System for venting cryogen from a cryostat

A system is provided for controllably venting cryogen of extremely low temperature from a cryostat, wherein a conduit formed of selected metal provides a path of flow for the cryogen from the cryostat to an exhaust system. A gauge is coupled to the conduit, along the path of flow, by means of a bushing which thermally isolates the gauge from the metallic conduit, which is at an extremely low temperature. Thus, the gauge, which is used to continually monitor cryogen pressure, does not become unreadable due to icing or frosting as a result of heat loss from the gauge to the conduit. The bushing is formed from a material such as GC-10, and includes an intermediate portion for spacing the bushing away from the metallic conduit, and also for enclosing a dead space which substantially reduces heat transfer directly between the gauge and cryogen in the conduit.

BACKGROUND OF THE INVENTION 
The invention disclosed and claimed herein pertains to a system for 
controllably venting cryogen of extremely low temperature from a cryostat, 
wherein the system includes a gauge for monitoring cryogen pressure. More 
particularly, the invention pertains to a system of such type which 
employs simple and inexpensive means to prevent the gauge from becoming 
unreadable due to the icing or frosting over thereof. 
In a superconducting magnet of the type commonly used in magnetic resonance 
imaging or spectroscopy, magnetic coils are contained in a cryostat and 
immersed in a liquid cryogen, such as liquid helium. The liquid helium is 
at a temperature on the order of 4.degree. Kelvin, and the extremely cold 
environment provided thereby maintains the conductors of the magnetic 
coils in a superconducting state. 
A "quench" occurs when a substantial amount of the liquid cryogen in the 
cryostat goes into a gaseous state. Quenches are generally unintentional, 
and result in a sudden rise in cryogen pressure. The pressure must be 
rapidly relieved, and a flow path must be provided for the gaseous 
cryogen, to safely direct it to an exhaust or ventilator system so that it 
can be removed from the vicinity of the cryostat. In a common arrangement, 
a main cryogen vent line is coupled between the cryostat and the exhaust 
system, and is sealed by a burst disk. In the event of a quench, the 
pressure in the vent line immediately exceeds a disk bursting level, such 
as 20 psi, whereupon the disk is rupured and the gaseous cryogen is 
enabled to flow through the main vent line to the exhaust system. 
When servicing operations are performed, such as to add additional cryogen 
to the cryostat (referred to as "filling"), or to couple electric current 
to the magnetic coils (referred to as "ramping"), some heat will be 
introduced into the interior of the cryostat. The amount of heat will not 
be enough to cause a quench, but will generate a small amount of gaseous 
cryogen. This small amount of cryogen must likewise be provided with a 
flow path from the cryostat to the ventilator or exhaust system. In some 
arrangements, the flow path is provided by coupling both ends of a disk 
bypass vent line to the main vent line, one end being coupled on either 
side of the burst disk. The small amount of gaseous cryogen generated by 
the servicing activity is thus routed around the burst disk, which is 
intended to seal the main vent line except for when a quench occurs. 
Typically, a valve is placed in the bypass vent line, to be opened during 
servicing activities of the above type, and otherwise to be kept closed. 
It will be readily apparent that if the pressure of the cryogen generated 
during servicing activity exceeds the disk bursting level, the disk will 
break, even though a quench has not occurred. This is very undesirable, 
since all the cryogen in the cryostat would thereby be vented and be lost. 
The bursting disk could also pose a safety hazard to a service operator or 
other personnel who happened to be in the area at the time. Accordingly, a 
pressure gauge is placed in the disk bypass line, and is continually 
monitored by an operator as he performs his servicing tasks. The gauge 
will show that the pressure in the vent line is rising toward the disk 
bursting level, so that corrective action can be taken before the bursting 
level is reached. 
The disk bypass vent line, as well as the fitting used to couple the 
pressure gauge into the bypass line, are typically made of a metal such as 
brass. While the cryogen flowing through the bypass line is in a gaseous 
state, it is still extremely cold, such as on the order of 20.degree. 
Kelvin. Accordingly, the fitting and bypass line become extremely cold, 
and heat is rapidly conducted away from the pressure gauge. The loss of 
heat causes the pressure gauge to frost or ice over. In some instances, 
frost collecting on the pressure gauge causes the gauge to resemble a 
"snow ball." The gauge thereby becomes unreadable, and is therefore 
unuseable for providing a warning of hazardous build-up in vent-line 
pressure. 
In the past, a heat gun has been employed to keep the face of the pressure 
gauge clear from frost. However, such solution requires a service operator 
to perform an additional activity, and heat from the heat gun can cause 
distortion of certain of the mechanical parts of the gauge. 
SUMMARY OF THE INVENTION 
The present invention provides a system for controllably venting cryogen of 
extremely low temperature from a cryostat, and includes a metallic conduit 
means for providing a path of flow for the cryogen from the cryostat to an 
exhaust system. The cryogen venting system further includes a gauge for 
indicating cryogen pressure in the conduit means, and a bushing or fitting 
means for coupling the gauge to the conduit means. The fitting means is 
formed from a material of high thermal impedance and has a first end 
joined to the gauge, a second end joined to the conduit means, and an 
intermediate portion between the first and second ends for spatially 
separating the gauge from the conduit means. The intermediate portion 
encloses an elongated dead space which substantially reduces heat transfer 
directly between the gauge and the cryogen in the conduit means. 
Preferably, the conduit means is configured to turn the path of cryogen 
flowing therethrough through an angle on the order of 90.degree.. The 
gauge and the fitting means are joined to the conduit means, proximate to 
the 90.degree. turn in the path of cryogen flow. 
An object of the invention is to improve efficiency and safety in servicing 
a cryostat, such as a cryostat containing a superconducting magnet. 
Another object is to provide an inexpensive and simple approach to prevent 
icing of a pressure gauge connected into the external plumbing of a 
cryostat of the above type. 
These and other objects and advantages will become more apparent from the 
following description, taken in conjunction with the accompanying drawings 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a cryostate 10 of a type used as a component for a magnetic 
resonance (MR) imaging system. Magnetic coils (not shown) are positioned 
around a bore 12 formed in the cryostat 10, and are kept immersed in a 
cryogen such as liquid helium. The coils are thus maintained at a 
temperature of 4.degree. Kelvin or less, and are therefore in a 
superconducting state. A current introduced into the superconducting coils 
will continuously circulate, to generate a magnetic field in the bore 12, 
which serves as the main magnetic field for MR imaging. FIG. 1 further 
shows cryostate 10 provided with a service turret 14 which contains ports 
and electrical connectors (not shown) for adding cryogen to the cryostat 
and for ramping the coils. 
FIG. 1 also shows a three-inch main vent line 16 having one end connected 
to internal cryogen vent plumbing (not shown) in turret 14 and the other 
end connected to a vent adapter 18. A burst disk 20 is positioned to 
tightly seal the main vent line 16. The disk will be ruptured by a 
pressure in the main vent line on the cryostat side of the disk, which 
exceeds 20 psi. Thus, if a quench occurs in cryostat 10, the resulting 
sudden increase in pressure will break the disk 20, thereby allowing the 
helium gas generated by the quench to be vented through the line 16 to 
adapter 18. 
Vent adapter 18 comprises the lower portion of a cryogen ventilator or 
exhaust system. A pipe 22 (only a portion of which is shown) is mated to 
the upper flange of adapter 18 and connects with a ventilator such as a 
rooftop ventilator (not shown). Such exhaust systems are well known in the 
art, and thus not described in further detail. 
FIG. 1 further shows a disk bypass vent line 24 having one end coupled to 
main vent line 16, between turret 14 and burst disk 20, and the other end 
coupled to the adapter 18. A valve 26 is coupled into bypass line 24, 
valve 26 normally being kept in a closed position, but being opened to 
allow gaseous helium to flow through bypass line 24 when servicing is 
being performed on the cryostat 10. Thus, small amounts of gaseous helium 
generated as a result of ramping, filling or other servicing operations 
flow through bypass line 24, and therethrough to vent adapter 18. A 
pressure gauge 28 of conventional design is coupled into bypass line 24 to 
enable a service operator to detect a rise in cryogen pressure in line 18, 
resulting from servicing activity. If such a rise was detected, corrective 
action could be taken to prevent disk 20 from rupturing. 
FIGS. 1 and 2 together show bypass line 24 comprising pipe segments 24a-d, 
joined together by conventional fittings. The respective segments and 
fittings are formed of a metallic material such as brass. 
FIGS. 2 and 3 show bypass line 24 including a conventional fitting 30 
referred to as a "plumber's cross." Cross 30 has four ports or openings 
30a-d, each provided with inside threads. Ports 30a and 30b are oriented 
with respect to each other so that pipe segments 24a and 24b, respectively 
coupled to ports 30a and 30b, are oriented at 90.degree. to each other. 
Accordingly, cryogen flowing through the plumber's cross 30 transits a 
path which makes a 90.degree. turn, as shown by Arrow A of FIG. 3. Port 
30c is in opposing relationship with port 30a, and port 30d is generally 
kept sealed by means of a plug 32. Plumber's cross 30 is likewise formed 
of a metallic material such as brass. 
FIG. 3 further shows gauge 28 having a threaded end member 28a engaging a 
bushing 34, which has a set of external threads mating with the inner 
threads of port 30c. Thus, bushing 34 is used to couple gauge 28 into 
bypass line 24, at a point where the path of cryogen flow turns through 
90.degree.. By positioning the gauge at a turn in the cryogen path of 
flow, as shown in FIG. 3, conduction of heat away from the gauge, directly 
by the cryogen flowing through line 24, is minimized. However, as stated 
above, the respective components of line 24, including cross 30, typically 
are formed of brass or other metal. Accordingly, line 24 becomes extremely 
cold within a short period of time after cryogen starts flowing through it 
from the cryostat 10. Accordingly, gauge 28 must be thermally isolated 
from line 24 to substantially reduce or eliminate heat flow from gauge 28 
into the cold metallic bypass line. 
Such thermal isolation is achieved, in part, by forming bushing 34 of a 
material such as GC-10, a plastic material which is known in the industry 
to have a very high thermal impedance. Alternatively, bushing 34 could be 
formed of a thermo-plastic material known as Xenoy, a trademark of the 
General Electric Company. To further thermally isolate gauge 28, and to 
therefore prevent the icing of the gauge, bushing 34 is formed as shown in 
FIG. 4. 
FIG. 4 shows bushing 34 provided with opposing end portions 34a and 34b, 
and an intermediate portion 34c between the two end portions. Outer 
threads 36 are formed around end portion 34b, threads 36 mating with the 
threads of port 30c of plumber's fitting 30. Also, a throughhole 38 is 
formed through bushing 34, inner threads 40 being formed in a portion of 
the throughhole which traverses end portion 34a. Threads 40 match the 
threads of end member 28a of pressure gauge 28. Thus, bushing 34 serves to 
join gauge 28 to plumber's cross 30, and at the same time, spaces the 
gauge 28 away from cross 30, which, as stated above, becomes extremely 
cold when cryogen is flowing therethrough. 
End member 28a, when received into threads 40 of end portion 34a, seals 
throughhole 38 to form a dead space 38a. By providing dead space 38a in 
bushing 34, comparatively little heat transfer takes place between gauge 
28 and cryogen which drifts into dead space 38a from bypass line 24. This 
is because cryogen which has moved into the dead space is well outside the 
cryogen flow path through line 24. 
FIG. 5 shows bushing 44 provided with wrench flats 42 for use in securing 
bushing 34 to plumber's cross 30. 
While a preferred embodiment of the invention has been shown and described 
herein, it will be understood that such embodiment is provided by way of 
example only. Numerous variations, changes and substitutions will occur to 
those skilled in the art without departing from the spirit of the 
invention. Accordingly, it is intended that the appended claims cover all 
such variations as are followed in the spirit and scope of the invention.