GB2461873A

GB2461873A - A method of and appratus for leak testing of a vacuum vessel

Info

Publication number: GB2461873A
Application number: GB0812831A
Authority: GB
Inventors: James Edward Price
Original assignee: Siemens Magnet Technology Ltd; Siemens PLC
Current assignee: Siemens PLC
Priority date: 2008-07-15
Filing date: 2008-07-15
Publication date: 2010-01-20
Anticipated expiration: 2028-07-15
Also published as: GB0812831D0; GB2461873B

Abstract

A method of leak testing of a vacuum vessel 1 comprises fitting a removable housing 12 between an inspection port 5 and test equipment (e.g. a mass spectrometer); coupling a plunger 21 in the housing 12 to a sealing plate 20 in contact with the inspection port 5; evacuating the housing 12; activating the plunger 21 to move the sealing plate 20 out of contact with the inspection port 5 allowing a sample into the test equipment; deactivating the plunger 21 and returning the sealing 5 plate to a sealed position; decoupling the plunger 21 and removing the housing 12.

Description

INSPECTION PORT SEALING DEVICE

This invention relates to an inspection port sealing device and a method of leak testing of a vacuum vessel using such a device, in particular for use in superconductor systems, such as a cryostat of a magnetic resonance imaging (MRI) system.

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.

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. The cryogen vessel is typically surrounded by one or more thermal shields, and a vacuum jacket completely enclosing the shield(s) and the cryogen vessel.

An access neck typically passes through the vacuum jacket from the exterior, into the cryogen vessel. Such access neck is used for filling the cryogen vessel with cryogenic fluids and for passing services into the cryogen vessel to ensure correct operation of the magnet system.

Cryogenic fluids, and particularly helium, are expensive and it is desirable that the magnet system should be designed and operated in a manner to reduce to a minimum the amount of cryogen consumed. An MRI magnet is typically housed in a vessel containing liquid helium at very low temperature. This helium vessel is encapsulated in an air-tight outer vessel (OVC). In order to reduce convective and conducted heat loads, a vacuum is generated inside the cryostat, in an outer vacuum chamber (OVC) surrounding a cryogen vessel. The volume between the cryogen vessel and OVC casing is evacuated prior to cooling the magnet. Evacuation is a slow process requiring many hours to complete. After evacuation has been completed and the vacuum has been generated, the port through which this is done is sealed, typically by use of blanking plates and 0-rings. In order to check the quality of this final seal, a mass spectrometer is attached, via an inspection port, to the volume between the seal and the main OVC, and helium is sprayed in the vicinity of the seal. Any leakage through the seal into the volume of the OVC is then evident on the mass spectrometer.

Finally, the inspection port through which the mass spectrometer is connected to the OVC is also sealed. Currently, this is done with use of a proprietary valve, but this increases the cost of manufacture.

The present invention provides a device and a method as defined in the appended claims.

Using the device of the present invention reduces the cost of the assembly used to seal an inspection port in the cryostat pump-out port, and eliminates reliance on high tech valves, or conventional alternatives, as currently fitted to the inspection port at the end of the test process, which add cost of the assembly due to complexity of the design and installation procedure.

Using a spring to achieve good final seal, also adds a failsafe to the system.

Using a non-ferrous material as a vacuum tight liner between the electro-magnetic coil of the solenoid and plunger gives a linear actuation, reducing the number of potential leak points, leaving only a flat sealing face.

An inspection port sealing device and a method of leak testing in accordance with the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a conventional device for connecting test equipment to an inspection port; Figure 2 illustrates a first example of an inspection port coupling according to the present invention; and, Figure 3 illustrates a second example of an inspection port coupling according to the present invention.

The present invention relates to a device used to open and reseal a port that is used to connect test equipment, such as a mass spectrometer, to the OVC vacuum and to a method of using such a device. A typical use of such a device is to ensure the integrity of the final OVC seals during assembly and in the field during specialist service operations. The tool of the present invention uses fewer sealing faces than conventional tools and so maintains a high vacuum integrity.

Using a sealing plate rather than a high tech valve in the inspection port reduces the overall cost per system manufactured, since the cost is taken from the magnet and added to service tooling, which is desirable since the number of service tools required in total, is only a fraction of the number of magnets that are produced every year. A blanking plate over the sealing plate adds security to the seal.

Fig. 1 illustrates a conventional arrangement for leak testing after completion of evacuation of the outer vacuum chamber. The 0VC casing 1 of a cryostat 2 is provided with a cryostat pump-out port 3, and a port closure 7, sealing the port via 0-rings seals 6. The cryostat pump-out port 3 is joined to the OVC by a relatively small volume 4 in which an inspection port 5 is provided. The inspection port 5 is sealed with a specialised vacuum valve 8, which is welded to the port 5 by means of welds 19. A grub screw 9 with 0-ring seal is inserted into the valve 8 and a rod 10, which passes through a coupling device 12, has an 0-ring 11 to hold the grub screw 9 onto the rod.

The far end of the rod 10 passes through an 0-ring sealed mechanical feedthrough 13. A knob 14 is turned to rotate the rod 10 and grub screw 9 and withdraw or insert the vacuum valve. The coupling device 12 has sealing surfaces 15, 16 which contact corresponding sealing surfaces 17 of the valve 8 and the pump-out and mass spectrometer port 18 of the test equipment (not shown).

Fig.2 illustrates a first embodiment of the present invention. As before, an inspection port 5 is provided in the volume 4 between the OVC casing 1 and the cryostat pump-out port 3. However, instead of a specialist vacuum valve being required in the inspection port 5 of the cryostat (which remains with the cryostat after the test has taken place), a sealing disc 20 is provided. The sealing disc 20, seals to surfaces 24 of the inspection port 5 with double 0-ring seals 22, 23 in contact with the surfaces of the port. In this invention the inspection port used when connecting the mass spectrometer to the OVC can be sealed effectively, with the capability of reopening/resealing in the field under service process control, as required, without using a costly vacuum valve. A vacuum valve is still required between the pump-out and mass spectrometer port and the test equipment itself, A vacuum is maintained during testing and the mass spectrometer test is carried out at a high vacuum, but this part of the equipment is reused, fewer proprietary valves have to be purchased than with the conventional method, where the valve is welded to the OVC inspection port and remains on the cryostat after testing. In this example, the sealing disc is moved in and out of the port 5 using a plunger 21 in the coupling device 12. The plunger may be purely mechanical, or it may be electromechanically controlled. For a mechanical plunger, suitable seals (not shown) are required to allow the plunger to pass though the outer casing of the tool, whilst maintaining the vacuum within.

An example of an electromechanically controlled plunger is illustrated in Fig. 3.

This example avoids the need for seals in the outer case. The sealing disc 20 in this example is a double 0-ring sealing disc, with 0-ring 37 contacting a surface 38 of the port 5 and 0-ring 38 contacting the sides of the port. Sealing flanges 25, 26 contact a sealing surface 27 of the inspection port 5. The sealing disc is attached to a rod 28 of plunger 29. At rest, the sealing disc is held in place by virtue of a resealing spring 31.

The plunger is mounted in a non-ferrous sleeve 32, such as stainless steel or aluminium, around which electromagnetic coils 33 are provided. The coils are fitted in an iron housing 34 and the end of the housing is closed with a soft iron plug 35. A void 36 at the top of the sleeve 32 allows the plunger to move when activated, pulling the sealing disc out of the port 5, against the spring force.

In the embodiment of Fig.3, opening and re-sealing the OVC seal inspection port is achieved through the use of an electro-magnetic coil 33 as a linear actuator, although types of actuation could be used. When excited this coil moves the plunger 28, which is made from a material that responds to the magnetic field generated by the coil, such as a ferrous material like soft iron or a permanent magnet. The magnetic field produced by the coil windings causes the plunger to move up the guiding cylinder 32 built into an aluminium, or stainless steel, chamber capable of holding a high vacuum.

This housing has two sealing flanges 25, 26, one of which connects to the OVC inspection port 5 and the other to a vacuum-out, or mass spectrometer system port.

The permeability of aluminium or non-magnetic stainless steel is such that it does not interfere greatly with the field produced by the electro-magnetic coil, allowing the plunger to move. The advantage with the use of stainless steel is that the radial gap between plunger and magnet could be reduced (while still maintaining the required mechanical properties to contain a high vacuum) which might be necessary when designing the solenoid coil. If however aluminium achieves the required mechanical properties without impeding the solenoid action, this may be preferable on a cost or design basis.

The solenoid is preferably of a continuous duty type so that when the plunger 28 is at the top of its stroke the solenoid is capable of holding the plunger in place indefinitely without the coil 33 overheating. When the coil is disconnected, the spring 31, placed between the vacuum sealed housing 30 and the sealing disc 20 pushes the plunger back into place and provides sufficient pressure so that a good seal between the disc 20 and sealing surface is achieved. This also acts as a fail safe, so that if the coil were to fail at any point during the opening procedure the integrity of the vacuum within the OVC remains.

This failsafe can also be extended, so that if a leak is detected during the mass-spectrometer reading procedure a signal can be sent to the solenoid to initiate a cut out and the sealing disc 20 pushed back into place. This can be used in the case of any of the seals used to connect the mass-spectrometer and vacuum-out equipment failing.

Additionally this helps to avert damage being caused to the mass spectrometer, which, due to its high sensitivity to pressure may be damaged sufficiently, by a sudden increase in pressure if a seal fails, to require a refurbishment costing in the order of hundreds of pounds.

An example of a method of operation according to the present invention is described below. When the OVC 1 is going through the evacuation procedure the sealing disc 20 is held in place with a blanking disc (not shown). During the OVC port inspection procedure the blanking disc is removed and the plunger 28 is attached to the sealing disc 20, e.g. by a screw thread or other means of attachment. The vacuum sealed housing 30 and electro-magnetic coil assembly 33, 35 are then placed over this plunger and evacuation is carried out using conventional equipment. This results in a negligible pressure drop from one side of the sealing disc to the other. The electro-magnetic coil 33 is then excited and the plunger 28 and sealing disc 20 move upwards, allowing the mass spectrometer to take a reading from the OVC volume during the leak test procedure. After this has been performed, the coil 33 is deactivated and the spring 31 pushes the sealing disc 20 back into position. The coil and vacuum sealed housing assembly are then removed and a blanking disc is screwed into place to hold in and protect the sealing disc.

This method also achieves advantages in its application, manufacture and installation. The installation of the system of the present invention requires that the sealing plate that is intended to be lifted can be attached, by a screw thread, or otherwise, to the plunger and has suitable sealing surfaces for the o-rings to come into contact with. Therefore cost per system is only that of the manufacture of the sealing disc, or plate and sealing surfaces. Another advantage to this system is that any potential leak risk is minimised by virtue of possessing only one potential leak point, around the sealing face o-rings, whereas the previous system had two potential leak points, around the sealing face 0-rings and the mechanical feedthrough 0-rings. This mechanical feedthrough is a significant leak risk as the o-rings within it can be susceptible to damage from grit. The system of the invention is also very versatile. By using appropriate 0-rings on the sealing faces this system can be used at any pressure, so long as the pressure drop over the sealing disc is minimal.

Claims

CLAIMS1. A method of leak testing of a vacuum vessel, the method comprising fitting a removable housing between an inspection port and test equipment; coupling a plunger in the housing to a sealing plate in contact with the inspection port; evacuating the housing; activating the plunger to move the sealing plate out of contact with the inspection port allowing a sample into the test equipment; deactivating the plunger and returning the sealing plate to a sealed position; decoupling the plunger and removing the housing.
2. A method according to claim 1, further comprising fitting a spring between the housing and the sealing plate to hold the sealing plate in contact with the inspection port when the plunger is deactivated.
3. A method according to claim 1 or claim 2, wherein the plunger is activated by means of a solenoid.
4. A method of leak testing of a vacuum vessel, the method comprising fitting an inspection port sealing device according to any of claims S to 15, between an inspection port and test equipment; coupling the plunger in the housing to the sealing plate in contact with the inspection port; evacuating the housing; activating the plunger to move the sealing plate out of contact with the inspection port allowing a sample into the test equipment; deactivating the plunger and returning the sealing plate to a sealed position; decoupling the plunger and removing the device.
5. An inspection port sealing device for coupling between an inspection port and test equipment, the device comprising a removable housing; a plunger within the housing and a sealing plate, wherein the plunger moves the sealing plate into and out of contact with the inspection port.
6. A device according to claim 5, further comprising a spring positioned between the housing and the sealing plate to hold the sealing plate in contact with the inspection port.
7. A device according to claim 5 or claim 6, wherein the inspection port is a port of a vacuum vessel.
8. A device according to any of claims 5 to 7, wherein the plunger comprises a solenoid activated plunger.
9. A device according to any of claims 5 to 8, wherein the plunger comprises soft iron or a permanent magnet.
10. A device according to any preceding claim, wherein the housing comprises a guide cylinder for the plunger.
11. A device according to claim 10, wherein the guide cylinder comprises non-ferrous material.
12. A device according to claim 10 or claim 11, wherein the guide cylinder comprises an aluminium or stainless steel chamber.
13. A device according to any of claims 5 to 12, wherein the test equipment comprises a mass spectrometer.
14. A device according to at least claim 8, wherein the solenoid is a continuous duty solenoid.
15. A device according to any of claims 5 to 14, wherein the plunger and sealing plate are removably coupled together.CLAIMS1. A method of leak testing of a vacuum vessel, the method comprising fitting a removable housing between an inspection port and test equipment; coupling a plunger in the housing to a sealing plate in contact with the inspection port; evacuating the housing; activating the plunger to move the sealing plate out of contact with the inspection port allowing a sample into the test equipment; deactivating the plunger and returning the sealing plate to a sealed position; decoupling the plunger and removing the housing.2. A method according to claim 1, further comprising fitting a spring between the housing and the seating plate to hold the seating ptate in contact with the inspection port when the plunger is deactivated.3. A method according to claim 1 or claim 2, wherein the plunger is activated by C means of a solenoid.4. An inspection port sealing device for coupling between an inspection port and test equipment, the device comprising a removable housing; a plunger within the C" 20 housing and a sealing plate, wherein the plunger moves the sealing plate into and out of contact with the inspection port.5. A device according to claim 4, ftirther comprising a spring positioned between the housing and the sealing plate to hold the sealing plate in contact with the inspection port.6. A device according to claim 4 or claim 5, wherein the inspection port is a port of a vacuum vessel.7. A device according to any of claims 4 to 6, wherein the plunger comprises a solenoid activated ptunger.8. A device according to any of claims 4 to 7, wherein the plunger comprises soft iron or a permanent magnet.9. A device according to any preceding claim, wherein the housing comprises a guide cylinder for the plunger.10. A device according to claim 9, wherein the guide cylinder comprises non-ferrous material.11. A device according to claim 9 or claim 10, wherein the guide cylinder comprises an aluminium or stainless steel chamber.12. A device according to any of claims 4 to 11, wherein the test equipment comprises a mass spectrometer. CO 1513. A device according to at least claim 7, wherein the solenoid is a continuous duty solenoid.14. A device according to any of claims 4 to 13, wherein the plunger and sealing plate are removably coupled together.15. A method of leak testing of a vacuum vessel, the method comprising fitting an inspection port sealing device according to any of claims 5 to 15, between an inspection port and test equipment; coupling the plunger in the housing to the sealing plate in contact with the inspection port; evacuating the housing; activating the plunger to move the sealing plate out of contact with the inspection port allowing a sample into the test equipment; deactivating the plunger and returning the sealing plate to a sealed position; decoupling the plunger and removing the device.