Abstract:
A hydraulic machine including an impeller which rotates with respect to a fixed structure and about an axis of rotation by a forced flow of water passing through it and at least one device for limiting water leaks is positioned between the impeller and the fixed structure so as to define an operating clearance between the impeller and the fixed structure and including at least one member that can be deformed or moved, while the impeller is rotating and being fed with water, in a redial direction with respect to the axis of rotation of the impeller.

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, said wheel being designed to pass a forced flow of water therethrough. Such a forced flow causes the wheel to be driven in rotation when the machine is a turbine. Such a flow results from said rotation when the machine is a pump. 
     2. Brief Description of the Related Art 
     Regardless of whether it is a turbine, a pump, or a pump turbine, a hydraulic machine is generally equipped with a device that is more often known as a “labyrinth” and that is designed to limit water leaks that result from the fact that a quantity of the water brought to the vicinity of a wheel tends to leak around said wheel. Such leaks give rise to loss of efficiency of the installation to which the machine belongs, and said labyrinth aims to limit that by defining operating clearances that are relatively narrow between the wheel and a stationary structure of the machine so that the flow path of the leaks is relatively narrow, thereby limiting the leakage flow-rate. 
     Labyrinths are generally designed in the form of a cylinder concentric with the axis of rotation of the wheel, with one or more stages imparting clearance that is as small as possible. In order to avoid premature wear of the labyrinth while the wheel is rotating, the operating clearance must be sufficient to take account of any radial movements of the wheel at transient speeds and to take account of any expansion of the wheel under the effect of the centrifugal force. In such a situation, the operating clearance of a current labyrinth must be determined by taking account of the most unfavorable operating conditions, namely transient speeds, and by taking account of the radial movements of the wheel. That leads to operating clearances being defined that are relatively large, and hence to non-negligible leakage around the wheel, in particular when the machine is a Francis turbine. 
     Analogous problems arise with pumps and with turbine pumps. 
     SUMMARY OF THE INVENTION 
     More particularly, an object of the invention is to remedy those drawbacks by proposing a hydraulic machine in which the operating clearance of the leak limiter device can be optimized in order to reduce the leakage flow-rate. 
     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, the wheel being designed to pass a forced flow of water therethrough, while at least one leak limiter device for limiting water leaks is disposed between the wheel and the stationary structure, said device defining an operating clearance between the wheel and the stationary structure. The leak limiter device has at least one deformable and/or movable member that is deformable and/or movable in a radial direction relative to the axis of rotation, while the wheel is rotating. This machine is characterized in that the deformable and/or movable member is deformable and/or mobile in the radial direction, between a first configuration corresponding to a first value of the operating clearance and a second configuration corresponding to a second value of the operating clearance and in that the machine includes means to control switching of the deformable and/or movable member from its first configuration to its second configuration depending on the operating speed of the machine. 
     By means of the deformable and/or movable member, it is possible to adjust the operating clearance of the leak limiter device that is more often known as a “labyrinth” even while the machine is being used, i.e. even while its wheel is rotating. This makes it possible to take account of the various operating clearance needs corresponding to the various possible operating speeds of the machine. In particular, it is possible to adopt maximum clearance during periods of use of the machine during which the wheel might move radially to a relatively large extent or might expand due to centrifugal forces. This applies, in particular, to start-up periods and to periods of excessive speed. Conversely, while the machine is in a generating period, e.g. while a turbine is coupled to an alternator in stabilized conditions, minimum clearance can be adopted insofar as the rotation of the wheel is also stabilized. This minimum clearance makes it possible to reduce the leakage of water around the wheel and thus to increase the efficiency of the machine. In addition, the movable nature of the member of the device of the invention makes it possible to consider moving it after an impact has been suffered by the wheel while said wheel is moving transversely relative to its axis of rotation. 
     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 leak limiter device has a deformable member that is deformable under the effect of the pressure exerted by a control fluid, between a first configuration corresponding to a first value for the operating clearance and a second configuration corresponding to a second value for the operating clearance. The deformable member is advantageously suitable for deforming by contracting radially to go from its first to its second configuration. This deformable member surrounds the wheel over the entire circumference thereof. 
     The leak limiter device has a plurality of movable members that are mounted to move radially relative to the axis of rotation of the wheel, between a first configuration corresponding to a first value for the operating clearance and a second configuration corresponding to a second value for the operating clearance. 
     The member or the members is or are mounted on the stationary structure with its/their radially inside surface(s) facing towards a radially outside surface of the wheel, while the operating clearance is defined between said radially inside and outside surfaces. 
     The machine has means for centering the member(s) relative to the axis of rotation of the wheel in each of the first and second configurations. 
     The member or each member co-operates with a portion of the stationary structure to define a chamber of variable volume that varies as a function of the configuration taken up by the member(s), this chamber being fed with control fluid under pressure. In this case, it is possible to provide means for controlling feeding the chamber of variable volume with control fluid under pressure. 
     In a variant, the machine has actuators for controlling movement of the movable members between the two configurations that they can take up. 
     In another variant, the machine has at least one means exerting a resilient return force on the movable members for urging them back into one of the two configurations that they can take up. 
     The machine has means for holding the or each member stationary in rotation about the axis of rotation of the wheel. 
     The control fluid is water tapped from a feed duct for feeding the wheel with water for forming the forced flow. In a variant, the control fluid may be a gas, in particular air, or oil. 
     The invention further provides an installation for converting hydraulic energy into electrical energy, or vice versa, said installation including a machine as mentioned above. Such an installation has better efficiency than state-of-the-art machines. 
     Finally, the invention further provides an adjustment method that can be implemented with a machine as described above, with a view to optimizing the operating clearance of a leak limiter device. According to this method, the adjustment takes place while the wheel is rotating about its axis by adopting a first value for the operating clearance during certain periods of rotation of the wheel, including at least transient speeds, and a second value for the operating clearance during certain other periods of rotation of the wheel, including at least one stabilized speed. 
     Advantageously, one of the values for the operating clearance is adopted by controlling the pressure and/or the quantity of a control fluid delivered to the leak limiter device. 
    
    
     
       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 six 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 a high-head energy conversion installation of the invention that includes a Francis turbine of the invention in a first operating configuration; 
         FIG. 2  is a view on a larger scale of the detail II of  FIG. 1 ; 
         FIG. 3  is a view analogous to  FIG. 2  when the turbine is in a second operating configuration; 
         FIG. 4  is a view on a larger scale of the detail IV of  FIG. 1 ; 
         FIG. 5  is a view analogous to  FIG. 4  when the machine is in a second operating configuration; 
         FIG. 6  is a view analogous to a portion of the right half of  FIG. 1 , for a low-head installation and a Francis turbine in a second embodiment of the invention; 
         FIG. 7  is a view on a larger scale of the detail VII of  FIG. 6 ; 
         FIG. 8  is a view analogous to  FIG. 7  when the machine is in a second operating configuration; 
         FIG. 9  is a view on a larger scale of the detail IX of  FIG. 6 ; 
         FIG. 10  is a view analogous to  FIG. 9  when the machine is in a second operating configuration; and 
         FIG. 11  is a fragmentary axial section view showing the principle of a Francis turbine in a third embodiment of the invention; 
         FIG. 12  is a fragmentary section view on line XII-XII in  FIG. 11 ; 
         FIG. 13  is a section view analogous to  FIG. 11  for a turbine in a fourth embodiment of the invention; 
         FIG. 14  is a section view analogous to  FIG. 11  for a turbine in a fifth embodiment of the invention; and 
         FIG. 15  is a section view analogous to  FIG. 11  for a turbine in a sixth embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The installation I shown in  FIGS. 1 to 5  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 vertical axis of rotation of the wheel  2  is stationary and is referenced X 2 . The turbine  1  is coupled via a shaft  11  to an alternator  5 . Between the casing  3  and the wheel  2  there are disposed two series of stay vane blades and of wicket gates (not shown) whose function is to guide a flow E that is coming from the duct  4  and that is to pass through the wheel  2  towards a discharge conduit  8 . 
     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 portion of the shaft  11  by means of screws  24  tightened into tapped holes  12  provided in the shaft  11 . 
     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 the figures, and that supports the rotary portions of the turbine  1 , in particular the shaft  11  and the wheel  2 . 
     When it reaches the vicinities of the leading edges  21   a  of the blades  21 , the flow  21  can enter the inter-blade spaces IA. It can also penetrate, through an annular slot f 1 , into an annular volume V 1  defined between the ceiling  22  and a portion  91  of the stationary structure  9 . The flow E can also penetrate through an annular slot f 2  into an annular volume V 2  defined between the belt  23  and another portion  92  of the stationary structure  9 . 
     The unwanted flows of water into the volumes V 1  and V 2  are indicated by arrows E 1  and E 2  in  FIGS. 2 to 5 . To avoid said unwanted flows E 1  and E 2  giving rise to large leaks and to a significant reduction in the efficiency of the turbine  1 , leak limiter devices  100  and  200  are interposed between the wheel  2  and the portions  91  and  92 . 
     The device  100  aims to limit the leaks from the volume V 1  to a volume V′ 1  situated above the ceiling  22 , radially between the volume V 1  and the axis X 2 . The device  200  aims to limit the leaks from the volume V 2  to the internal volume V′ 2  of the conduit  8 . The unwanted flows E 1  and E 2  thus flow from the inlet zone of the wheel  2  to the vicinities of the leading edges  21   a , respectively through the slots f 1  and f 2  and the volumes V 1  and V 2  towards the volumes V′ 1  and V′ 2 . 
     The device  100  has a housing  101  made up of two half-shells  101 A and  101 B. The housing  101  is held stationary on the portion  91  by means of screws  102  that pass through the two half-shells  101 A and  101 B and that are tightened into tapped holes  91 A provided in the portion  91 . The half-shells  101 A and  101 B are circular and centered on the axis X 2 . A sufficient number of screws  102  are provided to enable the housing  101  to be fastened effectively to the stationary portion  91 . 
     A ring  103  is mounted in the housing  101  and it co-operates with the housing to define a chamber of variable volume C 1  that is isolated from the outside by two seals  104 A and  104 B that are mounted in grooves  103 A and  103 E provided in respective ones of the top and bottom faces of the ring  103 . 
     Studs  105  are engaged in recesses  101 F and  103 F provided respectively in the housing  101  and in the ring  103  in such a manner as to hold the ring  103  stationary relative to the housing  101 , in rotation about the axis X 2 . 
     The half-shell  101 A has a wall  101 C that faces towards the half-shell  101 B. Similarly, the half-shell  101 E has a wall  101 D that faces towards the half-shell  101 A, and more particularly towards the wall  101 C. An opening O 1  is defined between the walls  101 C and  101 D, and the ring  103  is provided with a nose  103 C that is disposed between the walls  101 C and  101 D. 
     The ring  103  is made of an elastically deformable material, e.g. of steel or of a composite material. Said ring may be contracted towards the axis X 2 , in the direction indicated by arrow F 1  in  FIGS. 2 and 3 , under the effect of a force exerted on its radially outside surface  103 D. The contraction of the ring  103  towards the axis X 2  is controlled by injecting water under pressure into the chamber C 1 . 
     This water comes from the forced-flow duct  4  from which it is tapped by means of a tap-off  300  that constitutes the mouth of a pipe  301  making it possible to convey a secondary flow E s  towards a filter  302 , and then towards a pump  303  that makes it possible to increase the pressure of the flow E s . This pump feeds an accumulator tank  308  connected via a pipe  309  to a torus-shaped duct  304  connected to the chamber C 1  via tap-offs  305  opening out into the housing  101 . Said tap-offs  305  are distributed about the axis X 2  in planes offset angularly relative to the zones for receiving the screws  102 . 
     In addition, the ceiling  22  of the wheel  2  is equipped with a circular fin  221  that is formed integrally with the ceiling and that extends upwards relative to the top surface thereof. In a variant, the fin  221  can be removable. 
     When the turbine  1  is in the assembled configuration, the fin  221  has its radially outside surface  221 A disposed facing the device  100 . More precisely, the surface  221 A is disposed facing a portion of the radially inside surface  103 E of the ring  103 , which surface is actually the surface of the nose  103 C that is accessible through the opening O 1 . 
     While the turbine  1  is operating, the fin  221  turns about the axis X 2  with the wheel  2 , whereas the device  100  is stationary because it is mounted on the portion  91 . Operating clearance J must therefore be defined between the fin  221  and the facing portions of the device  100 , namely the half-shell  101 B and the ring  103 . 
     Because of this operating clearance J, the flow E 1  that penetrates into the volume V 1  can propagate towards the volume V′ 1 , as indicated by the arrows E 1  in  FIG. 2 . 
     In the configuration of  FIG. 2 , the chamber C 1  is not fed with water under pressure from the duct  304 , and the pressure P 1  of the water in the chamber C 1  is low, or indeed zero, so that, because of its resilience, the ring  103  takes up a relatively unstressed configuration in which the surface  103 E does not project from the walls  101 C and  101 D. The clearance J 1  between the surfaces  103 E and  221 A is thus sufficiently large to avoid impacts between the fin  221  and the device  100 , or to limit the contact forces, even when the wheel  2  is misaligned on the axis X 2  or when the wheel  2  expands under the effect of the centrifugal force. 
     Once the turbine  1  has reached a stabilized operating speed, the risks of the wheel  2  becoming misaligned relative to the axis X 2  are minimized, and the clearance J can be reduced to a value J 2  that is less than the value J 1 , while causing the nose  103 C to project from the housing  101  towards the fin  221 . This is obtained by increasing the pressure of water in the chamber C 1  to a value P 2  that results from the chamber C 1  being put into communication with the accumulator tank  308  through the duct  304  and through the tap-offs  305 . The pressure P 2  exerts a distributed force on the surface  103 D of the ring  103 , as indicated by the arrows F 2  in  FIG. 3 , which force causes the ring  103  to contract radially towards the axis X 2 , so that the surface  103 E moves towards the surface  221 A, while reducing the through cross section that is possible for the flow E 1 . Water leaks, from the volume V 1  to the volume V′ 1  are thus limited. 
     A solenoid valve  306 , mounted on the duct  301  between the pump  303  and the duct  304 , makes it possible to control putting the accumulator tank  308  and the chamber C 1  into communication with each other. This solenoid valve is controlled by an electronic control unit  307 . In a variant, solenoid valves can be installed on the tap-offs  305 , between the duct  304  and the chamber C 1 , for controlling feeding pressurized water to the chamber. 
     In addition, a solenoid valve (not shown) makes it possible to put the chamber C 1  into communication with an emptying line (not shown) that drains into the conduit  8  or into a drainage well (not shown). 
     In the event that turbine  1  is subjected to a sudden transient speed, in particular in the event of excessive speed, it is possible, by closing the solenoid valve  306  and by opening the other solenoid valve, to empty the chamber C 1  rapidly, so that the ring  103  returns resiliently to its  FIG. 2  configuration that makes it possible to limit the risks of accidental contact between the ring  103  and the fin  221 . 
     In the section plane of  FIGS. 2 and 3 , the housing  101  defines four reentrant corners  101 G,  101 H,  101 I and  101 J that are substantially complementary to respective ones of four salient corners  103 G,  103 H,  103 I, and  103 J of the cross-section of the ring  103  that can be seen in  FIGS. 2 and 3 . 
     The corners  101 G,  101 H,  103 G, and  103 H constitute centering means that act by means of co-operating shapes to center the ring  103  relative to the axis X 2  in the  FIG. 2  configuration. In the same way, the corners  101 I,  101 J,  103 I, and  103 J constitute centering means that act by means of co-operating shapes to center the ring  103  in the  FIG. 3  configuration. 
     By acting on the feed pressure of the chamber C 1  from the duct  304  and on the open time of the solenoid valve  306 , it is possible to cause the ring  103  to take up an intermediate configuration between the configurations shown respectively in  FIGS. 2 and 3 , which can be suitable for certain operating speeds of the turbine  1 . 
     The component elements of the device  200  that are analogous to the component elements of the device  100  bear like numerical references plus  100 . The device  200  has a housing  201  made up of two half-shells  201 A and  201 B mounted on the portion  92  and assembled together by means of screws  202 . An elastically deformable ring  203  is disposed in the housing  201  while being subjected to the pressure prevailing in a chamber C 2  of variable volume defined by the housing  201  and by the ring  203 . 
     Two O-ring seals  204 A and  204 B are disposed in respective ones of grooves  203 A and  203 B provided respectively in the top side and in the bottom side of the ring  203 . 
     Studs  205  are provided for indexing the ring  203  relative to the housing  201  in rotation about the axis X 2 . 
     As above, reentrant corners defined by the housing  201  and salient corners defined by the ring  203  in the plane of  FIGS. 4 and 5  make it possible to center said ring relative to the axis X 2  in the two configurations shown respectively in these figures. 
     The radially inside surface  203 E of the ring  203  is stepped in that it comprises three portions  203 B 1 ,  203 E 2  and  203 E 3 , each of which is formed by a cylindrical surface having a circular base, the diameter of the portion  203 E 2  being larger than the diameter of the portion  203 E 1 , and smaller than the diameter of the portion  203 E 3 . 
     Correspondingly, the radially inside surface  231 A of the downstream edge  231  of the belt  23  is made up of three portions  231 A 1 ,  231 A 2 , and  231 A 3  of increasing diameters. 
     The surfaces  203 E and  231 E define between them a sinuous passage through which a leakage flow-rate can flow, as indicated by the arrows E 2  in  FIG. 4 . As above, the operating clearance J′ of the turbine  1  at the belt  23  is defined as being the minimum distance between the surfaces  203 E and  231 A. 
     In the configurations in which the clearance J′ must be at a maximum, in particular in the event of transient speeds as mentioned above with regard to the device  100 , the clearance J′ is maintained at a maximum value J′ 1  corresponding to the configuration of  FIG. 4 , insofar as the water pressure in the chamber C 2  is maintained at a value P 1  that is low or zero. 
     Said chamber C 2  is fed with water under pressure by means (not shown) that are entirely comparable to the means used for feeding the chamber C 1 , and that comprise a duct  504  connected to the chamber C 2  via tap-offs  505 . The duct  504  may be connected to the pump  303  or to any other source of water under pressure. The water feed via the duct is controlled, e.g. by means of a solenoid valve analogous to the solenoid valve  306 , or indeed by the same solenoid valve  306  if it is chosen to cause the pressures in the chambers C 1  and C 2  to vary simultaneously. 
     While the turbine  1  is being used a steady speed, the clearance J′ may be reduced to a value J′ 2  shown in  FIG. 5 . For this purpose, the pressure of the water in the chamber C 2  is brought to a value P 2  that is greater than the value P 1 . The effect of this is to bring the surface  203 E closer to the surface  231 A, thereby limiting the through section area for the flow E 2  from the volume V 2  to the volume V′ 2 . In  FIG. 5  the arrows F 2  indicate the radial compression force of the ring  203  towards the axis X 2 , which force is due to the pressure P 2  of the water in the chamber C 2  and is exerted on the radially outside surface  203 D of the ring  203 . 
     The turbine shown in  FIGS. 1 to 5  is a high-head turbine for which the locations of the devices  100  and  200  have been optimized. 
     The invention is also applicable to low-head turbines, as shown in  FIGS. 6 to 10  for the second embodiment, in which elements analogous to the elements of the first embodiment bear like references. 
     In this embodiment, a leak limiter device  100  is mounted on a plate  91  belonging to the stationary structure  9  of the installation I, so as to generate operating clearance with the radially outside edge  222  of the ceiling  22  of the wheel  2 . The device  100  has a plate  101  that is held stationary on the portion  91  by means of a screw  102 . A recess is also provided in the plate  91 , in the vicinity of the edge  222  for the purpose of receiving an elastically deformable ring  103  whose top and bottom edges are provided with respective seals  104 A and  104 B engaged in grooves  103 A and  103 B provided for this purpose. 
     A seal  106  provides sealing for the assembly between the plate  101  and the portion  91 . 
     A plurality of studs, only one of which is visible in  FIGS. 7 and 8  with the reference  105 , make it possible to index the ring  103  relative to the portion  91  in rotation about the axis X 2  of rotation of the wheel. 
     The ring  103  has a radially outside surface  103 E disposed facing the radially outside surface  222 A of the edge  222 , while defining operating clearance J that makes it possible for the wheel  2  to move relative to the stationary portion  81 . An unwanted flow E 1  flows in the annular space defined by the surfaces  103 E and  222 A, from a volume V 1  in which the forced flow E flows at the inlet of the wheel  2 , in the vicinity of the ceiling  22 , towards a volume V′ 1  provided between the ceiling  22  and the portion  91 , above the wheel  2 . 
     A chamber C 1  of variable volume is defined between the portion  91  and the radially outside surface  103 D of the ring  103 . This chamber is connected via at least one tap-off  305  to feed means analogous to the feed means mentioned for the first embodiment, and comprise a pipe  301  fed from a tap-off provided on a feed duct of the casing  3  of the installation. This pipe  301  successively feeds a filter  302 , a pump  303 , and an accumulator tank  308 . This accumulator tank  308  is connected via a pipe  309  to an annular duct  304  from which one or more tap-offs  305  extend. A solenoid valve  306  controlled by an electronic control unit  307  controls the flow of secondary water E s  from the accumulator tank  308  to the duct  304 . It is thus possible to control the pressure of the water present in the chamber C 1 . 
     At transient speeds, the pressure P 1  of the water in the chamber C 1  has a value that is low or indeed zero, so that, under the effect of its resilience, the ring  103  takes up the position shown in  FIG. 7  in which the clearance J has a first value J 1  that is relatively large. 
     At a steady speed, an additional quantity of water is brought into the chamber C 1 , thereby bringing the pressure in this chamber to a value P 2  that is greater than the value P 1 . The effect of this is to deform the ring  203  radially, towards the axis X 2  and towards the surface  222 A, the ring then taking up the configuration of  FIG. 8 , in which the clearance J has a value J 2  less than the clearance value of  FIG. 7 . This makes it possible to limit the through section area of the unwanted flow E 1 , towards the volume V′ 1 . 
     An unwanted flow tends to flow between a volume V 2  in which the forced flow at the inlet of the wheel  2  flows, in the vicinity of the belt  23 , towards a volume V′ 2  provided around the belt, between said belt and a stationary cylinder  93 . 
     A device  200  is disposed in the vicinity of the upstream edge  232  of the belt  23  in order to limit the flow E 2 . This device  200  has a housing  201  made up of two portions  201 A and  201 B fastened to a plate  92  belonging to the stationary structure  9  of the installation I. A deformable ring  203  is mounted in the box  201  and, at its top edge, carries an O-ring seal  204 A mounted in a groove  203 A. In addition, an O-ring seal  204 B is mounted in the bottom portion  201 B of the body  201 , inside a groove  201 K. 
     A chamber C 2  of variable volume that can be fed with water under pressure through a tap-off  505 , such that the ring  203  takes up one or other of the configurations shown respectively in  FIGS. 9 and 10 , as a function of the value of the pressure, P 1  or P 2 , respectively prevailing in the chamber C 2 . 
     The chamber C 2  is connected to an annular duct  504  analogous to the duct  304 . 
     Thus, the clearance J′ between the radially inside surface  203 E of the ring  203  and the radially outside surface  232 A of the edge  232  can take one of the values J′ 1  and J′ 2  shown respectively in  FIGS. 9 and 10 . The configuration of  FIG. 10 , in which the clearance J′ is minimal, is selected for the steady operating speeds of the installation I, whereas the configuration of  FIG. 9  is preferred for transient speeds and for periods during which speed is changed, in particular start-up periods or periods of excessive speed. The unwanted flow E 2  is thus minimized while the installation is operating at a steady speed. 
     In the third embodiment of the invention shown in fragmentary manner in  FIG. 11 , elements analogous to the elements of the first embodiment bear identical references. 
     In the water leak limiter device  100 , a plurality of segments  103  are distributed around the outside peripheral edge  222  of the ceiling  22  of a Francis turbine wheel. Each segment  103  is equipped with a top groove  103 A and with a bottom groove  103 B in which a seal  104 A or  104 B is disposed. 
     The segments  103  overlap one another in a radial direction relative to the axis of rotation X 2  of the wheel  2 . More precisely, each ring  103  has a portion  103 M forming a rabbet  103 N in which a corresponding portion  103 P of an adjacent segment can be engaged. The portions  103 M and  103 P of two adjacent segments  103  thus overlap each other. 
     This overlap is achieved by providing a lateral gap E L  between a radially outside surface  103 Q of a portion  103 M of a segment  103  and a facing surface  103 R of an adjacent portion  103 S of another segment  103 . In the same way, a lateral gap E′ L  is provided between two radial surfaces  103 T and  103 V defined respectively by a portion  103 P of a segment  103  and by a facing portion  103 X of another segment  103 . 
     The segments  103  are disposed in a housing  101  against which the seals  104 A and  104 B bear and that has a rear partition  101 L that co-operates with the outside radial surfaces  103 D of the segments  103  and in a radial direction relative to the axis X 2 , to define a chamber C 1  of variable volume. 
     As above, it is possible to control the pressure of a quantity of water injected into the chamber C 1  via one or more tap-offs  305 , in order to control the movement of the segments  103  in radial directions indicated by arrows F 1  in  FIGS. 11 and 12 . This makes it possible to control the value of the radial clearance J between the radially inside surface  103 E of the various segments  103  and the radially outside surface  222 A of the edge  22 . 
     The lateral gaps E L , E′ L  make it possible to move the segments  103  closer together while they are moving towards the axis X 2 . 
     Seals  107  are disposed parallel to the axis X 2  and provide the sealing between the chamber C 1  and the interstice that exists between the surfaces  103 T and  103 V of the segments  103 . 
     As in the first embodiment, the housing  101  is provided with walls  101 C and  101 D that limit the movement of the segments  103  towards the axis X 2 . 
     In the fourth embodiment of the invention shown in  FIG. 13 , a plurality of segments  103  are used in a leak limiter device  100 , as in the embodiment of  FIGS. 11 and 12 , these segments partially overlapping one another and being equipped with seals  104 A (or equivalent seals) and  107 . 
     This embodiment differs from the preceding embodiment in that the radial positioning of the segments  103  is controlled not by acting on the pressure in the chamber situated radially outside these segments, but rather by using double-acting actuators  400  that can be controlled hydraulically or pneumatically. 
     In a variant, the actuators used are single-acting actuators, in which case the pressure of the flow between the segments  103  and the edge  222  is used for pushing said segments back towards a spaced-apart configuration in which the clearance J is increased. 
     In the embodiment shown in  FIG. 14 , various segments  103  are used, in a leak limiter device  100 . These segments overlap one another as in the embodiment of  FIGS. 11 to 13 . In this embodiment, springs  500  are distributed around the segments  103  and they make it possible to exert thereon a spring force F 5  that is directed towards the axis X 2 , thereby making it possible to giving a predetermined value to the clearance J between the radially inside surfaces  103 E of the segment  103  and the radially outside surface  222 A of the edge  222  of the ceiling of a turbine wheel. 
     In the event of imbalance of the wheel  2 , said wheel can hit one of the segments  103  that can then be pushed backed in opposition to the force F 5  to which it is subjected, without any significant damage being done to the ceiling of the turbine. This propensity to damp impacts is also present in the machines of the embodiments of  FIGS. 1 to 13  because the chambers C 1  make it possible for the rings  103  and  203  and the segments  103  to move radially outwards in the event of an impact. These chambers are connected to the accumulator tank  308  (and equivalent reservoirs), thereby making it possible to remove a fraction of the control liquid in the event that a ring or that a segment moves radially outwards under the effect of an impact. 
     In the variant shown in  FIG. 15 , the springs  500  may be replaced with a single spring  600  disposed around the segments  103  in a leak limiter device and also exerting a spring force F 5  directed towards the axis of rotation X 2  of the wheel  2 , and distributed over the various segments  103 . 
     In the embodiments of  FIGS. 14 and 15 , each segment  103  can slide in a radial direction F 1  towards the axis X 2  and the force F 5  tends to urge the segments back into a configuration in which the clearance J is minimal. 
     The invention is described as implemented in a Francis turbine. However, it is applicable with other types of turbine, with pumps, and with pump turbines. The technical characteristics of the embodiments described and mentioned above may be combined with one another within the ambit of the invention. 
     The various modes of controlling the positions of the segments  103  that are considered in the third, fourth, fifth, and sixth embodiments may be mutually combined. In particular, springs may be provided in the embodiments of  FIGS. 11 to 13 . 
     In the first, second, and third embodiments, a fluid other than water coming from the duct  4  can be used to control the positions of the members  103  or  203 . In particular, it is possible to use oil or air under pressure.