Hello All:
Thanks a lot to everyone who responded. I really appreciate this. I have
learnt a LOT from all the responses. It is nice to have a such a
knowledgeable group, now I feel secure :-)
Regards,
Dipanjan
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Original Message:
Currently we are going through a renovation in the Department. Several
magnets in the labs have to be blocked off from the renovation workers. We
are planning to cover up our 200, 300 and 600 MHz magnets with 3/4" plywood
planks all around. The plywood pieces would be put together using 2X4s and
stainless steel/alumunium screws. They also have to be covered up with
plastic sheets around the plywood covers to ensure that dust doesn't get in.
1. My concern is that if we seal off the plywood boxes will we harm the
magnets in anyway by creating a micro-environment of low/high pressure area?
2. We will obviously have the plywood on the top and front detachable so
that we can do the cryogen fills.
3. Workers will be working there for couple of weeks, can we keep the
magnets "sealed" with plastic and plywood for so long?
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The biggest problem with sealing-up the magnets isn't so much pressure
(although that would be a concern if it were really sealed tightly), but
rather a high concentration of He gas in the local atmosphere. Helium
can penetrate the O-ring seals, and will soften the vacuum in the
cryostats. You need to vent the enclosures. My plan was to construct a
fan/filter system, so fresh, filtered air could be blown into the
enclosure, and vented out the top. It would be easy to construct a
system using good filters (I would use the 3M-Filtrete furnace filters),
with an inexpensive box-fan. Of course, there would be lots of DUCT
TAPE involved !! <grin>. Seriously, some flexible duct-work (available
at Home Depot or Menards), a fan, and a filter, would provide fresh air,
and would prevent a buildup of helium gas in the area around the magnet.
If it's going to be several weeks, you don't want to have to cold-pump
the magnets to get your boiloff back down.
Otherwise, I think your plan sounds pretty good.
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You MUST have the tubing on the He and nitrogen ports come out trough
the box so that pressure does not build up. As long as those are venting
to the room, covering the rest of the magnet should be fine. I would turn
off the current to the shim coils, but presumably you are going to turn off
the entire console anyway.
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We did this a few years ago. I would _definitely_ crate the magnets. I
am positive they would have been trashed on our site if we had not boxed
them securely. Painters left steel scaffolds right next to the box.
Plastic sheeting would never be enough, in my experience.
Just pay attention to venting your helium boiloff well outside the box.
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I hope someone tells you to be sure that your He boil off is vented outside the
boxes. I have heard (no experience myself) that some folks had a problem when
they put a tarp during painting of the lab over a magnet and over the boil off
outlet. The helium gas built up under the tarp and diffused into the o-rings.
The magnet went soft soon after. It is always best (so I have been told) to
make sure you keep helium gas away from the joints of the magnet.
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I don't think that pressure would be the problem as much as a build-up of helium
around the magnets. There would be enough leakage through the
plywood that the pressure probably wouldn't get much above atmospheric.
Helium, however, can diffuse through the Dewar O rings if it builds up in
great concentration . During normal operation most rooms have enough
ventilation that this is not a problem. You may want to run a hose from the
helium vent (one-way valve) to a space outside the box. - possibly to a
convenient fume hood.
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It is helpful to maintain a slight positive pressure in the magnet
enclosure that will help keep small magnetic dirt out.
Asbestos abatement people are very good at creating containment
shelters... in this case HEPA filtered air would be blown into the shelter
rather than being drawn out. Hi-Tack mats (Lab Safety Supplies),
carpet mats and shoe coverings can help minimize the amount of dirt
tracked into the magnet area.
Whenever you open the containment you risk allowing magnetic debris to
enter. A magnetic nail sweeper is useful to help minimize the free
floating stuff in the general construction area outside the enclosure. The
amount of ***ubiquitous*** Magnetic crud at all construction jobs is
simply amazing.
3/4 plywood is much too heavy. I would use 1/4" or 5/16" plywood... or
even heavy plastic sheeting where the risk of puncture is not great.
Ordinary drywall screws are fine for assembly. Stainless & aluminum
fasteners break too easily.
You must keep tools, door frames, ducts etc etc well away from the magnet
even if it is shrouded. The magnet coil would be pulled on its flimsy
mounts as strongly as the foreign object is attracted. Try to keep ALL
construction activity outside the 20 gauss limit.
If you must do cryogen fills while the magnet is in a shelter special
precautions must be taken to monitor oxygen levels and to prevent
suffocation. Consult your environmental health/safety people on this
issue. Always have another person stationed outside the shelter.
We have gone through several remodels... they are always an exercise in
patience & diligence and need careful planning and attention to detail.
Note that remodeling projects ALWAYS take much longer than expected.
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I would worry about a build-up of helium near the magnets; the helium
could diffuse into the vacuum of the dewars. Perhaps vent the helium to
outside the plywood box.
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The most important thing to remember is to vent the helium boiloff outside
the plastic sealed enclosure. Without that you will be asking for trouble
with
your cryostats.
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10 years ago when we had building renovations, the way we handled it was to
FIRST -- wrap the magnet with plastic sheets taped to floor & to magnet.
Next we had the plywood box (with ample caulking to seal gaps) surrounding
magnet area built & also wrapped the box with plastic. For the He & N2
boil off, we ran some tubing from boil-off ports thru a tiny hole in
plywood to vent those gases away from the magnet. Then once a week for 8
weeks, I would come in at 5 in the morning & open an access panel & do a L.
N2 fill. The workers were required to sweep & vacuum up the majority of
the dust prior to day I would do the fills.
When unwrapping the magnet, we had a vacuum cleaner nearby, & slowly
unwrapped multilayers of plastic around the magnet while having the end of
vacuum near to gather up any dust or magnetic material that may have worked
its way into the boxed of area.
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I'd vent the He outside the box so it doesn't soften your vaccum.
Vent the N2 inside the box to keep it clean & dry.
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Be careful to vent the helium boil-off outside the cover or it may
build up enough to make a helium atmosphere near the upper magnet
seals. Helium easily permeates the seals and destroys the vacuum, since
it does not freeze. An increased boil-off will result. It may be
necessary to have the Dewar pumped. It can be done while energized, but
is tricky to avoid a quench. the high boil-off may cause a quench if not
attended to promptly. We had a GEM-200 quench during construction for
this reason. A plastic tarp was used to cover the magnet and the boil-off
was not sent outside the tent.
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Make sure helium boiloff is not contained by the plastic sheets or your
dewar vacuums will soften as helium penetrates the seals.
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Dipanjan, you will want to vent the helium boil-off outside of the container
to prevent the helium from being absorbed into the high-vacuum dewer seals
and softening the vacuum in the cryostat.
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What a nightmare of mothballing magnets: my sympathies.
The great caveat which I am sure you will hear from others
as well is that the helium MUST be vented elsewhere. If the
He concentration goes up in your 'enclosures' it will diffuse
through your O-Rings and weaken the vacuum.
I suspect you cannot make a good enough seal to create micro
environment for pressure unless you have a complete unibody
plastic bag.
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I think it is great idea to box them up but remember if the boiloff ports
are not vented to the exterior of the box, it may be possible to have a
helium gas environment. this can cause a magnet to go soft because helium
has the second smallest molecule in nature it is the hardest to seal against
... other than hydrogen. The much larger O2 and N2 molecules in air will
not squeeze through the tiny spaces around O rings.
Of course you will power down the room temperature shims and maintain good
air-flow through the box.
I was actually thinking about a similar aesthetically pleasing box system
that one could control the micro-environment. Sound, vibration,
temperature, electromagnetic shielding etc.
Keep an Eye on everyone
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I should think this would be OK. I had a similar situation in my last
job. We constructed large wooden boxes with brass screws - made a 3
sided box, slid it into place and fixed the last side. We built in
access flaps for nitrogen fills and small grills to allow boiloff to
escape. A small positive pressure from magnet boiloff will help to
keep dust off, but if it will be really dirty near by I would tape
covers securely over each end of the magnet bore, and make sure that
these are smooth enough to be able to clean off any magnet debris
before removal.
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We had some renovations done in our lab about a year ago. A Bruker engineer
informed us not to seal off the magnets since the N2 and He boil-off can
cause the temperature to severely drop and potentially freeze the o-rings,
and quench the magnet. He mentioned that this actually happened when one
customer did not consult them on the "storage" of their magnets during
construction. We put a wall up, instead of boxing in the magnets, then
covered them, very loosely with plastic. I have attached a picture of our
temporary wall. We put kimwipes over and under the bore openings to block
any entry of dust, instead of tape, since we did not want frost building up
during this time.
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We did this a few years ago when we had an R2D2 300 MHz system. We put
up the plywood "box" to prevent hammers, wrenches, etc getting stuck to the
magnet.
One side was left open, but covered with plastic (not sealed) for access to
change samples and etc.
Regular steel nails were used in the construction of the box.
We experienced no problems over the course of about three months with this
configuration.
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vent the helium outside the plywood enclosure to prevent long term
softening of vacuum
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Dipanjan Nag, Ph.D.
Director of Instrumentation
University of Nebraska-Lincoln
Department of Chemistry
Lincoln, NE 68588-0304
Phone:402-472-6255
Fax: 402-472-6964
Received on Mon Apr 08 2002 - 18:10:52 MST