Abstract:
A clearance control apparatus providing compressed cooling air to a turbine casing in a gas turbine, the apparatus including: a cooling gas passage extending through an inner annular shell of the turbine casing; a cooling gas conduit connected to a compressor of the gas turbine and to the turbine casing, wherein the cooling gas conduit receives compressed air from the compressor and delivers the compressed air to the turbine casing, and wherein the cooling gas conduit is in fluid communication with the cooling gas passage, and a heat exchanger connected to the cooling gas conduit and to a fuel conduit delivering fuel to a combustor of the gas turbine, wherein the heat exchanger transfers heat from the cooling gas to the fuel.

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 dimension of gaps between the rotor and the stator casing that surrounds the rotor. The rotor is typically an axial turbine having rows of buckets each mounted on a turbine wheel. The wheels are mounted on a shaft of the turbine. The stator casing houses the rotor and includes rows of stationary nozzles positioned between the rows of buckets. Clearance often refers to the annular gap between the tips of the buckets and the stator casing. 
         [0003]    Clearance is needed to allow the buckets to rotate without rubbing against the stator casing. If the clearance is too great, combustion gases may leak over the tips of the buckets and do not drive the rotation of the turbine. If the clearance is too small, the tips may rub against the stator casing and may cause vibrations that damage the turbine. 
         [0004]    The clearance varies as the turbine is heated and cools during its various operational phases. The variations in clearance are due to thermal expansion and contraction of the components of a turbine. A turbine is typically formed of metal components having different heat expansion rates. The turbine wheels, buckets on the wheels and annular shells around the buckets expand and contract at different rates. Due to different rates of thermal expansion, clearance could increase or shrink as the gas turbine heats and cools. 
         [0005]    Clearance is needed whenever the turbine buckets rotate including: while the turbine heats up during startup, as the gas turbine transitions from full speed no load (FSNL) operation to and during full speed full load (FSFL) operation, and as the turbine shuts down. Maintaining adequate clearance during any and all operational phases of a gas turbine is achieved by a clearance control system. 
         [0006]    Clearance control systems and techniques provide adequate clearance during all phases of gas turbine operation. Conventional clearance control systems and techniques include cooling systems mounted on external skids adjacent the gas turbine, complex sensing and actuation systems for regulating cooling flow bled from the compressor and used for turbine cooling and external heat transfer systems to heat or cool the cooling air before it enters the turbine. 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. Some active conventional clearance systems are actuated in response to a certain operating conditions, such as at pinch points which occur when clearance is at its narrowest. For example, heating cooling air may be added to the turbine casing to increase the thermal expansion of the casing and thus increase the clearance at a pinch point. 
         [0007]    Active clearance control systems are often mechanically complex, expensive and require computer or hydraulic controllers. Passive clearance control systems do not require controllers and tend to be relatively mechanically simple and inexpensive. However, passive controllers typically do not have the ability to adjust the cooling capacity of the cooling gas fed to the turbine. Even in view of the conventional clearance control systems, there remains a long felt need for clearance control systems and techniques that are robust, economical, assure adequate clearance at all phases of gas turbine operation and avoid excessive clearances especially at steady operating phases such as FSFL. 
       SUMMARY OF INVENTION 
       [0008]    An approach to clearance control has been conceived in which fuel is used to cool the coolant used in clearance control. As the coolant flows through the turbine, such as the stator casing and nozzles, the coolant affects the thermal expansion or contraction of the turbine. The amount of thermal expansion or contraction depends on the temperature of the coolant and specifically the difference between the coolant temperature and the temperature of the turbine. Clearance control can be achieved, at least in part, by adjusting the temperature of the coolant. The conceived approach is to adjust the temperature of the coolant by transferring heat between the coolant and the fuel flowing to the combustors of the gas turbine. 
         [0009]    The conceived approach may be embodied with a heat exchanger through which passes the coolant and a portion of the fuel flow. The coolant is generally cooled by the fuel flow because the fuel is at lower temperature than the coolant—which may be compressed air extracted from one or more states of the compressor in the gas turbine. The rate of fuel flowing through the heat exchanger affects the amount of cooling of the coolant flowing through the heat exchanger to the turbine. The greater the fuel flow the lower the temperature of the coolant flowing to the turbine. 
         [0010]    The flow rate of fuel increases to transition the gas turbine from full speed, no load (FSNL) to full speed, full load (FSFL). As the fuel flow rate increases, there is a similar increase in the cooling, e.g., reduced temperature, of the coolant. The increased cooling reduces the capacity of the coolant to heat and thermally expand the turbine casing during FSFL operation. The reduced capacity results in a reduction in the clearance at FSFL that would otherwise occur if the coolant was not cooled by the fuel. 
         [0011]    While the fuel flow is relatively low, the coolant is cooled to a much lesser extend than when the fuel flow is high. During low fuel flow, the coolant remains relatively hot and thus causes the turbine casing to thermally expand to a greater extent than if the coolant had been cooled by a greater fuel flow. Allowing the turbine casing to thermally expand during low fuel flow may be used to increase the clearance while the fuel flow is low such as during FSNL. 
         [0012]    By using fuel flow to cool the coolant flowing to the turbine casing, a passive clearance control system may be configured that allows for increased clearance during low fuel flow operations and reduced clearance during high fuel flow operations. This ability may be useful for gas turbines having clearance pinch points during low fuel flow operations. 
         [0013]    The clearance control system may be formed by providing a heat exchanger through which passes the compressed coolant flow extracted from the compressor and a portion of the fuel flow. The clearance control system relies on variations in fuel flow to change the amount of cooling of the coolant flow. The clearance control system may be embodied without valves, actuators or other active control devices. 
         [0014]    A clearance control apparatus has been conceived to provide compressed cooling air to a turbine casing in a gas turbine, the apparatus including: a cooling gas passage extending through an inner annular shell of the turbine casing; a cooling gas conduit connected to a compressor of the gas turbine and to the turbine casing, wherein the cooling gas conduit receives compressed air from the compressor and delivers the compressed air to the turbine casing, and wherein the cooling gas conduit is in fluid communication with the cooling gas passage, and a heat exchanger connected to the cooling gas conduit and to a fuel conduit delivering fuel to a combustor of the gas turbine, wherein the heat exchanger transfers heat from the cooling gas to the fuel. 
         [0015]    A gas turbine has been conceived that includes: a turbine casing enclosing a rotating turbine in the gas turbine; a fuel conduit connectable to a supply of fuel and to a combustor of the gas turbine, wherein fuel flows through the fuel conduit from the fuel supply to the combustor; a cooling gas conduit connected to a compressor of the gas turbine and to the turbine casing, wherein the cooling gas conduit receives compressed air from the compressor and delivers the compressed air to an internal passage in the turbine casing, and a heat exchanger connected to the fuel conduit and to the cooling gas conduit, wherein the heat exchanger transfers heat from the cooling gas to the fuel. 
         [0016]    A method has been conceived for clearance control in a gas turbine having a compressor, combustor, turbine and a turbine casing housing the turbine, the method comprising: extracting compressed air from the compressor; cooling the compressed air in a heat exchanger wherein heat from the compressed air is transferred to fuel flowing to the combustor, and modulating a gap between the turbine casing and a rotating section of the turbine by the passing the cooled compressed air through a passage turbine casing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic diagram of a gas turbine showing a cut-away view of the turbine and turbine shell. 
           [0018]      FIG. 2  is a perspective view of a portion of an exemplary inner annular shell of a turbine casing. 
           [0019]      FIG. 3  is a chart showing an exemplary transition from full speed, no load (FSNL) to full speed, full load (FSFL) in an industrial gas turbine. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 1  shows a gas turbine  10  having a compressor  12 , combustor  14  and turbine  16 . An inlet duct  17  provides a passage for air to enter the gas turbine and be directed to the inlet to the compressor. 
         [0021]    Gas turbines generate power by compressing air, mixing the compressed air with fuel  15 , combusting the mixture and driving a turbine with combustion gases. The turbine includes an annular casing  18  that houses rows of turbine buckets  20  (also referred to as blades) 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 stationary nozzles  24  (also referred to as guide vanes). Hot combustion gases  27  (see arrow) flow in an annular hot gas 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  28  is near the outer rims of the wheels  22 . 
         [0022]    A conduit  23  for compressed air extracted from the compressor directs air from the outlet (last stage) of the compressor to the inlet duct  17 . The conduit  23  is included in an inlet bleed heat (IBH) system used while the gas turbine is operating under a no-load or low load condition. By diverting compressed air from the combustor and to the inlet to the compressor, the IBH system reduces the compressor efficiency so that the gas turbine will operate in a self-sustaining manner with no or minimal load. A gas turbine is self-sustaining when the turbine is driven solely by the hot combustion gases formed in the combustor. If the compressor efficiency is not reduced, a much greater load would need to be applied to a self-sustaining gas turbine to avoid undesired increases in the speed of the turbine. Adding the compressed air to the inlet air, the IBH system (conduit  23 ) heats the compressed air entering the compressor. 
         [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 and nozzles. The nozzles  24  are mounted to the inner annular shell  26 . 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. 
         [0024]    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. 
         [0025]    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 delivered by compressed air conduit  37  to cooling passages  40  in the inner annular shell  26  and extending through the nozzles  24 . The compressed air is extracted from one or more stages of the compressor  12  and flows through the conduit  37 . The plenum  34  around the earlier stage buckets receives air compressed to a higher degree and from a later stage of the compressor than the compressed air received by the plenum  36  surrounding a 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. 
         [0026]      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 an outer wall of the plenums  34 ,  36  ( FIG. 1 ). An inner wall of the plenums is formed by the inner annular shell  26 . Radially inward surfaces of the inner annular shell include rows of slots  38  to receive hooks of the shrouds  30 . 
         [0027]    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 a cooling passage, such as 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 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. 
         [0028]    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. The turbine shell is typically hotter than the compressed air because hot combustion gas flows through the hot gas passage  28  and heats the inner turbine shell. 
         [0029]    The compressed air from conduit  37  flows from the compressor  13  may through a compressor extraction modulation valve  48  that regulates the flow of compressed air taken from the compressor and directed to the turbine casing. The modulation valve may be a conventional parallel arrangement of an adjustable valve and a fixed orifice. The adjustable valve may be set to a fixed position during operation of the gas turbine. Or, the adjustable valve  48  may be controlled during gas turbine operation to provide more or less cooling air to the turbine casing. Compressor extraction modulation valves are conventional. 
         [0030]    The compressed air in conduit  37  passes through a heat exchanger  50  as the air flows to the turbine casing. Fuel  15  also flows through the heat exchanger. Heat is transferred through the heat exchanger from the compressed air  37  to the fuel. Heating fuel, especially gas fuel, is conventional and is used to improve the efficiency of a gas turbine by reducing the amount of fuel needed to reach a desired firing temperature in the combustor. It is not conventional to heat fuel with compressed air used for clearance control in a turbine. 
         [0031]    The heat exchanger  50  may be a double tube heat exchanger which ensures that any leakage of fuel does not allow fuel to mix and potentially combust with the compressed air flowing through the heat exchanger. Double tube heat exchangers are conventional and well suited to transfer heat between fuel for a gas turbine and compressed air. In a double tube heat exchanger, the fuel and compressed air may flow through separate tubes  52 ,  54  within the exchanger. A conductive material, such as a fluid or metal, within the heat exchanger transfers heat between the tubes. For example, a condensing fluid in the heat exchanger may be vaporized by the compressed air and condensed by the fuel. A condensing fluid provides an effective and high capacity media for transferring heat between the compressed air and fuel. 
         [0032]    A double walled heat exchanger may also be used as the heat exchanger  50 . A double walled heat exchanger has two walls separating the fuel from the compressed air. Due to the double walls, a leak in one of the walls will not result in the compressed air and fuel mixing in the heat exchanger. The volume of the heat exchanger between the walls may be filled with a solid, liquid or gas that conducts heat between the tubes having fuel and the tubes having compressed air. 
         [0033]    A portion of the fuel  15  is directed through the heat exchanger  50 . Fuel flows from the fuel supply  15  through a conduit  55  to the combustor  14  of the gas turbine. A portion of the fuel is diverted by a flow diverter  56 , such as a one inlet/two outlet valve. The flow diverter may be set at a fixed operating position such that a constant portion of the fuel is diverted through the heat exchanger during all operational phases of the gas turbine. A second flow diverter  58  is downstream of the heat exchanger and merges the fuel flowing through the heat exchanger with the fuel flowing through the conduit  52 . The first and second flow diverters  56 ,  58  divert a portion of the fuel flow through a fuel conduit  60  that extends from the fuel conduit  55 , through the heat exchanger  50  and back to conduit  55 . 
         [0034]      FIG. 3  is a chart showing the rate of fuel flow in an exemplary industrial gas turbine transitioning from full speed, no load (FSNL) to full speed, full load (FSFL). This transition is typically included in the operation of a gas turbine. The transition occurs after the gas turbine has been started and is self-sustaining by combusting fuel. In an exemplary transition, the rotational speed  60  of the gas turbine is accelerated from about twenty percent (20%) to one-hundred percent (100%) of full speed operation. During the initial portion of the transition, no load  62  is applied to the gas turbine. The load  62  may be applied by an electrical generator coupled to the drive shaft. The electrical generator applies a torque to the drive shaft that is overcome by the torque generated by the gas turbine. Shortly after the gas turbine reaches full speed  64 , the load  62  may gradually increase until the load reaches  100  percent  66 . Just before  67  or as the load is applied to the gas turbine accelerates, the inlet bleed heat (IBH) system reduces or shuts off the flow of compressor air through conduit  37  ( FIG. 1 ) to the inlet of the gas turbine. 
         [0035]    The increase in speed  60  is a result of increasing the rate  68  of fuel flow. The fuel flow rate  68  also increases as the load  62  is applied to the gas turbine. The fuel flow rate reaches a maximum in conjunction with the gas turbine reaching a FSFL condition. The rate  70  of coolant flow also increases with the increase in the fuel flow rate  68 . The coolant flow is extracted from the compressor. As the compressor speed increases and the load on the gas turbine increases, the amount of working fluid, e.g., air, flowing from the compressor into the combustor increases to cause the turbine to produce more work to drive the load. When the load is not applied during FSFL, a portion of the compressor outlet air is diverted to the front of the compressor as inlet bleed heat (IBH). As the IBH is turned off (point  67 ), the rate  70  of the coolant extracted from the compressor increases. In this example, the coolant flow rate  70  increases at approximately one-half of the rate  68  at which the fuel flow increases. This difference in rates affects the amount of cooling of the coolant in the heat exchanger  50 . The amount of cooling of the coolant increases as the rate of fuel flow increases more rapidly than the rate of the coolant. 
         [0036]    The turbine may have a “pinch point”  72  during the transition from FSNL to FSFL. A pinch point is where the clearance between the thermal expansions of the rotating and stationary components becomes narrowest during an operation of the gas turbine. Using the fuel to cool the coolant is an effective and simple technique to provide clearance control at the pinch point  72  during the FSNL to FSFL transition and reduce the clearance during FSFL. 
         [0037]    A gas turbine is designed to provide a suitable clearance gap at a pinch point, even if the pinch point occurs for a short period during a transition from FSNL to FSFL. The design requirement that a suitable clearance at the pinch point may result in a larger than desired clearance at operation conditions other than at the pinch point. For example, the clearance at a FSFL may be greater than is desired so that the clearance is adequate at a pinch point. A greater than necessary clearance during a FSFL or other steady state operating condition potentially could reduce the efficiency of the gas turbine for extended periods of operation. 
         [0038]    The greater rate of fuel flow  68  at FSFL provides greater cooling of the coolant at FSFL as compared to the cooling at FSNL and at the pinch point  72 . The clearance at FSFL and other operating conditions may be reduced by cooling the compressed air with fuel is to reduce the clearance between the stationary and rotating components of the gas turbine. Cooling the compressed air  37  flowing into the cooling passage  40  results in the greater cooling of the inner annual shell. The greater cooling will reduce the thermal expansion of the annular shell and shrink the clearance between the annular shell and the rotating components of the turbine. 
         [0039]    The amount of cooling of the compressed air depends on the amount of fuel flowing through the heat exchanger. The greater the fuel flow, the greater amount of cooling of the compressed air. The relationship between the fuel flow and the amount of cooling of the compressed air may be exploited for the benefit of clearance control. The exploitation may be embodied by cooling of the coolant with the fuel flow to modulate the clearance gap between the tips of the buckets and the shrouds attached to the inner annular shell. The modulation may include minimizing the cooling of the coolant while the fuel flow is relatively low, such as during FSNL. Minimizing the cooling of the coolant during FSNL may be used to ensure that at a pinch point the gap does not become too narrow. The modulation may also include increasing the cooling of the coolant while the fuel flow is increased to ensure that the gap does not become too great during FSFL. 
         [0040]    For example, the fuel flow increases dramatically as the gas turbine transitions from full speed, no load (FSNL) to full speed, full load (FSFL). The cooling of the compressed air in the heat exchanger increases in coordination with the increase in the fuel flow. The increasing cooling is applied to provide greater cooling of the inner annular shell as the gas turbine approaches and runs at FSFL as compared to the cooling of the inner annular shell while the gas turbine is at and near FSNL. 
         [0041]    The increased cooling of the inner annular shell due to an increase in fuel flow may be applied to maintain a desired clearance at and near FSFL while providing sufficient clearance at a pinch point at or near FSNL. At FSNL, the rate of fuel flow is relatively low and the amount of cooling in the heat exchanger of the compressed air is small. The small amount of cooling of the compressed air is desired if the gas turbine has a pinch point at or near FSNL. To provide adequate clearance at the pinch point, it is helpful to allow the inner annular shell to thermally expand while the gas turbine is at the pinch point. As the gas turbine transitions beyond the pinch point, the increasing fuel flow results in greater cooling of the compressed air that cools the inner annular shell. The greater cooling slows the thermal expansion of the inner annular shell and minimizes the clearance as the gas turbine reaches FSFL. 
         [0042]    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.