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!

Cryogenic Liquid Cylinders- Safe Handling

I recently wrote about safety concerns related to lifting cryogenic liquid cylinders overhead. A number of questions have been received related to safe methods for moving and handling liquid cylinders. Rightfully so as there are a lot more liquid cylinders moved along the ground than lifted overhead. In turn, there is a lot more opportunity for injury and damage resulting from improper handling.

What are the risks related to moving liquid cylinders?

- Back, muscle and joint strain
- Broken bones
- Crushed toes, feet, fingers and hands
- Cuts from cracked or broken handling rings and site glasses.
- Damage to liquid cylinders that are dropped or tipped over including broken necks and inner leaks
- Broken site gauge and covers
- Damage to other equipment or fixtures hit by a dropped liquid cylinder

There are a number of resources for information on proper care and handling of liquid cylinders including manufacturer’s operating instructions and CGA Publication P-30 which can be purchased and downloaded on line. We’ll also continue to hit items here.

One for today is the suggestion that cryogenic liquid cylinders be moved using a safe cart or carrier. This is especially true for full liquid cylinders that commonly weigh from 650 to over 1000 pounds. Carts designed specifically for moving liquid cylinders have a hook that catches the hole in the handling ring support bracket. They also have wheels that the liquid cylinder rests on when tilted back to avoid reliance on brute force to keep the cart from dropping.

Nothing is ever a problem until if becomes a problem and then it is usually too late to get the toothpaste back in the tube. I get enough calls on this issue to know it is a frequent issue. This might be the best week yet to go do a little audit of how your folk are handling liquid cylinders and what improvements you might be able to make in your methods.

Cryogenic Liquid Cylinders- Safe Lifting Method

A few weeks ago we discussed some of the potential hazards and risks associated with lifting liquid cylinders to elevated locations along with ways not to lift them. It is agreed among all manufacturers that the handling ring around the top of a liquid cylinder is not to be used as a lifting point. While some instruction manuals show cryogenic liquid cylinders being lifted by the holes in the handling ring support brackets this is not a universally recommended practice. This is especially true regarding full cylinders being lifted overhead. The risk of lifting any liquid argon, oxygen or nitrogen vgls by the support brackets are that:

a) It is unlikely if you know the specific cylinder you are lifting is one with supports designed for lifting;
b) The liquid cylinder supports could have been weakened over time by being bent and straightened or through metal fatigue or corrosion.

In any case- the support brackets can tear away from the vessel allowing it to drop. A liquid cylinder full of product that is dropped from an overhead location is a clear hazard.

So- What are the options?

1) Consider running feed lines up from your vessel to the work area first and keeping all liquid containers at ground level;
2) If the distance from the ground to a work area is prohibitively long then use a safe carrier to raise the vessel to the area needed.

A photograph of an enclosed carrier for lifting liquid cylinders is shown. These can also be fitted with lifting eyes on all four corners for 4-point lifting. Be sure that whatever you use that the device is regularly inspected for damage or corrosion and that any lifting cables are in-spec for the intended purpose.

Cryogenic Vessels- Vacuum and Moisture- Part 2

So- How do you get frozen moisture molecules out of the annular space of a cryogenic vessel?

And- If they are frozen, they are immobile right? So are they hurting anything?

The first answer is heat. The only simple way to get molecules freed up and moving so they can be sucked out of a cryogenic vessels annulus is by applying energy. The problem is that this must be done indirectly. More specifically, we cannot blow hot air into the annulus to free up the molecules as this just puts molecules in and can tear up the super-insulation in a SI vessel. This means that you have to apply heat to the inside of the inner vessel and possibly also to the outside of the outer vessel.

With super-insulated cryogenic liquid cylinders this is fairly easily accomplished by inserting a thermostatically controlled electric heat rod into the inner vessel. With larger vessels and cryogenic bulk tanks it is necessary to blow in filtered, hot air or pure nitrogen. This is pretty inefficient and can be expensive, but might be the only option. Heat lights or strips can also be used for heating the outer vessel.

In any case, it is super-critical to avoid overheating in order to stay within specifications and avoid damage or stress to the inner vessel which will weaken it. By the way- this is one more reason why there is a DOT program for inspecting and authorizing DOT-4L vessel repair facilities. And- if you want to know if your vendor is actually a DOT 4L authorized repair center just ask them for their DOT “K” number. Real prudence will prompt you to also look and make sure they are affixing their “K” tag to all serviced vessels as required.

As far as hurting anything...... next post.....

Oxygen + Fuel Cutting- Starving for Oxygen

Good news- We have unprecedented access to information.
Bad news- There is too much information.

Frustration with large tip cutting torch operations is common. Tips burn up and results just aren’t what are expected.

Enter easy-to-use gas flow guides like those published by Victor. It only took two of us who knew what we were looking for about 30 minutes to find them today online. They are probably easier to find if you are one person with less presupposed knowledge…….. or not. You can also just drop by your local welding supply and pick them up.

Anyhow- check out the flow requirement for a #8 tip and you’ll see the oxygen flow needed for a type 303M, GPM, GPN, GPP is 450 to 500 scfh. Go to a Type 1-101, 3-101 (Oxy-Acetylene) and the oxygen flow requirement is 900-1050 scfh. Back in the day when your #4 tips system was fed by oxygen gas 16- packs everything was fine. The only problems were the weight, size and cost so you go to a liquid cylinder and all is fine. That is until a #8 tip is needed and no one ever mentioned that the maximum 8-hour flow rate from a 180 liter liquid cylinder is 350 scfh +/- depending on the manufacturer.

Result- A big powerful torch that is now starving for air which is kind of like a Ferarri with a wet air filter….. all show and no go.

Solution: Additional vaporization.
New Result: More air; More cut. Maybe even a speeding ticket….