Taylor-Wharton EF-450 - Manual (Page 20)

User adjustment of the pressure gauge or liquid level gauge is not possible. If the gauges are malfunctioning,
they must be replaced. Empty the container of liquid and completely depressurize it before replacing either
gauge.
Note that the liquid level gauge may read erratically while the safety valve is venting. This is normal and will
stop when the safety valve reseats.
Checking Vacuum
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 heat from entering the inner container. 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 that gather trace gas molecules from the
vacuum space. The maintenance system can perform its function for years, however it has a limited capacity.
When the vacuum maintenance system becomes 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.
With liquid in the container, the outer casing will be much colder than comparative containers.
2.
Frost, indicating the liquid level, may be visible on the outer casing of the container.
3.
Condensation may form on the container. Note that some icing or condensation is normal around
the piping connections of the vessel. Condensation may also occur on the vessel outer surface as
a result of high humidity.
4.
The relief valve will open continuously until the container is empty.
If a loss of vacuum integrity is suspected, the container’s normal evaporation rate (NER) should be checked.
The test procedure explained below measures the actual product lost over time.
1.
Fill the container with approximately 200 pounds of liquid nitrogen.
2.
Close the liquid valve and the pressure building valve. Leave the vent valve open for the duration
of the test.
3.
Allow the container to stabilize for 24 hours after filling. Weigh the container. Record the
weight, date, and time.
4.
Move the container as little as possible during the test. After 48 hours, weigh the container a
second time. Record the weight, date, and time.
The following formula will provide the actual normal evaporation rate in pounds per day. An actual NER that
exceeds 20 pounds per day indicates a vacuum problem.
20