Maintenance Read the Safety Precautions in the front of this manual before attempting any repairs
on these containers. Also, follow these additional safety guidelines while performing
container maintenance.
Never work on a pressurized container. Open the vent valve as a standard
practice during maintenance to guard against pressure build-up from residual
liquid.
Use only repair parts for oxygen service. Be certain your tools are free of oil and
grease. This is a good maintenance practice and helps ensure you do not introduce
any contaminants to the plumbing of the container.
Leak test connections after every repair. Pressurize the container with an appro
priate inert gas for leak testing. Use only approved leak test solutions and follow the
manufacturers recommendations. “Snoop” Liquid Leak Detector is one approved
solution. It is available from Nupro Co., 4800 E. 345
th
St., Willoughby, Ohio 44094
USA.
CHECKING CONTAINER PERFORMANCE
Cryogenic containers are two containers, one within the other. The space between the
containers acts as a highly efficient thermal barrier including high technology insulation,
and a vacuum maintenance system. Each serves a very important part in the useful life
of the container. The high technology insulation is very effective in preventing radiated
heat or solid conduction from entering the container; the vacuum prevents heat con-
vection from reaching the stored product. Unfortunately, the perfect vacuum cannot be
achieved since trace gas molecules begin to enter the vacuum space from the moment
of manufacture. The vacuum maintenance system consists of materials which gather
gas molecules from the vacuum space. The maintenance system can perform its
function for years, but has a limited capacity. When the vacuum maintenance system is
saturated, it can no longer maintain the vacuum integrity of the container. The change
will be very gradual and may go unnoticed for several years. When the vacuum in the
insulation space is no longer effective, the following symptoms may appear.
1. When the container is filled with liquid, the outer casing will be much colder than
normal.
2. The container may appear to “sweat” if the air surrounding the container is hot and
humid.
3. The relief valve will open continuously until the container is empty.
4. The container will hold pressure for several days, but will not hold liquid.
NER TESTING
If a loss of vacuum integrity is suspected, the container’s Normal Evaporation Rate
(NER) should be checked. The test measures the actual product lost over time so you
can compare the results obtained to the NER value in the SPECIFICATIONS table. A
test period of 48 hours is recommended, after the container is allowed to stabilize, but
the formula given produces a Daily NER over any time period.
1. Fill the container with 150 pounds (68 kg) of liquid nitrogen.
2. Close the LIQUID valve, open the VENT valve and allow it to remain open during
the test.
(18 pages)
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