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PROCEDURE
FOR RE-EVACUATION OF CRYOGENIC EQUIPMENT
Introduction
If a cryogenic vessel or vacuum insulated transfer line develops
condensation on the jacket, or is cold to the touch during
use, the insulating vacuum level is inadequate and must be
restored. In order to determine if this vacuum degradation
is due to a vacuum leak or normal outgassing, a vacuum leak
test should be performed if a helium mass spectrometer leak
detector is available. If no leak is detected, then the proper
operating vacuum must be reestablished using the procedures
below. If a leak is discovered, repairs must be made, and
then the vacuum must be restored according to the procedure
below, before the equipment is put into service again.
In
order to gain access to the vacuum space for leak testing
or for re-evacuation, an appropriate pump-out operator must
be used. On equipment manufactured by Cryofab, Inc. the pump-out
will more than likely be either one of our standard sizes,
1/2 inch or 1 inch. The pump-out operators for these sizes
are slightly different, and operating instructions for them
are given below.
1/2"
Operator
To
attach, remove plastic cap on pump-out and extract snap ring,
if present. Push the operator down over the pump-out body,
then push the stem down until it touches the sealing plug.
Turn the stem clockwise to engage the threads all the way.
Tighten the hex nut on the bottom to complete the seal. Attach
a vacuum hose to the side branch of the operator.
After
evacuation is complete, reseal the pump-out by pushing the
stem all the way down. Disengage the stem by turning counterclockwise
until free. Close the isolation valve on the vacuum pumping
system or turn the pump off. Loosen the hex nut and pull the
operator off the pump-out. Replace the snap ring and plastic
cap.
1"
Operator
To
attach, remove the plastic cap on the pump-out, and push the
operator down over the pump-out until it bottoms. Push the
stem down until it touches the sealing plug. Turn the stem
clockwise to engage the threads completely. Tighten the coupling
nut on the top to complete the seal. Attach a vacuum hose
to the side branch of the operator.
After
evacuation is complete, reseal the pump-out by pushing the
stem all the way down. Disengage the stem by turning counterclockwise
until free. Close the isolation valve on the vacuum pumping
system or turn the pump off. Loosen the coupling nut and pull
the operator off the pump-out. Replace the plastic cap.
Re-Evacuation
The
other end of the vacuum hose should be connected to the vacuum
pumping system. The vacuum pumping system must be capable
of at least 1 millitorr ultimate pressure, and have an appropriate
gauge for monitoring the vacuum level.
Pump
the vacuum hose down first, and then open the pump-out operator
by holding the operator body with one hand while pulling up
on the stem with the other. Pump on the vacuum jacket for
at least 4 hours, but preferably longer. Heating either the
inner wall or vacuum jacket by any suitable means will enhance
the quality of the vacuum obtained. The method of heating
must be carefully controlled, however, so that none of the
surfaces reach temperatures in excess of 250°F. In time,
depending upon the size of the vacuum space and the condition
of the insulation, the vacuum level should reach a stable
value of less than 20 millitorr. If it does not, check the
assembly for leaks, and repair. Allow the equipment to remain
on the vacuum pumping system for a number of hours after the
minimum vacuum level has been reached.
After
evacuation is complete, reseal the pump-out as described above,
and disconnect the vacuum hose. If a high vacuum pumping system
is available (10–3 millitorr),
transfer the equipment to it, and pump for an additional 8
hours. Reseal the pump-out and return the equipment to service.
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