Abstract:
An abstract for a quench valve of a cryostat, in particular for use in a magnetic resonance imaging system, is attachable to the quench valve so as to raise the cracking pressure of the quench valve without changing the operability of the quench valve. Such an accessory device is usable to enable the cryostat, containing a cryogen, to be safely transported by air transportation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Field of the Invention 
         [0002]    This invention relates to an accessory device for a quench valve of a cryostat, in particular for use in a magnetic resonance imaging (MRI) system. Furthermore, this invention relates to a method of enabling a cryostat containing a cryogen to be safely transported by air transportation. 
         [0003]    Description of the Prior Art 
         [0004]    Superconducting magnet systems are used for medical diagnosis, for example in magnetic resonance imaging systems. A requirement of an MRI magnet is that it produces a stable, homogeneous, magnetic field. In order to achieve the required stability, it is common to use a superconducting magnet system which operates at very low temperature. The temperature is typically maintained by cooling the superconductor by immersion in a low temperature cryogenic fluid, also known as a cryogen, such as liquid helium. 
         [0005]    The superconducting magnet system typically comprises a set of superconductor windings for producing a magnetic field, the windings being immersed in a cryogenic fluid to keep the windings at a superconducting temperature, the superconductor windings and the cryogen being contained within a cryogen vessel. 
         [0006]    Superconducting magnets are susceptible to quench events, in which, for one of a number of reasons, part of the superconducting magnet ceases to be superconducting. The resulting resistance in part of the magnet causes heat due to the current flowing through it. This rapidly causes further parts of the superconducting magnet to cease superconducting. The result is that all of the energy which was stored in the magnetic field of the magnet is suddenly released as heat. In a superconducting magnet cooled by a liquid cryogen, this typically results in rapid boil-off of a large volume of the cryogen, with gaseous and liquid cryogen being expelled from the cryostat at high speed. During a quench, it is essential that the escaping cryogen gas is allowed to exit the cryostat in a safe manner. The exit point typically opens by responding to an increase in the pressure within the cryostat. It is known to provide a quench valve to control the exit point. The quench valve is closed until a certain pressure is reached within the cryostat. Once the cryostat pressure reaches the certain value, the quench valve is opened by the pressure acting upon it. 
         [0007]    During transportation of an already assembled system, filled with cryogen, no cooling can be provided to the cryogen, which leads to a heat input into the cryostat, leading to a boil-off of cryogen. Therefore, during air transportation, relief devices must be available in order to guarantee a pressure-relief to protect against overpressure. In other words, a significant build-up of pressure within the cryostat shall be prevented. 
         [0008]    However, the change of atmospheric pressure during an air shipment, even in a pressurized compartment, can cause a problem with the relief devices employed. Ordinary relief valves can freeze and plug up following rapid ejection of cold gas following altitude changes. For this reason, for air transportation, each magnet system has to be fitted with an absolute pressure relief valve, which is unaffected by atmospheric pressure. In addition, in order to comply with safety regulations, an independent second device has to be present, which second device can be a gauge device. 
         [0009]    It is permissible to use the existing quench valve as the gauge device. However, the differential pressure required to crack the quench valve is less than the differential between the pressure within the magnet system and the pressure within the hold of the air craft during air transportation. Therefore, the quench valve would lift and vent excessive cryogen gas. In order to overcome this, it is known to blank off the outlet of the quench valve by an air tight plate fitted with e.g. a 13 PSIG valve. Additionally, a hand valve is fitted, which may also be used to relief pressure before removing the plate. The whole assembly needs to be leak tight and fully tested, making this an expensive solution. Furthermore, the assembly is discarded after arrival on operational site. 
       SUMMARY OF THE INVENTION 
       [0010]    It is therefore an object of the present invention to provide a simple and reliable technique to ensure a safe air transportation of a cryostat containing a cryogen. 
         [0011]    A core idea of the invention is to enable the existing quench valve of the cryostat to serve as a pressure-relief device during air transportation of the cryostat, in a way that the quench valve remains fully operable. In other words, the operating ability of the quench valve is not restricted. Merely the cracking pressure of the quench valve is temporarily raised for the purpose of air transportation. By this means, a safe air transportation of a cryostat containing a cryogen is achieved in a simple, reliable and very effective way, thereby following safety regulations. 
         [0012]    Instead of removing parts of the existing quench valve, and installing an additional hand valve in case of air transportation, as is conventional in the prior art, the invention raises the valve cracking pressure in order to improve the capability of the existing quench valve. The cracking pressure of the quench valve is raised such that the expected differential pressure between the inside of the cryostat and the air craft hold is less than the raised cracking pressure. No additional valve is required. The accessory device, which is used for raising the cracking pressure of the quench valve, may be used several times. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a schematic illustration of a cryostat (prior art). 
           [0014]      FIG. 2  shows a schematic illustration of a quench valve of the cryostat in a sectional view (prior art). 
           [0015]      FIG. 3  shows a schematic illustration of a quench valve of the cryostat, according to the present invention, in a sectional view. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    A cross-section of a superconducting magnet system for use in an MRI system is illustrated in  FIG. 1 . Superconductive magnet coils (not shown) are provided in a cryogen vessel  2  of a cryostat  1 . The coils are immersed in a liquid cryogen  3 , e.g. liquid helium. A central bore  4  is provided to accommodate a patient for examination. An access neck  5  with vent tube  6  is provided at the top of the cryostat  1  to allow access to the cryogen vessel  2 . For clarity reasons, other parts of the cryostat  1 , e.g. the refrigerator for providing active refrigeration to cool the cryogen  3 , the outer vacuum chamber, or the thermal radiation shields, are not shown. 
         [0017]    As illustrated in  FIG. 2  in more detail, a turret outer assembly  7  encloses upper extremities of the access neck  5 , and provides a normal exit path for cryogen gas from cryogen vessel  2 . Turret outer assembly  7  is joined to the cryogen vessel  2  in a leak-tight manner and defines an interior volume which is separated from atmosphere by a protective valve and/or burst disc, in this case by a quench valve  8 . The quench valve  8  is closed until a certain pressure is reached within the cryogen vessel  2 . Once the cryostat pressure reaches the certain value, the quench valve  8  is opened by the pressure acting upon it. 
         [0018]    Quench valve  8  includes a valve plate  9  which is held against valve seat  10  by a first spring arrangement  11 . In case of overpressure within cryogen vessel  2 , a corresponding pressure of cryogen gas acting on the inner side  12  of the valve plate  9  will exceed the pressure acting on the outer side  13  of the valve plate  9  sufficiently to overcome the force of the first spring arrangement  11  and open the quench valve  8 . Cryogen gas will escape, maintaining the pressure within the cryogen vessel  2  at an acceptable level. Once the pressure in the cryogen vessel  2  drops below the pressure needed to keep the quench valve  8  open, first spring arrangement  11  will press the valve plate  9  back into contact with valve seat  10 . Part of the valve plate  9  may be formed by a burst disc, not visible in  FIG. 2  as it lies in the plane of the valve plate  9 . In case the differential pressure across the valve plate  9  becomes much higher than the pressure at which the quench valve  8  should open, for example if the quench valve  8  sticks, or the pressure increase within the cryogen vessel  2  is extremely rapid or severe, the burst disc will rupture and cryogen gas will then escape through a hole left by the burst disc and out of the cryogen vessel  2 . This burst disc is typically a declared regulatory pressure relief safety device, provided to rupture in the event of quench valve failure. 
         [0019]    An embodiment of the invention is depicted in  FIG. 3 . The existing quench valve  8 , as shown in  FIG. 2 , is modified prior to air shipment, without thereby loosing the valve operability of the quench valve  8 . During modifying no part is removed from the quench valve  8 . Instead, an accessory device  14  is installed to the quench valve  8 , which temporarily raises the cracking pressure of the quench valve  8 . 
         [0020]    The accessory device  14  comprises a main body  15  forming a cylindrical or box-shaped container  16  with walls  17 , with an open front  18  and a back plate  19 . The main body  15  is provided with a number of small vent holes, which serve as openings to allow cryogen gas originating from the quench valve  8  to escape from the container  16  in case of a quench. An exemplary position of the vent holes is indicated in  FIG. 3  by arrow  20 . The main body  15  is fitted to the outer flange  21  of the quench valve  8  by means of removable fastening elements  22 , e.g. screws. For this purpose, the front end of the main body  15  is extended to form mounting flanges  23 . 
         [0021]    The back plate  19  is arranged parallel to the valve plate  9  of the quench valve  8 , when the accessory devices  14  is mounted. A second spring arrangement  24  comprising four spring-loaded plungers  25  is provided within the container  16 . In  FIG. 3  only two plungers  25  are illustrated. The plungers  25  bear on the valve plate  9 , by this means raising the cracking pressure of the quench valve  8 . The second spring arrangement  24  comprises four spring elements  26  in the form of compression springs. The spring elements  26  are employed to act on the plungers  25 , in order to provide the spring load, as required. The back plate  19  of the main body  15  acts as counter bearing for the spring elements  26 . For each spring element  26  an internal guiding rod  27  is provided. All guiding rods  27  are mounted to the back plate  19  of the main body  15 . 
         [0022]    By means of the accessory device  14 , using the second spring arrangement  24 , the cracking pressure of quench valve  8  may be raised for example from 6 to 13 PSIG. In case of overpressure during air shipment, the pressure of cryogen gas acting on the inner side  12  of the valve plate  9  has to overcome the force of the second spring arrangement  24  in order to open the quench valve  8 . In this event, cryogen gas exits the cryogen vessel  2  and enters the container  16 , from which the gas escapes through the number of small vent holes. 
         [0023]    When mounted, the main body  15  of the accessory device  14  is adapted to serve as a protective enclosure both for the first and second spring arrangement  11 ,  24 , as well as for the surroundings of the cryogen vessel  2  in case of a rupture of a burst disc. 
         [0024]    On arrival in the hospital or any other operational site, the accessory device  14  is removed, bringing the quench valve  8  back into its normal operation mode. 
         [0025]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant&#39;s contribution to the art.