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.

Customer Vacuum Question- Part 1

A client called yesterday with a couple of good questions. Here is the first:

“Do you pull vacuums on liquid cylinders when you rehab them and if so to what level?”

Yes. Cryogenic liquid oxygen, argon and nitrogen have very low boiling points. Nitrogen for instance boils (converts to gas) at
-195.8 °C or -320.44 °F. Keeping argon, nitrogen and argon cold enough to maintain them in a liquid state requires specialized vacuum insulated containers. Cryogenic liquid cylinders are double walled vessels with a super-insulation wrapped inner container that holds the liquid and is inserted into a slightly larger outer vessel. The annular space (annulus) between the two vessels is used to insulate the liquid. After both vessels are sealed leak tight a vacuum is pulled on the annulus to remove virtually all atmospheric molecules and impede the transfer of energy.

To be an effective insulator this vacuum has to be pulled to very low levels. Leaks will result in a loss of vacuum and the need for rehab. West Cryogenics performs a helium mass-spec leak investigation at rehab to check for molecular level leaks. Then the vacuum is pulled to < 10 microns. The vacuum pull is done in two phases (hot and room temperature) to assure all moisture and other stray molecules are removed. A final inspection is performed after refinishing and other testing to verify that the vacuum has not risen due to latent moisture or a leak that needs to be found and repaired. If final inspection shows a weakening of the vacuum then the liquid cylinder goes back to mass-spec and vacuum restoration.