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.