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.