Abstract:
A method to design a turbine including: estimating rates of thermal radial expansion for each of a stator and a rotor corresponding to a period of operation of the turbine; estimating a clearance between the rotor and the stator based on the rates of thermal radial expansion, and determining a mass or surface area of the stator or rotor based on the clearance.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to clearance control in a turbine, such as a gas turbine. 
         [0002]    Clearance in a turbine typically refers to the between the rotor of the turbine and the stator that surrounds the rotor. In a gas turbine, the rotor is typically an axial turbine having rows of buckets each mounted on a turbine wheel. The stator in a gas turbine is a casing that includes an inner annular shell supporting annular shrouds that surround the rows of buckets and rows nozzles between the bucket rows. Clearance is between the tips of the rotating buckets and annular shrouds. 
         [0003]    Clearance is needed to allow the buckets to rotate without rubbing against the shrouds. If the clearance is too great, combustion gases leak over the tips of the buckets and do not drive the rotation of the turbine. If the clearance is too small, the tips of the buckets rub against the shroud and may cause vibration that damages the turbine. 
         [0004]    Clearance is needed whenever the turbine buckets rotate, including while the turbine heats up during startup, while the gas turbine is hot during full speed, full load (FSFL) operation, and as the turbine cools as it shuts down. The turbine is typically formed of metal components having various heat expansion rates. In particular, the turbine wheels, buckets on the wheels and annular shells around the buckets expand and contract at different rates as the turbine heats up and cools down. Due to different rates of thermal expansion, clearance could increase or shrink as the gas turbine heats and cools. 
         [0005]    Control systems and techniques are conventionally used on gas turbines to ensure that clearance never becomes too small during all stages of operation and does not become too large during extended periods of operation, especially at FSFL. Conventional clearance control systems and techniques may include cooling systems mounted on external skids adjacent the gas turbine, complex sensing and actuation systems for the cooling system, flow rerouting of compressed air form the compressor, and other assemblies. Conventional clearance control systems and techniques tend to be active in that they adjust the amount of a cooling fluid flowing through the shell or buckets. 
         [0006]    Some conventional clearance systems are actuated in response to a certain operating conditions, such as at pinch points which occur when clearance is the smallest. For example, additional heating of the casing shell may be used to increase the clearance at a pinch point. Despite these conventional systems, there remains a long felt need for a clearance control system and scheme that is robust and economical. 
       SUMMARY OF INVENTION 
       [0007]    An approach to clearance control for a turbine has been conceived in which the components of the turbine are designed to thermally expand and contract during operation such that sufficient clearance is maintained throughout all stages of operation. As part of the design of the turbine, the thermal expansion and contraction of the turbine components are predicted for all operating conditions. Knowing the thermal expansion and contraction of the turbine components, the clearance is predicted for each the operational phases of the gas turbine. If the predicted clearance is insufficient, adjustments are made to the design of components of the turbine, such increasing or decreasing the mass of the stator, and increasing or restricting cooling passages in the stator. After the adjustments, the clearance is again predicted to confirm that the clearance is adequate overall operating conditions. The cycle of designing the turbine to achieve a desired clearance and predicting the clearance of the current turbine design can be repeated until the clearance is acceptable at all operating conditions. 
         [0008]    The clearance control system may be passive. The clearance control system relies on the thermal expansion of the components of the turbine. The clearance control system may be embodied without valves, actuators or other control devices for regulating the flow of cooling or heating fluid through the turbine shell. 
         [0009]    A method has been conceived to design a turbine including: estimating rates of thermal radial expansion for each of a stator and a rotor corresponding to a period of operation of the turbine; estimating a clearance between the rotor and the stator based on the rates of thermal radial expansion, and determining a mass or surface area of the stator or rotor based on the clearance. The estimation of the clearance may include determining closure of the clearance during the period and identifying a peak value of the closure and the determination of the mass or the surface area reduces the peak. 
         [0010]    The stator may include an inner annular shell housing the rotor, and the rotor may include a turbine wheel on which is mounted an annular row of buckets, and the estimation of the clearance may include determining a difference between a thermal radial expansion of the inner annular shell and a sum of the thermal radial expansion of the wheel and the buckets. The determination of the mass or the surface area may include determining a volume or internal surface area of a cooling passage in the inner annular shell. 
         [0011]    The method may include estimating a peak in the clearance and reducing the peak by the determination of the internal volume or the internal surface area, the portion of the operation is a startup stage. 
         [0012]    A method has been conceived to design an inner annular shell which houses a rotating axial turbine comprising: estimating rates of thermal radial expansion for each of the inner annular shell and the axial turbine which includes a turbine wheel and a row of buckets mounted to the wheel; estimating a clearance between tips of the buckets and an interior surface attached to the inner annular shell aligned with the tips, wherein the clearance is estimated based on the rates of thermal radial expansion, and determining a mass or internal surface area of the inner annular shell based on the clearance. The interior surface may be a surface of a shroud. 
         [0013]    The estimation of the clearance may include determining closure of the clearance during a period of operation of the turbine and identifying a peak value of the closure, and the determination of the mass or the surface area reduces the peak. The estimation of the clearance may include determining a difference between a thermal radial expansion of the inner annular shell and a sum of the thermal radial expansion of the turbine wheel and the buckets. The determination of the mass or the internal surface area may include determining a volume or internal surface area of a cooling passage in the inner annular shell. A peak in the clearance may be reduced by the determination of the internal volume or the internal surface area. 
         [0014]    A method has been conceived for clearance control in a gas turbine including an inner annular shell housing a turbine wheel supporting a row of turbine buckets, the method comprising: during a startup stage of the gas turbine, thermally expanding in the inner annular shell at a rate faster than thermally expanding the turbine wheel and the row of turbine buckets; directing compressed gas through an interior passage of the inner annular shell during the startup operation, and controlling a clearance between tips of the turbine buckets and an inner surface of the inner annular shell or connected to the inner annular shell, wherein the control of the clearance is achieved based on a surface area or volume of the interior passage sized to cause the inner annular shell to achieve the faster thermal expansion. 
         [0015]    A clearance control system has been conceived for a turbine comprising: a stator; a rotor housed within the stator; a clearance between the stator and the rotor, and a cooling fluid passage internal to the stator having an internal surface area or an internal volume sized to cause the stator to expand radially at a faster rate than the radial expansion of the rotor during a startup stage of the turbine. The stator may include an inner annular shell and the rotor includes a turbine wheel and buckets mounted to the wheel. The fluid passage may include an inlet proximate to an outer surface of the stator and an outlet proximate to an inner surface of the stator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic diagram of a gas turbine showing a cut-away view of the turbine and turbine shell. 
           [0017]      FIG. 2  is a cut-away view of a portion of an inner annular shell of the gas turbine shown in  FIG. 1 . 
           [0018]      FIG. 3  is an exemplary chart showing predicted variations in the radial displacement due to thermal expansion of components of the turbine, and the closure of clearance as these components expand. 
           [0019]      FIG. 4  is a cross-sectional view of an enlarged portion of the inner annular shell shown in  FIG. 2 . 
           [0020]      FIG. 5  is an exemplary chart showing predicted heat rates acting on the portion of the inner annular shell shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 1  shows a gas turbine  10  having a compressor  12 , combustor  14  and turbine  16 . Gas turbines generate power by compressing air, mixing the compressed air with fuel, combusting the mixture and driving a turbine with combustion gases. The turbine includes an annular casing  18  that houses rows of turbine buckets  20  that rotate about a shaft  23 . The buckets in each row are mounted on a turbine wheel  22 . Between the rows of buckets are rows of nozzles  24  (guide vanes). 
         [0022]    Hot combustion gases  27  flow in an annular passage  28  through the rows of buckets  20  and nozzles  24 . The turbine casing  18  forms the outer surface of the hot gas passage  28 . The inner wall of the passage is near the outer rims of the wheels  22 . 
         [0023]    The turbine casing  18  includes an outer annular shell  32  that houses and supports an inner annular shell  26 . The inner annular shell surrounds the rows of buckets. The nozzles  24  are mounted to the inner annular shell  26 . 
         [0024]    Annular rows of shrouds  30  are mounted to the inner turbine shell  26  and aligned with the tips of the buckets. The gap between the shrouds  30  and the tips of the buckets  20  is referred to as the “clearance” or “clearance gap” of the gas turbine. 
         [0025]    A small clearance ensures that minimal amounts of hot combustion gases leak over the tips of the buckets. If the clearance becomes too small, the tips of the bucket scrape against the shrouds which causes wear to the buckets and shrouds, and can create vibrations in the turbine. Wear is generally not desired as it increases the clearance gap and can lead to damage to the buckets and shrouds. Vibrations are generally not desired because they can damage the turbine. 
         [0026]    Annular plenums  34 ,  36  are formed between the outer and inner annular shells  32 ,  26  of the turbine casing  18 . These plenums  34 ,  36  distribute compressed air to cooling passages in the inner annular shell  26 . The compressed air is extracted from one or more stages of the compressor  12 . The plenum  34  around the earlier stage buckets receive air compressed to a higher degree and from a later stage of the compressor than the plenum  36  surrounding the later stage turbine. The arrangement and number of plenums in the turbine shell  18  and the selection of compressor stages to be coupled to each of the plenum is a matter of design choice. 
         [0027]      FIG. 2  is a perspective view of a section of the annular inner annular shell  26 . The inner annular shell is typically formed of a metal material. The outer surface of the shell has annular ledges and ribs that engage the outer annular shell  32 . The outer surface of the inner annular shell forms a wall of the plenums  34 ,  36  ( FIG. 1 ) for the compressed air. The inside surface of the inner annular shell includes rows of slots  38  to receive hooks of the shrouds  30 . 
         [0028]    Internal cooling passages  40  (see dotted lines) are arranged within the inner turbine shell. Compressed air from one of the annular plenums  34 ,  36 , enters an inlet  42  to the cooling passages  40 . Air flows through the passages (see serpentine arrow  44 ) and exits  45  into a slot  38 . The cooling passages  40  may be arranged to extend longitudinally along the rotational axis of the gas turbine. The cooling passages may follow a serpentine, e.g., switch-back, course by reversing direction at a cross-over pocket chamber  46  near an axial end of the inner annular shell. Several cooling passages  40  may be arranged symmetrically around the circumference of the turbine shell. The cross-over pocket chamber may be sealed by a plate  47  ( FIG. 4 ) on the forward face  62  of the inner annular shell. The arrangement of the cooling passages in the shell is a matter of design choice and within the skill of an engineer experienced in design turbine shells. 
         [0029]    The cooling passages  40  allow compressed air from the plenums  34 ,  46  to pass through the inner turbine shell  26  and vent into the hot combustion gas path. Heat transfer occurs between the inner turbine shell  26  and the compressed air as the air flows through the cooling passages  40 . The compressed air cools the inner turbine shell if the shell is at a higher temperature than the compressed air. While hot combustion gas flows through the turbine the inner turbine shell will normally be hotter than the compressed gases. If the compressed air is warmer than the inner turbine shell, the air will heat the shell. The compressed air may be warmer before combustion occurs in the combustor during the early steps of the startup operation of the gas turbine. 
         [0030]    The amount of heat transfer from the cooling gas into the inner annular shell depends on the internal surface areas of cooling air passages  40  and pocket chambers  46 . The amount of heat transfer affects the thermal expansion and cooling of the inner turbine shell  26 . Further, the volume of the cooling passages  40  and pocket chambers  46  affects the mass of the inner annular shell. The mass of the shell affects the thermal expansion of the shell. 
         [0031]    During the design of the gas turbine, the inner annular shell is designed to have desired thermal expansion characteristics. The desired thermal expansion characteristics may be achieved, at least in part, by designing the shape, length and cross-sectional area of the cooling passages  40  and cross-over pocket chambers  46  to provide certain levels of heat transfer at the different stages of operation of the gas turbine. The desired thermal expansion characteristics may also be achieved, at least in part, by selecting volumes of the cooling passages  40  and the cross-over pocket chambers  46  to adjust the mass of the inner annular shell. 
         [0032]    A step towards determining a desired thermal expansion of the turbine and particularly the inner annular shell is to predict the clearance gap during the different operating stages of the gas turbine. The clearance gap can be predicted by estimating the thermal expansion and contraction of the turbine components. For example, the expansion, in a radial direction, of the turbine wheel and buckets is estimated and combined to estimate the radial displacement of the tips of the buckets due to thermal expansion and contraction. Similarly, the radial displacement due to thermal expansion for the inner annular shell can be estimated. The difference between radial displacement of the tips of the buckets and that of the inner annular shell indicates the clearance gap. 
         [0033]    The clearance gap is estimated over all normal operational conditions of the gas turbine, including cold start, fast (warm) start, steady state (such as full speed, full load) and shutdown. The estimated thermal expansions for each of the turbine components and the estimated clearance gap may be plotted to show the clearance graphically during the operational stages of the gas turbine. 
         [0034]      FIG. 3  is an exemplary graph showing radial displacements of turbine components during a cold start stage of a gas turbine. The graph plots time during a cold start stage versus the radial displacement of the turbine components at a particular stage of the turbine and gap closure. The estimated radial displacement is plotted of the turbine wheel (solid line), the buckets attached to the wheel (lines marked with “Δ” and “x”). The thermal displacement of the inner annular casing is represented by the line marked with “∘”. The clearance closure (dotted line) represents the difference of the displacement of the inner annular casing and the sum of the displacements of the turbine wheel and the buckets. 
         [0035]    In the example shown in  FIG. 3 , the radial displacements for the turbine wheel (solid line) and one buckets (line marked by “x”) increase more rapidly than the radial displacement (line marked by “∘”) of the inner annular shell (line marked with “x”). Because the displacements of the turbine wheel and bucket increases more rapidly than the inner annular shell, the clearance between the tip of the bucket and the shrouds begins to close as is indicated in the rapid increase in the clearance closure plot (dotted line). The clearance closure remains generally at a steady value as the rate increases of radial displacement of the inner annular shell. 
         [0036]    The clearance closure plot indicates the dimension of the clearance gap during operation of the gas turbine. As the clearance closure plot increases in value the amount of the clearance gap is reduced. As is shown in  FIG. 3 , the increase in the clearance closure plot shows that clearance closes during startup as the rotating components (turbine wheel and buckets) of the turbine heat faster than the stationary components (inner annular shell). 
         [0037]    The smallest clearance occurs at the peak  48  in the clearance closure plot. Abrupt and narrow peaks in the clearance closure plot indicate short periods of operation during which the clearance is the smallest. Abrupt and narrow peaks are to be reduced and minimized to avoid having to adjust the clearance to accommodate only a short period of the operation of the gas turbine. 
         [0038]    Peaks in the clearance closure line plot may be reduced by altering the design of the turbine components. For example, increasing the surface area of the cooling passages  40  could cause more rapid heating of inner annular shell during startup. The more rapid heating may reduce a peak in the closure line plot. The closure line plot in  FIG. 3  has a peak  50  than is small and not abrupt which resulted from designing the inner annular shell to heat at least as fast as the turbine wheel and buckets. 
         [0039]    An approach to determining the thermal expansion of the stationary components involves accounting for the heat inputs to the components of a portion  60  of the inner annular shell. This approach is illustrated in  FIGS. 4 and 5 . Knowing the heat transfer rates through the portion  60 , the thermal expansion of the inner annular shell can be estimated. 
         [0040]      FIG. 4  shows a cross-section of a portion  60  of the stationary components of a turbine. The portion may correspond to a stage  1  of the turbine. The portion  60  may be selected as the portion of the inner annular shell that is most prone to closing clearance gaps. The portion  60  may be used to estimate the clearance gap based on an assumption that the other portions  62  ( FIG. 3 ) of the inner annular shell are less prone to closing clearance gaps. 
         [0041]    The components include the inner annular shell  26  and a shroud  30 . The tip of a turbine bucket  20  is shown with a small clearance gap (c) between the tip and the inner surface of the shroud. The clearance gap (c) in  FIG. 4  is exaggerated for purposes of illustration. If the clearance gap (c) becomes too small, the tip of the bucket will rub against the shroud. If the gap is too large, hot combustion gases leaking through the gap will be excessive and reduce the efficiency of the turbine to convert the hot gases to work. 
         [0042]    The thermal expansion of the stationary components, e.g.,  26  and  30 , may be estimated based on the heat transfer rates across the surfaces of these components. For example, heat transfer through a portion  60  of the inner annular shell proximate to a stage in the turbine may be estimated based on the heat transfer a forward face  62  and an aft face  64  of the portion  60 . The portion  60  of the shell also has a radially outer surface  66  and a radially inward surface that corresponds to the slots for hooks  70  of the shroud. The radially inward surface may be treated as a hot gas passage (HGP) interface because this surface will be exposed to hot combustion gases that leak into the shroud. The portion  60  is connected by a ligament  72  to another portion  74  ( FIG. 2 ) stage, e.g., stage  2 , of the inner annular shell. While the ligament is not a surface of the portion  60 , the heat transfer rate through the ligament can be estimated. The heat transfer rates through the surfaces of the cooling passages  40  and cross-over pocket chambers  46  is also considered in determining the thermal expansion of the inner annular shell. 
         [0043]      FIG. 5  is a chart showing the heat transfer rates for each surface and ligament of the portion  60  of the inner annular surface. The heat transfer rates are shown at different stages of operation of the gas turbine. 
         [0044]      FIG. 5  shows a graphical bar representing the heat rates  76 ,  78 ,  80 ,  82 ,  84 ,  86  and  88  for each stage of operation of the gas turbine. The order left-to-right of the bars ( 76 ,  78 ,  80 ,  82 ,  84 ,  86  and  88 ) is the same in  FIG. 5  for each of the cold start, steady state, shutdown, turning gear and re-start (hot start) stages of the operation of the gas turbine. 
         [0045]    During a cold start stage of the operation of the gas turbine, the portion  60  of the inner annular shell heats quickly because of the high rates of heat entering the shell through the forward face (rate  76 ), the HGP interface (rate  78 ) and the outer surface (rate  80 ). During a cold start, there is little heat transfer due to the cross-over pocket chamber (rate  82 ) and cooling tubes (rate  84 ). Also, the rates are relatively low of heat transferred to the portion  60  of the inner annular shell due to the aft face (rate  86 ) and through the ligament (rate  88 ) connection in the inner annular shell. 
         [0046]    During the cold start stage, the inner annular shell, turbine wheel and buckets heat and expand rapidly. A person designing the turbines may prefer that the inner annular shell expand radially as fast as or faster than the turbine wheel and buckets to avoid a lack of sufficient clearance during a cold start. To increase the expansion rate of the inner annular shell, the designer may reduce the mass in the shell by, for example, increases the volume of the cooling passages  40  and the cross-over pocket chambers  46 . The designer may also reduce the mass of the inner annular shell by reducing the thickness of the shell or making other adjustments to the design of the shell. 
         [0047]    During the steady state stage, the heat rates  82 ,  84  for the cooling passage and cross-over pocket chamber cool the inner annular shell, while the heat rates through the forward face (rate  76 ), HGP interface (rate  78 ) and outer surface (rate  80 ) continue to heat the shell. A designer may want to achieve a balance between the heating rates and cooling rates of the shell during the steady state stage to minimize thermal expansion or contraction of the inner annular shell during this stage. The designer may adjust the cooling heat rates  82  and  84 , for example, by changing the internal surface areas of the cooling passages  40  and the cross-over pocket chambers  46 . 
         [0048]    During the shutdown stage, the inner annular shell contracts radially inward due to the cooling heat rates. The largest cooling heat rates  82 ,  84  during shutdown are due to the cooling passage  40  and cross-over pocket chambers  46 . The radially inward contraction of the inner annular shell during shutdown potentially could reduce the clearance if the shell contracts faster than the turbine wheel and buckets. The designer may need to consider the heat rates due to the cooling passages and cross-over pocket chambers during the shutdown stage when determining the volumes and surface areas of the cooling passages and cross-over pocket chambers. These volumes and surface areas may need to be reduced to avoid excessively fast contraction of the inner annular shell during the shutdown stage. 
         [0049]    During the turning gear stage, there is little heat transfer through the portion  60  of the inner annular shell. Accordingly, the inner annular shell should not significantly expand or contract during the turning gear stage. 
         [0050]    During a re-start of the gas turbine stage the portion  60  of the inner annular shell is heated due large rates of heat coming through the forward face (heat rate  76 ), the HGP interface (heat rate  78 ), and the outer surface (heat rate  80 ). Smaller but significant rates of heat transfer into the portion  60  come from the cross-over pocket chambers (heat rate  82 ), the cooling passages (heat rate  84 ), the aft face (heat rate  86 ) and the ligament (heat rate  88 ). The heat transfer rates during the re-start stage cause the inner annular shell to expand radially. The designer may confirm that the rate of radial thermal expansion of the inner annular shell is at least as great as the rate of radial expansion of the turbine wheel and buckets during the re-start. If a the rate of radial thermal expansion for the inner annular shell is not sufficient, the designer may reduce the mass of the shell or increase the surface areas of the cross-over pocket chambers and cooling passages. 
         [0051]    Clearance control may be based solely on the design of the turbine. For example, the inner annular shell may be designed to thermally expand radially at least as fast as the turbine wheel and buckets during the cold start and re-start stages, and to minimize pinch points in the clearance during all operational stages. 
         [0052]    To achieve clearance control, the designer may adjust the area of the surfaces the cooling passages and cross-over pocket chambers and change the volume of these passages and pocket chambers to adjust the mass of the inner annular shell. The surface area and volume of the cooling passages may be adjusted by changing the internal diameter of these passages. Similarly, the surface area or volume of the cross-over pocket chambers may be adjusted by changing the internal dimensions of the pocket chambers, such as the height, width or depth. 
         [0053]    To determine whether adjustments are needed to the surface area and volume of the cooling passages and cross-over pocket chambers, the designer may consider the thermal radial displacements for the turbine wheel, buckets and inner annular shell during various operational stages, including a cold start, steady state, shutdown, turning gear and re-start stages. For each of these stages, the designer may estimate the closure of the clearance during the stage and identify a pinch point(s) where the clearance is smallest. The adjustments may be made to minimize the pinch point or the rate of change of the clearance closure near the pinch point. 
         [0054]    In addition, the designer of a turbine may consider the rates of heating or cooling of a component of the turbine, such as an inner turbine shell. These rates may correspond to the surfaces or ligaments of a portion of the shell near a particular stage, e.g., stage one (1), of the turbine. If the combined heat rates through the surfaces or ligaments result in a thermal expansion of the shell that is not commensurate, e.g., match or slightly exceed, with the combined thermal radial expansion of the turbine wheel and buckets, the design of the inner annular shell may be changed, such as by adjusting the surface area or volume of the cooling passages and cross-over pocket chambers in the shell. 
         [0055]    The cooling passages and cross-over pocket chambers may be designed to cause the inner annular shell to thermally expand radially faster than the turbine wheel and buckets during a cold start and a re-start operation. Similarly, the cooling passages and cross-over pocket chambers make be designed to thermally contract radially slower than the turbine wheel and buckets during a shutdown operation. Achieving these design goals should reduce the pinch points in clearance closure during operation of the gas turbine. 
         [0056]    Clearance control in a gas turbine may be achieved by matching the thermal radial expansion and contraction of the inner annular shell with that the thermal radial expansion and contraction of the turbine wheel and buckets. The matching expansions should occur during the various operational stages of the gas turbine. The clearance control may be achieved without active devices such as cooling flow valves that adjust the flow of compressed air through passages  40  to modify the thermal expansion or contraction of the inner annular shell or with controllers to operate such valves. 
         [0057]    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.