HELIX CTI-CRYOGENICS CRYO-TORR 100 Installation, Operation And Servicing Instructions page 36

7.5 Helium Circuit
Decontamination
Contamination of the helium-gas circuit is indicated by
sluggish or intermittent operation (ratchetting) of the
cold head drive mechanism. With severe contamination
the cold head drive may seize and fail to operate. One of
the major sources of contamination is using helium gas
of less than the required purity. When perfonning the
decontamination process, use only 99.999% pure-helium
gas, and the regulator and charging line must be properly
connected and purged.
This decontamination procedure will remove contam-
inants from the cold head and/or compressor, thereby
restoring system perfonnance. The cold-trapping of
contaminants inside the cold head during this procedure
also decontaminates the compressor if the contam-
ination is not severe. (Separate decontamination of the
compressor is required whenever the compressor has
been opened to atmosphere, or the pressure dropped to
zero.)
7.5.1 Cryopump Decontamination
Procedures
I. Cool down the cryopump and operate it for one to
three hours. (If the system will not cool down,
proceed to step 2.) Operating the cryopump will
isolate the contaminants by coldtrapping them in the
cold head. The longer the cryopump is operated
beyond the one-hour period, the greater is the
amount of contamination that becomes isolated
inside the cold head.
2. Shut down the cryopump per Section 5.10, page 5-5.
3. Immediately disconnect the helium-gas supply and
helium-gas return lines from the gas-supply and
gas-return connectors at the rear of the compressor.
Leave them attached to the cold head.
4. Attach the maintenance manifold (PIN BOB0250KOO3)
to the disconnected ends of the helium-gas return and
helium -gas supply lines.
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5. Reduce the pressure in the cold head to a level of 45
psig by using the maintenance manifold.
6. Allow the second stage of the cold head to warm up
to room temperature. Warmup time can be reduced
by purging the cryopump with warm dry argon
or nitrogen gas. Using the gas heater, CTI PIN
BOB0250K020,
will reduce warm-up time about
50 percent, and will maintain the gas temperature
below the 150"1> (66"C) limit.
7. Once the cryopump has reached room temperature,
attach a two-stage regulator (0-3000/0-400 psig) and
charging line to a helium bottle (99.999% pure). DO
NOT OPEN THE BOTILE AT TIllS TIME. Purge
the regulator and charging lines as instructed in
steps a through d below. Do not use helium gas
that is less than 99.999% pure.
a. Open the regulator a small amount by turning the
adjusting knob clockwise until it contacts the
diaphragm; then turn approximately
I/B
to 1/4
turn more, so that the regulator is barely open.
b. Slowly open the bottle valve, and purge the
regulator and line for 10 to 15 seconds. Turn the
regulator knob counterclockwise until the helium
stops flowing.
c. Loosely connect the charge line to the l/B-inch
valve on the maintenance manifold.
d. Purge the charge line again, as in step a, for 30
seconds, and tighten the charge line flare fitting
onto the valve while the helium is flowing.
This procedure is required to ensure that both the
regulator and the charging line will be purged of
air.
For
best results, CTI suggests a dedicated helium bottle,
regulator, and line, which are never separated, for adding
helium.
B. Perfonn in sequence:
a. Backfill the cold head with helium to a static
charge pressure of 245-250 psig (1690-1725 kPa),
by adjusting the regulator to the required
pressure, and opening the valve on the manifold.
Close the valve when the pressure is correct.