Patent Publication Number: US-9897379-B2

Title: Shaft furnace charging device equipped with a cooling system and annular swivel joint therefore

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/389,483 filed on 9 Feb. 2012 which is the U.S. National Phase of International Application Number PCT/EP2010/062494 filed on 26 Aug. 2010 which claims priority to Luxembourg Patent Application Number 91601 filed on 26 Aug. 2009, all of which said applications are herein incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to a rotary charging device for charging a metallurgical reactor, in particular a shaft furnace, such as a metallurgical blast furnace. Such a charging device usually comprises a suspension rotor with a charge distributor, typically a pivotable distribution chute, and a stationary housing supporting the suspension rotor so that the rotor—and therewith the distributor—can rotate about an axis, which is typically the furnace central axis. The present invention relates more particularly to a cooling system configured to warrant cooling on the suspension rotor using an annular swivel joint for coupling a stationary portion of the cooling system to a rotary portion that is arranged on the suspension rotor. The invention also relates to the proposed annular swivel joint itself (per se). 
     BRIEF DISCUSSION OF RELATED ART 
     It is well known in the art that cooling the suspension rotor, which is exposed to high internal furnace temperatures, by means of liquid coolant is most effective in extending the service life of mechanical components, has a lower initial investment cost and is less energy-consuming, when compared to pure inert gas cooling as suggested e.g. in Japanese patent application JP 55 021 577. 
     Therefore, as early as 1978, PAUL WURTH proposed water cooling of the charging device of a BELL LESS TOP® installation, as described in detail in U.S. Pat. No. 4,273,492 (see FIG. 8 of this patent). In this device, a lower screen, which protects against radiant heat from inside the furnace, has an associated cooling circuit, which is supplied with liquid coolant via an annular swivel joint arranged coaxially around the central feed channel above the distribution chute. This joint comprises a rotating and a fixed part, which are generally annular i.e. ring-shaped. The rotary part is an extension of the suspension rotor and forms an integral part thereof that extends above the housing. The fixed part is fastened to the housing with a clearance coaxially around the rotary part. Two cylindrical roller bearings centre the rotary part in the fixed part. The fixed part comprises two annular grooves, one above the other, which face ports in the external cylindrical surface of the rotary part to define connection passages for coolant. Watertight seal packings or gaskets have to be mounted to both sides of each groove in between the fixed and rotary parts. In practice a revolving fluid joint of this kind has not proven successful. Indeed, the watertight seals as suggested in U.S. Pat. No. 4,273,492 deteriorate rapidly, among others because they are in contact with a very hot moving part. Moreover, due to the relatively large diameter of the revolving joint and consequently of the watertight seals, considerable friction is inevitable. This limits the service-life of the seals and, besides, also increases required driving power for driving the rotor. Accordingly, a rotating joint of the type described in U.S. Pat. No. 4,273,492 has not proven practically viable for feeding a cooling circuit portion on the suspension rotor. 
     Therefore, in 1982, PAUL WURTH proposed a cooling system with a revolving joint that works without any watertight seal packings or gaskets. This cooling system, as described in U.S. Pat. No. 4,526,536, now equips numerous blast furnace charging devices throughout the world. It includes an upper annular trough, i.e. a narrow upwardly open receptacle, which is mounted on an upper sleeve of the suspension rotor to rotate therewith. The stationary circuit portion has one or more ports above the upper trough for feeding the latter by gravity. The upper trough is connected to a number of cooling coils installed on the suspension rotor. These coils have outlet pipes discharging into a lower annular fixed trough that is mounted on the bottom of the housing. Cooling water therefore flows from a non-rotating supply into the rotary upper trough of the suspension rotor, then passes purely by gravity trough the cooling coils on the rotor, and from there into the fixed lower trough from where it is discharged. Whilst having the major benefit of avoiding wear-prone watertight seals, a first disadvantage of this cooling system is that pressure available to force cooling water through the cooling coils on the suspension rotor is limited by the difference in height between the upper and lower troughs, which height in turn is inherently limited by constructional constraints. The suspension rotor must therefore be fitted with low-loss cooling coils, which is a considerable disadvantage in terms of cost, occupied space and/or cooling efficiency. A second disadvantage is that dust-laden gases from the blast furnace come into contact with the cooling water in both troughs so that dust inevitably passes into the cooling water. A particular problem is caused by the resulting sludge formed in the upper trough, because the latter passes through the cooling coils of the suspension rotor and may cause blocking i.e. plugging of the coils. 
     To achieve higher cooling capacity, German patent application DE 33 42 572 proposes to fit the rotary circuit portions on the rotor with an auxiliary pump. This auxiliary pump on the suspension rotor is driven by a mechanism which takes advantage of the rotation of the rotor to drive the pump. It follows that the pump only works when the rotor is rotating. Moreover, such a pump is rather sensitive to sludge passing through the cooling coils on the rotor. 
     International patent application WO 99/28510 by PAUL WURTH presents a method for operating a cooling system fitted with an annular swivel joint. Contrary to previous principles, no attempt is made to ensure that the joint is watertight, as proposed by U.S. Pat. No. 4,273,492 for example, nor to avoid coolant loss from the joint by means of level controls, as specified in U.S. Pat. No. 4,526,536. Instead, a supply of liquid coolant is provided to the annular swivel joint in such a way that a leakage flow passes through annular separation apertures between the rotating and fixed parts of the joint. This leakage flow forms a “liquid seal”, which prevents dust penetrating into the joint. The leakage flow is then collected and drained, without passing through the rotary portion of the circuit. Accordingly, dust-laden sludge no longer passes through the rotary circuit portion so that the risk of clogging is eliminated. WO 99/28510 proposes a number of embodiments for putting into practice the suggested method. Each embodiment comprises an annular fixed part mounted on the stationary housing and an annular rotary part mounted on the suspension rotor. The parts have mating configurations that allow relative rotation. The rotary part, similar to the teaching of U.S. Pat. No. 4,526,536, includes an annular trough that defines an annular volume, via which the stationary and rotary circuit portions are in fluidal communication. The leakage flow passes through annular separation apertures between sidewalls of the trough and sidewalls of an insert that protrudes into the trough and belongs to the fixed part. A first drawback of this system is the loss of cooling water through the “liquid seal”, which requires constant topping-up. Furthermore, the system and method proposed in WO 99/28510 still comes with a lower collecting trough (see FIG. 1 of WO 99/28510), similar to that proposed in U.S. Pat. No. 4,526,536, and thus involves additional dust contamination at this level. The lost water fraction and the fraction recovered from the lower trough thus both require treatment before reuse. 
     International patent application WO 03/002770 by PAUL WURTH presents a further configuration of an annular swivel joint. This joint partially reverts to the initial principles of 1978 since it does not use open collecting troughs connecting the stationary and rotary circuit portions and thereby prevents dust contamination. It comprises a ring-shaped fixed part mounted to the housing and a ring-shaped rotary part rotating with the suspension rotor. The fixed and rotary parts together form a cylindrical interface in which one or more annular grooves allow transferring pressurized liquid coolant between the fixed and rotating rings. To this effect, watertight seals are provided in between the grooves and between the grooves and the open ends of the interface. The rotary part is supported in floating manner solely on the fixed part by means of roller bearings. Selective mechanical coupling means connect the ring-shaped rotary part with the suspension rotor so as to transmit only rotational torque, while at the same time preventing other forces from being transmitted from the rotor to the rotary ring. Liquid coolant is transferred from the rotary part to the circuit portion on the suspension rotor by means of a deformable flexible connection. In the design of WO 03/002770, as opposed to that of U.S. Pat. No. 4,273,492, the rotary ring is supported by the fixed ring. Therefore, the joint in general, and the watertight seals more specifically, are less subject to problems of excessive friction and hence of short service-life. Whilst having the advantages of allowing pressurized forced circulation through cooling coils on the rotor and of significantly increasing the seal service-live, watertight seals arranged between the fixed and rotary ring-shaped parts are still required. Even though subjected to reduced strain, these seals will unavoidably wear-off so that a costly replacement operation is inevitable. 
     International patent application WO 2007/071469 by PAUL WURTH proposes another joint design for a cooling system as generally set out above. In the latter design, a heat transfer device includes a stationary part configured to be cooled by a cooling fluid flowing through a stationary cooling circuit and a rotary part configured to be heated by separate cooling fluid circulated in the rotary cooling circuit. The parts are arranged in facing relationship and have there between a heat transfer region for achieving heat transfer through the heat transfer region without mixing of the separate cooling fluids in the rotary and stationary circuits. Accordingly, this revolving coupling is not a true fluidal swivel joint but rather a purely thermal coupling. Whilst a thermal coupling according to WO 2007/071469 eliminates both the need for watertight seals and the risk of dust contamination altogether, one drawback of this coupling is that it requires a certain size of facing surfaces forming the heat transfer region in order to warrant a given thermal coupling capacity. In practice, when compared to fluidal swivel joints, this design thus requires more constructional space in case of high thermal loads, e.g. with large diameter blast furnaces. Moreover, means for forced circulation on the suspension rotor, e.g. a pump as disclosed in DE 33 42 572, are required when using conventional cooling coils on the rotor. 
     In conclusion, although a variety of approaches are known today, the prior art still leaves room for improving the swivel joint required to couple the fixed portion of the cooling system to the rotating portion. 
     BRIEF SUMMARY 
     The invention provides an improved cooling system for a shaft furnace charging device and more specifically, an improved annular swivel joint therefore, which eliminates the need of using fluid tight seals while at the same time enabling a pressurized forced circulation of cooling fluid through the rotary part of the cooling system. 
     The present invention generally relates to a cooling system in a charging device for a metallurgical reactor such as a shaft furnace, especially a blast furnace. The device comprises, in typical manner, a suspension rotor with a charge distributor, e.g. a pivotable chute, and a stationary housing, which supports the suspension rotor so that the latter is rotatable about an axis. 
     The cooling system comprises a stationary circuit portion, which remains at rest with the housing and a rotary circuit portion that is arranged on the suspension rotor to rotate with the latter. Furthermore, the cooling system comprises an annular swivel joint, which is arranged coaxially on the rotation axis and connects the stationary circuit portion with the rotary circuit portion. In the present context, the expression “swivel joint” refers to a fluid-communicating connector that permits full rotations between the connected circuit portions. In a manner known per se, e.g. from patent application WO 99/28510, the fluidal/hydraulic swivel joint comprises a fixed part supported by the housing and a rotary part mounted on the suspension rotor. The parts have conjugated configurations that allow relative rotation and either one of them includes an annular trough that defines an annular volume, through which cooling fluid can pass from one circuit portion to the other. 
     According to the disclosure, the proposed fluidal/hydraulic swivel joint presents the following main features:
         at least four connections, including a pair of a forward and a return connection to the stationary circuit portion, and a pair of a forward and a return connection to the rotary circuit portion;   a partition structure that divides the volume inside the annular trough into an annular external cavity and an annular internal cavity in such a way that the internal cavity is at least partly surrounded by the external cavity and so that the forward path passes through the internal cavity and the return path passes through the external cavity or vice-versa;   two flow restrictors, each arranged in one of two clearances, through which the two separate cavities communicate and which are provided between the fixed and rotary parts of the joint to allow relative rotation.       

     As will be appreciated, the proposed fluidal/hydraulic swivel joint is configured so that cooling fluid can circulate in forced circulation from the stationary circuit portion, through one of the first and the second cavities, to the rotary circuit portion and, through the other of the first and the second cavities, back to the stationary circuit portion. 
     While providing dual coupling of both the forward and return paths and even as it enables forced circulation, the proposed swivel joint is not based on a side-by-side arrangement to achieve the dual coupling nor does it require liquid-tight seals to enable forced circulation through the rotary circuit portion. In fact, both rotary-stationary interfaces on the forward side and on the return side are configured as open connections devoid of liquid-tight seals. More notably however, by virtue of the partition structure according to the invention, the proposed joint integrates one of both open connections to its counterpart i.e. “inside” the other open connection. Thereby, the circuit is truly “open” to the ambient atmosphere only at one of both connections, i.e. at one specific pressure potential of the circuit. Having a circuit open only at one specific pressure potential, the system can provide forced circulation through any kind of rotary circuit, even high-pressure loss circuits, without the need for any wear-prone liquid-tight seal. All that is required is maintaining a pressure differential between the cavities. To this effect, any suitable kind of flow restrictors can be used, such as non-contact labyrinth seals. As another benefit compared to the widespread design of U.S. Pat. No. 4,526,536 it will be noted that the need for a lower collecting trough is eliminated, where most of the dust contamination of the coolant water occurs in the conventional prior art design. Accordingly, construction of the charging device itself can be simplified and, moreover, hitherto provided filtering devices may become unnecessary. This is achieved because the proposed swivel joint functions as a dual coupling of for both paths, i.e. forward and return, and—by virtue of its configuration—it has much less exposed water surface compared to a conventional design according to U.S. Pat. No. 4,526,536. 
     The present invention also relates to the annular fluidal/hydraulic annular swivel joint itself (per se), for use as a retrofitting component in existing charging devices or for newly equipping other kinds of metallurgical installations or metallurgical reactors, in which cooling of a rotating part of the installation is required. The proposed swivel joint can be used e.g. in the cooling system of the rabbling arms of a multiple hearth furnace. The swivel joint may, of course, also have any of the preferred features set out below when used independent of a shaft furnace charging device. 
     In a preferred configuration, each of the first and second flow restrictors is respectively configured as non-contact labyrinth seal. In a simple construction, the partition is a multi-part structure that preferably comprises an annular stationary partition member supported by the stationary housing and an annular rotary partition member supported by the suspension rotor. The internal cavity and the clearances can then defined in between and by the shape of the stationary and rotary partition members. To achieve symmetrical pressure drop through both restrictors, the stationary and rotary partition members are advantageously configured generally mirror-symmetric with respect to a vertical bisecting axis, when seen in vertical cross-section. Similarly, the annular first clearance and the annular second clearance are beneficially generally mirror-symmetric with respect to a vertical axis with the annular first flow restrictor being a non-contact labyrinth seal arranged radially outward and the annular second flow restrictor being a non-contact labyrinth seal arranged radially inward. In order to provide substantially equal pressure drop, the difference in radius between the flow restrictors is preferably taken into account and may be compensated e.g. by a difference in effective flow restrictor length. 
     In a preferred and relatively simple construction of the swivel joint, the rotary part comprises the annular trough, which is mounted on or partially formed by the suspension rotor coaxially on the axis and is preferably of generally U-shaped cross-section; and the fixed part comprises an annular hood, which is mounted on the stationary housing so as to protrude at least partially into the trough and is preferably of generally inverted U-shaped cross-section. In this construction, the trough and the hood are preferably also configured mirror-symmetric with respect to a vertical bisecting axis. 
     In a particularly preferred embodiment, the stationary partition comprises a hood-shaped ring assembly, preferably of generally inverted U-shaped cross-section, that is arranged inside the hood of the stationary part and has a radially inner side and a radially outer side. In this embodiment, the rotary partition comprises at least one Teflon ring arranged to protrude into the ring assembly, the Teflon ring having a radially inner face and a radially outer face that cooperate with the radially inner side and the radially outer side of the ring assembly so as to provide the first and second clearance there between respectively and so as to form the first and second flow restrictors in the clearances respectively. Teflon is preferred because of its resistance to heat and wetting and its wear-resistance (self-lubricating). In order to easily achieve a certain effective length of the flow restrictors, the swivel joint preferably comprises a plurality of stacked Teflon rings, each having a cross-section of a truncated wedge shape and/or corrugated inner and outer faces so as to form comparatively long first and second flow restrictors, e.g. of the labyrinth seal type. 
     When using a hood-and-trough configuration, the hood and the trough preferably each have annular inner and outer sidewalls, the sidewalls of the hood being separated from the sidewalls of the trough by narrow substantially vertical gaps, which communicate freely through the external cavity. This configuration minimizes the exposed water surface while also enabling an inherent venting function with an appropriate forward/return connection scheme. To enhance venting through the substantially vertical gaps, the vertical gaps preferably communicate with the external cavity via transverse apertures provided in the sidewalls of the hood or in between the annular hood and the stationary partition member. 
     In a simple manner of connecting the pairs of forward and return connections, the stationary partition member comprises an upper plate, at which one of the stationary forward and the stationary return connections is provided, whereas the annular hood comprising a top plate, at which the other of the stationary forward and the stationary return connections is provided. Furthermore, the rotary partition member comprises a lower plate, at which one of the rotary forward and the rotary return connections is provided, the annular trough comprising a bottom plate, at which the other of the rotary forward and the rotary return connections is provided. In this configuration the external cavity preferably has an upper portion located between the upper plate and the top plate and a lower portion located between the lower plate and the bottom plate. 
     Irrespective of the connecting scheme used, the external cavity preferably substantially surrounds the internal cavity. Accordingly, the external cavity beneficially comprises an upper portion arranged above the internal cavity and a lower portion arranged below the internal cavity, both portions communicating, e.g. through the lateral gaps mentioned hereinabove. 
     As additional enhancements, the fixed part may comprise a coolant level detection device that is connected to control a replenishing valve in the stationary circuit portion. Similarly, the fixed part preferably comprises a venting device for venting any gas inclusions, e.g. from the external cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  is a partial vertical cross-sectional view of a charging device equipped with a cooling system and with an annular swivel joint according to a first embodiment; 
         FIG. 2  is a schematic diagram of a simple first variant of a cooling system for use with the device of  FIG. 1 ; 
         FIG. 3  is a view composed of a schematic diagram of a second variant of a cooling system for use with the device of  FIG. 1 , including a venting device as shown in  FIG. 9 , and an enlarged schematic vertical cross-sectional view of the annular swivel joint of  FIG. 1 ; 
         FIG. 4  is a perspective vertical section of the annular swivel joint of  FIG. 1 ; 
         FIG. 5A  is a top view of a second embodiment of an annular swivel joint; 
         FIG. 5B  is a bottom view of a second embodiment of an annular swivel joint; 
         FIG. 6A  is a vertical cross-sectional view of the second embodiment of an annular swivel joint according to lines A-A of  FIG. 5A ; 
         FIG. 6B  is a vertical cross-sectional view of the second embodiment of an annular swivel joint according to lines B-B of  FIG. 5A ; 
         FIG. 6C  is a vertical cross-sectional view of the second embodiment of an annular swivel joint according to lines C-C of  FIG. 5B ; 
         FIG. 6D  is a vertical cross-sectional view of the second embodiment of an annular swivel joint according to lines D-D of  FIG. 5B ; 
         FIG. 7  is a perspective vertical section of the annular swivel joint of  FIGS. 6A-C ; 
         FIG. 8  is vertical cross-sectional view of an annular swivel joint according to  FIGS. 1-4  illustrating a first embodiment of a venting device; 
         FIG. 9  is vertical cross-sectional view of an annular swivel joint according to  FIGS. 1-4  illustrating a second embodiment of a venting device; 
         FIG. 10  is a vertical cross-sectional view of an annular swivel joint according to a third embodiment, which corresponds to a view taken along coinciding lines A-A and C-C of  FIGS. 5A-B ; 
         FIG. 11  is a vertical cross-sectional view of an annular swivel joint according to a third embodiment, which corresponds to a view taken along coinciding lines B-B and D-D of  FIGS. 5A-B ; 
         FIG. 12  is a vertical cross-sectional view of an annular swivel joint according to a fourth embodiment, which corresponds to a rotational position with coinciding lines B-B and D-D in  FIGS. 5A-B . 
     
    
    
     Identical reference signs or reference signs with incremented hundreds digits are used to identify similar or identical parts throughout the drawings. 
     DETAILED DESCRIPTION 
       FIG. 1  partially illustrates a shaft-furnace-charging device, generally identified by reference numeral  10 . The charging device  10  is configured for distributing bulk charge material (burden) in targeted manner into a blast furnace. The rotary charging device  10  is equipped with a cooling system  12 , illustrated in  FIGS. 2-3 , for cooling components of the device  10  that are heated by the process temperature inside the furnace. In the charging device  10 , a rotatable structure, hereinafter called suspension rotor  14  supports a distribution chute  16 . The distribution chute  16  is attached to the suspension rotor  14  by means of a mechanism configured for varying the tilt angle of the chute  16  about a horizontal axis. The rotary charging device  10  further comprises a stationary housing  18  within which the suspension rotor  14  is supported. The stationary housing  18  comprises a fixed tubular central feed channel  20 , which is arranged coaxially on the central axis A of the furnace. During the charging procedure, in a manner known per se, bulk material is fed via the feed channel  20 , through the stationary housing  18  and the suspension rotor  14 , onto the distribution chute  16 . The distribution chute  16  distributes charge material radially and circumferentially inside the furnace according to its inclination and rotation. 
     Except for the cooling system  12 , the configuration of the charging device  10  may be of a well-known type. Various well-known components of the charging device  10 , such as drive and gear components, are not shown in  FIG. 1 . These are described in more detail e.g. in U.S. Pat. No. 3,880,302. As seen in  FIG. 1 , the suspension rotor  14  is supported on the stationary housing  18  by means of an annular bearing  22  so as to be rotatable about axis A. The suspension rotor  14  has an essentially annular configuration with a central passage for bulk material in prolongation of the central feed channel  20 . It comprises a cylindrical inner wall portion  24  adjacent the central feed channel  20 , a lower flange portion  26  for supporting the chute  16  and protecting the drive and gear components and an upper flange portion  28 , which is mounted to the bearing  22 . The stationary housing  18  and the suspension rotor  14  constitute the casing of the rotary charging device  10  that typically forms the top closure on the throat of a blast furnace (not shown in  FIG. 1 ). 
     The cooling system  12  comprises a cooling circuit with a rotary circuit portion  30  fixed on the suspension rotor  14  and a stationary circuit portion  32 , which is best seen in  FIGS. 2-3 , that remains immobile with the stationary housing  18 . During operation, the rotary circuit portion  30  rotates with the suspension rotor  14  whereas the stationary circuit portion  32  remains immobile with the housing  18 . The rotary circuit portion  30  comprises any suitable heat exchanger, e.g. a heat exchanger comprising several cooling pipe coils, e.g. two coils  34 ,  36  as shown in  FIG. 1 , that are arranged on the suspension rotor  14 . The coils  34 ,  36  are in thermal contact with the inner wall portion  24  and the lower flange portion  26 , on their inside in order to cool parts of the charging device  10 , which are most exposed to the furnace heat. In addition, the rotary circuit portion  30  also provides cooling of the drive and gear components (not shown) provided for rotating and pivoting the chute  16 . Although not shown in  FIGS. 1-3 , the rotary circuit portion  30  may comprise additional cooling pipes/coils, e.g. for cooling the distribution chute  16  itself, as disclosed e.g. in U.S. Pat. No. 5,252,063, or any other suitable kind of heat exchanger configuration. 
     As will be understood, during operation, the cooling system  12  carries away heat collected by the rotary circuit portion  30  via the stationary circuit portion  32 . To this effect, as seen in  FIGS. 1-3 , the cooling system  12  comprises a heat exchanger  38  and a circulation pump  40 , which are part of the stationary circuit portion  32 . As further seen in  FIGS. 2-3 , the stationary circuit portion  32  further comprises a replenishing valve  42  connecting a replenishing conduit, fed e.g. by a public main or local water supply, to the stationary circuit portion  32  for initial filling and for topping up. Liquid coolant, especially water, possibly distilled water, is preferred, although use of other cooling fluids, including gases is possible. In the variant of  FIG. 3 , the stationary circuit portion  32  further comprises a vent tank  44  for use in combination with the venting device of  FIG. 9 , which allows for venting the circuits  30 ,  32 . 
     As will be appreciated, the cooling system  12  is configured to achieve forced circulation of coolant from the stationary circuit portion  32  to the rotary circuit portion  30  and vice-versa, while the latter portion  30  rotates relative to the former portion  32 . To this effect, the cooling system  12  includes an annular swivel joint  100 , which fluidally couples both circuit portions  30 ,  32  as schematically seen in  FIGS. 1-3 . As seen in  FIG. 1 , the annular swivel joint  100  is provided in an upper portion of the stationary housing  18 , e.g. on the upper flange portion  28  and underneath the top plate of the housing  18 , other locations being possible. The swivel joint  100  is of generally annular configuration and arranged coaxially on axis A, e.g. so as to surround the feed channel  20  as seen in  FIG. 1 . 
     As shown in  FIGS. 2-3 , the fluidal swivel joint  100  according to the invention comprises a stationary forward connection  102  (stationary inlet), through which it receives coolant from the stationary circuit portion  32 , and a rotary forward connection  104  (rotary inlet), through which it supplies coolant to the rotary circuit portion  30 . Moreover, the fluidal swivel joint  100  includes a rotary return connection  106  (rotary outlet), through which it receives coolant from the rotary circuit portion  30 , and a stationary return connection  108  (rotary outlet), through which it returns coolant to the stationary circuit portion  32 . Accordingly, the single fluidal swivel joint  100  serves as dual coupling in both forward (inlet) and return (outlet) directions. As will be understood, the fluidal swivel joint  100  may comprise several pairs of rotary forward and return connections  104 ,  106 , e.g. a pair for each separate coil  34 ,  36  connected in parallel to the fluidal swivel joint  100 . For more equal pressure distribution, the fluidal swivel joint  100  may also comprise several pairs of stationary forward and return connections  102 ,  108  (see  FIGS. 5A-B ). 
     As seen in  FIG. 1  and  FIG. 4  (in which annular curvature is not shown), the fluidal swivel joint  100  comprises an annular rotary part  110  that is attached to the suspension rotor  14  and an annular fixed part  112  that is attached to the stationary housing  18 . These rotary and fixed parts  110 ,  112  have conjugated mating configurations that allow fully revolving (&gt;360°) relative rotation. In the embodiment of  FIGS. 1-4 , the rotary part  110  includes a generally annular trough  114 , i.e. a ring-shaped narrow and upwardly open receptacle having the form of a gutter. Although the trough  114  preferably belongs to the rotary part of the joint  100 , with parts and connections appropriately inverted, the trough could likewise belong to the fixed part. The trough  114  delimits an annular volume by means of which the circuit portions  30 ,  32  are in fluidal communication as illustrated in  FIG. 3 . 
     As best seen in  FIGS. 3-4 , a main feature of the fluidal swivel joint  100  is a partition  120  arranged inside the trough  114 . More specifically, the partition  120  is a structure that divides the inner volume of the trough  114  into separated regions, namely an annular external cavity  122  and an annular internal cavity  124 . In the first embodiment, as best seen in  FIG. 3 , the partition  120  is configured so that the return connections  106 ,  108  communicate, i.e. they are fluidally coupled, via the internal cavity  124 . Conversely, the forward connections  102 ,  104  communicate via the external cavity  122 . A reversed arrangement of forward and return connections, as described below in relation to  FIGS. 5-7  and  FIGS. 10-11  is also possible. The partition structure  120  is shaped so that the upper portion of the external cavity  122  partially surrounds the internal cavity  124 . With its upper portion taken together with an optional lower portion, the external cavity  122  fully surrounds the internal cavity  124 . The lower portion serves as an annular collector for the rotary forward connection(s)  104  and is therefore optional. Similarly, the internal cavity  124  has a certain volume content serving as collector for the stationary return connection  108 . 
     Turning to  FIG. 4 , purely exemplary constructions of the fluidal swivel joint  100  and of the partition structure  120  will be detailed below. The trough  114  is of generally rectangular U-shaped cross-section and made e.g. of profiled metal sheet sectors, whereas it may also be formed in part by the suspension rotor  14  itself. The fixed part  112 , as a main component, comprises an annular hood  126 , which is of generally rectangular inverted U-shaped cross-section and also made e.g. of profiled metal sheet sectors. The annular hood  126  is mounted on the stationary housing  18  and protrudes into the trough  114 . The rotary trough  114  and the stationary hood  126  each respectively have vertical inner and outer sidewalls  134 ,  136 . The sidewalls  134 ,  136  are separated by narrow vertical gaps  138 , the width of which slightly exceeds the radial tolerance of the bearing  22 . The orientation of the gaps  138  may also be slanting, e.g. in V-shape. The upper portion of both sidewalls  136  of the hood  126  is recurved around the upper end of the sidewalls  134  of the trough  114  in order to provide a chicane or labyrinth-like seal that reduces exposure of the gaps  138  to the dust-laden atmosphere from inside the housing  18 . To the same effect, the sidewalls  134  of the trough  114  are provided with swellings  137 . In order to substantially eliminate exposure to dust, the hood  126  is further provided at the upper recurved end of each sidewall  136  with circumferentially distributed injection pipes  139  connected to an appropriate gas supply. The injection pipes  139  are operated to inject inert gas, e.g. N 2 , at a pressure that slightly exceeds the pressure inside the housing  18  in order to displace the dust-laden atmosphere out of the gaps  138 . The partition  120  on the other hand comprises a ring-shaped rotary partition member  140  and a cooperating ring-shaped stationary partition member  142 . The stationary partition member  142  has a cross-section with a Π-shaped (greek “Pi”, capital letter) concave central part and horizontal lateral disk flanges on either one side. Furthermore, the annular stationary partition member  142  is provided with interrupted circular arc-shaped apertures  144  arranged circumferentially in each lateral end portion of the horizontal flanges. At its extremities, the partition member  142  is fixed to the lower ends of the sidewalls  136  of the hood  126 . The annular partition member  142  can be assembled of correspondingly shaped sectors of punched and profiled sheet metal. The rotary partition member  140  of  FIGS. 1-4  is a simple ring-shaped plate having interrupted circular arc-shaped apertures  146  arranged circumferentially in its radially inward and outward end regions so as to face the apertures  144 . The rotary partition member  140  is fixed at its extremities to the sidewalls  134  of the trough  114  at a certain height inside the trough  114 . As will be understood, each pair of facing apertures  144 ,  146  warrants unrestrained free communication between the upper and lower portions of the external cavity  122  and thus between the forward connections  102 ,  104 . The partition members  140 ,  142  are spaced by a vertical distance that slightly exceeds the axial tolerance of the bearing  22 . 
     In order to allow unimpeded relative rotation between the fixed part  112  and the rotary part  110 , the joint  100  has an annular first clearance  150  and an annular second clearance  152  provided between the partitioning members  140 ,  142 . Due to this required clearance, the external cavity  122  and the internal cavity  124  are necessarily in leakage permitting communication. As will be appreciated however, the partition  120  is configured to provide a double and substantially symmetrical communication through both clearances  150 ,  152 . To this effect, the stationary and rotary partition members  140 ,  142  are configured mirror-symmetric, i.e. left-right symmetric, with respect to an imaginary vertical bisecting axis of the joint  100  (see dashed line in  FIGS. 6A-D ) in general and of the annular trough  114  in particular. Similarly, the trough  114  and the hood  126  are both generally mirror-symmetric. Thereby, despite leakage between the cavities  122 ,  124 , largely spatially uniform, left-right symmetrical pressure conditions exist inside the external cavity  122 . As a result, essentially equal water levels are warranted inside the gaps  138 , which both communicate freely with each other through the external cavity  122 . The crosswise width of the clearances  150 ,  152  corresponds to the spacing between the partition members  140 ,  142 , i.e. a distance that slightly exceeds the axial tolerance of the bearing  22 . As may also be noted, the width of the apertures  146  in the rotary partition member  140  is preferably larger than the crosswise width of the clearances  150 ,  152 , whereas the width of the apertures  144  in the stationary partition member merely needs to warrant free communication between the upper and lower portions of the external cavity  122 . 
     In order to enable forced circulation of coolant through the rotary circuit portion  30 , e.g. through the coils  34 ,  36 , by action of the stationary pump  40 , short-circuiting of coolant flow through the clearances  150 ,  152  should be minimized. To this purpose, annular first and second flow restrictors  160 ,  162  are provided in the first and second clearances  150 ,  152  respectively. The flow restrictors  160 ,  162  are configured to minimize leakage between the external and internal cavities  122 ,  124 , i.e. to minimize short-circuiting of the coolant flow through the clearances  150 ,  152 . In other words, since the clearances  150 ,  152  physically form “parasitic conduits” connected in parallel to the rotary circuit portion  30 , the flow restrictors  160 ,  162  are provided to significantly increase the flow resistance of these undesired parallel “parasitic conduits”. Preferred flow restrictors  160 ,  162  are non-contact labyrinth seals formed e.g. by conjugated protrusions and/or recesses on both or either one of the facing portions of the partition members  140 ,  142  that form the clearances  150 ,  152 . A major advantage of this type of flow restrictor  160 ,  162  is that they do not wear off. 
     Returning to  FIG. 3 , an arrangement for controlling the coolant level inside the fluidal swivel joint  100  comprises a level sensor  50 , schematically illustrated in  FIG. 3 . The level sensor  50  is arranged in one of the gaps  138  ( FIG. 4 ) and used to detect whether the coolant falls below the minimum level, indicated at  51 . When the minimum level  51  is reached, the level sensor  50 , e.g. by use of a controller of suitable known configuration (not shown), triggers opening of the motorized replenishing valve  42  for topping up a loss of coolant, typically caused by evaporation. The level sensor  50  also detects reaching of the maximum level, indicated at  53 , in order to trigger closing of the replenishing valve  42 . The maximum level  53  is set above the top plate of the hood  126  so that, during normal operation, the external cavity  122  is substantially filled with coolant.  FIGS. 2-3  further show a venting device  60 , which will be described below with reference to  FIG. 9 . 
     A second embodiment of an annular swivel joint  200  will now be described by reference to  FIGS. 5-7 . Main features being identical to those of the previous embodiment, only the differences will be set out below. The plan views of  FIG. 5A  and  FIG. 5B  best illustrate the annular configuration (which applies analogously to  FIGS. 1-4 ) of the swivel joint  200 . 
     As seen in  FIG. 5A , illustrating the fixed part  212  in top view, the fluidal swivel joint  200  comprises four stationary forward connections  202  and four stationary return connections  208 , which respectively connect forward (supply/flow) and return (runback) manifolds (not shown) of the stationary circuit portion  32  to the joint  200 . The stationary connections  202 ,  208  are arranged equi-circumferentially and centrally in the radial sense for maintaining circumferentially uniform pressure conditions within the generally left-right symmetric joint  200 . 
       FIG. 5B  illustrates the rotary part  210  in bottom view. As seen in  FIG. 5B , the fluidal swivel joint  200  is configured for supplying two parallel parts of the rotary circuit portion  30 , e.g. two cooling pipe coils  34 ,  36  as illustrated in  FIG. 1 . Accordingly, the joint  200  comprises two pairs of diametrically opposite rotary forward connections  204  and rotary return connections  206 . 
     In  FIGS. 6A-6D  only main reference signs are provided for alleviation of the drawings. As seen in  FIGS. 6A-6D  and as opposed to  FIGS. 1-4 , in the swivel joint  200 , the forward connections  202 ,  204  are coupled through the internal cavity  224 , i.e. on the inside of the partition structure  220 , whereas the return connections  206 ,  208  are coupled through the external cavity  222 , i.e. on the outside of the partition  220 . More specifically: As shown in  FIG. 6A , the stationary forward connections  202  debouch into the internal cavity  224  at the upper plate in the H-shaped central portion of the stationary partition member  242 . As seen in  FIG. 6C , the rotary forward connections  204  spring from the internal cavity  224  at the central part of the rotary partition member  240  that forms a lower plate. On the other hand, concerning the return connections  206 ,  208 , the rotary return connections  206  debouch into the lower portion of the external cavity  222  at the bottom plate of the trough-shaped rotary part  210 , whereas the stationary return connections  208  spring from the upper portion of the external cavity  222  at the top plate of the hood-shaped rotary part  212 . A configuration according to  FIGS. 1-4 , in which the forward path passes through the external cavity  122  and the return path passes through the internal cavity  124 , maximizes the volume of coolant that may evaporate and thus minimize the frequency of replenishing through replenishing valve  42 . The connection scheme and circulation sense of  FIGS. 5-7  however enables integrating a simpler self-venting solution into the swivel joint  200 , which will be detailed below with respect to  FIGS. 10-11 . 
     As further seen in  FIGS. 6A-D  and  FIG. 7 , the fluidal swivel joint  200  comprises first and second annular gas distributor pipes  270 ,  272  connected to a suitable supply of gas, especially of inert gas such as N 2 . Each gas distributor pipe  270 ,  272  is respectively associated to one annular clearance  250 ,  252 . Each gas distributor pipe  270 ,  272  is provided equi-circumferentially with injector nozzles or simple bores that communicate through a corresponding hole or bore in the stationary partition member  242  with the associated clearance  250 ,  252  for injecting a bubbling gas, into the liquid coolant on the forward (upstream) side of the clearances  250 ,  252 . With the higher forward coolant pressure in the internal cavity  224 , each distributor pipe  270 ,  272  thus injects gas for bubbling the coolant on the upstream side of the flow restrictors  260 ,  262 . By virtue of the resulting effervescence, the flow resistance created by the labyrinth seal-type flow restrictors  260 ,  262  is further enhanced. As seen in  FIGS. 6A-D , the configuration of the gas distributor pipes  270 ,  272  is symmetrical in order to equally enhance the effectiveness of both flow restrictors  260 ,  262 . As will further be appreciated, the bubbling gas injection through the distributor pipes  270 ,  272  also assumes the function of creating a displacement pressure inside the vertical gaps  238  between the fixed part  212  and the rotary part  210  to avoid dust contamination. To this effect, the downstream end of each clearance  250 ,  252  debouches directly into the corresponding gap  238 . In order to avoid inclusion of gas bubbles in the coolant that returns through the external cavity  222 , the communication between the upper and lower portions of the external cavity  222  is established through horizontal apertures  244  arranged in the horizontal sidewalls of the hood  226 , as best seen in  FIG. 7 . The horizontal apertures  244  enable general venting of the circuits  30 ,  32  and venting of inclusions of bubbling gas injected via the distributor pipes  270 ,  272 , since gases tend to rise upwards through the gaps  238 , which act as annular uptakes communicating with the ambient atmosphere. Accordingly, the upper and lower portions of the external cavity  222  communicating freely trough the gaps  238  and apertures  244 , bubbling gas rises upwards in the gaps  238  and is only minimally included in the return flow from the external cavity  222  to the stationary circuit portion  32 . 
     In the perspective view of  FIG. 7 , the illustrated fluidal swivel joint  200  is provided with additional reference sings with an incremented hundreds digit compared to  FIG. 4 , which identify features that are identical or similar to those described above in relation to  FIG. 4 .  FIG. 7  further illustrates respective feed pipes  274 ,  276  of the gas distributor pipe  270 ,  272 , which feed inert gas for injection into the clearances  250 ,  252 . 
       FIG. 8  illustrates the fluidal swivel joint  100  of  FIGS. 1-4  equipped with a first embodiment of a venting device  59 . The venting device  59  is a venting valve of the float valve type and is arranged in the top plate of the hood-shaped fixed part  112  so as to vent the upper portion of the external cavity  122  in case the coolant level drops below a predetermined level, e.g. a venting level  56  as indicated in  FIG. 8 . 
       FIG. 9  illustrates the fluidal swivel joint  100  of  FIGS. 1-4  equipped with a second embodiment of a venting device  60 . The venting device  60  is designed in particular for venting residual air and vapour locked in the circuits  30 ,  32 . It comprises a small-diameter venting pipe  61  bridging the uppermost region of the external cavity  122  to the stationary return connection  208  and a ventilating valve  63  provided in the venting pipe  61  for adjusting the venting rate of gas/vapour. The ventilating valve  63  allows only a minimal amount of liquid coolant to pass through the venting pipe  61  into the return connection  208 . Due to the draught caused by forced circulation, gases in the external cavity  122  are automatically evacuated through the return connection  208 , and may then be de-aerated by means of an auxiliary venting device  65  provided on the vent tank  44  (see  FIG. 3 ), in which residual air and vapour bubbles up. 
     Referring now to  FIGS. 10-11 , a preferred third embodiment of a fluidal swivel joint  300  will be described below. 
     In the joint  300  of  FIGS. 10-11 , the rotary part  310  comprises an annular trough  314  of substantially rectangular U-shaped cross-section that is formed, on one side, by the upper part of the cylindrical inner wall portion  24  of the suspension rotor  14 , and on the other side, by a cylindrical ring  313  fixed to the wall portion  24  by means of a disc-shaped bottom plate  315 . The fixed part  312  comprises an annular hood  326 , of inverted substantially rectangular U-shaped cross-section, which protrudes approximately halfway into the annular volume defined by the annular trough  314 . The trough  314  and the hood  326  are dimensioned so that narrow vertical gaps  338  between the sidewalls  24 ,  313  of the trough  314  and the sidewalls  336  of the hood  326  have minimal width required for unimpeded rotation of the trough  314  relative to the hood  326 . As seen in  FIGS. 10-11 , the upper end portions of the sidewalls  24 ,  313  of the trough  314  protrude into conjugated recesses provided in the top plate of the stationary housing  18  so as to form a chicane or labyrinth-like joint reducing exposure of the gaps  338  to dust. 
     As best illustrated in  FIG. 11 , the fluidal swivel joint  300  also comprises a partition structure  320  that divides the inner volume of the trough  314  into an annular external cavity  322  and an annular internal cavity  324 . The stationary partition member  342  of the partition  320  mainly comprises two annular downwardly tapering machined parts  342 - 1 ,  342 - 2  fixed to a disc-shaped upper plate  342 - 3 . Similarly, the rotary partition member  340  mainly comprises two annular upwardly tapering machined parts  340 - 1 ,  340 - 2  fixed to a lower disc-shaped plate  340 - 3 . The stationary partition member  342  is fixed to the stationary housing  18 , whereas the rotary partition member  340  is fixed to the wall portion  24  of the suspension rotor. As will be appreciated, both partition members  340 ,  342 , as well as the trough  314  and the hood  326  are generally left-right symmetrical in cross-section. 
     Each stationary machined part  342 - 1 ,  342 - 2  defines a respective oblique inner labyrinth surface  343  facing a respective conjugated oblique outer labyrinth surface  345  defined by either one of the rotary machined parts  340 - 1 ,  340 - 2 . The annular surfaces  343 ,  345  may be simple stepped surfaces, simple corrugated surfaces or surfaces with alternating protrusions and recesses that are arranged to interdigitate, similar to the labyrinth seal disclosed in FIG. 4-5 of WO 99/28510. Between the surfaces  343 ,  345 , the rotary and stationary partition members  340 ,  342  define annular clearances  350 ,  352  of minimal width as required to permit rotation. As will be understood, the external and internal cavities  322 ,  324  communicate through these clearances  350 ,  352 . Accordingly, similar to the previous embodiments, the labyrinth surfaces  343 ,  345  form flow restrictors  360 ,  362  in each clearance  350 ,  352  respectively in order to minimize short-circuiting flow between the cavities  322 ,  324 . 
     As seen in  FIGS. 10-11 , the rotary partition member  340  is shaped and arranged to protrude into the stationary partition member  342  with the labyrinth surfaces  343 ,  345  facing each other so that the clearances  350 ,  352  form branches of a generally inverted V-shape in cross-section. This oblique arrangement allows increasing the length of the flow restrictors  360 ,  362  i.e. the non-contact labyrinth seals defined by the surfaces  343 ,  345 , without increasing the overall height/width of the partition  320 . As will be appreciated, in the joint  300 , the flow restrictors  360 ,  362  extend substantially over the entire length of the oblique clearances  350 ,  352 , which exceeds the height (greatest sectional dimension) of the internal cavity  324 , in order to maximize achieved flow resistance/pressure drop. 
     As further seen in  FIGS. 10-11 , the upper and lower portions of the external cavity  322  communicate unrestrictedly through annular vertical channels  348  between the cylindrical outer surfaces of the stationary machined parts  342 - 1 ,  342 - 2  and the sidewalls  336  of the hood  326  and via the lower portions of the vertical gaps  338  into which the channels  348  debouch through transversely, e.g. horizontally, arranged apertures  344 . Accordingly, any general gas inclusions, including optionally gas injected by optional gas bubbling upstream of the clearances  350 ,  352 , can be largely prevented from entering the upper portion of the external cavity  322 , i.e. from entering the return path through the stationary return connection  308 . 
     Venting works in substantially identical manner as in swivel joint  200  of  FIGS. 5-7 : Any included gas preferentially passes by the apertures  344  and rises upwardly through the upper portion of the gaps  338  to be vented to the atmosphere, e.g. to the inside of housing  18 . Returning coolant, on the other hand, is forced from the lower portion of the external cavity  322 , through the lower portion of the gaps  338 , to turn laterally through the horizontal apertures  344  into the channels  348  to pass into the upper portion of the external cavity  322 . Accordingly, by virtue of the horizontally arranged apertures  344  and the chosen flow sense, i.e. the return flow passing upwardly through the external cavity  322 , the swivel joint  300  has an integrated self-venting configuration, venting air/gas through the inherent gaps  338 . An advantage of the self-venting solutions of  FIGS. 5-7  and  FIGS. 10-11 , resides in that a vent tank arrangement as in  FIG. 3  and venting devices as in  FIGS. 8-9  can be omitted so that a simpler cooling circuit  12  as in  FIG. 2  can be used. As will be understood, proper venting of residual air and vapour locked in the coolant enables complete filling of the circuit portions  30 ,  32  and warrants uninterrupted forced circulation through the rotary and stationary circuit portions  30 ,  32  by action of pump  40 . 
       FIG. 10  also illustrates the minimum and maximum water levels  351 ,  353 , between which coolant is maintained during normal operation by means of an appropriate level detection device that controls replenishing via the replenishing valve  42  (see  FIG. 2 ) to avoid suction of ambient air into the return connection  308  and overflow of coolant out of the gaps  338 . 
     In operation, the fluidal swivel joint  300  works as follows: 
     As illustrated in  FIG. 10 , cooled liquid coolant is supplied under pressure by the pump  40  from the stationary circuit portion  32  through the stationary forward connection  302  into the internal cavity  324 . To this effect, the stationary forward connection  302  passes trough the upper plate  342 - 3  of the stationary partition member  342 . From the pressurized internal cavity  324 , most of the coolant is supplied to the “forward side” of the rotary circuit portion  30 , e.g. to a coil  34 ,  36 , through the rotary forward connection  304  (only incidentally located in the same plane as the stationary forward connection  302  in the position shown in  FIG. 10 ). To communicate with the internal cavity  324 , the rotary forward connection  304  passes trough the lower plate  340 - 3  of the rotary partition member  340 . Accordingly the rotary circuit portion  30  is provided with pressurized coolant, i.e. subjected to forced circulation through the fluidal swivel joint  300 . Short-circuiting coolant flow through the clearances  350 ,  352  on the other hand is minimized by the facing pairs of surfaces  343 ,  345  which form a labyrinth seal. 
     As best illustrated in  FIG. 11 , heated liquid coolant that has absorbed heat, e.g. at one of several coils  34 ,  36 , is returned from the rotary circuit portion  30  via the rotary return connection  306 , which debouches into the lower portion of the external cavity  322  through a central bore in the bottom plate  315 . From there, coolant is forced upwardly through a lower region of the gaps  338 , laterally into and upwardly through the annular vertical channels  348 , into the upper portion of the external cavity  322 . From there, liquid coolant passes via the stationary return connection  308 , which takes source in the upper portion of the external cavity  322  through a central bore in the disc-shaped top plate  327  of the annular hood  326 , back to the return side of the stationary circuit portion  32 . 
     As will be understood, operation of the fluidal swivel joint  200  of  FIGS. 5-7  is substantially identical, whereas operation of the fluidal swivel joint  100  of  FIGS. 1-4  differs mainly in the inverted forward and return connections  102 ,  104 ;  106 ,  108  and therewith the opposite coolant circulation sense and, moreover in the manner by which the circuits  30 ,  32  are vented. 
     Referring now to  FIG. 12 , a most preferred fourth embodiment of a swivel joint  400  will be described. The swivel joint  400  of  FIG. 12 , whereas it provides the same benefits as the embodiment of  FIGS. 10-11 , is more cost-efficient in manufacture and considered more reliable. 
     As will be appreciated, the rotational position illustrated in  FIG. 12  corresponds to that illustrated in  FIG. 10 , i.e. a position where the section lines A-A and C-C of  FIGS. 5A-B  would coincide. Accordingly, in  FIG. 12 , a stationary forward connection  402  and a rotary forward connection  404  are shown in axially aligned position. The rotary part  410  also comprises an annular U-shaped trough  414  into which an annular U-shaped hood  426  of the fixed part  412  similarly protrudes downwards. Between the sidewalls of the hood  426  and of the trough  414  there are similar but longer respective narrow gaps  438  that permit venting and unimpeded rotation. Venting is favored by downwardly slanting apertures  444 , through which an upper portion of the external cavity  422  communicates with a lower portion thereof. The apertures  444  are provided in the lowermost region of the sidewalls of the hood  426  and define the minimum operational water level. Even though not shown in  FIG. 12 , it will be understood, that the stationary and rotary return connections are provided similarly as in  FIG. 11 , i.e. in the bottom plate  415  of the trough  414  and in the top cover of the stationary housing  18  respectively. Accordingly, as illustrated in  FIG. 12 , the forward path passes through the internal cavity  424 , whereas the return path (not shown) passes through the external cavity  422 . As in the previous embodiments, the rotary part  410  and the stationary part  412  have a generally mirror-symmetric configuration. 
     As will be noted when compared to  FIGS. 10-11 , the embodiment of  FIG. 12  mainly differs in terms of the structure of the partition structure  420  and, in particular, the configuration of its rotary and stationary partition members  440 ,  442  and, consequently, of the first and second flow restrictors  460 ,  462  there between. 
     As seen in  FIG. 12 , the stationary partition member  442  comprises a hood-shaped ring assembly of inverted U-shaped cross-section that is arranged inside the hood  426 . The hood-shaped ring assembly has a radially inner side  442 - 1 , a radially outer side  442 - 2  and an upper plate  442 - 3  and can be build in simple manner, e.g. as a welded steel plate assembly. Similar to  FIGS. 10-11 , vertical channels  448  are provided between the sidewalls of the hood  426  and the inner and outer sides  442 - 1 ,  442 - 2  of the stationary partition member  442  in order to connect the upper and lower portions of the external cavity  422 . Accordingly, the channels  448  form part of the external cavity  422  so that the external cavity  422  surrounds the internal cavity  424 . In the embodiment of  FIG. 12 , the length of the channels  448  is increased however to increase the filling level. 
     The rotary partition  440  on the other hand comprises a plurality of vertically stacked Teflon rings  441  that protrude into the ring assembly of the stationary partition member  442 . A single ring of increased height is also possible, whereas a certain minimum height is desired in order to achieve sufficient flow restriction (pressure drop). In the embodiment of  FIG. 12 , the Teflon rings  441  have a truncated wedge shaped cross-section that widens downwards, i.e. the rings have a radially inner face  441 - 1  and a radially outer face  441 - 2  that are oblique. Alternatively or in combination, the faces of the Teflon rings  441  can be corrugated. Each face  441 - 1 ,  441 - 2  is arranged with a small radial clearance, in the order of several tenths of a millimeter wide, adjacent the corresponding adjacent side  442 - 1 ,  442 - 2  of the stationary ring assembly  442 , i.e. with the required first and second clearances  450 ,  452  there between in order to permit relative rotation. As will be appreciated, by virtue of the configuration of the Teflon rings  441 , turbulence is created within the leakage-permitting clearances  450 ,  452 . Accordingly, the faces  441 - 1 ,  441 - 2  in cooperation with the closely adjacent inner and outer sides  442 - 1 ,  442 - 2  of the stationary partition member  442  respectively form first and second flow restrictors  460 ,  462  of the labyrinth seal type. Teflon is preferred as material for the rings  441  since it has so-to-speak “self-lubricating” properties in case of accidental contact between the rotary and stationary partition members  440 ,  442 . The rings  441  can by made one-piece and configured fully circumferential with corresponding bores for receiving tubes of the rotary forward connections  404  as seen in  FIG. 12 . 
     As will be understood, despite an improved structure, operation of the swivel joint  400  of  FIG. 12  is generally identical to that of  FIGS. 10-11  as described hereinbefore.