Abstract:
Method of minimizing the gap between a wheel and a casing of a turbine, a turbine, and a method of determining the wear behavior of a wheel of a rotor. To minimize the gap between a wheel and a casing in a turbine, optical methods are also often used in order to minimize the gap. However, this is very expensive. The method according to the invention proposes that the wheel and the casing (be part of an electric circuit, so that an electrical parameter, such as resistance for example, can be measured, the value of which shows whether a contact is present.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority of the European application No. 03023207.8 EP filed Oct. 13, 2003 under the European Patent Convention and which is incorporated by reference herein in its entirety. 
   FIELD OF THE INVENTION 
   The invention relates to a method of minimizing the gap between a wheel and a casing of a turbine according to the preamble the claims and to a turbine according to the preamble of the claims and to a method of determining the wear behavior of a wheel of a rotor as claimed in the claims. 
   BACKGROUND OF THE INVENTION 
   In a turbine, such as a steam or gas turbine for example, a rotor having at least one disk and a plurality of blades rotates inside a casing. There is a gap between the blade end and the casing. 
   Methods of displacing rotor and wheel have been disclosed by DE 42 23 495 and WO 00/28190. 
   In order to achieve a high efficiency, the gap between blade end and casing is to be minimal. 
   Methods of minimizing the gap have been disclosed by DE 39 10 319 C2 and DE 39 01 167A1. 
   However, the methods require considerable outlay in terms of equipment and/or are not very accurate, so that further optimization is desirable. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to show a method with which the gap between wheel and casing is minimized in a simple manner. 
   The object is achieved by a method as claimed in the claims by the wheel and the casing being part of an electric circuit, so that a mechanical contact is determined with the establishing of an electrical contact. 
   It is likewise an object of the invention to show a turbine in which the gap between wheel and casing is minimal. 
   The object is achieved by a turbine as claimed in the claims by the wheel and the casing being part of an electric circuit. 
   It is also an object of the invention to show a method of determining the wear behavior of a wheel. 
   The object is achieved by a method as claimed in the claims by the wheel and the casing being part of an electric circuit, so that a mechanical contact is determined with the establishing of an electrical contact. 
   If the end of a blade of a wheel wears, the distance between blade end and casing increases. This can be determined by the method as claimed in the claims by the characteristic curve of the change in distance with respect to time being determined. 
   Further advantageous measures are listed in the subclaims. 
   The measures listed in the subclaims may be advantageously combined with one another. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawing: 
       FIG. 1  shows a gas turbine, 
       FIGS. 2 ,  3  show a casing with a wheel as part of an electric circuit, and 
       FIGS. 4 ,  6  show determined measuring curves, and 
       FIG. 5  shows part of a turbine blade. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a gas turbine  100  in a longitudinal partial section. 
   In the interior, the gas turbine  100  has a rotor  103  which is rotatably mounted about a rotation axis  102  (axial direction) and is also designated as turbine wheel. Following one another along the rotor  103  are an intake casing  104 , a compressor  105 , a, for example toroidal, combustion chamber  110 , in particular an annular combustion chamber  106 , having a plurality of coaxially arranged burners  107 , a turbine  108  and the exhaust-gas casing  109 . The annular combustion chamber  106  communicates with a, for example annular, hot-gas duct  111 . The turbine  108  is formed there by, for example, four turbine stages  112  arranged one behind the other. Each turbine stage  112  is formed from two blade rings. As viewed in the direction of flow of a working medium  113 , a row  125  formed from moving blades  120  follows a guide-blade row  115  in the hot-gas duct  111 . 
   In this case, the guide blades  130  are fastened to the stator  143 , whereas the moving blades  120  of a row  125  are attached to the rotor  103  by means of a turbine disk  133 . A generator or a driven machine (not shown) is coupled to the rotor  103 . 
   During the operation of the gas turbine  100 , air  135  is drawn in through the intake casing  104  and compressed by the compressor  105 . The compressed air provided at the turbine-side end of the compressor  105  is passed to the burners  107  and is mixed there with a fuel. The mixture is then burned in the combustion chamber  110  while forming the working medium  113 . From there, the working medium  113  flows along the hot-gas duct  111  past the guide blades  130  and the moving blades  120 . The working medium  113  expands at the moving blades  120  in an impulse-transmitting manner, so that the moving blades  120  drive the rotor  103  and the latter drives the driven machine coupled to it. 
   The components exposed to the hot working medium  113  are subjected to thermal loads during the operation of the gas turbine  100 . The guide blades  130  and moving blades  120  of the first turbine stage  112  as viewed in the direction of flow of the working medium  113 , in addition to the heat shield blocks lining the annular combustion chamber  106 , are subjected to the greatest thermal loading. In order to withstand the temperatures prevailing there, said guide blades  130  and moving blades  120  are cooled by means of a coolant. Likewise, the blades  120 ,  130  may have anti-corrosion coatings (MCrAlX; M=Fe, Co, Ni, X=Y, rare earths) and heat-resistant coatings (thermal insulating layer, for example ZrO2, Y2O4-ZrO2). 
   The guide blade  130  has a guide-blade root (not shown here) facing the inner casing  138  of the turbine  108  and a guide-blade tip opposite the guide-blade root. The guide-blade tip faces the rotor  103  and is secured to a fastening ring  140  of the stator  143 . 
     FIG. 2  schematically shows an electric circuit between the wheel  120  and a casing  138 . 
   In order to produce an electric circuit between a wheel  120 , in particular a turbine blade  120 , and a casing  138  of a steam or gas turbine  100 , an electrical connection is made between the turbine blade  120  and the casing  138  by means of electric lines  60  or by means of electromagnetic transmission, for example via the shaft. An electric resistance and/or other electrical parameters can be measured by means of a corresponding measuring instrument  63  (voltmeter, ammeter, ohmmeter or capacitance meter). 
   For example, the electric resistance between at least one turbine blade  120  (shown schematically here) and the casing  138  can be measured. 
   If there is no contact between the turbine blade  120  and the casing  138 , the electric resistance is very high or infinitely high. 
   If touching occurs between a blade tip  87  of the turbine blade  120  and the casing  138 , an electrical contact is made between the turbine blade  120  and the casing  138 , as a result of which the resistance is greatly minimized and the electric circuit is closed. 
   Depending on how large the contact area is between turbine blade  120  and the casing  138 , the electric resistance changes. The measured electrical quantity is therefore a measure of the existing size of a gap d between blade tips and casing. 
   On account of the conicity of the tip of the wheel  120  and of the casing  138  relative to one another (FIG. 1, WO 00/28190), the gap d is reduced or increased by an axial displacement of the wheel  120  or of the casing  138 . 
   Further electrical quantities which may be measured are the voltage or capacitance (direct current, alternating current, which is generally inversely proportional to the gap d) between both elements  120 ,  138 . If a voltage is applied between wheel  120  and casing, no electric current flows as long as there is no mechanical contact. 
   If contact occurs between wheel  120  and casing  138  by axial displacement, a current flows, which can be measured, or a voltage drop is recorded. 
     FIG. 3  shows a further exemplary embodiment of a turbine which is designed according to the invention and with which the method according to the invention can also be carried out. The conicity of the tip of the wheel  120  and of the casing  138  is not shown here. 
   The turbine blade  120  and the casing  138  are as a rule made of metallic material, so that they can conduct electric current. 
   However, the turbine blade  120  often has a ceramic coating, so that a flow of electric current would not be possible between turbine blade  120  and the casing  138 . In these cases, an electric path between casing  138  and turbine blade  120 , in particular the blade tip  87 , must be made possible by other measures. 
   This is done, for example, by electrically conducting projections  69  which produce an electrical connection ( FIG. 5 ) through the coating of the turbine blade  120  from the casing  138  to the turbine blade  120  and the electric line  60 . 
   The projection  69  on the turbine blade  120  constitutes an electrical contact area  66  and is, for example, of triangular or conical design and can be worn by contact with the casing  138 . 
   The projection  69  may be present on one or more turbine blades  120  of one or more turbine stages  112 . 
   The at least one projection  69  is, for example, aligned with at least one electrical contact area  66  of the opposite casing  138 . 
   The casing  138  may likewise have separately designed electrical contact areas  66  which have, for example, a high electrical conductivity and/or high wear resistance. 
   The turbine blades  120  may likewise have blade tips  87  according to the prior art which are designed for wear (abradables). 
   In  FIG. 4 , the electric resistance R is plotted against an axial displacement of the moving blade  120  relative to the casing  138 . The electric resistance R (or capacitance) stands for a certain gap d between casing  138  and the turbine blade  120 . 
   The axial displacement is effected, for example, hydraulically by displacement of the rotor  103  together with the moving blades  120  in axial direction  102 . On account of the conicity of the wheel tip and of the casing  138  (FIG. 1, WO 00/28190), the gap d is reduced as a result. 
   At the start, the electric resistance R has, for example, a certain value or is infinitely high. 
   By an axial displacement of the rotor  103  relative to the casing  138 , the existing gap is narrowed and finally an electrical contact is produced, so that the resistance R drops. Depending on the axial displacement of the moving blades  120  relative to the casing  138 , a more or less large contact area is produced between the turbine blades  120  and the casing  138 , as a result of which the magnitude of the electric resistance R (or the capacitance) is also determined. Thus various measuring points  81  are obtained as a function of the value of the axial displacement. 
   The greater the axial displacement, the smaller the electric resistance. If an electrical contact has been produced, the moving blades  120  are shifted back again just until there is no longer any electrical contact (point  85  of the curve  84 ). A minimum gap is then set. 
   This setting of the minimum gap may be effected during operation, but also before start-up. 
   A curve  84  which serves to readjust the wheel  120  if the blade tip  87  wears can also be determined from the measured resistance values  81 . 
   A final time at which a wear coating  75  ( FIG. 5 ) on the turbine blade  120  is worn out can thus likewise be established. 
   This is done by the distance x over which the rotor  103  has been readjusted relative to the casing  138  in order to set a certain minimum gap being determined continuously or intermittently by the time t. This results in a curve as shown in  FIG. 6 . This distance x corresponds to a certain loss of coating thickness. Since the coating thickness h of the coating  75  is known, the total distance of the readjustment x can determine when the coating  75  is worn out or how thick it still is. 
     FIG. 5  shows a turbine blade  120  of a turbine  100  designed according to the invention. The turbine blade  120  has a metallic substrate which (not shown) has a ceramic coating  75  and/or an outer wear coating  75 . The outer wear coating  75  is, for example, porous and/or ceramic, so that there would actually be no electric path between the blade tip  87  and the metallic core  72  of the turbine blade  120 . 
   At least one continuous electric path  78  is therefore produced in the anti-wear coating  75 . The electric path  78  may be present in one or more turbine blades  120  of one or more blade rows.