Patent Application
20110036101
Superconducting magnets are well known and are used for several applications, for example magnetic resonance imaging (MRI); nuclear magnetic resonance (NMR) spectroscopy; particle acceleration and energy storage to name a few. Commonly, the magnets include a coil of superconducting wire which is cooled in a bath of liquid cryogen. Liquid helium and liquid nitrogen are commonly used cryogens, but others are known.
A difficulty experienced with superconducting magnets is that of quench. In operation, a large current (typically several hundred amperes) circulates around a closed superconducting loop, comprising many turns of wire. If, for any reason, any part of the superconducting wire should be heated, or subjected to an intolerably intense magnetic field, it will quench, reverting to a resistive state. The current through that resistive part will cause heating, and cause quench of adjacent parts of the magnet. The quench will propagate, and the resultant heating of the magnet will cause much of the liquid cryogen to be boiled off. Adequate protection systems must be provided to allow the boiled off cryogen to escape from the cryogen vessel without reaching a dangerously high pressure, and without risking contact with surrounding personnel. The risks of contact with the cryogen include cold burns and asphyxiation.
A conventional protection system will be described below. It consists of a burst disc, and/or valve, closing a relatively wide-bore path to atmosphere or to a cryogen recovery system. When the pressure within the cryogen vessel exceeds a certain limit, the valve will open or the burst disc will fracture, allowing the safe egress of cryogen from the cryogen vessel.
Access neck 20 allows access to the cryogen vessel 12 for filling with cryogen, and to allow electrical connections to the magnet to be led out of the cryostat. Turret cover 24 encloses the access neck 20 and provides a seal of the cryogen vessel to atmosphere. The access neck 20 may be closed by a valve or burst disc.
A quench valve 26 is typically provided, sealing a quench path exit from the turret cover 24. In case of a quench, the increasing pressure within the cryogen vessel 12 will cause any valve or burst disc closing the access neck 20 to open. The turret cover 24 will fill with cryogen gas and, if the pressure of the gas is high enough, the quench valve 26 will open. Cryogen gas, and possibly also liquid cryogen, will be lost out of the quench valve 26 to atmosphere or to a cryogen recovery facility.
It is important that the cryogen has a clear escape path, so that it can escape from the cryogen vessel rapidly in case of a quench. If the cryogen escape path were blocked and a quench occurred, dangerously high pressures would build up within the cryogen vessel, and could lead to an explosion. To avoid this risk, it has been conventional to provide a separate auxiliary vent as a fail-safe cryogen egress path in case of blockage of the access neck 20. If the main quench path through access neck 20 is blocked for any reason, the cryogen can still escape through the auxiliary vent, although at a higher pressure than through the access neck 20.
The turret cover 24 is at approximately ambient temperature, while the access neck 20 descends into a cryogen vessel containing a cryogen at a very low temperature. If a helium cryogen is used, the helium gas inside the cryogen vessel is at a temperature little above its boiling point of 4.2K. Thermal stratification will occur within the access neck under steady-state conditions. If there is a leak into the turret cover, air will enter. Components of air, such as water vapor and nitrogen, will circulate, entering the access neck 20. When such air components reach a point within the access neck below their freezing point, they will freeze onto the surface of the access neck. If the leak is severe, or the leaking takes place over an extended period of time, the access neck 20 may become blocked, or at least severely constricted. In order to alleviate this danger, the current lead 21 may be formed as a hollow tube, which extends into the cryogen vessel so far that its lower extremity sits in a thermal stratification at a temperature below the freezing point of common air components such as water vapor and nitrogen. It is essential that the burst disc or equivalent closing the auxiliary vent path through the current lead 21 should be effectively sealed to the current lead to prevent ingress of any air into the current lead. Any air leaking into the access neck 20 will freeze before it reached the lower extremity of the current lead, and so air components cannot enter the inside of the current lead 21.
In case of quench, cryogen boils in the cryogen vessel and the pressure increases. Normally, the cryogen gas will escape through the access neck 20 into the turret cover 24. The pressure in the turret cover will build up until it is sufficient to open the quench valve 26. Once the quench valve has opened, the cryogen can safely escape to atmosphere or to a cryogen recovery facility. Conventionally, the quench valve 26 is arranged horizontally, as illustrated, and a 90° elbow is provided downstream of the quench valve to direct the flow of cryogen gas upwards so that the quench recovery pipe goes up into the ceiling void along with all the other pipes and cables. The outlet of the quench valve sometimes has a horizontal pipe fitted, for example on mobile systems and in sites with very low ceiling height.
However, if the access neck 20 is blocked, or severely constricted, the cryogen gas will not be able to escape through the access neck. The pressure within the cryogen vessel will increase, until it reaches a pressure which will cause the burst disc or equivalent closing the auxiliary vent path to open. Cryogen may then escape the cryogen vessel through the current lead 21 and the auxiliary vent path 281. The cryogen will escape into the atmosphere or into a cryogen recovery facility. The egress path 281 through the current lead is more constricted than the access neck 20, so the pressure within the cryogen vessel will remain higher than in the case of the access neck 20 being used for cryogen egress.
It is accordingly desired to provide an egress path for cryogen in the case of a quench, which will be free of solid deposits even in the case of a turret cover leak, which provides effective sealing and which is relatively simple to re-seal following use.
The present invention accordingly provides methods and apparatus as defined in the appended claims.
The above, and further, objects, characteristics and advantages of the present invention will become more apparent from consideration of the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein:
The present invention provides a pre-assembled, pre-tested quench outlet assembly, which may be replaced after use and which is not susceptible to blockage by deposit of frozen air components.
According to an aspect of the present invention, the quench outlet assembly is tested prior to assembly into the cryogen vessel, and is known not to leak. If air should leak into the turret cover 24, it will not be able to reach the inside of the cryogen egress tube 32. As the cryogen egress tube has been tested prior to assembly into the cryogen vessel, it is known not to leak. The only possible route for air components to reach the inside of the cryogen egress path is by descending through the access neck 20 and entering the lower end of the cryogen egress tube 32. However, the cryogen egress tube is designed to have a length such that, in normal conditions, the thermal stratification of gas within the cryogen vessel means that any air components which might enter the access neck 20 will condense and solidify on the surface of the access neck before they reach the lower end of the cryogen egress tube 32. The freezing temperature of nitrogen may appear at level 42 in a typical thermal stratification. One may therefore be sure that the cryogen egress tube 32 will not become blocked by frozen air components. Due to the increased confidence that the cryogen egress tube 32 will not become blocked, there is no need to provide a secondary cryogen egress path. This saves space and reduces build complexity.
As illustrated, the quench outlet assembly may be arranged such that escaping cryogen gas travels along an essentially vertical egress path 40, and that no elbows are required to direct the escaping cryogen to a vertical path. This further simplifies the build, and reduces the back-pressure caused in the cryogen egress path.
The quench outlet assembly is tested, and assembled into a cryogen vessel. Preferably, the quench outlet assembly is re-tested for leaks after installation in the cryogen vessel. It must remain absolutely leak tight until a quench occurs. Preferably, the burst disc(s) is/are of metal, welded to the flange 28 of the quench outlet assembly, to ensure leak-tightness. Typically, the quench outlet assembly would be fitted after magnet testing, and before the cryostat is prepared for shipping. The flange 28 may be of stainless steel, or aluminum, for example, to simplify welding. The cryogen egress path 32 may be of a composite material such as glass-fiber reinforced plastic, or stainless steel. The cryogen egress path should be of a material having a low thermal conductivity.
The burst disc is designed to burst at a differential pressure equivalent to the maximum gauge pressure tolerable within the cryogen vessel. In arrangements having a burst disc upstream of the quench valve, the quench valve itself may be set to open at a lower pressure than the burst disc. This will ensure that the quench valve opens as soon as the burst disc opens. In such arrangements, the quench valve serves principally to seal the cryogen vessel from the atmosphere once the quench is over. In such arrangements, the quench valve may be of simple, low-cost design as it does not need to provide an effective long-term seal.
As compared to the existing solution, the current lead 21 may be increased in diameter, to provide a wider cryogen egress path 40. The access neck 20 may be reduced in diameter, reducing the thermal heat load into the system. These modifications are made possible by the fact that it is not necessary to provide two cryogen egress paths, one through the current lead and one through the access neck 20.
After a quench, the whole quench path assembly may be removed and replaced. The removed assembly may be repaired or discarded. While the quench path assembly is removed, the interior of the current lead 21 may be checked for solid deposits.
In a variant of the present invention, the quench valve 26 is removable from the remainder of the quench path assembly. For example, as shown in
The present invention accordingly provides improved cryogen egress paths for cryogen egress in case of quench, in which a single egress path is provided, which is leak-tight, which is relatively simple to install and to replace. The improved egress path may be straighter, reducing back-pressure in case of quench.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0914005.4 | Aug 2009 | GB | national |
