag on every vessel they service in accordance with the guidelines outlined in 49CFR. By the way, their ‘K’ number is ______.”

Anyone answering “C” has a commendable understanding of at least some of the requirements for compliance with 49CFR guidelines for 4L liquid cylinder repairs.

A beautiful thing about the internet is how easy it makes it to search and find information. Those with “A” or “B” answers might just want to dig a little deeper into the subject before........

Lifting Cryogenic Liquid Cylinders?

Try getting a consensus on issues related to lifting liquid cylinders up into overhead work areas. Good luck. As few manufacturers of cryogenic liquid cylinders as there are, the only consistent answer I could get is “Do Not Lift by the Handling Ring.” And- this is extremely important. There are many cases where lifting devices were attached to handling rings and the cylinders lifted up with bad consequences. Clearly, everyone I talked with agreed that handling rings are not a lifting point.

Beyond that, I was curious on the various manufacturers’ position related to using the eyes in the handling ring support posts for lifting filled vessels and made calls to all. Responses varied and the reasons became obvious pretty quickly. Some of the original manufacturers either no longer exist at all or have been acquired by others. This is important as there are still liquid cylinders that are in use since the 1970s and the differences is designs for liquid oxygen, nitrogen, argon and carbon dioxide cylinders are subtle, but meaningful. Almost all include holes in the handling ring support eyes that are used as catch points for cylinder handling carts. These carts have a hook which slips into the hole and grabs it so the liquid cylinder can be tilted back in the cart and rolled. This is a widely agreed method for moving liquid cylinders as long at the user has a cart designed for the purpose and is using it in the proper method.

The real question comes up when it is necessary to get a liquid cylinder up to a 5th floor work area at a site under construction or demolition. Without going into the variety of responses related to engineering of supports for lifting and potentially proper devices I’ll take a different tack.

- Do you know where that liquid cylinder was last month, last year, last decade?
- Do you have a document in front of you clearly stating that the lifting support stanchion holes for the exact model of vessel you have are intended for lifting full liquid cylinders overhead including instructions on the proper method and device?
- Do you have an inspection report certifying the current condition of the welds and metal?
- If not- Why risk it?

The picture attached shows a common condition of cryogenic liquid cylinders received by West Cryogenics for repair. Note the bent supports and associated stress to the head. Corrosion of outer vessels is also frequent. All of these change the game from original design and might not ever be discernable after a good rehab job. The safer approach for lifting would be to use an approved lifting cradle, freight elevator or other device rather than a direct connection to the vessel. Personally, I’d rather have the excitement of seeing my wife and kids and watching “House” or “Top Chef” than dealing with the aftermath of a liquid cylinder that tore loose from its lifting hook 40 or 50 feet in the air.