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World Leader in Remanufactured Analytical Instruments,
Service, and Accessories
NMR
Quenched Magnet Services:
If the magnet is not filled
in the proper manner it will quench causing significant "down time"
for your lab, not to mention the exorbitant costs associated with
bringing the magnet back up to field. MR Resources is fully equipped,
staffed and prepared to deal with any quenched magnet situation.
Quenches, although infrequent, create costly down time and pose
a serious impediment to your research.
In the event
of a quench, time is of the essence; please call us as soon as possible.
MR Resources will:
Determine the status of your magnet
Recommend a plan of action
Disperse a qualified magnet engineer to your location
Disperse all needed pumps, and power supplies to service the magnet
Replace O' rings and reseal your magnet
Pump down to high vacuum specification
Cool the LN2 and LHe temperature
Re-energize and Re-cryoshim the magnet
For
price and complete details on our Quech Services:
Contact
Sales at (800) 443- 5486 ext. 330 or E-mail:
Sales
NMR
Quench Fill Safety Tip:
Since the possibility of a quench is higher when filling the magnet,
and since the transfer involves manual operations, there is a remote
possibility that an operator could be rendered unconscious around
the time of a quench. Fills should only be done by a single operator
when the fill cannot be deferred, and exceptional caution should then
be used.
The
following steps should be observed to avoid accidents:
Magnetic
fields may permanently damage watches, calculators and certain
types of credit cards. Keep those items more than 2.5 m away from
center of magnet.
Failure
to refill or de-energize the magnet when low levels are indicated
by cryogen level sensors may result in a magnet quench with possible
magnet damage. If low level warning lights on sensors are continuously
lit, immediately inform the NMR operator.
Use
only carbon dioxide fire extinguishers to avoid equipment damage.
In
case of serious flooding or other situations where there is risk
of electrocution, turn off the equipment circuit breakers.
Quench
Awareness Tutorial:
What
is a quench?
A cryomagnet is basically just a closed loop of superconducting wire
that allows an electric current to flow perpetually through it without
any resistive losses, thus generating a stable, "permanent"
magnetic field. The wires that are used to wind high field cryomagnets
are only superconducting when cooled to very low temperatures. For
this reason, cryomagnet coils are submerged in liquid helium at 4.2
degrees Kelvin.
Although there is no resistance in a cryomagnet coil when properly
cooled and energized, there is a great deal of energy stored in the
electric current. If, for any reason, a very small portion of the
superconducting wire stops being a superconductor (goes "normal"),
then the resistive heat generated in that section of wire will cause
neighboring sections of wire to also go normal, resulting in a chain
reaction that swiftly warms the entire coil and causes all of the
stored energy to be dissipated as heat. This process takes only a
few minutes, and can result in an impressive display of rapidly boiling
liquid helium, similar to a geyser.
What can cause a quench?
Quenches are frequently due to improper liquid helium filling procedure,
or failure to fill with helium at the required interval (a "dry"
quench). Another common cause is the attraction of a large, ferromagnetic
object, such as a chair, ladder or mop bucket, against the side of
the magnet. While these are the most common causes, magnets sometimes
quench for no apparent cause, sometimes even after years of uneventful
service.
What are the consequences of a quench?
Fortunately, magnets manufactured after 1980 are well protected against
quenches, and are rarely damaged by them. However, an elaborate and
time consuming process is needed to bring them back to field. Typically,
this is what must be done:
1.
Fill the vacuum space with nitrogen gas and warm the magnet to
room temperature.
2. Take the magnet apart
3. Reposition the magnet coil and concentric bore tubes inside
the magnet
4. Replace the helium "getters"
5. Reseal the magnet with new 0-rings
6. Re-pump the vacuum
7. Fill the inner and outer dewars with liquid nitrogen to pre-cool
the magnet
8. Remove the liquid nitrogen from the inner dewar and fill with
liquid helium
9. Energize the magnet to the proper field (can take up to three
days)
10. Adjust the magnet's homogeneity by putting current in the
cryo-shims.
The entire
procedure requires two weeks or more of on-site labor, in addition
to parts and special equipment such as a pump, magnet power supply,
and magnet lifting device. Typical cost is $18,000, and typical
down time for the NMR system (including waiting for a magnet engineer
to be available) can be four weeks or more. If significant time
is required to obtain the needed funds, then the instrument down
time can be much longer.
What can I do to protect against a quench?
1. Use care to exclude magnetic items from the area of the magnet.
2. Monitor the helium level, and fill regularly and well before
the magnet runs out of helium.
3. Use proper equipment when filling the magnet, including a proper
pressure regulator
and gage for the helium gas used to pressurize the liquid helium
storage vessel.
What should I do if the magnet is about to quench?
If you feel
that the magnet is about to quench, due to a sudden increase in
the boil-off rate, or because the magnet is nearly out of liquid
helium:
1. Fill the magnet with liquid helium if available.
2. Otherwise, DO NOTHING!!!
Many NMR systems
are delivered from the manufacturer with a complete magnet "de-energization"
kit. In our opinion, this is not a good idea, since a magnet can
be easily damaged if an attempt is made to remove the field by someone
without proper training. Even a skilled engineer will not attempt
to remove the field unless the magnet is nearly full of liquid helium.
It is very rare for a cryomagnet to be damaged if it is allowed
to quench without interference. However, if it quenches during a
hurriedly organized "'rescue attempt," the chances of
damage to the magnet go up sharply.
What should I do after a quench?
If possible,
fill the magnet with liquid helium within 24 hours after the quench.
This will save considerable time and cost in restoring the magnet
to field. If this is not possible within 24 hours, then simply leave
it alone and let it warm up. Either way, don't worry too much. The
chances of anything actually being damaged are very low.
Why purchase a QRA?
A quench recovery
agreement protects you in two ways. First, against the unforeseen
expenses associated with a quench. Second, against lost productivity
associated with long delays in obtaining funding, issuing a purchase
order, and waiting for a magnet engineer to be available. With an
MRR QRA, just give us a call and you will have our top priority
with no hassles and no red tape in the way.
NMR
Quench Safety Tip: In the unlikely event of the magnet quenching or
of a cryogenic failure, up to 100 m3 of helium gas may evolve over
a period of several minutes. Although inert, lighter than air and
non-toxic, there could be a risk of asphyxiation in a confined space.
Personnel should evacuate the area in such a situation. A quench warranting
evacuation would be obvious by the noise of the escaping gas and clouds
of vapor. When transferring liquid nitrogen or helium, the following
steps should be observed to avoid accidents:
Gloves,
eye protection, and closed shoes (NOT STEEL CAPPED) must be worn.
Doors should
be propped open to increase ventilation.
Tanks on
wheels must be chocked or held by another person.
The transfer
must be continuously attended.
Since the possibility
of a quench is higher when filling the magnet, and since the transfer
involves manual operations, there is a remote possibility that an
operator could be rendered unconscious around the time of a quench.
Fills should only be done by a single operator when the fill cannot
be deferred, and exceptional caution should then be used.
For
price and complete details on our Quench Services:
Contact Sales
at (800) 443- 5486 ext. 330 or E-mail:
Sales
Determine the status of your magnet