Abstract:
A cover is provided to conceal the forward-facing step defined by the protrusion of the first stage diaphragm into the LP inlet. The cover presents a smoothly contoured surface to the flowing fluid in both radial and circumferential directions and thus reduces energy loss of the working fluid, thereby to increase performance of the LP turbine.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method and device for improving inflow to the first stage diaphragm of an axial flow turbine. 
     A typical axial flow turbine includes a rotor, a casing, and one or more turbine stages. Each stage is a cooperating combination of stationary and rotational components. 
     The rotational component is a wheel with a plurality of blades or buckets attached to its outer circumference. The wheel is coaxially mounted to the turbine shaft which is disposed to rotate about its axis. The assembly of the shaft and wheels is referred to as the turbine rotor. 
     The stationary component of the axial turbine stage is typically comprised of a diaphragm or a nozzle ring. A diaphragm of an axial flow turbine typically comprises inner and outer circumferential rings and a plurality of spaced apart vanes or nozzles. Each nozzle is fixedly secured at each end thereof to the inner and outer rings, respectively, for forming fluid flow passages therebetween. The outer ring is generally fixedly mounted to the inner shell of the turbine casing and the inner ring is spaced from and surrounds the turbine rotor. The nozzles control and direct flow of working fluid onto the energy extracting turbine blades or buckets. Thus, the nozzle passages are positioned radially about the first axis to line up with the buckets of the turbine rotor that are disposed axially behind the nozzle passages. 
     Each of the parts of the turbine discussed above has an upstream and a downstream side with respect to the flow of working fluid through the turbine. The turbine casing surrounds the turbine stage components. The casing also provides an inlet and exhaust for the working fluid. The inlet portion of the casing, called the inlet casing, is where the working fluid initially comes into the turbine. This inlet casing directs working fluid to the first stage stationary component. 
     In order to obtain maximum power from energy available from the working fluid, it is necessary for the flow of fluid be precisely controlled. The flow of working fluid must impinge the buckets at a predetermined optimum angle and the optimum fluid flow distribution from the radially inner portion of the root of the nozzle to the radially outer portion of tip of the nozzle must be maintained to accommodate a broad range of operation conditions. 
     On several externally fed LP steam turbines, the length of the assembled turbine stages is actually longer than the casing  26  that contains them. In such a case, the first stage nozzle diaphragm  18 , 24  often protrudes into the inlet casing, presenting a forward facing step  28  to the working fluid flow path from the inlet casing  16  into the first stage diaphragm  18 , 24 , as illustrated in FIG.  1 . As the inflowing steam encounters the step, it separates from a smoothly flowing pattern. This separation increases the energy loss of the working fluid (steam). If the energy loss associated with this separation can be reduced, more energy may be extracted by the turbine and converted to electricity. 
     Known methods for reducing energy loss in the LP inlet include metering the flow upstream of the inlet, contouring of inlet walls, and reducing flow velocity. While it may reduce losses in the inlet proper, metering of the flow anywhere in the flow path generates energy loss. For other reasons, it may be necessary to meter the flow upstream of the inlet step and thereby mitigate losses at the step. In such a case, there will be no additional loss. However, metering the flow is not an effective method if used specifically to target energy loss associated with the forward facing step presented by the protruding first stage nozzle diaphragm. 
     As noted, it is also possible to contour the inner surface of the inlet to redistribute the velocity profile at the first stage nozzle. However, there is no method of re-acquiring the lost energy associated with the diaphragm step protruding into the LP inlet. Also, this contouring method is very sensitive to surface contour and is often impractical. 
     Also noted above, an effective method of reducing steam flow velocity is to increase the volume of the LP inlet since losses are proportional to the square of velocity. The general disadvantage of this method, however, is that the cost of the larger inlet increases with added material. Furthermore, this method may influence the velocity profile at the first stage nozzle. 
     Therefore, a need remains for a device that can conceal the forward facing step from the working fluid flow without negatively disturbing the distribution of flow to the first stage nozzle. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention is embodied in a cover that conceals the forward-facing step defined by the protrusion of the first stage diaphragm into the LP inlet. The cover presents a smoothly contoured surface to the flowing fluid in both radial and circumferential directions and thus reduces energy loss of the working fluid, thereby to increase performance of the LP turbine. The cover of the invention thus provides a solution to the energy loss problem while having no negative impact on the flow distribution to the first stage. 
     The invention is thus embodied in a cover for e.g., the LP first stage diaphragm, comprising: a generally flat plate main body having an outer peripheral edge and an inner peripheral edge, said outer peripheral edge being circular and said inner peripheral edge defining a generally discontinuous circle having a plurality of cutouts for receiving protruding structure of the inlet casing, said cover being pre-formed to define a generally curved contour from a plane of said outer peripheral edge to a plane of said inner peripheral edge, said plane of said outer peripheral edge and said plane of said inner peripheral edge being parallel and spaced apart according to said contour. 
     The invention may thus be embodied in an axial flow turbine including a rotor and turbine blades coupled to said rotor for converting at least a portion of energy available from a working fluid into mechanical energy, comprising: a diaphragm for circumferential disposition around the rotor for directing at least a portion of said working fluid onto said buckets, said diaphragm including a plurality of spaced apart nozzles forming a respective plurality of channels therebetween, an inlet casing for conducting working fluid to an upstream end of said diaphragm, said diaphragm protruding into said inlet casing to define a step, and a cover extending between a wall of said inlet casing and an upstream end of said diaphragm, said cover defining a gradual transition between the inlet casing wall and the upstream end of the diaphragm. 
     The invention may also be embodied in a turbine comprising: a shaft which extends along and rotates about a central axis; a rotor wheel mounted to the rotor; a plurality of buckets mounted to and around said rotor wheel; a diaphragm upstream of said rotor wheel, said diaphragm including a plurality of circumferentially arrayed nozzle vanes housed in a stationary flow path component and configured to direct fluid against an upstream side of the buckets to effect rotation of the rotor wheel; an inlet casing for conducting motive fluid to said diaphragm, said diaphragm projecting axially into an inlet chamber defined by said inlet casing so that said diaphragm defines a step from a wall of said inlet casing to an upstream end of said diaphragm, and a cover extending between said wall of said inlet casing and said upstream end of said diaphragm, said cover defining a gradual transition between the inlet casing wall and the upstream end of the diaphragm. 
     The invention is further embodied in a method for improving inflow from an inlet casing of a turbine into a first stage diaphragm thereof when said first stage diaphragm protrudes into said inlet casing, the method comprising: providing a cover that has a generally flat plate main body having inner and outer peripheral edges and a generally circular shape; mounting the cover to a wall of the inlet casing adjacent said outer peripheral edge; and disposing the inner peripheral edge of the cover at an upstream end of the first stage diaphragm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic, partial cross-sectional view of a conventional LP turbine and inlet; 
     FIG. 2 is a schematic, partial cross-sectional view of the LP turbine and inlet of FIG. 1 with a diaphragm cover embodying the invention mounted thereto; 
     FIG. 3 is a partial, schematic cut-away perspective view of the LP inlet casing showing a diaphragm cover embodying the invention; 
     FIG. 4 is an elevational view of a diaphragm cover embodying the invention; and 
     FIG. 5 is a view taken along line  5 — 5  of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the illustrated turbine, the flow of working fluid (motive steam) is provided through an opening  10  in an outer casing  12  to an inlet chamber  14  in an inlet casing  16  whereupon the steam is directed onto the first annular row of nozzles  18 . Thus, the steam entering the turbine  20  is directed transverse to the rotor axis  22 , i.e., radially inward. As the steam reaches the area of the nozzles  18 , it must be turned through 90° and then redirected by means of the first stationary circumferential blades or nozzles  18  onto a first stage of rotating blades  24 . 
     FIG. 1 schematically illustrates an externally fed LP steam turbine in which the length of the assembled turbine stages is longer than the casing  26  that contains them such that the first stage nozzle diaphragm structure  28  protrudes into the inlet casing. Thus, the first stage nozzle diaphragm ring presents a forward facing step to the inlet flow path. Consequently, steam encounters the step and separates from a smooth flowing pattern as illustrated. 
     To conceal the forward facing step  28  generated by the protrusion of the first stage diaphragm  18 , 24  into the LP inlet, a diaphragm cover  30  is provided to extend from the wall  32  of the inlet casing  16  to the upstream end  34  of the first stage diaphragm. As illustrated in FIGS. 2 and 3, the diaphragm cover has an appropriate contour to define a smoothly contoured surface to the flowing fluid in both radial and circumferential directions. As illustrated in FIGS. 3 and 4, the diaphragm cover has a generally flat plate main body that is of generally circular shape as defined by an outer peripheral edge  36  and an inner peripheral edge  38 . The outer peripheral edge is defined as a substantially continuous circle whereas the inner peripheral edge defines a plurality of cutouts  40  so that it is a generally discontinuous circle. 
     The diaphragm cover may be comprised of a plurality of part circular segments  42  to accommodate protruding parts of the LP inlet and/or first stage diaphragm and/or the casing structure  16 . In the presently proposed embodiment, the diaphragm cover  30  is provided as four segments  42 , the upper two segments associated with the upper casing half and the lower two segments associated with the lower casing half. Also, the upper two segments and the lower two segments of the diaphragm cover define circumferential gaps at 12:00 and 6:00 to accommodate, e.g., the LP inlet structure, as illustrated in FIG.  3 . Cut outs may also be provided if necessary or desirable to accommodate other features of the inlet structure, such as stiffeners  44 . 
     The nozzle diaphragm cover  30  may be constructed of any material that can withstand the environment and attachment criteria for the LP inlet and that can define the appropriate contour. In an exemplary embodiment, the diaphragm cover is fabricated from plate steel formed to fit the appropriate contour and is welded to the inlet. It will be appreciated that the actual configuration and profile of the diaphragm cover will vary depending on the LP inlet dimensions and the diaphragm protrusion. 
     As will be appreciated, a diaphragm cover  30  embodying the invention may used on any turbine application where a smooth flow of steam is interrupted by the first stage diaphragm protruding into the inlet. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.