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.

4L DOT Cryogenic Liquid Cylinders - Age Coding

Client X has a few dozen liquid cylinders that they are considering repairing and want to be sure that they aren’t too old to be worth the trouble. So- Their very good question:

“Is there any way to tell from the data plate what year a DOT 4L- class vessel was manufactured?”

I’ll give you what I got from some quick targeted research and will appreciate any information you might have to share with the rest of us:

TAYLOR-WHARTON- The last letter in the serial number identifies the year of manufacture.
A=1985 to H=1992
J=1993 to N=1997
P=1998 to X=2005
BB=2005 to FF=2009

UCAR- Purchased by Taylor-Wharton in 1985 so anything UCAR is 1985 or before

CHART/MVE- Beginning in 1995 the Year of Manufacture was inserted right in the middle of the serial number. So this is the most obvious… 97, 99, 02, etc. If the unit is before 1995 then the only way to know is to call the number and ask.

CSI- Purchased by Chart in 1995 (?). No defined date code system.

CYLTEC- No date code imbedded in the serial number, but virtually all are 2005 or newer.

Cryogenic Bulk Tanks- Vacuum & Leaks (d)

The principle differences between Bulk Tank and 4L-Liquid Cylinder vacuum investigations?

A) Size and B) More parts and Penetrations

Bulk Tank size matters a great deal when it comes to the ability to conduct and effective vacuum investigation. A liquid cylinder can typically be thoroughly investigated in less than an hour. A perlite bulk tank can take weeks just to get a vacuum good enough on it to perform an effective investigation. Investigating in place will also frequently require the use of a manlift or other device to get all the way up and around the vessel seams, lift plate, etc. Add just a little breeze and it can be well nigh impossible to get a accurate investigation completed due to helium dispersal. The only answer is tenting or bagging section of the vessel which further increases the time required.

Add to Size issues related to additional parts- especially the probe tube. Probe tubes frequently leak through causing vacuums to go bad just by trying to read them. This has been true in many cases even with new probe tubes. There have also been a number of leaks in the probe tube isolation valve handles that are sometimes only present with the valve open. Then there is the vacuum lift plate used on many bulk tanks. This plate is o-ring and lube sealed by the vacuum pulled on the tank. And unlike 4L- Liquid Cylinders which have all piping run through a single head boss, bulk tanks are loaded with line penetrations. None of these issues are overly challenging for a capable investigator using helium mass spec equipment. But they do help explain why those not performing full investigations can get a good vacuum pulled on a tank and leave believing the tank is fixed.

We go back to size and semi-permanent installation. Unlike a liquid cylinder it is a huge undertaking to remove a bulk tank, send it in for vacuum investigation and restoral and reinstall. And, there is typically not a ready supply of spares to pop in place while a bulk tank is being repaired. Hence- It is almost always easiest to send 4L- Liquid Cylinders in for repair while it is frequently justifiable to perform bulk investigations in the field. Justifiable…… but often costly nonetheless making it all the more critical that the right tools and right people are used.

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?