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, a
vacuum, 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 hear from entering the inner container. The vacuum prevents heat convection or
conduction from reaching the inner container. Unfortunately, the perfect vacuum cannot
be achieved since trace gas molecules being to enter the vacuum space from the moment
of manufacture. The vacuum maintenance system can perform its function for years, but
it 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
my ago unnoticed for several years. When the vacuum in the insulation space is no longer
effective, the following symptoms may appear:
1. With liquid in the container and pressure building/vaporizer coil not in use, the outer
casing will be much colder than comparative containers.
2. Frost, indicating the liquid level, may be visible on the outer casting of container.
3. The container may appear to sweat if the air surrounding the container is hot and
humid.
4. The relief valve will open continuously until the container is empty.
5. The container will hold pressure for several days but will not hold liquid.
NER Testing
If a loss of vacuum integrity is suspected, the containers 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 recommended, after the container is allowed to stabilize, but the for-
mula 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 and the PRESSURE BUILDING valve, leave the VENT valve
open and allow it to remain open during test.
3. Allow the container to stabilize for 24 hours, then reweigh it. Record the weight, time,
and date.
4. Reweigh 48 hours later. The test is most effective if container is not moved during this
period. Record the second test date, time and weight.
The following calculation will provide Normal Evaporation Rate in pounds-per-day. Daily
normal evaporation is simply half the loss over 48 hours.
Daily NER =
Weight (Step 3) Weight (Step 4) x 24
Time between Step 3 and Step 4 in hours
Compare the results of your test to the as manufactured NER value in the SPECIFICA-
TIONS section of this manual. A container in service should maintain an NER value of less
than two times the new specification. Any test result greater than two times the listed
value is indicative of a failed, or failing, vacuum. If NER is found to be high, contact Taylor-
Wharton Customer Service at (334) 443-8680 for disposition.
NOTE:
Fill through the LIQUID
valve with the VENT
valve open. The
Pressure Building
valve must be closed
during the NER rest or
P.B. operation will
increase evaporation
and invalidate test
results
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