Re: Varain R2D2 repair

From: John Tomaszewski <jtomasze_at_indiana.edu>
Date: Mon, 06 Feb 2006 14:09:15 -0500

Hi all,
    Below is my original question and a listing of the responses (at least those that didn't have a privacy disclaimer, or offers for new magnets, etc.) to my recent query.  Many thanks to: Jerry (both of them), David, Rich, Steven, Ben, Charles, Tom, Ulli, and Robert.
    It seems that there are three R2D2 seal variants (from oldest to newest): 1) 4 Indium seals, 2) 1 O-ring and 3 Indium seals, and 3) 4 O-rings, and two different retrofits: 1) provided by Varian which required sending the magnet back to Palo Alto (not sure if they will still do this or what the cost is), and 2) a "kit" that MR Resources sells.  Our magnet is at the 1 O-ring/3 Indium seal stage.  It sounds like the Varian retrofits do work well, but I'm sure must be quite expensive.  No specific feedback on the performance of the MRR kit.
    Based on comments regarding the small surface area of the seal provided by the 4 O-ring solutions, and the cost, we're going to stick with our current configuration.  With a little web surfing and/or a polite AMMRL query you can obtain Indium suitable for sealing the magnet (~ 1m x 0.064" - 0.08" diameter) at a reasonable cost.
    Thanks again for the education and assistance.

John T.

	
     Our R2D2 just quenched again and won't hold a vacuum so this time
 we're going to replace the seals.  I understand that we can refit the
 magnet to use O-rings in place of the Indium seals but I've been
 cautioned that this might not be a good move.  Does anyone out there
 have experience with the conversion/refit they could possibly share?
 Thank you.

1) We did that in 1996 and it totally cured our vacuum problems; the magnet was fine until we retired it in 2002.
Dave Vander Velde, University of Kansas

2) Our 200 at Nebraska had the original Indium seals, and they had to be replaced every 4-5 years! This was because we naively followed Varian's protocol for tightening the seals every 6 months with our little torque
wrench. Once I stopped tightening them (and squeezing the Indium out of the seal like toothpaste from a tube), our magnet lasted for years (until it was finally decommissioned after I left Lincoln).

Enough reminiscing... Varian stopped manufacturing these with Indium seals just a few years after the first ones shipped (realizing that this was a bad idea from the start). I believe that my counterparts at the U. Nebraska Medical School did the retro-fit to O-rings early on (while Varian was still manufacturing these magnets and providing full service for them). The process of switching to O-rings required shipping the magnet back to Palo Alto, and they had to do major work (I believe they had to cut off the old flanges, and re-weld new ones onto it). It finally worked fine when they got it back.

As you well know, only a few old-timers left at Varian have a clue how to service these magnets, let alone do such a major modification. I'd be surprised if Varian even claimed that could do it for you (at any price).

3) *MANY* years ago, we did a seal upgrade on an original R2D2 with the original design 4 indium seals. The upgrade consisted of new flanges for the bottom of the RT bore tube, replacing the inner indium seal with an O-ring seal. The other three seals remained indium. This went well and easy, no particular problems, and it eliminated the seal failures with VT. I believe that newer design R2D2 systems had 4 O-ring seals, and that perhaps performance with these seals was not quite as good. I had one or two and didn't notice any particular difference.
The main question in my mind is whether or not you can get the new flange set to replace one of the indium seals with an O-ring. I expect that this can only be done to one (or both) of the inner seals - the outer seal mating piece is part of the dewar (OVC) asssembly and cannot be replaced.
Good Luck.
Steve

4)
 I personally experimented with new top and bottom flange plates which would use "O" rings.  The result was that you had to use "O" rings with pretty small surface areas for sealing.  Would advise using Indium. 

Regards,

Ben Hamilton

5)
I worked for Varian field service for 23 years and I've never heard of such a conversion. In field service, we used 4 orings, 1 oring/3 indium seals or all four indium seals depending on the age of the magnet we encountered.

Regards,
Tom

6) I used the o-ring seals on our R2D2 when ours lost vacuum but it had already
been retrofitted from early in its life so I know it lasts at least 10
years.  It's easy, seal are replaced in the top plate, bottom plate and
valve.  Your friends are right to worry about the rubber if you don't have
the supplies to do the He fill.  Most especially a sound rubber mat on top
for doing He fills.  Also you must depend on TA's to fill N2 properly
because the seals on top are very vulnerable to cracking when liquid gas is
spilled on them.

Our boiloff dropped to less than 2cc/hr L He after and it's now nearly a 3
month magnet but would be longer if I filled closer to lowest recommended
levels. 

Have fun;)
Robert Harker
  

--
 
John W. Tomaszewski, Ph.D.
IU, Dept. of Chemistry
800 E. Kirkwood Ave.
Bloomington, IN  47405-7102
(812) 855-4478
jtomasze_at_indiana.edu

Received on Mon Feb 06 2006 - 16:11:54 MST