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.

Cryogenic Bulk Tanks- Vacuum & Leaks (c)

How do bulk cryogenic vacuum investigations differ from liquid cylinder vacuum investigations?

The principle differences relate to the size of the vessels and the more permanent nature of bulk cryogenic tank installations. There is also an issue with added penetrations and connections on a bulk tank which mean more potential leak points. There are also a couple of other nuisances related to probe tubes, lift plates, vacuum valve and perlite insulation. First- The principles of helium mass-spec leak detection are the same for almost all double-walled, vacuum-insulated cryogenic liquid storage vessels and include:

A) A vacuum evacuation port of some type must be installed on the outer vessel allowing a vacuum pump to be connected to pull molecules from the annular space. These are specialized connections capable of pulling down to very low atmospheric (high vacuum) levels without leaking past. On smaller 4L-DOT type vessels this is typically a port and plug assembly that are pulled on using a fixture that allows the plug to be pulled and replaced under vacuum. A larger vacuum diaphragm valve is used with bulk tanks

B) A helium mass-spec leak detector is set up on a slip stream of the vacuum line to allow sampling for helium molecules. The leak detector can only be used effective after vacuum has been pulled down far enough to get a helium reading. If the vacuum will not come then the leak will have to be found by other means and some repair performed prior to performing the full helium mass spec leak investigation. Vacuum valves, vacuum probe tubes and probe tube isolation valves are often replaced prior to performing an investigation due to their propensity for being large leak points. It is also common to break the vacuum seal on a vessel and reseal lift plates where present (including o-ring replacement and vacuum lube addition). Perlite is also added at this point for perlite insulated vessels. It is worth noting that breaking vacuum improperly can be a major source of moisture contamination and can also cause permanent damage to the vessel.

C) The initial leak investigation consists of spraying helium molecules around the outside of the vessel and all penetration points. This process can be painstakingly slow and tedious as a leak can both provide a positive indication and flood the annular space with helium that can take hours (or days) to evacuate. Also- getting a positive reading is only a start. It is often necessary to work through a series of tests where small portions of the vessel seams or other penetrations are masked off to ultimate isolate the leak point. And- repairs requiring welding will likewise require careful breaking of the vacuum, completion of the repair, and then pulling the vacuum back down on the vessel to start testing all over again.

D) Once the outside of a vessel has been determined to be leak-free then helium is added to the inner vessel and some pressure added behind it to look for possible inner leaks. Fortunately, inner leaks are not that common on cryogenic bulk tanks- especially stationary tanks- since they are not subjected to the kind of rough handling that 4L Liquid Cylinder and smaller open top nitrogen dewars are. I say “fortunately” as isolating and repairing inner leaks is most complicated and worthy of another full set of articles.

But what about the differences between Liquid Cylinder and Bulk Tank vacuum investigations?

We’ll hit more on this next.

Cryogenic Bulk Tanks- Vacuum & Leaks (b)

Shortly after posting yesterday’s article I saw an advertisement for cryogenic equipment noting, “We Do Revacs.” And….. so do lots of people. But- Does this include full vacuum investigation and repair?

As mentioned previously, there are very frequent occurrences of people putting a vacuum pump on a tank and pulling the vacuum down within specification limits. Because the vacuum pulls down to where it needs to be the assumption is made that the vessel must be good now. The fly in the ointment here is that a tank with a small leak can easily be pumped on to the point that the vacuum reading is good. This is fairly simple as all you have to do is have a pump that pulls molecules out of the vessels annular space faster than they come in through the leak.

This is very much like a leaky boat. My good friend, John, told the story of an off-shore fishing trip where they struck a piece of driftwood or some other debris and started to take on water. This was particularly problematic since they were catching lots of fish and did not want to quit. So, they turned on the bilge pumps and fortunately were able to pump out water faster than it came in. Returning home the leak grew and they had to pull the plugs in the back of the boat to allow water to flow out. All was OK until they stopped at the boat ramp and jumped out to go get the trailer. Once stopped, the boat sank almost immediately and had to be floated up from the bottom with lift bags.

Interestingly enough, we never talked about it, but the final excess flow might have come from a failure to get the plugs in fast enough once stopped. This has a parallel to cryogenic vessels that will be touched on in a moment.

But, first- If we assume that a cryogenic vessel was originally constructed in a leak tight fashion and this was proven out over years of operation then where did a leak come from? There are several primary sources of vessels leaks:

a) The Vacuum Rupture Disk and/or Lift Plate
b) The Vacuum Probe Tube and/or Probe Tube Isolation Valve
c) The Vacuum Evacuation Valve
d) Piping Penetrations, Doublers and Other External Welds
e) Occasional Inner Leaks

Time, corrosion and/or damage (intentional or otherwise) can cause leaks in each of these areas. Unlike a boat leak, though, it is rare that they are visible with the naked eye. In most cases, these are leaks that have developed very slowly over time and are allowing just a few molecules to seep in little by little. And- as vacuum loss is typically a boiled frog progression (slow and with almost imperceptible progress) it is also difficult to link the leak to any cause or event.

So how do you find the cause and location of a leak?

Helium Mass Spec Leak Detection.

This testing utilizes mass spectrometry equipment specially calibrated for detection of helium molecules along with a vacuum pump to sniff out leaks. Helium is used since the molecules are so small they will travel through the tiniest leaks.

Next we’ll do some review of Helium Mass Spec Testing and the principle causes of leaks in tanks.

Cryogenic Bulk Tanks- Vacuum and Leaks

"Nature abhors a vacuum."

When conducting cryogenic vessel classes we often discuss the fact that, other than occassional flashes between my ears, natural vacuums do not exist within the earth's atmosphere. Given time and opportunity, all vacuums will draw in molecules to the point that atmospheric balance is restored.

It is no great wonder then that vacuum insulated cryogenic bulk tank and other liquid cryogenic vessels lose their vacuums and related insulation capacity over time. And, when the vacuum is lost, the vessel loses all or the greatest part of its insulating capacity and can no longer protect the liquid from heat resulting in excessive loss of oxygen, argon, or nitrogen liquid product. The mystery (and fun CSI part) of the job comes from determining the how, where, and why of a lost vacuum.

All too often, vessels with bad vacuums are hooked up to a vacuum pump until a good reading is once again achieved. The pump is then disconnected and the vessel returned to use only to find that it is again losing product within days, weeks or months. The process is repeated with the unsurprising recurrence of the problem.

There is another way and we'll discuss it next.....

Oxygen Effects on Wine Production

This article http://www.practicalwinery.com/sepoct09/impact2.htm "IMPACT of oxygen level in wine when bottling" was recently forwarded to me. It provides some specific technical background that might be interesting not only for those in wine production, but also in other industries utilizing nitrogen and CO2 inert gas blanketinig.

Cryogenic Liquid Cylinder Filling- The Venting Dividend

Standard cryogenic liquid cylinder filling procedure for many facilities is to:

1) Connect the source

2) Open the Vent valve; Open the Liquid valve; open the Source

3) Fill by weight

4) Close the Source; Close the Liquid valve; Close the Vent Valve

Others vary Step 4 to:

4+) Close the Source; Close the Liquid valve; Leave the Vent valve open until venting stops (15 minutes to one hour) and then: Close the Vent valve.

The benefits gained from 4+) are significant:

a) High Pressure Liquid Stabilized- When liquid cylinders are filled from a bulk tank with over 40psi there is gas entrapped in the liquid. The higher the pressure of the source tank the more gas is transferred in the liquid. Venting allows this gas to escape instead of crowding up the the liquid cylinders head space building pressure.

b) Hot Liquid Cylinders are Cooled- Liquid Cylinders that sit empty for awhile get warm inside. Allowing the vessel to cool fully prior to closing the vent also reduces the buildup of gas in the head space.

Both of these benefits share the effect of lengthening the flattened, left-hand tail of the pressure building curve. Put another way- Pressure building can be expressed as an accelerating curve. The higher pressure gets the faster the pressure builds steepening the curve to the venting point. The more liquid is allowed to stabilize and cool the longer and flatter the curve.

The net result of all this is that initial time to vent is lengthened and product life is extended.

Sense and Insensibility in Medical Oxygen

Homecare Magazine recently had two articles that might be interesting to those interested in homecare oxygen delivery. The first, "Providing Oxygen Sensibly" http://homecaremag.com/oxygen/select-right-oxygen-technology-200907/ gives some overview of alternative delivery methods, costs, and returns.

The second article, "Vladeck: Home Oxygen Is 'Nothing but Air'" http://homecaremag.com/oxygen/vladeck-congress-reimbursement-cut-20090831/ highlights some of the misperception challenges faced by homecare oxygen providers.

And- Don't forget to mark your calendars for the upcoming MedTrade conference in Atlanta October 12th - 15th. http://www.medtrade.com/ It is one of the few places where you can find so many of the key homecare oxygen manufacturing and service players all together. If you do make it to MedTrade be sure to drop by booth 1131 (West Cryogenics) and say hi.