NOT PORTABLE- Skid Mounted Vertical Cryogenic Tanks.

What do you say when someone asks you to install a Road Valve on a vertical cryogenic bulk tank that is mounted on a fork pocket style palletized base and designed for stationary use only?

No.

There are tanks that are made to hold liquid for stationary applications only and are, in turn, not suitable for transporting cryogenic liquids. Many small vertical bulk tanks are now being fitted with fork pocket bases because it makes them so easy to move to a new installation site and set in place. In many applications the base also serves as the pad for the tank to support the legs. They also allow for pre-plumbing of final line manifolds and vaporizers. But- your typical 300 gallon or larger vertical cryogenic bulk tank is not made to be moved with liquid in it which is the common differentiator for “portable.”

True portable tanks are typically horizontal vessels that are built under DOT specs, exemption or exception and are designed to be moved full of liquid. Putting a base on a stationary tank does not make it all of the sudden qualified to be portable. And putting a Road Valve on a liquid oxygen, nitrogen or oxygen tank designed specifically for stationary use does not make it legal for transporting cryogenic liquids.

Granted- There are vertical vessels that are designed for transporting liquid including your typical DOT 4L cryogenic liquid cylinders and some of the micro-bulk vessels that are larger than the DOT 4L class. But- only certain specifically designed and classified vessels are for liquid transport. And the manufacturers of these vessels justifiably charge more for them so they provide very clear information on those vessels that are designed for transport.

There is significant risk associated with moving stationary-only vessels that are full of cryogenic liquids. Hazards include injury caused by tipping and turning over forklifts or other handling equipment. Inner vessel supports on stationary vessels are also not made to sustain movement of full vessels creating a lot of potential for damage to equipment and personnel. Stationary micro bulk vessels not designed for movement with liquid are regularly seen with broken neck tubes and other damage caused by someone improperly moving them full of liquid. These repairs are very expensive and not always even possible.

Let’s also not forget that improper transport of cryogenic liquids can result in very large fines. If you are not positive that your equipment is designed for liquid transport it is suggested that you ask a reliable source. There is nothing wrong with clarifying. And-You might just save someone from a serious injury.

Overfilling Portable Cryogenic Liquid Vessels

There are several negative consequences that can result from overfilling portable cryogenic liquid vessels used for transport or delivery of liquid oxygen, nitrogen or argon. While there are also concerns with 4L-DOT liquid cylinders and stationary bulk tanks this article is more focused on horizontal vessels from approximately 119 gallons up that are used for medical oxygen delivery, construction and demolition or other portable applications.

If you fill portable horizontal cryogenic tanks do you always stay in close attendance while filling?Do you use the Full Trycock as your indicator for when to stop? Or do you have a “full” mark on your liquid level indicator to tell you when to stop filling that is well below the point that liquid would come out the vent?

If you answered “yes” to these questions then you are less likely to have some of the problems that are common with overfilling.

First, let’s note that most portable horizontal cryogenic liquid vessels are fitting with a Full Trycock. This line and the vent are to be fully open when transfilling liquid into the portable vessel. The trycock and vent lines run into the top of the vessel. The Trycock line is lower inside the vessel than the vent so that liquid will come out the trycock before reaching the vent. Shutting off the fill as soon as liquid starts to come out the trycock reduces the incidence of liquid getting all the way up to the fill line.

It is not uncommon to hear of situations where the trycock was not used or the liquid source was not shut off quickly enough. Liquid was then allowed to flow out the vent. This can result in:

a) Liquid running up through a roof or side vent and ruining paint or cracking the metal or windshield of a vehicle in hot weather.
b) Uncontrolled liquid oxygen flow running to areas which will be saturated increasing fire risks;
c) Liquid being trapped in the vent line when the fill line and vent line are closed with inadequate head space left in the vessel.

Incident “C” is a common reason for vessels building excessive pressure. Liquid trapped in the line will rapidly vaporize creating pressure in the vessel and causing it to vent excessively. It can also result in blown head safety rupture disks. Users often mistake this overfilling for a vacuum related problem as the net result (over pressuring) is very similar.

A lot of discussion occurs in this post about the importance of fully venting cryogenic liquid vessels when they are filled. This means that after shutting off the liquid fill source you need to vent until there is no evident positive air flow from the vent line prior to closing it. There are few more important things that can be done to help prevent premature and excessive venting of cryogenic liquid vessels. Venting will also help correct some of the ills of over filling and hot filling.

But- Is product wasted when we vent fully?

Are Bulk Cryogenic Spot Drops a Good Thing?- Part II – Supplier Perspective

In the last episode it was determined that it is costly for cryogenic bulk transports to return home with any of the liquid they departed with. One way to make it home empty is to drop off any liquid remaining after scheduled deliveries at some other customer tank that has some room available. Ideally for the supplier, this would convert the left over liquid from a cost to income. It was asked, “Is this a good thing?” First, we’ll look at the supplier side with the question, “Did the revenue generated offset the cost of an 800 gallon delivery considering that Company X typically hates to deliver under 1500 gallons at a time?”

Here, again, comes The Millennium Answer. The Millennium Answer came into vogue shortly after some high placed U.S. politico said something to the effect of “It depends on your definition of the word ‘is.’” Ever since then it seems everything is contingent. i.e. “It depends.” In this case- It depends on the cost of getting to a drop location and making the fill compared to the revenue that might be generated. Pretty simple, eh? But, not necessarily as simple as it appears on the surface and especially when asking about dropping off 800 gallons where 1500 is preferred.

The dependant aspect of whether 800 gallons is a good drop or not relates to whether it is the (first/only) drop or whether it is the last drop. 800 gallons as a first or only drop is not necessarily a profitable proposition for a bulk supplier. Costs associated with running a 4000 or 6000 gallon transport out on a unscheduled single drop delivery might just exceed any revenue that could be made. This is especially true with lower priced products like liquid nitrogen.
On the other hand, delivering 800 gallons (or even significantly less) as a convenience drop at the end of a route is markedly different. In this case, the costs associated with getting the transport loaded and out into the market place have already been incurred and the liquid is out there. Now, there is just the incremental cost of getting to another location and offloading versus the revenue that could be generated from the drop. Playing to the profit side of this equation are the opportunity costs of not doing the delivery:

- Liquid that would just vent off if left in the transport or be partially lost transferring back in and out of plant storage;
- Slightly higher fuel costs with the load,
- The need to return to the convenience drop site sooner by not having made the drop and not having enough liquid to completely fill the tank when the delivery is made.

All things considered-convenience drops are usually good for the bulk cryogenic liquid supplier as long is the drop is within relatively close proximity of the last scheduled stop or the return route home.
But then……. Are convenience drops good for the customer?

Temperature Effects on Gas Pressure

It is often hard to comprehend just how big an impact increased temperature can make of the gas pressure within a cryogenic liquid cylinder or bulk tank containing oxygen, nitrogen or argon.

My good friend Greg Cain at Oxylance provided the following information which is just slightly edited:

“One of the things that you may want to put in there to show people the gas volume / temperature is as follows.

Take a plastic bottle. The 1 liter size makes a bigger impression but a 16 or 20 oz will also work.

Get it warm and dry and put the top on it very tight. Stick it in the freezer for about 15 minutes and pull it out. As the air inside the bottle cools down the air molecules become smaller (more condensed) and the bottle will pull a vacuum on itself and when you take it out it will be half its normal size. The temperature difference will only be about 40 degrees (from 70 down to 30).

Now let the bottle warm back to room temperature by taking the lid off and get the bottle out to its normal size. Put the top on it tight and try squeezing it and see how soft it is. Remove the top and put the warm bottle and top back in the freezer. Leave it in there for 15 minutes. Reach in the freezer and put the top back on very tight and set it out in the sun. After it warms up try squeezing it and see how stiff the bottle is. A difference of 40 degrees will make the bottle so stiff you can’t squeeze it. Now think about how much difference it would make if you were going from -296 to +70 degrees.”

This last sentence is in reference to liquid nitrogen, oxygen or argon which have boiling points nearly 400 degrees F from what we consider to be room temperature.

Thanks, Greg!

Why is CO2 Service a One-Way Trip?

Liquid Nitrogen, Oxygen and Argon are typically produced by cooling atmosphere in air separation units to levels below their boiling points. In a form of reverse distillation, the gases liquefy and are collected as relatively pure products. Due to their purity and absence of residual contamination it is common for liquid cylinders or bulk tanks to be used for one product and switched to another following purging and testing.

Liquid carbon dioxide (CO2), on the other hand, is usually collected as a byproduct of manufacturing other products. Depending on the specific grade of CO2, trace levels of non-volatile organic residue (oil and grease) and total volatile hydrocarbons are common and tend to linger after the CO2 product is vaporized and used.

Conscientious gas producers, distributors and service companies go to great pains to avoid the potential for mixing oils, grease and hydrocarbons with pure oxygen. In turn, it is generally deemed unacceptable to convert a liquid CO2 vessel to any other service. It is just not possible to verify complete cleanliness of a vessel after it has been in CO2 service and there is not an acceptable level of risk above zero when it comes to oxygen possibly coming in contact with flammable and/or explosive residue.

Medical Oxygen Wakeup Costs Comments

A couple of really good comments came in today:

Medical Oxygen Wakeup Costs Reduction B You do not mention the fact that owning a bulk tank makes the homecare provider a "manufacturer" in the eyes of the FDA (compared with a "distributor") and opens them up to increased inspections by the FDA or the state board of pharmacy/dept of health.

Medical Oxygen Wakeup Costs The bulk supply company should work with their clients to stagger the refill time. They should also invest in a larger capacity fill hose to get these trucks "in and out" as the faster their customers trucks are on the road making deliveries, the less time their employees are having to spend filling these vans.