Abstract:
This hydraulic machine ( 1 ) has a wheel ( 2 ) mounted to move in rotation relative to a stationary structure ( 9 ) and about a stationary axis of rotation (X 2 ), the wheel ( 2 ) being designed to pass a forced flow (E) of water therethrough. A hydrostatic bearing ( 100 ), provided between firstly an element ( 111 ) constrained to rotate with the wheel ( 2 ) and, secondly a portion ( 91 ) of the stationary structure ( 9 ), is disposed between a first zone (Z 1 ) of the machine, which zone is in fluid communication with the forced flow (E) and in which zone, during operation, a pressure prevails that is similar to the pressure of the forced flow, and a second zone (Z 2 ) of the machine that is isolated from the forced flow by said bearing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hydraulic machine that has a wheel mounted to move in rotation relative to a stationary structure and about a stationary axis of rotation, which wheel is designed to pass a forced flow of water therethrough. Such a flow is at a relatively high pressure that depends, in particular, on the geometry of the machine and on the general configuration of the installation of which the machine is part, in particular on the head when the machine is a turbine. The pressure of said flow generally lies in the range 3 bars to 80 bars. The forced flow through the wheel of the machine causes said wheel to be driven in rotation when the machine is a turbine. Said flow results from said rotation when the machine is a pump. The invention is applicable to hydraulic machines of the turbine type, of the pump type, or of the pump-turbine type. 
     2. Brief Description of the Related Art 
     In hydraulic machines, e.g. Francis-type water turbines, it is known that a labyrinth can be disposed between firstly a portion of the turbine that is subjected to a water pressure similar to the pressure of the flow and secondly a “low-pressure” chamber from which the water that flows through the labyrinth can be removed towards the downstream end of the installation. The flow-rate of water that passes through the labyrinth constitutes a leak, which can be non-negligible, relative to the quantity of water brought to the turbine wheel. Said leak is not a driving flow, i.e. it is not used to drive the turbine wheel in rotation about its axis. In general, a Francis turbine also has a bearing designed to center a shaft supporting the wheel relative to its axis of rotation. Usually, a shaft seal is also provided that is designed to prevent water from flowing towards an alternator or towards some other portion of the power plant. Those various items of equipment, namely the labyrinth, the bearing, and the shaft seal, are relatively costly, and each of them requires regular maintenance, which further adds to the costs of operating a prior art Francis turbine. 
     Analogous problems arise with pumps, in particular with centrifugal pumps, and with turbine pumps. 
     SUMMARY OF THE INVENTION 
     More particularly, an object of the invention is to remedy those drawbacks by proposing a novel hydraulic machine in which the leaks that exist at the labyrinths in known machines are reduced significantly, thereby making it possible to increase the overall efficiency of an installation incorporating such a machine. 
     To this end, the invention provides a hydraulic machine having a wheel mounted to move in rotation relative to a stationary structure and about a stationary axis of rotation, the wheel being designed to pass a forced flow of water therethrough. This machine is characterized in that a hydrostatic bearing adapted to fulfill a centering function of the wheel with respect to its axis of rotation and provided between firstly an element constrained to rotate with the wheel and, secondly a portion of the stationary structure, is disposed between a first zone of the machine, which zone is in fluid communication with the forced flow and in which zone, during operation, a pressure prevails that is similar to the pressure of the forced flow, and a second zone of the machine that is isolated from the forced flow by said bearing. 
     In the meaning of the invention, the pressure in the first zone is similar to the pressure of the forced flow in that those pressures are of the same order of magnitude. In particular, the pressure in the first zone is substantially equal to the mean pressure of the forced flow, ignoring head loss. The pressure in the first zone is greater than 60% of the pressure in said forced flow, and preferably greater than 80% of the pressure in said forced flow. 
     By means of the invention, the hydrostatic bearing can perform simultaneously the functions of the labyrinth, of the bearing, and of the shaft seal of the state-of-the-art machines. In particular, the hydrostatic bearing isolates the first zone of the machine effectively from the second zone, with leaks that are considerably smaller than the leaks obtained with a state-of-the-art labyrinth. This thus makes it possible to recover a larger fraction of the quantity of water brought to the vicinity of the wheel of a turbine so that said water does indeed drive the wheel in rotation. When that machine is a pump, a larger proportion of the water moved by the wheel can be recovered at the outlet of the machine, compared with state-of-the-art machines. 
     In advantageous but non-essential aspects of the invention, such a machine may incorporate one or more of the following characteristics, taken in any technically feasible combination:
         the hydrostatic bearing is provided, along the axis of rotation of the wheel, at or in the vicinity of a zone of junction between the wheel and a shaft supporting the wheel;   water injection means fed from a feed duct for bringing water to the machine, open out into the bearing, through the stationary structure portion; in which case, means are advantageously provided for increasing the pressure of the injected water relative to the pressure of the water in the duct;   the water injection means include some series of diaphragms installed one after the other and adapted to, in conjunction with the pressure increase means, control the pressure of the water injected into the bearing;   in a first embodiment of the invention, the element constrained to rotate with the wheel is formed by or mounted on the end of a shaft supporting said wheel; in a second embodiment, the element constrained to rotate with the wheel is formed by or mounted on a member that is integral with said wheel; in a third embodiment, the element constrained to rotate with the wheel is mounted removably on said wheel;   water injection means open out into several cavities distributed uniformly about the axis of rotation of the wheel and provided in one of the facing surfaces of the element constrained to rotate with the wheel and of the portion of the stationary structure, between which surfaces the hydrostatic bearing is provided; and   the radial clearance of the bearing is less than 0.5 millimeters (mm), and preferably lies in the range 0.1 mm to 0.2 mm, for a bearing of diameter lying in the range 1.5 meters (m) to 2 m; this small clearance of the bearing makes it possible to limit very considerably leakage of water through said bearing.       

     The invention also provides an installation for converting hydraulic energy into electrical energy, or vice versa, which installation includes a machine as mentioned above. Such an installation is simpler to manufacture and offers better technical and economical performance than state-of-the machines, in particular because maintenance is simplified. 
     The invention also provides the use of a hydrostatic labyrinth bearing under the conditions mentioned above, i.e. for performing: a function of providing fluid isolation between firstly a first zone of the machine that is in fluid communication with a forced flow flowing through a wheel of the machine and in which, during operation, a pressure prevails that is similar to the pressure of the forced flow, and secondly a second zone of the machine, in which zone a pressure prevails that is less than the pressure prevailing in the first zone; and a function of centering the wheel relative to its axis of rotation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood and other advantages of the invention appear more clearly from the following description of three embodiments of a machine and of an installation that comply with the principle of the invention, the description being given merely by way of example and with reference to the accompanying drawings, in which: 
         FIG. 1  is a section view showing the principle of an energy conversion installation of the invention that includes a Francis turbine of the invention; 
         FIG. 2  is a view on a larger scale of the detail II of  FIG. 1 ; 
         FIG. 3  is a half-section view corresponding to the bottom right portion of  FIG. 1 , for an installation and a machine in a second embodiment of the invention; and 
         FIG. 4  is a half-section view analogous to  FIG. 3  for an installation and a machine in a third embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The installation             shown in  FIG. 1  includes a Francis turbine  1  whose wheel or “runner”  2  is fed from a casing  3  into which a forced-flow duct  4  opens out. The turbine  1  is coupled via a shaft  11  to an alternator  5 . Between the casing  3  and the wheel  2  there is disposed a series of stay vane blades  6  and of wicket gates  7  whose function is to guide a flow E that is coming from the duct  4  and from the casing  3  and that is to pass through the wheel  2  towards a discharge conduit  8 .
     The axis of rotation of the wheel is stationary and is referenced X 2 . 
     The wheel  2  is provided with blades  21  that extend between a ceiling  22  and a belt  23 . The blades co-operate with one another and with the ceiling  22  and the belt  23  to define inter-blade spaces IA through which the flow E passes while it is flowing through the wheel  2 . 
     The wheel  2  is fastened to the bottom end  111  of the shaft  11  by means of pins  12  that pass through orifices  112  provided in the end  111  and that are engaged in tapped holes  221  provided in the ceiling  22 . A nut  13  is mounted around each pin  12  once said pin has been tightened into the corresponding tapped hole  221 , by means of its polygonal head  121 . 
     The casing  3 , the duct  4 , and the conduit  8  are part of a stationary structure  9  that is shown in fragmentary manner only in  FIGS. 1 and 2 , and that supports the rotary portions of the turbine  1 , in particular the shaft  11  and the wheel  2 . 
     A hydrostatic bearing  100  is formed around the end  111  of the shaft  11 . This bearing  100  is defined between a ring or band  113  mounted around the end  111  and an annular element  91  disposed around said ring, with a small amount of clearance. The annular element  91  is part of the stationary structure  9 . It is thus stationary in rotation relative to the axis X 2 . 
     Reference d indicates the distance, measured radially relative to the axis X 2 , between the radially outside surface  113   a  of the ring  113  and the radially inside surface  91   a  of the element  91 . The distance d is the radial clearance of the bearing  100  that is defined between these surfaces. It has a value of less than 0.5 mm for a bearing whose diameter D 100  lies in the range 1.5 m to 2 m. The value of the thickness d advantageously lies in the range 0.1 mm to 0.2 mm under the above-mentioned conditions. This value is exaggerated in  FIG. 2 . The value of the radial clearance d is determined in order to ensure that the bearing  100  operates hydrostatically. 
     This value is substantially less than the usual clearance in a labyrinth, which clearance lies approximately in the range 1 m to 3 mm. 
     The bearing  100  is fed with water from the forced-flow duct  4 . A pipe  150  connects a tap-off  151  provided in the duct  4  to a filter  152  that is designed to remove the impurities from the water coming from the duct  4 , such impurities being, in particular, any grains of sand that said water might be carrying. The flow of water through the pipe  150  is referenced E 1 . This flow has a flow-rate considerably less than the flow-rate of the flow E that passes through the elements  6 ,  7 , and  2 , as explained above. In practice, the flow-rate of the flow E 1  has a value less than one third of the flow-rate of the flow through a conventional labyrinth of a prior art turbine. 
     The pipe  150  is extended downstream from the filter  152  to a pump  153  that makes it possible to increase the pressure of the water of the flow E 1 . The pipe  150  is connected to a torus-shaped duct  154  centered on the axis X 2 . The duct  154  makes it possible to feed the bearing  100  through the element  91  at a plurality of points distributed about the axis X 2 . In the example shown, twelve feed points are provided for the bearing P, which points are distributed uniformly about the axis X 2 , with an angular offset between two adjacent feed points of 30°. 
     The pump  153  is optional in the sense that the pressure of the flow E 1  at the tap-off  151  can be sufficient to feed the bearing  100 . 
     At a feed point, the duct  154  is connected, via a tap-off  155 , to a series of diaphragms  92  mounted one behind another in a conduit  93  provided inside the element  91 . This conduit  93  extends in a direction D 1  that essentially radial relative to the axis X 2  and opens out into a second conduit  94  whose main direction D 94  is perpendicular to the direction of the conduit  93  and parallel to the axis X 2 . This conduit  93  is closed off, at the top portion of the element  91  by a screw  95 . 
     A third conduit  96  extends parallel to the conduit  93 , i.e. in a direction D 96  that is radial relative to the axis X 2 , and it connects the conduit  94  to a cavity  97  provided in the surface  91   a . Thus, the water coming from the duct  4  can be injected into the bearing  100  at the twelve cavities  97  at a pressure controlled by means of the pump  153  and by means of the series of diaphragms  92 . 
     This injection of water distributed about the axis X 2  makes it possible to lubricate the hydrostatic bearing  100  while the wheel  2  and the shaft  11  are moving in rotation relative to the structure  9 . 
     This mode of injecting water into the hydrostatic bearing makes it possible to balance the movement in rotation of the wheel  2  about the axis X 2 . Considering the two water injection points shown respectively on the left and on the right of  FIG. 1 , it is possible to imagine the situation when the wheel  2  tends to shift towards the left of  FIG. 1  relative to the axis X 2 , e.g. under the effect of transient imbalance. In such a situation, the thickness d of the bearing  100  tends to increase on the same side as the injection point situated on the right of  FIG. 1 , and the throttling of the water exiting from the cavity  97  situated on the right of  FIG. 1  is lower, so that the flow-rate of water exiting from that cavity tends to increase. As a result the head loss in the series of diaphragms  92  increases, so that the pressure of the water flowing through the conduits  94  and  96  and then into the cavity  97  decreases. In other words, a shift of the wheel  2  leftwards in  FIG. 1  tends to reduce the pressure at which the water is injected into the bearing  100 , at the injection point shown on the right of this figure. 
     Conversely, the same shift reduces the thickness d of the bearing at the injection point shown on the left side of  FIG. 1 . This induces a reduction in the flow-rate through the series of diaphragms  92  of this injection point. This reduction in flow-rate induces a reduction in the head loss through this series of diaphragms  92  and, as a result, an increase in the pressure of the water injected at said injection point into the bearing  100 . 
     Thus, the wheel  2  shifting leftwards in  FIG. 1  reduces the pressure in the bearing  100 , at the injection point shown on the right of this figure, and increases the pressure at which water is injected into the bearing, at the injection point shown on the left of this figure. These pressure variations tend to re-align the wheel  2  on the axis X 2 . The hydrostatic bearing  100  thus performs a function of centering the wheel  2  on its axis of rotation X 2 . 
     Reference P E  designates the pressure of the flow E at the inlet of the wheel  2 , i.e. in the vicinities of the leading edges  21   a  of the blades  21  immediately downstream from the wicket gates  7 . Reference  98  designates a support that is part of the stationary structure  9  and on which the element  91  is mounted. In order to enable the wheel  2  to rotate relative to the structure  9 , an interstice I N  is provided between the radially outer edge  222  of the ceiling  22  and the support  98 . Due to the presence of this interstice, water flows from the inlet zone of the wheel  2 , as indicated by arrow F 1 , into a zone Z 1  that it fills to a pressure P′ E  that is of the same order of magnitude as the pressure P E . In practice, the zone Z 1  is situated immediately below the bearing  100  and the value of the pressure P′ E  corresponds to more than 60% of the value of the pressure P E , and preferably to more than 80% of the value of the pressure P E . 
     A second zone Z 2  of the turbine  1  is defined that surrounds the shaft  11  in the vicinity of the heads  121  of the pins  12 . This zone is normally not subjected to the pressure P E  of the flow E. In practice, a minimum quantity of water is necessary in the zone Z 2 . The pressure in the zone Z 2  is less than the pressure in the zone Z 1  and is preferably equal to atmospheric pressure. 
     The bearing  100  thus performs a function of isolation between the zone Z 1  in which water is present at a pressure P′ E  that is relatively high, and the zone Z 2  in which a residual quantity of water is present at low pressure. The bearing  100  can thus be said to be a “labyrinth bearing”. This isolation function is obtained by means of the fact that, in operation, the bearing is fed with water through the pipe  150 , the duct  154 , and the various conduits  95 ,  96  and cavities  97 . The water injected under pressure into the cavities  97  is distributed in the bearing  100  and thus prevents the water that is coming from the zone Z 1  from flowing towards the zone Z 2 . This is to be compared with the fact that the pressure P I  at which the water is injected into the cavities  97  is, by means of the pump  153  or by means of the pressure at the tap-off  151 , greater than the pressure P′ E . 
     Thus, the hydrostatic bearing  100  prevents a fraction of the flow E that goes via the stay vane blades  6  and the wicket gates  7  from being lost due to a non-negligible leak at the zone Z 1 , as it would be if a labyrinth were installed as in the state-of-the-art hydraulic machines. On the contrary, the presence of the hydrostatic bearing  100  between the zones Z 1  and Z 2  makes it possible to avoid such a leak or to limit such a leak very considerably. The quantity of water injected through the various cavities  97  is distributed in the bearing  100  and a fraction of said quantity flows by gravity into the zone Z 1 . In other words, a fraction E 2  of the flow E 1  flows towards the zone Z 1  and, through the interstice I N , can join the flow E to take part in driving the wheel  2  in rotation. This fraction E 2  of the flow is thus a driving flow for driving the machine. 
     Another fraction E 3  of the flow E 1  flows towards the zone Z 2  this fraction constituting the only genuine leak from the sum of the flows E and E 1 . In view of the small radial clearance d of the bearing  100 , this fraction of flow E 3  has a flow-rate that is very low compared with the flow-rate of the flow E, which constitutes progress compared with the state-of-the-art machines. 
     A cap  160  is disposed around the zone Z 2  and makes it possible to retain the small quantity of water that builds up therein due to the flow E 3 . A drain (not shown) makes it possible to direct this small quantity of water towards a drainage well. At the interface between the shaft  11  and the cap  160 , a set of baffles  161  is provided to limit the risks of splashing. 
     The surfaces  113   a  and  91   a  are treated to withstand wear and seizing for extreme situations of water feed interruptions. The presence of a coating or lining prevents the bearing  100  from being damaged in such extreme situations. 
     In a first embodiment, the ring or band  113  is not essential and a coating as mentioned above could be deposited directly on the radially outside surface of the end  111 . 
     In the second embodiment of the invention shown in  FIG. 3 , the elements analogous to the elements of the first embodiment bear like references. In the description below, only what distinguishes the second embodiment from the first embodiment is described. 
     The hydrostatic bearing  100  is provided between an annular element  91  belonging to the stationary structure and a fin  25  that is integral with the ceiling  22  of the wheel  2 . A band  113  is mounted around the fin  25  and its radially outside surface  113   a  co-operates with the radially inside surface  91   a  of the element  91  to form the bearing  100 . A zone Z 1  is defined immediately under the element  91  and it is in fluid communication, through an interstice I, with the flow E flowing through the wheel  2 . A zone Z 2  is defined radially between the fin  25  and the bottom end  111  of the shaft  11 . The hydrostatic bearing  100  makes it possible to isolate the zone Z 1 , in which the water is at a relatively high pressure, from the zone Z 2 , in which the water is at a low pressure, the pressure in the zone Z 2  preferably being equal to atmospheric pressure. 
     As in the first embodiment, the band  113  may be omitted. In which case, the bearing  100  is defined between the radially outside surface of the fin  25 , optionally coated with an appropriate coating, and the surface  91   a.    
     In the third embodiment of the invention shown in  FIG. 4 , the elements analogous to the elements of the first embodiment bear like references. In the description below, only what distinguishes the third embodiment from the first embodiment is described. 
     A ring  26  is mounted on the ceiling  22  of the wheel  2  by means of screws  27  distributed around the periphery of said ceiling. This ring  26  is made of a material comparable to the material of the bands  113  of the first and second embodiments. It is optionally coated with a suitable coating on its radially outside surface  113   a  that faces towards the radially inside surface  91   a  of an element  91  of the stationary portion with which the ring  26  defines the hydrostatic bearing  100 . As above, said bearing  100  makes it possible to isolate from each other a high-pressure zone Z 1  situated under the element  91  and in fluid communication through an interstice I N  with the flow E going via the wheel  2 , and a low-pressure zone Z 2  situated between the ring  26  and the bottom end  111  of the shaft  11  of the turbine  1 . 
     The bearings  100  of the second and third embodiments perform a centering function, like the bearing of the first embodiment. They operate in the same manner and are therefore not described in any more detail. 
     Regardless of the embodiment in question, reducing the radial clearance, i.e. reducing the thickness d, of the hydrostatic bearing, compared with the clearance of labyrinths of machines of conventional design, makes it possible to limit considerably the losses by leakage, and to increase the efficiency of the turbine  1 . Such losses are limited to the flow E 3  of the first embodiment and to the corresponding flows of the other embodiments. In addition, the positioning of the hydrostatic bearing  100 , which is relatively close to the axis X 2  with a diameter of less than 2 m, makes it possible for the wheel to be centered properly while it is operating, and to achieve a reduction in the movements of shafts perpendicular to the axis X 2 . This reduction in unwanted movements facilitates design of the labyrinth  170  installed between the belt  23  and the stationary structure  9  in order to limit the leakages in the downstream zone of the wheel. This also improves the efficiency of the turbine  1 . 
     By its design and its positioning in the machine  1 , the hydrostatic bearing  100  performs the respective functions of labyrinth, of bearing, and of shaft seal of the state-of-the-art machines, which is advantageous in terms of manufacturing cost, and of maintenance cost. 
     The invention is not limited to the examples described and may be implemented with turbines, with pumps, in particular centrifugal pumps, or with pump turbines other than a Francis turbine.