Abstract:
The zero-backflow vent assembly ( 100 ) of the present invention prevents backflow into the magnet cryogen vessel ( 12 ) and therefore eliminates magnet icing. In general, the present invention employs a spring loaded valve in the magnet vent turret ( 38 ) to prevent the influx of air after a magnet quench event. The magnet vent turret ( 38 ) is the interface between the liquid helium vessel ( 12 ) in the magnet and the atmosphere ( 40 ). A vent stack is employed to channel any cryogenic exhaust gas out of the room, normally to the outside atmosphere ( 40 ).

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to superconducting magnets, and more particularly to an assembly for preventing ambient air from flowing back into a superconducting magnet following magnet quench or burst disk failure.  
         [0002]     As is well known, a magnet can be made superconductive by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing a cryogen. This extremely cold environment effectively reduces resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed, thereby maintaining a magnetic field. Such a magnetic field finds wide application in the field of magnetic resonance imaging (MRI). The cryogen most often used with MRI superconducting magnets is helium, which exists in a liquid state at approximately 4.2° K. The liquid helium cools the superconducting wire so that the magnet can be energized, or “ramped” up to full operational field. Once the magnet is ramped, it maintains the particular magnetic field until the magnet is deenergized. Deenergizing of the magnet, or ramping down, is a planned event which occurs in the normal course of operations. The magnet may also be quenched, which is an unplanned event.  
         [0003]     During normal superconducting operation of the magnet, the cryostat must be a closed or sealed system so as to prevent leakage of helium liquid and helium gas from the cryostat, both of which would deplete liquid helium from the cryostat reservoir. In the event of an undesired magnet quench or reversion of the magnet to a non-superconductive state, however, the rapid and potentially dangerous high pressure buildup of helium gas in the cryostat requires pressure relief through rapid venting of the gas to the atmosphere outside of the magnet. When the magnet quenches, the magnetic field energy can be represented by the equation P=V*I wherein V is the voltage in the coil, I is the current in the coil, and P is the amount of power that is converted to heat. That heat, in turn, boils the liquid helium. As the liquid helium heats, and then boils, it rapidly expands. The ratio of liquid to vapor expansion for helium is approximately 770:1, resulting in a nearly instantaneous pressure increase in the magnet vessel. Obviously, this is a potentially hazardous condition for medical staff and the patient. Less obvious is the potential for damage to the system, which can be extremely expensive.  
         [0004]     A replaceable rupture disk, or burst disk, may be interposed within a magnet vent assembly, which disk is designed to rupture at a predetermined pressure thereby opening the cryostat to an atmospheric vent. Even in normal operation, however, the liquid helium vessel always operates slightly above atmospheric pressure. The burst disk performs the function of remaining sealed to maintain pressurization of the liquid helium vessel and to prevent air inflow. If the maximum safe pressure is exceeded, the burst disk fractures, thereby allowing the magnet to vent, normally to the outside. The atmospheric vent may be a vent stack which extends from the roof of a building or from the roof of a motor vehicle which is used to transport a portable MRI system contained within it.  
         [0005]     Once ruptured, the burst disk must be replaced. More immediate, however, is the need to seal the system once the pressure in the cryostat has dissipated. That is, in the event of a magnet quench, the burst disk performs its function by preventing damage to the magnet caused by the sudden pressure increase. After the liquid helium vessel in the magnet vents, and the pressure inside the magnet vessel returns to atmospheric level, the liquid helium vessel remains open, thus allowing for the inflow of air into the magnet. That is, once the pressure in the helium vessel has dissipated and the helium vessel pressure reaches equilibrium with atmosphere, the magnet stops venting. At this point, the magnet begins to function as a cryogenic pump, that is, it draws in air to the cool surfaces of the helium vessel. The air is then frozen on the cool surfaces, leading to a condition known as magnet icing which continues until the magnet warms further or the burst disk is replaced. Air consists primarily of nitrogen and oxygen, which have freezing temperatures of 63° K. and 54° K., respectively. When air is permitted to flow into the liquid helium vessel, the air freezes and magnet icing occurs.  
         [0006]     Thus there is a need for a system that prevents magnet icing in a superconducting magnet after a magnet quench or a burst disk rupture. The present invention provides such a device.  
       SUMMARY OF THE INVENTION  
       [0007]     The zero-backflow vent system of the present invention prevents backflow into the magnet liquid helium vessel and therefore eliminates magnet icing. The present invention opens in the event of a magnet quench, but closes after pressure is relieved. In general, the present invention employs a magnet vent turret. The magnet vent turret is the interface between the liquid helium vessel in the magnet and the atmosphere. A burst disk is located downstream of the helium vessel in the vent turret. A vent stack is employed to channel any cryogenic exhaust gas out of the room, normally to the outside.  
         [0008]     In order to prevent magnet icing, the present invention provides a zero-backflow vent system to prevent ambient air from entering the helium vessel after a quench. The zero-backflow vent system is generally comprised of a plunger assembly with a valve face, the valve face being designed to form a seal against a valve seat. In general, a load spring provides the closing force to maintain a sealed system during normal operation. The amount of load force required to keep the valve face sealed against the valve seat can be adjusted using a load adjustment screw to increase or decrease the length, and thus the overall force applied by, the load spring.  
         [0009]     When the magnet quenches, the pressure within the helium vessel rises, which subsequently increases the amount of force acting against the plunger face. In operation, the load spring is adjusted to allow the magnet to vent during a quench. Therefore, when the force generated by the pressure of the helium gas expanding is greater than the spring force, the plunger assembly opens and allows the magnet to vent. After a short duration of venting, the magnet pressure is relieved, the spring load overcomes the pressure load, and the plunger face reseats on the valve seat, sealing the magnet against icing.  
         [0010]     The foregoing and other features of the system of the present invention will be apparent from the detailed description that follows.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a simplified cross-sectional view of a superconducting magnet and burst disc assembly.  
         [0012]      FIG. 2  is a simplified cross-sectional view of a superconducting magnet incorporating an embodiment of the present invention in the closed position.  
         [0013]      FIG. 3  is a simplified cross-sectional view of a superconducting magnet incorporating an embodiment of the present invention in the open position. 
     
    
     DETAILED DESCRIPTION  
       [0014]     Referring now to the drawings in detail, wherein like numbered elements correspond to like elements throughout,  FIG. 1  illustrates a recondensing superconducting magnet system, generally identified  10 . The system  10  includes a cryostat or helium pressure vessel  12  (when liquid helium is the cryogen), which vessel  12  is shown schematically in reduced size and which encloses a plurality of magnet coils  14 ,  16  in liquid helium  18 . The helium pressure vessel  12  is enclosed within a surrounding vacuum vessel  20  and intermediate members such as a thermal radiation shield  22 . Helium gas  21  forms above the liquid helium  18  through the boiling of the liquid helium  18  in providing cryogenic temperatures to the superconducting magnet system  10 . In this fashion, the extreme cold maintains current flow through the magnet coils  14 ,  16  after a power source, which source is initially connected to the coils  14 ,  16 , is disconnected due to the absence of electrical resistance of the cold magnet coils  14 ,  16 , thereby maintaining a strong magnetic field in the bore of the magnet. Helium gas  21  which forms may be recondensed back to liquid helium  18  by a mechanical refrigerator (not shown) or be vented to the atmosphere.  
         [0015]     In an embodiment of the system described herein, a service turret  28  is bolted to a collar  30  by a plurality of bolts. The collar  30  is connected to the interior of the helium vessel  12  by a first section of vent pipe  35  which provides external access for electrical leads (not shown) and for service purposes. A second section of vent pipe  34  connects between the magnet vent/service turret  28  and a burst disk assembly  36  to an exhaust vent assembly  38  which is connected to the outside atmosphere  40  through vent piping  41 . The burst disk assembly  36  provides a barrier between the vent pipe  34  and the exhaust vent assembly  38  during normal operation of superconducting magnet  10 . The vent pipe  35  is of relatively large diameter, approximately 3 inches, for example, with section of vent pipe  34  being of even larger diameter.  
         [0016]     During an undesired quenching or discontinuance of superconducting operation of the superconducting magnet assembly  10 , as much as 1,800 liters of liquid helium can be boiled off in as little as 20 seconds. This creates tremendous pressure which must be vented to the atmosphere  40  outside the building that houses the superconducting magnet assembly  10  in order to prevent damage to the superconducting magnet assembly  10 . The rapid venting of helium gas  21  to the atmosphere  40  is made possible by the presence of a burst disk. The burst disk is designed to rupture at a predetermined pressure above that produced during normal superconducting magnet operation.  
         [0017]     As shown in  FIG. 1 , the burst disk is interposed between an o-ring face seal (magnet side) and Teflon gasket (vent side) that are used to properly seal the burst disk against leakage of helium gas  21  during normal operation. Bolts  49  around the circumference of the burst disk are used to secure the burst disk although use of other mean for securing the burst disk are also possible.  
         [0018]     Unfortunately, and as discussed above, while the burst disk provides protection against damage to the superconducting magnet assembly  10  due to the extreme pressure increase caused by liquid helium  18  boiling off, it cannot prevent magnet icing. Magnet icing occurs when the burst disk has ruptured, the liquid helium  18  has boiled off and the pressure within the helium vessel  12  has been equalized with that of the atmosphere  40 . The helium vessel  12  is then open to the ambient atmosphere  40  and, because of its extremely cold state, functions as a cryogenic pump, drawing in air until the burst disk can be replaced. The air then freezes as it comes in contact with the super-cooled surfaces within the liquid helium vessel  12 .  
         [0019]     Referring now to  FIGS. 2 and 3 , The present invention provides a zero backflow vent assembly, generally identified  100 , generally comprised of a valve seat  110 , a valve face  120 , a plunger assembly  130 , a spring  140  and a load adjustment screw  150 . In location, the zero backflow vent system is located in the vent assembly  38  slightly downstream of the burst disk, or on the atmosphere side  40  of the burst disk.  
         [0020]     In operation, the zero backflow vent system  100  simply prevents the backflow of air into the liquid helium vessel  12  by closing the vent assembly  38  after pressure is equalized following a magnet quench. During a magnet quench, pressure behind the burst disk would increase to the point that the burst disk is ruptured. At that point, pressure on the valve face  120  would be so high as to overcome the force provided by the load spring  140  and helium gas  21  would be permitted to vent. At some point, the pressure in the liquid helium vessel  12  would diminish, and thus the force exerted on the valve face  120  would likewise diminish and the valve spring  140  would overcome the force exerted on the valve face  120 . The valve spring  140  would then return the valve face  120  to the closed position, thus preventing air from entering the liquid helium vessel  12  and freezing.  
         [0021]     The zero backflow vent system  100  of the present invention is located within the vent assembly  38  on the atmosphere side  40  of the burst disk. The vent assembly  38  of the present invention differs from prior vent assemblies in that a spring recess area  160  is provided opposite the burst disk but within the vent assembly  38 . The vent stack  41  then remains the same with the only addition being the zero backflow vent assembly  100 . In general, the vent assembly  38  will be designed having a valve seat  110  on the atmosphere side  40  of the burst disk. The valve seat  110  simply provides a sealing resting point for the valve face  120 . The valve face  120  abuts the valve seat  110  and is the object exposed to the increase in pressure of the helium gas  21  and to the remnants of the ruptured burst disk.  
         [0022]     Behind the valve face  120  is a load spring  140 , enclosed in a plunger assembly  130 . The load spring  140  is designed to provide enough resilience to push the valve face  120  back into the valve seat  110  when the pressure inside the liquid helium vessel  12  has returned to a safe level. The valve face  120  then prevents air from the atmosphere from entering the liquid helium vessel  12  and freezing. The load spring  140  is also calibrated such that in the event of a rise in pressure in the liquid helium vessel  12 , it would open again to permit liquid helium  18  to boil off.  
         [0023]     The load spring  140  is enclosed in a plunger assembly  130  in order to prevent pieces of the burst disk from becoming tangled in the load spring  140 . In general, the spring  140  is compressed between the back of the valve face  121  and a spring backing plate  141 . Circumferentially, the spring  140  is enclosed by the plunger assembly  130 . A portion of the load spring  140  and the spring backing plate  141  is housed in the spring recess area  160 . The length of the load spring  140 , and thus the force exerted by it, is adjustable using the load adjustment screw  150 . The load adjustment screw  150  rotates within a threaded aperture  161  in the spring recess area  160 , permitting magnet engineers to adjust the length/force of the spring  140  by threading the load adjustment screw  150  further in, or unthreading it, thus permitting the user to ensure that the valve face  120  is always resting firmly against the valve seat  110 .  
         [0024]     In summary, the present invention provides a zero backflow vent assembly for a cryostat pressure relieving vent system  100  for a cryogen cooled superconducting magnet  10  having a cryogen gas vent attached to the cryostat and connected to an exhaust vent  41 , said cryogen gas vent being installed to vent cryogen gas  21  from the cryostat to the atmosphere in the event of an undesired pressure buildup comprising; a spring recess area  160  within the cryogen gas vent; a valve seat integrated with the cryogen gas vent opposite the spring recess area  160 ; a threaded aperture  161  within the spring recess area  160  opposite the cryogen gas vent; a threaded rod  150  threaded into the threaded aperture  161 ; a spring backing plate  141  at the end of the threaded rod  150 ; a spring  140  having a first end attached to the spring backing plate  141  and a second end  142 ; and a valve face  120  attached to the second end of the spring  142 . The spring  140  of the present invention may alternately permit the valve face  120  to move towards the exhaust vent in the event of an undesired pressure buildup and seal the valve face  120  against the valve seat  110  when the pressure inside the cryostat has subsided to safe levels. The spring of the present invention may also be enclosed in a plunger assembly  130 . The present invention may also provide for adjustable spring  140  by either moving the spring backing plate  141  either closer to or further away from the valve face  120  by threading or unthreading the threaded rod  150 . Normally, the zero backflow vent system  100  of the present invention is positioned downstream of a burst disk assembly.  
         [0025]     Accordingly, an improved device for preventing the backflow of air into a liquid helium vessel has been presented. While the applicants believe they have provided a full and complete disclosure of the invention has been made, additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details disclosed and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.  
       Parts List  
       [0000]    
       
         
           
               10  superconducting magnet system  
               12  helium pressure vessel  
               14  magnet coil  
               16  magnet coil  
               18  liquid helium  
               20  vacuum vessel  
               21  helium gas  
               22  thermal radiation shield  
               24  vent pipe  
               28  service turret  
               30  collar  
               32  bolts  
               34  vent piping  
               35  pipe  
               36  burst disk assembly  
               38  exhaust vent assembly  
               40  outside atmosphere  
               41  vent piping  
               49  bolts  
               100  zero backflow vent system  
               110  valve seat  
               120  valve face  
               121  back of  120   
               130  plunger assembly  
               140  load spring  
               141  spring backing plate  
               150  load adjustment screw  
               160  spring recess area  
               161  threaded aperture