GB2563426B

GB2563426B - Cryostat precooler interface

Info

Publication number: GB2563426B
Application number: GB1709542.3A
Authority: GB
Inventors: Alan Charles Walton Philip; Mortensen Anders; Barber David
Original assignee: Siemens Healthcare Ltd
Current assignee: Siemens Healthcare Ltd
Priority date: 2017-06-15
Filing date: 2017-06-15
Publication date: 2019-10-02
Anticipated expiration: 2037-06-15
Also published as: GB201709542D0; GB2563426A

Description

CRYOSTAT PRECOOLER INTERFACE

The present invention relates to precooling equipment for cryogen vessels. The invention may particularly be applied to precooling equipment for cryogen vessels used for cooling superconducting magnets for MRI systems.

In conventional closed-loop precooling equipment, cold cryogen heat transfer fluid is provided to the cryogen vessel, acts to cool the cryogen vessel and its contents, and is drawn from the cryogen vessel for re-cooling and recirculation. For cryogen vessels intended to be filled with liquid helium, helium gas is typically used as the cryogenic heat transfer fluid at a gauge pressure of around 1 BAR. The cryogen vessel and its contents should typically be cooled to 40K or below, in an acceptable time-frame, to allow efficient filling with liquid helium. In this way, the temperature of the cryogen vessel and its contents is lowered to near the boiling point of liquid cryogen before the cryogen vessel is filled, or partially filled, with the cryogen. This technique minimises the boil-off of liquid cryogen on filling, as the cryogen only has to cool the cryogen vessel and its contents over a very limited temperature range from 40K to 4K, in which range the heat capacity of solids is low.

In conventional closed-loop precooling equipment, cryogenic heat transfer fluid is circulated through supply- and returnhoses to and from the cryogen vessel, being cooled by a cryogen cooler external to the cryogen vessel, and circulation is provided by cryogen pump. The cryogen cooler and cryogen pump are typically combined into a single piece of equipment: a cryogenic pre-cooler.

The cryogen cooler typically has only a limited cooling power and therefore the objective of an efficient cooldown can only be achieved if heat in-leak and flow restriction are considered in the overall design of the helium gas circuit. This is conventionally achieved by means known in themselves, such as vacuum insulation, multi-layer insulation, charcoal getter materials, low thermally-conductive assemblies, and analysis by computational fluid dynamics.

Closed-loop cooling may be performed in a factory environment, or a portable arrangement may be employed, where a portable cryogenic pre-cooler is taken to a site to perform a precool, and is later moved on to a next site. The present invention may be used in all of these applications.

The present invention is a tool - an interface, and a method for its use. The tool links a cryogenic pre-cooler to a cryogen vessel by replacing a turret flange of the cryogen vessel.

An external, and preferably portable, cryogenic pre-cooler provides cooled cryogen gas to the interface of the invention, and receives warmed cryogen gas from the interface of the invention, for re-cooling and re-circulation.

Fig. 1 schematically represents a conventional arrangement 10 for pre-cooling a cryostat 12 housing a superconducting magnet (not illustrated). Cryogenic pre-cooler 14 provides cooled cryogen along a vacuum-insulated supply hose 16 to cryostat 12, and receives warmed cryogen along a vacuum-insulated return hose 18 from the cryostat 12. Cryostat 12 comprises a cryogen vessel 12a located within an outer vacuum container 12c. A thermal radiation shield 12b is located between the cryogen vessel 12a and the outer vacuum container 12c. The cryostat 12 has a turret 19 providing access to the cryogen vessel 12a for electrical conduction, and for adding and removing cryogen.

In use, thermal stratification of cryogen within the cryogen vessel means that the turret is at, or near, ambient temperature. Typically, the turret 19 is closed by a turret cover 20 which is not insulated. Couplings 22 are provided to join supply hose 16 and return hose 18 between cryogenic pre-cooler 14 and turret cover 20. Supply hose 16 and return hose 18 are typically vacuum insulated hoses.

Supply hose 16 is connected to a coupling 22 which in turn provides connection to a supply tube 24. Return hose 18 is connected to a coupling 22 which in turn provides connection to a return pipe 26. Supply pipe 24 extends vertically within the cryogen vessel, inside a positive tube 27. Positive tube 27 is typically used as a positive electrical connection for equipment such as superconducting magnet coils located within the cryogen vessel. Supply pipe 24 extends to a level vertically below a lower extremity of return pipe 26.

Return cryogen gas is drawn from an interior volume of turret cover 20. Turret cover 20 is not typically vacuum insulated, so a significant thermal influx is received through the wall of turret cover 20. The return cryogen gas could be drawn from the top or the side of the turret. The thermal influx through the turret is easily dealt with by the cooling power of a static, factory-based cryogenic cooler 14. However, a portable cryogenic cooler may not be powerful enough to cope.

To attach the arrangement 10 for precooling to the cryostat 12, supply pipe 24 and return pipe 26 are passed through corresponding openings in the turret cover 20. It is therefore necessary to have sufficient height clearance above the turret cover 20 to be able to introduce supply pipe 24 and return pipe 26. Supply pipe 24 is longer than return pipe 26 so that cooled cryogen is introduced towards the bottom of the cryostat 12, while return pipe 26 is shorter than the supply pipe, and the lower extremity of the return pipe 26 is at a location higher than the lower extremity of the supply pipe 24.

The magnet service neck 29, which is the entry point to the cryogen vessel, is exposed to the turret cover 20 which is not vacuum insulated. In use, thermal stratification of cryogen gas means that gas at the top of the turret 19 is at approximately ambient temperature, so that insulation is not required.

In such a conventional closed-loop precooling system cryogen from the cold supply hose 16 enters through uninsulated turret cover 20 and is pumped through the positive tube 27 by the cryogenic pre-cooler 14. The return cryogen gas is collected from a flange on the turret housing 20. The return cryogen gas is in direct contact with the un-insulated turret cover 20, which increases the heatload on the cryogenic precooler 14. This may be acceptable in a large, factory-based cryogenic pre-cooler, but may be intolerable in the case of a smaller portable cryogen cooler 14.

In many MRI installations, the ceiling height is insufficient to allow introduction of a conventional supply pipe 24 and return pipe 26 through corresponding openings in the turret cover 20. This means that conventional portable precooling of the cryogen vessel 12 and its contents cannot be employed in such arrangements.

It is therefore desired to enable closed-loop precooling of cryogen vessels and their contents even for cryogen vessels installed under relatively low ceiling heights.

It is also desired to provide supply and return cryogen gas connections to the cryogen vessel through the service neck 29 tube that avoid excessive heatloads associated with non-vacuum-insulated parts of the turret and also avoids excessive flow restriction of cryogen in these connections.

It must be possible to make- and unmake- connections between cryogen supply- and return- hoses 16, 18 and the cryostat 12 with a cryostat in its installed position, where there is often limited room between the turret and a ceiling.

The present invention accordingly provides methods and/or apparatus as defined in the appended claims.

The above, and further, objects, characteristics and features of the present invention will become more apparent from the following description of certain embodiments thereof, given by way of non-limiting example only, in conjunction with the appended drawings, wherein:

Fig. 1 schematically represents a conventional arrangement for closed-loop pre-cooling of a cryostat housing a superconducting magnet;

Fig. 2 schematically illustrates a closed-loop precooling equipment for precooling a cryogen vessel and its contents, according to an embodiment of the invention;

Fig. 3 schematically shows a cross-sectional representation of an interface of the present invention in use;

Fig. 4 illustrates a cross-sectional representation of an interface of the present invention, including the optional addition of a venting pipe enabling pressure relief in case of excess pressure of cryogen gas.

Fig. 2 schematically illustrates the use of an interface of the present invention as part of a closed-loop precooling arrangement. In this arrangement, a portable cryogenic precooler 14 is employed, connected by vacuum-insulated supply hose 16 and vacuum-insulated return hose 18 as described with reference to Fig. 1, to an interface 100 according to an embodiment of the present invention. The interface 100 is mechanically attached and tightly sealed on to a turret outer 19a which may be used to provide electrical connections to magnet coils in the cryogen vessel, for example, and is itself mounted atop the turret 19.

Fig. 3 schematically shows a cross-sectional representation of an interface of the present invention in use. Cryogenic pre-cooler 14, vacuum-insulated supply hose 16 and vacuum-insulated return hose 18 are as described with reference to Figs. 1, 2 but are not shown in Fig. 3 for ease of representation. In this embodiment, vacuum-insulated supply hose 16 and vacuum-insulated return hose 18 are connected to in-line venting assembly 102. This provides a coupling path 104 to a venting arrangement 106 (Fig. 2) which may, for example, comprise a pressure relief valve and/or burst disc assembly, to provide a limitation on pressure within the cryogen vessel during operation of the precooling arrangement.

In-line venting assembly 102 may connect to interface 100 by a bayonet coupling. As will be explained below, it is preferred to connect interface 100 to cryogen vessel 12 before connecting in-line venting assembly 102 to interface 100 .

The interface 100 of the present invention makes a connection between supply- and return- cryogen hoses and the cryogen vessel 12 through the service neck tube 29.

The interface 100 comprises a vacuum-insulated insert which is mounted on an assembly flange 115 that may replace a standard turret cover 20. The lengths of supply pipe 110 and return pipe(s) 112 may be selected as compromise values: shorter pipes would enable installation under more restricted ceiling height, but longer pipes may provide improved thermal properties. In preferred embodiments, the pipes have a length in the range 20cm - 80 cm.

In this embodiment, the interface 100 and the in-line venting assembly 102 include a cryogen supply path 107 insulated by a vacuum jacket 108. A similar vacuum jacket surrounds a cryogen return path 109, which in this embodiment is located behind the supply path 107. Cryogen supply path 107 joins in to supply pipe 110 and cryogen return path 108 joins in to return pipe 112. Bayonet couplings may be used to effect these joins.

Supply pipe 110 and return pipe 112 form a set of vacuum insulated pipes which extend below an assembly flange 115, which is used to mount the interface 100 in position, in place of a conventional turret cover 20.

As will be discussed in more detail below, return pipe 112 may in fact comprise a plurality of pipes connected in parallel. In such embodiments, a return pipe manifold 114 is provided, to join the plurality of return pipes 112 into a single cryogen return path 108.

The interface 100 of the present invention is vacuum insulated, that is to say, the supply pipe 110 and return pipe(s) 112 are dual-walled with an evacuated region 120 extending between the walls. The evacuated region 120 also extends within the housing 116, within which housing the cryogen supply path 107 joins the supply pipe 110, and the cryogen return path 108 joins the return pipe(s) 112. By providing vacuum insulation of the pipes, the feed and return cryogen gas flows are not warmed by direct contact with noninsulated walls.

As is common in some conventional cryostats, a positive tube 122 is provided. This tube serves as a cryogen inlet pipe, and may also serve as an electrical connection into the cryostat. For example, the positive tube 122 may serve as a positive electrical connection to a superconducting magnet within the cryogen vessel. The positive tube 122 in this embodiment is cooled by a thermal intercept 124 partway along its length, for example cooling to 50K. In this embodiment, the positive tube 122 extends into the cryogen vessel by a certain amount below an opening 126 which allows warmed cryogen gas in the cryogen vessel to reach the return pipe(s), driven by the cryogenic pump 14.

The cryogenic pre-cooler 14 (Fig. 2) circulates and cools cryogen gas in a closed circuit with the cryogen vessel via the interface of the invention. The cryogenic cooler cools the cryogen gas returned from the cryogen vessel via the interface and the cryogenic pump supplies the cooled cryogen gas back to the cryogen vessel via the interface.

The interface 100 is vacuum insulated, unlike the turret cover 20 it replaces. Vacuum-insulated cryogen supply- and return- hoses, completing the closed circuit for circulation of gas, carry cryogen gas to and from the interface, from and to the cryogenic pre-cooler.

The interface includes vacuum-insulated supply and return pipes 110, 112, which extend below the assembly flange 115. The vacuum-insulated supply and return pipes 110, 112 are used for the delivery and return of cryogen gas for precooling of the cryogen vessel and its contents.

In use, the vacuum insulated supply pipe 110 extends into a positive tube 122, itself located within a service neck 29. The positive tube is a permanent feature of the cryogen vessel and may also serve as an electrical connection. A thermal intercept 124 may be thermally linked to the service neck 29, providing active cooling in use, for example to 50K. Preferably both inlet 110 and return 112 pipes extend below the location of the thermal intercept 124. The service neck 29 may serve as an electrical connection, for example a negative connection to a superconducting magnet coil. The positive tube guides the cryogen supply gas into the cryogen vessel. The cryogen supply gas cools the cryogen vessel and its contents by a heat exchange process. By providing the incoming cold cryogen through the positive pipe 122, it is directed to the lower part of the cryogen vessel. Return cryogen through return pipe(s) 112 is drawn from a region of warmer cryogen.

In use, the vacuum insulated return pipe 112 extends into the service neck 29, outside of the positive tube 122. The vacuum insulated return pipe 112 may have a kidney-shaped cross section to extend within the service neck, outside of the positive tube 112. This may be achieved by using a single tube of kidney-shaped cross-section, a single tube of circular cross-section or a plurality of smaller tubes of circular cross-section, which fit within the available kidney-shaped opening and maintain low flow resistance within the return pipes despite their smaller respective diameter, by providing parallel paths in the respective tubes. Where multiple return pipes 112 are used, they may be brought together at a manifold 114 for connection to the return cryogen path 109. While one supply pipe 110 and three return pipes 112 provide an effective fit for certain embodiments, the number and size of each type of pipe may be selected as most appropriate for any desired installation.

The open ends of inlet pipe 110 and outlet pipe 112 should be kept separated to avoid direct flow of cryogen between inlet pipe 110 and return pipe 112. Longer inlet pipe 110 and outlet pipe 112 may be beneficial from the point of view of thermal efficiency, but the inlet pipe 110 and outlet pipe 112 should in some cases be kept short to deal with any height restriction due to limited ceiling height.

In a preferred embodiment, the supply pipe 110 and the return pipe(s) 112 extend to below the thermal intercept 124 linked to the positive tube 122. The vacuum-insulated supply pipe 110 and the return pipe(s) 112 should extend sufficiently into the cryogen vessel that advantage may be taken of thermal stratification outside of the positive tube to ensure that warmer cryogen gas remains within the cryogen vessel. Return cryogen gas is directed into the return pipe(s) 112, directly from the cryostat volume and is driven by the flow of cryogen gas provided by the cryogenic pre-cooler. Since, according to the invention, a non-insulated turret cover 20 is replaced by a vacuum-insulated interface 100, excessive thermal load on the cryogen gas near the supply 110 and return pipe(s) 112 is avoided. Turbulence in the cryogen gas in the service neck 29 should be avoided, as turbulence would bring cold cryogen into contact with the uninsulated turret outer. Turbulence may be reduced by slowing the speed of cryogen gas provided that the mass-flow rate of cryogenic heat transfer fluid remains sufficient for efficient heat transfer between cryogen vessel 12 and the cryogenic precooler 14. A lower volume flow rate produces less turbulence and may give a lower cooling power. A very low volume flow rate may provide very little cooling effect. Since colder cryogen has a higher density, it may be possible to maintain a sufficient mass-flow rate even though the volume flow rate is reduced, when the cryogen vessel is cooled to cryogenic temperatures .

In certain embodiments, inlet pipe 110 should be a fairly tight fit into the positive pipe 122. In an alternative preferred embodiment, a loose plate 131 may be provided around the inlet pipe 110, which blocks a gas path between the inlet pipe 110 and positive tube 122, for reducing flow of cryogen gas from the inlet pipe up inside the positive tube. Such a flow would have the effect of "short-circuiting" the cooling of the cryogen vessel by allowing cold cryogen from inlet pipe 110 to reach return pipe 112 without cooling the cryogen vessel and its contents. The loose plate 131 is biased onto an upper extremity of the positive tube 122 by a spring, or by gravity. Where loose plate 131 is provided, the inlet pipe 110 need not be a tight fit into the positive tube 122.

The vacuum-insulated return pipe(s) 112 are situated outside of the positive tube 122, and collect cryogen gas from the cryogen vessel. The flow from the inlet pipe 110 to the outlet pipe(s) 112 through the interior volume of the cryogen vessel is driven by a pressure differential that is provided by the cryogen pump.

The vacuum insulated supply pipe 110 and the vacuum insulated return pipe 112 may be shorter than the positive tube 122. They should reach far enough in that the cryogen gas is supplied and returned with a tolerable amount of heatload from the turret outer, for example 10W.

It is not necessary for the supply pipe 110 to extend beneath the lower extremity of the return pipe 112, as the positive tube 122 provides constriction of flow of cryogen gas around the supply pipe 110. It is therefore possible to provide an interface 100 with a supply pipe 110 which is remarkably shorter than the supply pipe 24 in conventional arrangements. In turn, this means that it is possible to install and remove the interface 100 under a much lower ceiling height than was possible using the supply pipe 24 in conventional arrangements. Positive tube 122 directs incoming cryogen deep into the cryogen vessel, while return pipe 112 draws return cryogen from an upper region of service neck 29, which cryogen is warmer due to heat exchange with the cryogen vessel and its contents.

Installation of an arrangement as shown in Fig. 3 proceeds according to the following steps. The turret cover is removed. The interface 100 is installed onto the turret 20, then the in-line venting assembly 102 is mounted to the interface. This allows use of a vertically-extending coupling path 104 and a vertically extending inlet- and outlet- pipes 110, 112 despite limited ceiling height. A vertically-extending coupling path retains a stratification such that the cryogen gas at the burst disc and/or pressure relief valve is at approximately ambient temperature, limiting thermal influx through the burst disc and/or pressure relief valve.

Fig. 4 illustrates an alternative embodiment of the present invention. In this embodiment, a venting pipe 150, is tee-d into the vacuum insulated supply pipe 107. The venting pipe 150 is also surrounded by a vacuum insulation arrangement 152. The venting pipe 150 is closed by a pressure relief valve and/or burst disc assembly (conventional in itself and not represented in Fig. 4), to provide a safety venting path close to the magnet cryostat. In such an embodiment, it is not necessary to provide in-line venting assembly 102 (Fig. 3) . Such embodiments may be found appropriate for use in a factory environment, for example, where ceiling height is not restricted. A burst disc and/or pressure relief valve should be used close to the venting pipe 150.

The return cryogen hose 18 may connect to the cryogen return path 109 by a bayonet coupling.

The vacuum insulation in region 120 may be provided by active pumping using a vacuum pump or, if sufficiently sealed, may rely on a cryopumping effect from the cryogen pipes. Activated charcoal may be used as a getter in the region 120, and/or MLI may be placed round the gas paths.

The invention provides an interface 100 comprising a vacuum insulated housing 116 which interfaces a portable cryogenic pre-cooler 14 to a cryogen vessel 12 to enable precooling by circulation of cryogen. The vacuum insulated housing is provided with vacuum insulated supply and return pipes 110, 112 to provide a vacuum insulated cryogen path into the cryogen vessel. The interface is so designed that it may be installed and removed despite restricted vertical access to the cryogen vessel 12. Use of the present invention provides a fully vacuum insulated path from cryogenic pre-cooler to the interior of the cryogen vessel.

By introducing a supply of cryogen into the positive tube 122, the cold cryogen is introduced further down inside the cryogen vessel than would be provided by the length of supply pipe 110 alone. This leads to a more effective cooling of the cryogen vessel and its contents. By locating the return pipes 112 outside of the positive tube 122, the cryogen gas introduced by the supply pipe 110 will not directly mix with return gas which is collected into the return pipe(s) 112. The return gas which is collected into the return pipe(s) 112 is effectively collected at its warmest point due to thermal stratification in the cryogen gas. The use of multiple parallel return pipes 112 may be easier to produce than a single return pipe of kidney cross-section. The flow restriction in the return cryogen path is reduced by a large free cross-sectional area in the return pipes 112, and the non-circular geometry of the service neck 29 outside of the positive tube 122 is accommodated.

Preferably, vacuum-insulated supply pipe 110 and return pipes 112 are arranged vertically, which utilises gas stratification to reduce thermal influx from the turret housing, since warmer cryogen gas, which has absorbed heat from the outer turret housing will remain in the turret.

The shape and dimensions of the interface 100 of the present invention enable the interface to be introduced horizontally to a position over the turret of the cryogen vessel, then lowered into the operational position, even in case of restricted ceiling height. Then, in-line venting assembly 102 may be offered horizontally into coupling with the interface 100. This is important as the vent coupling path must extend vertically above the inlet path 107, such that thermal stratification ensures that thermal influx through the vent coupling path is minimised. A constrained ceiling height might impede installation if interface 100 and in-line venting assembly 102 were formed as a single piece.

As compared to known arrangements, the interface of the present invention allows return gas to be collected closer to the cryogen vessel, which offers improved safety in that excess pressure in the cryogen vessel may be vented without having to pass through the flow restriction provided by cryogen hoses 16, 18. Pressure relief valves and/or burst discs may be provided both at the turret and at the cryogenic cooler .

The interface 100 of the present invention may be constructed of stainless steel, or another material of relatively low thermal conduction. The material used must be thick enough to stand a differential pressure of a few BAR, and robust enough to handle without risk of damage. In some embodiments, vacuum-insulated supply pipe 110 and vacuum-insulated return pipe 112 may be flexible vacuum-insulated pipes, conventional in themselves.

The interface of the present invention may be used in filling the cryogen vessel with liquid cryogen, in addition to its function in the precool process.

Multiple interface 100 tools may be used in series or in parallel to cool or fill multiple cryogen vessels.

In alternative embodiments of the invention, where a cryogen vessel has two service necks 29, the interface 100 of the invention may be presented as two separate interfaces, each comprising an assembly flange as described, carrying one of the inlet pipe (110) and the return pipe (112) .

Claims

CLAIMS :

1. An interface comprising an assembly flange (115) provided with a vacuum insulated housing (116), for mounting to a turret (19) of a cryogen vessel in place of a turret cover (20), comprising a vacuum-insulated inlet pipe (110) and a vacuum insulated return pipe (112), said vacuum-insulated inlet pipe (110) and said vacuum insulated return pipe (112) protruding into the turret of the cryogen vessel when the assembly flange is mounted onto the turret, the interface further comprising a cryogen supply path (107) joined to the inlet pipe (110) and a cryogen return path (108) joined to the return pipe (112) .

2. An interface according to claim 1 wherein the vacuum-insulated inlet pipe (110) and vacuum insulated return pipe (112) are dual-walled, with an evacuated region (120) also extending within the housing (116).

3. An interface according to claim 1 or claim 2, wherein the vacuum insulated return pipe (112) comprises a plurality of pipes in parallel, with a return pipe manifold (114) provided to join the plurality of return pipes (112) to a single cryogen return path (108).

4. A precooling arrangement comprising a cryogen vessel (12) provided with an interface according to any preceding claim sealed to the cryogen vessel, wherein the vacuum-insulated inlet pipe (110) and the vacuum insulated return pipe (112) are connected for circulation of cryogen into, and out of the cryogen vessel.

5. A precooling arrangement according to claim 4 wherein the cryogen vessel (12) is provided with a vertically- oriented positive tube (122), and the vacuum-insulated inlet pipe (110) protrudes into the positive tube (122), while the vacuum insulated return pipe (112) does not protrude into the positive tube (122) .

6. A precooling arrangement according to claim 5 wherein the cryogen vessel comprises a thermal intercept (124) in thermal contact with the positive tube (112), and the vacuum-insulated inlet pipe (110) protrudes vertically down into the positive tube (112) beyond the thermal intercept (124) .

7. A precooling arrangement according to claim 5 or claim 6 wherein a loose plate (131) is provided around the vacuum-insulated inlet pipe (110), the loose plate (131) being biased onto an upper extremity of the positive tube (122) by a spring, or by gravity.

8. A precooling arrangement according to any of claims 4 to 7, wherein the vacuum-insulated inlet pipe (110) and the vacuum insulated return pipe (112) each protrude into the cryogen vessel by distance of 20 cm - 80 cm.

9. A precooling arrangement according to any of claims 4 to 8, further comprising an in-line venting assembly (102) connected to the interface.

10. A precooling arrangement according to any of claims 4 to 9, wherein the cryogen vessel has two service necks (29) and wherein the interface (100) is presented as two separate interfaces, each comprising an assembly flange as described, carrying one of the inlet pipe (110) and the return pipe (112) .

11. A method for precooling a cryogen vessel (12) comprising the steps of: - removing a turret cover (20) of the cryogen vessel, - mounting an interface (100) according to claim 1 in place of the turret cover, - connecting the vacuum-insulated inlet pipe (110) and the vacuum insulated return pipe (112) for circulation of cryogen into, and out of the cryogen vessel; - connecting a supply hose (16) between a cryogenic precooler (14) and the interface; - connecting a return hose (18) between the cryogenic precooler (14) and the interface; and - operating the cryogenic pre-cooler (14) to cause cryogen to circulate in a closed loop, the closed loop comprising the cryogenic pre-cooler (14), the supply hose (16) and the return hose (18) and the cryogen vessel.

12. A method for precooling a cryogen vessel (12) according to claim 11 further comprising the step of: connecting an in-line venting assembly (102) to the interface, after the interface is mounted in place of the turret cover; - connecting a supply hose (16) between a cryogenic precooler (14) and the in-line venting assembly (102); and - connecting a return hose (18) between the cryogenic precooler (14) and the in-line venting assembly (102).