Thank you for the helpful responses regarding how to clear the large ice
plug out of the power port of our 200 MHz Varian R2D2 magnet. There
were many replies, including a picture of a cut-away magnet with its
internal dimensions.
We have cleared the ice plug and the procedure was successful.
Following the procedure below, after we had topped off the magnet with
liquid helium, clearing the ice plug took about 7 minutes (including
some flow rate adjustments), with a loss of about 0.5 cm of liquid
helium in the full magnet dewar (we had no need to top off the liquid
helium afterwards) and the use of a minimal amount of helium gas (hardly
noticeable reduction in helium gas tank pressure). Note: This procedure
worked fine on our older 1990's vintage magnet but may not necessarily
be applicable to more updated magnet designs.
What we did / learned:
1. We kept the helium gas outlet pressure around 1 psi, and
adjusted the flow rate as needed. Started out with a flow rate that
could barely be felt on the side of the face.
2. We wanted to have a flow rate that would melt the ice plug at a
sufficient rate that the open port would not suck in more air. At first
there was very little vapor coming from the top of the port and the plug
seemed very solid so we turned up the helium gas flow rate a bit. With
this increased flow rate we could hear the hissing and popping as the
surface of the ice plug liquefied and boiled away. I gauged how deep I
was into the neck port by comparing to a mark on the blow-out tube.
After we began making serious progress into the ice plug and the plug
was almost melted away (became really slushy), we turned the flow rate
back down to melt the remainder of the ice.
3. When we re-inserted the now-warm port cap along with its
attached safety tube, one of the release valves inside the magnet leg
started hissing, probably due to residual ice in the neck port
vaporizing because of the warm safety tube (apparently in our eagerness
to button up the magnet again, we hadn't quite gotten all of the ice).
The helium flow meter also went up to about 8 for awhile (probably
because of the extra ice vapor coming off) but came back down after
about 15 minutes. If we had it to do over again, we would have taken a
bit more time with the low flow rate at the end to try to melt more of
the residual ice once the large plug was gone, and we would have
re-inserted the safety tube a little more slowly when re-capping the
port. But the magnet has come back to normal boiloff rates and so far
there haven't been further problems.
Based on information from Varian phone support and AMMRL replies, we had
documented a procedure for our own records which is copied below.
Thanks to all of those who contributed.
Procedure for removing ice from power port of Varian R2D2 magnet (200
MHz):
For reference: See also pp. 49-52 of Varian NMR Superconducting Magnets
Installation and Maintenance Manual, including diagram on p. 51.
1. This works better as a 2-person job.
2. Obtain a clock and put it where you can see it while melting the
ice plug.
3. Measure 18.5 inches from the long end of the "L" shaped tube and
mark off with permanent marker. This serves as a "depth gauge" for
approximately how far down the neck to insert the "L" tube. The power
connector is probably about an inch deeper.
Tidbit: The length of the safety tube attached to the port cap (18.5
inches) is a good guide to the length of the neck tube.
4. Fill magnet with liquid helium as usual. May want to verify the
helium level with the flutter tube.
Purging:
5. Determine a suitable helium flow rate through the L-tube. Page
51 of the manual indicates similar flow rates to those used for helium
fills: about 4 to 8 LPM. Phil Louthan says the flow rate will be very
low - so low that you can barely feel the helium on the side of your
face or ear. Tank outlet pressure will be low (Phil said so low that he
couldn't really give a number). Last time 1 psi worked for the outlet
pressure. Keep this constant and adjust the flow rate as needed using
the last needle valve on the tank.
6. Using a higher flow rate, purge the Tygon tubing and L-tube with
room-temperature helium (with the short end of the "L" attached to the
helium).
7. After the "L" tube has purged, adjust the pressure and flow rate
as determined previously.
8. Tie off the tubing to the helium fill port, keeping magnet bib
on.
9. Make sure helium flow is maintained as both adequately low and
steady.
Going into the magnet:
10. Depressurize and remove cap from power port.
11. If this hasn't been done before, check the position of the mark on
the "L" tube to verify that it reflects the length of the safety tube on
the magnet's power port side.
12. Insert the "L" tube into the magnet, holding it against the side of
the neck tube, and go back and forth and around and around to melt the
ice plug.
a. Immediately note the time and the approximate depth of the ice
plug so you can gauge progress.
b. Keep a positive pressure of vapor coming out of the neck tube to
avoid drawing in more air. If there's not enough vapor coming out of
the neck tube, you may need to turn up the helium flow rate a little.
To turn up the flow rate, pull out the "L" tube, ask the other person to
cover the fill port with a gloved hand, and adjust the flow rate while
feeling it. When flow rate is turned up a bit higher, you will hear the
"popping" sound of the liquefied ice plug boiling off. May want to turn
the flow rate back down once ice plug is mostly melted. Don't use the
"L" tube as an ice pick; the neck tubes are fragile.
c. Watch the pressure of the helium tank (if low) to make sure it
doesn't run out.
d. Stop after 10-15 minutes to check helium level and ice on the
other side. Last time we lost 0.5 cm helium after about 5 minutes.
Avoid going below 35 cm, and below 31 cm may expose the solenoid.
e. Important: Stop when the residual ice is gone, or when you
reach the electrical connector for the power stick, which should be at
or beyond a depth close to that indicated by the tape on the "L" tube.
f. When re-inserting the safety tube on the port cap, do this
slowly. Remaining ice crystals in the neck will quickly vaporize
because of the warm tube and may cause the magnet to pressurize quickly
(causing the release valve in the magnet leg to go off).
g. If magnet should quench, leave the area immediately to avoid
suffocation, then refill with cryogens after magnet stops venting.
Finishing up:
13. Check the helium level and if necessary, re-fill the magnet with
liquid helium.
If the magnet should quench:
Leave the area to avoid suffocation; but ASAP after the magnet stops
venting, refill the nitrogen, then the helium if possible in order to
keep the magnet cold (and avoid the further waste of cryogens).
Other information:
* If no pressure is released when the port is depressurized,
this means the area around the outside of the safety tube is blocked.
The inside of the safety tube leads to the boiloff tubing, while the
area outside the safety tube leads to the port cap.
* Phil Louthan says that you cannot get electrocuted from
inserting the "L" cleanout tube into the power port.
Possible causes for ice formation in magnet
1. O-rings on port caps damaged (if O-ring is leaking, flow meter
will be inaccurate)
2. Release valve on port cap(s) leaking (also helium boiloff cap
inside magnet leg)
3. Not remembering to tie off Tygon boiloff tubing while doing a
fill
4. Ice from bottom of liquid helium dewar getting accidentally
pumped into the magnet during a fill will eventually vaporize and
re-condense in the neck tubes.
* Cryopumping can then occur in which the magnet pulls in and
freezes more air
* Usually only a very small amount of air goes in while doing a
flutter tube check
* Is the helium gas dry enough?
Ruth E. Nalliah, Ph.D.
Associate Professor of Chemistry
Huntington University
Huntington, Indiana
rnalliah_at_huntington.edu
________________________________
From: Ruth Nalliah [mailto:rnalliah_at_huntington.edu]
Sent: Monday, December 12, 2005 10:41 AM
To: ammrl_at_chemnmr.colorado.edu
Subject: Removing large ice clog from power port
We have a Gemini 2000 NMR spectrometer (Varian R2D2 magnet) which has
the magnet's helium power port severely clogged with ice. We pulled the
cap off the port (to replace the O-ring in the cap) and found that it
was very difficult to pull the hollow safety tube out and get it back in
due to ice down in the power port neck. We knew it was ice because we
had to chip away at it in order to get the rod back in - it looks like
there could be ice in several inches of the power port neck tube. Also
when we had initially vented the port through the top, no pressure was
released indicating that the portion of the neck on the outside of the
safety tube was clogged with ice. Tying off the Tygon tubing of the
helium fill port has still resulted in normal flow rates, though,
indicating that the safety tube itself is not clogged. Helium boiloff
rates are still normal. But it looks like we need to attend to this as
soon as we can.
We were recommended by Varian phone support to top off the magnet with
liquid helium and use the L-shaped cleanout tube to melt the ice in the
power port by using helium gas at a flow rate that's barely detectable
when we hold the tube against our ear (manual indicates similar flow
rates to those used for helium fills: about 4 to 8 LPM). We are to
lower the cleanout tube into the port by lowering it against the side of
the neck tube and once we reach the ice plug go "back and forth and
around and around" until the ice plug is melted. Has anyone ever done
this before with a Varian R2D2 magnet? (Yes, we know the risks of
quenching - the adrenaline will be pumping hard that day.)
Questions:
1) About how long might we expect to take to melt this ice plug which
may be several inches long in the power port neck tube? Should we stop
and check the helium level after a couple of minutes, or can we afford
to go longer?
2) As we melt the ice plug, it looks like it might be hard to avoid
blowing some of the fairly large volume of air vaporizing from the ice
plug back into the magnet. We plan to leave the helium fill port cap on
during the procedure and tie off the Tygon tubing of the fill port,
which may help force most of the expanding air from the large ice plug
back out of the power port neck. It's possible, though, that we could
at least partially cause the ice plug to "trade places." Perhaps we
should stop periodically to make sure that the helium fill port is not
forming an ice plug from all the air that is being vaporized from the
power port neck tube?
3) Regarding how far down to go in the neck tube, I'm told to insert the
clean-out tube until I've reached the electrical connector for the power
stick, which may be about 17 inches from the top of the neck tube (also,
the length of the safety tube on the port cap is a rough guide for how
far down to go in the neck tube). From the magnet diagram in one of the
manuals, the power connector lies on a "shelf" on top of the solenoid
and blocks further access into the magnet dewar through the power port.
Obviously if we get this far we want to stop immediately. I'm planning
to estimate how far down to go in the neck tube by marking off the
length of the port cap's safety tube with tape on the L-shaped cleanout
tube.
Any further pointers from someone familiar with doing this type of
procedure on the Varian R2D2 will be helpful.
Thanks -
Ruth
Ruth E. Nalliah, Ph.D.
Associate Professor of Chemistry
Huntington University
Huntington, Indiana
rnalliah_at_huntington.edu
Received on Mon Dec 26 2005 - 07:53:45 MST