Abstract:
The invention relates to an axial flow turbine and method of operating thereof. The turbine comprises a last stage of rotating blades located towards a downstream end of the turbine having a distal region at an end of the airfoil of the blades. A monitoring control system has at least one sensor in the distal region of at least one last stage blade for measuring at least one physical property of the airfoil and a control element that is capable of influencing at least one physical property of the distal region. The control system further includes a controller that adjusts the control element based on at least measured physical property so by controlling the at least one physical property.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to EP Application 12170178.3 filed May 31, 2012, the contents of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     This invention relates generally to axial flow turbine control systems, including low-pressure steam turbines, and more specifically to control systems for last stage blades, in particular last stage hybrid composite blades. 
     BACKGROUND INFORMATION 
     The rotating blades of low-pressure steam turbines induce tremendous centrifugal forces into the rotor. This can be a limiting factor in designing the turbine for maximum efficiency. A solution is to use lower density blade materials as such blades exert less force into the rotor. This solution can, however, only be applied if the low-density material has adequate mechanical properties. While Titanium is one existing solution, in some circumstances it may be preferable to use alternatives with even better strength to weight ratios. Another alternative is composite materials, an example of which is disclosed in Swiss Patent Number CH547943. 
     A disadvantage of composite materials is that they typically have less temperature tolerance than metals. This can be a problem, in particular during low volume flow operation and full speed conditions when last stage blades are susceptible to windage heating of the blade tip area. Normal blade temperatures typically do not exceed 65° C. However, without corrective means, due to this condition, last stage blade tip temperatures can exceed 250° C. At such temperatures, the mechanical properties of composite material are significantly impacted and they may undergo permanent degradation including deformation and reduced strength. 
     A solution to windage heating is provided by Patent application No. US2007/292265 A1. The solution comprises injecting a cooling medium in the vicinity of the last stage tip region. The medium, which includes either steam or water, may be injected from the casing either fore or aft of the blade tip. As an alternative, or in addition, a small extraction groove for extracting flow through the outer sidewall may be provided near the blade tip just forward of the blade. 
     Typically these control measures are taken based on predictive assessment using means such as computational fluid dynamic calculation methods. As the optimum control measure typical varies between installations and further with operating conditions, simple universally applied threshold values are likely to lead to suboptimal control. In particular, the control action may inject more steam/water than is actually required. This may lead to excessive erosive attack, in particularly along the blades leading or trailing edges, dependent on how and where the injection occurs. In other case, the control action may not inject enough steam/water, resulting in inadequate cooling. In yet further cases, too much working fluid may be extracted via extraction grooves detrimentally affect steam turbine efficiency. It is therefore desirable to provide a control system that ensures acceptable blade life while maximising turbine efficiency and minimising the consumption of the cooling fluid. 
     SUMMARY 
     The disclosure is intended to provide a turbine that overcomes the problem of sub-optimal control of the detrimental effects of windage on the last stage blade distal region resulting in reduced blade life. 
     It attempts to address this problem by means of the subject matters of the independent claims. Advantageous embodiments are given in the dependent claims. 
     The disclosure is based on the general idea of providing a controller that uses direct measurement of a blade&#39;s physical property to initiate and modulate corrective action against blade overheating. 
     An aspect provides an axial flow turbine comprising a casing that defines a flow path for a working fluid therein. Within the casing, coaxial to the casing, is a rotor. A plurality of stages is mounted in the flow path. The stages each comprise a stationary row of vanes circumferentially mounted on the casing; and a rotating row of blades that are circumferentially mounted on the rotor wherein airfoils of the blades each extend into the flow path therein. Of the plurality of stages, the last stage blade row, which is defined as the blade flow located towards a downstream end of the turbine in the flow direction, includes a distal region at an end of the airfoil distal from the rotor. The turbine further comprises a monitoring control system having three main elements:
         at least one sensor configured and arranged in the distal region of at least one last stage blade, for measuring at least one physical property of the airfoil in the distal region;   a control element, configured and arranged to influence at least one of the physical properties of the distal region of the last stage blades; and   a controller, configured to adjust the control element based on at least one of the measured physical properties so by controlling at least one of the physical properties.       

     The feedback controller ensures that the control action can be optimised, ensuring that neither too much or too little control action is taken thus making it possible to optimise turbine efficiency in view of blade life. 
     In a further aspect, the sensor is embedded in the airfoil. Sensors located on the outer surface of the blades are typically operable only for a limited time, for example for the duration of a test run, before they are damaged and/or eroded. In additional located sensors on the outer surface of a blade can have a negative impact on a blades aerodynamic efficiency. This makes them unsuitable for commercial use. However, by embedding the sensor, these problems can be overcome. 
     In another aspect, the airfoil has a composite core body and further comprises a covering that covers at least a portion of the airfoil wherein the sensor is embedded between the composite core body and covering or alternatively between composite layers. In these locations, the sensor is located close to the outer surface of the airfoil so it makes it possible to estimate the extremes to which the airfoil is exposed to, without the disadvantages of having sensors located on the airfoil&#39;s surface as well as reducing the signal response time. 
     In a further aspect, the sensor is configured to measure one or more physical properties from a selection of strain, temperature and moisture content. While in particular temperature and the combined relationship of temperature and strain, has a universal influence on the longevity of most blade materials, moisture is of particular concern for blades made of composite materials that contain fibre material embedded in a polymer based matrix. Higher moisture content of the matrix material typically results in decreased e-moduli, strength and glass transition temperature. While a certain level of moisture absorption typically occurs at atmospheric condition, for example, during storage, wet conditions in the steam turbine may result in additional absorption, in particularly if excess wetness is created by the over injection of cooling water. 
     In an aspect the airfoil comprises a plurality of sensors distributed along the extensional length of the airfoil from the rotor wherein the controller is configured to adjust the control element based on the measurement profile of the plurality of sensors. 
     In an aspect, where the airfoil compromises conductive material, the conductive property is exploited by using the conductive material as a ground for the sensor. In this way, the sensor requires only one wire, which has the advantage that the wire can be laid so as to minimise strain, thus prolonging the life of the sensor. 
     In an alternative aspect, the sensor and the controller communicates, at least partially, by any know wireless means, thus simplifying sensor installation. 
     In an aspect, the control element is a means for adjusting at least one of a selection of water injection, steam injection, working fluid extraction and mass flow through the turbine. 
     A further aspect provides a method for controlling a physical property of a distal region of a last stage blade of an axial flow turbine. The method comprises the steps of:
         providing a control element configured and arranged to influence a physical property of the distal region;   measuring a physical property of the distal region; and   adjusting the control element in response to the measured physical property so by controlling the measured physical property.       

     The measured physical property of this aspect may be a selection of one or more of temperature, moisture, strain and moisture. Where the last stage blade is at least partially made of composite material, the step of measuring the physical property includes measuring the physical property at a point between the composite layers. The combination of temperature and strain sensors has the additional benefit that these temperature sensors can also be used for temperature compensation for the strain measurements. 
     In a further aspect, each of the aspects is incorporated into a low-pressure steam turbine. 
     It is a further object of the invention to overcome or at least ameliorate the disadvantages and shortcomings of the prior art or provide a useful alternative. 
     Other aspects and advantages of the present disclosure will become apparent from the following description, taken in connection with the accompanying drawings, which by way of example illustrate exemplary embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic of an exemplary axial flow turbine; 
         FIG. 2  is an expanded view of section I of a last stage blade of the turbine of  FIG. 1 , additionally showing an exemplary control system; 
         FIG. 3  is a cross sectional view through II-II of the blade tip of  FIG. 2  showing an embedded sensor; 
         FIG. 4  is a cross sectional view through II-II of the blade tip of  FIG. 2  showing a sensor embedded in the covering of an airfoil; and 
         FIG. 5  is a cross sectional view through II-II of the blade tip of  FIG. 2  showing a sensor embedded in recess and covered by a covering; and 
         FIG. 6  is a schematic view of an exemplary airfoil with a plurality of sensors. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present disclosure are now described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiments disclosed herein. 
     Throughout this specification reference to a controller is taken to mean a system for receiving an input, comparing the input with a set value, using an algorithm to calculate manipulate value and finally applying the manipulate value to a control element in order to achieve a control objective. 
       FIG. 1  shows an exemplary multiple stage  16  axial flow turbine  10 . The turbine  10  comprises a casing  15  enclosing stationary vanes  14  that are circumferentially mounted thereon and rotating blades  12  that are circumferentially mounted on a rotor  11  wherein the rotor is located coaxially to the casing  15 . The casing  15  itself defines a flow path for a working fluid therein. Each blade  12  has an airfoil  13  extending into the flow path from the rotor  11  to a distal region  19  wherein the distal region  19  is defined as the top one third of the airfoil  13 . The airfoil  13  may be made of metal, including metal alloys, composites including layered composites that comprise layered carbon fibre reinforced polymers or metal matrix composites. The multiple stages  16  of the turbine  10  are defined as a pair of stationary vane  14  and a moving blade  12  rows wherein the last stage  18  of the turbine  10  is located towards the downstream end of the turbine  10  as defined by the normal flow direction through the turbine  10 . The exemplary turbines  10  of this type includes steam turbines  10  and in particularly low pressure steam turbines  10 . 
       FIG. 2  shows an exemplary monitoring control system that may be applied to the turbine  10  shown in  FIG. 1 . The control system comprises at least one sensor  22  located in the distal region  19  of the airfoil  13  of at least one last stage  18  blade  12 . The sensor  22  is preferably located so that is does not interfere with the aerodynamics of the blade  12 , for example between the covering  27  and composite material as shown in  FIG. 4  or else embedded in the composite material as shown in  FIG. 3 . In an exemplary embodiment, the covering  27  is a coating. In another exemplary embodiment, the covering  27  is a metal sheet. In an exemplary embodiment the blade  12  is made of metal and the sensor  22  is located in a cavity within the airfoil  13 . In an exemplary embodiment the blade  12  is at least partially made of composite material and the sensor  22  is embedded between composite layers  26  of the composite, as shown in  FIG. 3 . In an exemplary embodiment, this is achieved by the sensor  22  being laid-in before the curing or resin infiltration process starts. The sensor  22  may alternatively be embedded before curing, in particularly when using pre-impregnation or resin infiltration (e.g. Resin Transfer Moulding) methods so that the sensor  22  forms an integrally part of the composite structure. In an exemplary embodiment, only after curing is a covering  27  applied. 
     As shown in  FIG. 4 , in an exemplary embodiment, the airfoil  13  has a covering  27  and the sensor  22  is embedded between the composite core body  25  of the airfoil  13  and the covering  27 . In this exemplary embodiment, the airfoil  13  may be made of layered composite material, as shown in  FIG. 4  or of unlayered material (not shown). This embodiment enables particularly effective measurement of the boundary conditions of the airfoil  13  and thus is particularly effective when the sensor  22  is a temperature sensor  22 . 
     In another exemplary embodiment shown in  FIG. 5 , the composite core body  25  is cured with a recess. The sensor  22  is then fitted in the recess and then the covering  27  is then applied over the sensor  22 . 
     The means of embedding a sensor  22  in an airfoil  13  is not limited to the provided exemplary embodiments but also include known methods that neither interfere with the aerodynamics of the airfoil  13  nor compromise its mechanical integrity to an extent that significantly impacts the service life of the blade. 
     In separate exemplary embodiments, the sensor  22  is a temperature sensor  22 , a strain sensor  22 , a temperature and strain sensor  22  or a moisture sensor  22 . In an exemplary embodiment, a plurality of either the same or different sensors  22 , i.e. temperature, strain or moisture, is embedded in the distal region  19  of the airfoil  13  of at least one last stage  18  blade  12 . 
     As shown in  FIG. 2 , an exemplary embodiment further includes a control element  24  that is configured and arranged to influence localised temperature, strain, moisture or any combination of localised temperature, strain or moisture of the last stage  18  blades  12  as measured by the sensor  22 . This can be achieved by several means, each of which may be equally effective. 
     In an exemplary embodiment, this is achieved by the control element  24  being configured to inject water or steam from a cavity into the casing  15 , upstream of the distal region  19  of the last stage  18  blades  12  in a region prone to windage. The injected water or steam provides a cooling means to overcome localised heating of blades  12 . 
     In another exemplary embodiment, this is achieved by the control element  24  being configured to inject water or steam from a cavity in the casing  15  from the casing  15  at a point downstream of the last stage  18  blades  12 , for example in a diffusor of the turbine. The injected water or steam provides a cooling means to overcome localised heating of blades  12  while reducing possible erosion effects caused by injecting water or steam upstream of the blades  12 . 
     In another exemplary embodiment, water or steam as cooling medium is injected downstream of the last stage  18  blade  12  where it mixes with the circulating flow and as a result is drawn upstream into the last stage  18 . 
     In another exemplary embodiment, this is achieved by the control element  24  being configured to bleed, extract and/or withdraw working fluid from around the distal region  19  of the last stage  18  blades  12 . By this means windage is reduced thus reducing localised overheating. 
     In another exemplary embodiment, the control element  24  is configured to adjust the mass flow through the turbine and thus control windage based on the principle that significant windage occurs only below above a minimum turbine mass flowrate. 
     In addition to a sensor  22  and control element  24 , exemplary embodiments further include a controller  20  as shown in  FIG. 2 . The controller  20  is configured by means of programming as well as connection to both the sensor  22  and control element  24  to avoid localised heating of the distal region  19  of the last stage  18  blades  12  by adjusting the control element  24  in response to the physical property measured by the sensor  22 . 
     In an exemplary embodiment shown in  FIG. 4 , the last stage  18  blade  12  is made of electrically conductive materials, for example, from metal or electrically conductive fibres such as carbon fibre, and the signal transmission means is achieved by a single wire  17 . This is achieved by utilising the conductive parts as earthing/grounding for the sensors  22 . The single wire  17  arrangement of this embodiment enables greater flexibility in the placement of the transmission wire  17  so as to minimise strain on the wire  17  during turbine  10  operation, thus improving the sensor  22  reliability. 
     In an exemplary embodiment, the communication between the sensor  22  and the control occurs, at least partially, by wireless means. This may be realised, for example, by the use of telemetric systems or contact rings and includes the use of RFID (radio frequency identification devices) that are configured to be read during operation by the controller  20 . 
     In an exemplary embodiment shown in  FIG. 6 , the airfoil  13  comprises a plurality of sensors distributed along the extensional length of the airfoil  13  from the rotor  11 . In a further exemplary embodiment, the control action of the controller  20  is based on the relative measurement and/or measurement profile along the extensional length of the airfoil  13 . 
     Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, it will be appreciated by those skilled in the art that the present disclosure can be embodied in other specific forms. For example, the invention may also be applied to axial compressors used in gas turbines  10 . In addition, the location of the sensors is not restricted to the distal region  19  but could be distributed along the entire length of the airfoil  13 . The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein.