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PROCEDURE
FOR ICE PLUG REMOVAL
Introduction
If
an ice plug should develop in the neck of a liquid helium
storage dewar, there are a number of procedures which can
be used to free it. First determine if the container is equipped
with a redundant relief passage (as are most storage dewars),
and if one or both are blocked. If only the center one is
blocked, the liquid access port assembly may be removed from
the dewar (after depressurizing) and defrosted separately
outside. During this process the dewar must be sealed from
the atmosphere until the access port assembly is once again
ready for use.
WARNING
Keep
hands and face away from the dewar opening while performing
any of the procedures below. A sudden release of vapor could
cause severe burning or propel particles or objects out of
the dewar neck. Observe all precautions pertaining to the
handling of cryogens as outlined in the manufacturer's literature.
If both
passages are blocked, or if only one is present, use the following
procedure:
1.
Using a piece of tubing positioned just above the blockage,
direct a flow of room temperature (or heated) helium gas onto
the blockage until an opening is formed to relieve the helium
space.
2.
Once an opening is formed, the rest of the blockage may be
carefully chipped away if it is accessible, or melted away
with additional gas if it is not.
WARNING
Use only
a blunt object inside the neck of a dewar; the neck material
is very thin and is easily damaged or punctured.
If the
blockage has just formed, and or it is determined that very
little pressure build-up is present in the helium space, the
following alternate procedure may be used.
1.
Obtain a suitable length of copper or aluminum rod and affix
a stop at a point such that the rod can not accidentally drop
into the reservoir when the blockage is pierced.
2.
Warm the rod somewhat, if convenient.
3.
Lower the rod into the neck tube to contact the blockage,
and allow it to remain there until its heat capacity has been
completely transferred. Repeat until an opening is formed.
4.
Carefully chip away the remaining ice.
After
an ice plug has been cleared and the liquid helium is consumed,
the remains (ice or water) of the blockage at the bottom of
the reservoir should be removed before the dewar is refilled
again.
November
30, 1999
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