Patent Document

FIELD OF THE INVENTION 
     The invention relates to an apparatus and method for cooling a turbine. 
     BACKGROUND OF THE INVENTION 
     A gas turbine engine conventionally includes a compressor for compressing ambient air for being mixed with fuel and ignited to generate combustion gases in a combustor. A turbine receives the hot combustion gases and extracts energy therefrom for powering the compressor and producing output power, for example for powering an electrical generator. The turbine conventionally includes one or more stages of stator nozzles or vanes, rotor blades and annular shrouds around the turbine blades for maintaining appropriate clearances therewith. As the turbine inlet temperatures have increased to improve the efficiency of gas turbine engines, it has become necessary to provide a cooling fluid, such as air, to the turbine vanes, blades and shrouds to maintain the temperatures of those components at levels that can be withstood by the materials thereof, to ensure a satisfactory useful life of the components. Cooling is typically accomplished by extracting a portion of the air compressed by the compressor from the compressor and conducting it to the components of the turbine to cool the same. Any air compressed in the compressor and not used in generating combustion gases necessarily reduces the efficiency of the engine. Therefore, it is desirable to minimize the amount of cooling air bled from the compressor. 
     Turbo-machinery performance and reliability are impacted by the clearances between rotating and static hardware. Tighter clearances produce higher efficiencies, but also increase the likelihood of damage from rubs. During operation, the casing of the gas turbine cools off much faster than the rotor on a typical turbine rotor. During a warm or hot restart, the thermal mismatch between the casing and the rotor may cause the rotor to have a greater initial component of thermal growth than the stator and then, as the unit increases in speed, the rotor experiences an additional component of mechanical growth. This causes a transient clearance pinch point. As time progresses and the stator heats up, the casing grows away from the rotor and results in more open full speed full load (FSFL) clearances. The build clearances of a unit must be set in such a way as to avoid a rub during the transient pinch point and still be tight at FSFL. The difference in minimum clearance to FSFL clearance is defined as “entitlement.” The entitlement is determined by the thermal mismatch between rotor and casing. 
     Previous attempts to address this problem have included active clearance control systems. For example, an inner turbine shell may be heated with a medium (e.g. air, N 2 , steam) during startup to grow the stator away from the rotor or to be cooled at FSFL to bring the shell closer to the rotor. As another example, a hydraulic ram may be used to move the rotor axially into position after the unit has reached FSFL. The angle of the bucket tips and the casing shrouds in the turbine are greater than the associated angle and the compressor and this angle mismatch enables the elimination of rub between the bucket tips and the casing shrouds during the transient pinch point. 
     The prior attempts to avoid a rub during the transient pinch point require relatively large clearances between the buckets and the casing and/or the use of an expensive system to be continuously run to achieve clearances during operation of the gas turbine at FSFL. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment, a cooling system for a turbine, comprises a blower configured to generate a cooling gas flow to be passed through a rotor cavity of the turbine; piping configured to deliver the cooling gas flow to the turbine; and at least one valve configured to control the cooling gas flow. The piping is operatively connected to the rotor of the turbine. 
     A method of cooling a gas turbine comprises method of cooling a turbine comprises generating a cooling gas flow with a blower to be passed through a rotor cavity of the turbine; and delivering the cooling gas flow to the turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  schematically depicts a turbine including an apparatus for cooling the gas turbine according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a gas turbine  2  comprises a compressor section  4  and a combustor  6 . The compressor may be an axial compressor having alternating rows of stator vanes and rotor blades arranged in a plurality of stages for sequentially compressing the air, with each succeeding downstream stage increasing the pressure higher and higher until the air is discharged from a compressor outlet at maximum pressure. The combustor  6  receives the compressed outlet air from the compressor portion  4 . Conventional fuel supply conduits and injectors (not shown) are further provided for mixing a suitable fuel with the compressed outlet air for undergoing combustion in the combustor  6  to generate hot combustion gases. 
     The turbine section  8  is downstream from the combustor  6  and the energy of the hot combustion gases is converted into work by the turbine section  8 . The hot gases are expanded and a portion of the thermal energy is converted into kinetic energy in a nozzle section of the turbine section  8 . The nozzle section includes a plurality of stator blades, or nozzles,  28 ,  30 ,  32 . For example, a first stage nozzle includes a stator blade  28 , a second stage nozzle includes a stator blade  30 , and a third stage comprises a stator blade  32 . 
     The turbine section  8  also includes a bucket section. In the bucket section, a portion of the kinetic energy is transferred to buckets  40 ,  42 ,  44  that are connected to rotor wheels  34 ,  36 ,  38 , respectively, and is converted to work. The wheel  34  and the bucket  40  form the first stage, the wheel  36  and the bucket  42  for the second stage, and the wheel  38  and the bucket  44  form the third stage. Spacers  50 ,  52  may be provided between each pair of rotor wheels. 
     During a shutdown of the turbine  2 , a blower  12  is provided to cool down the rotor of the turbine section  8 . The blower  12  may be connected to the inner diameter of an aft shaft  26  of an aft disk by stage  1  piping  14  that is configured to deliver a flow of air between the first and second stages, and by stage  2  piping  18  that is configured to deliver a flow of air between the second and third stages. A first set of check valves, including a blower check valve  15  and a piping check valve  17 , may be provided in the stage  1  piping  14 . A second set of check valves, including a blower check valve  21  and a piping check valve  19 , may be provided in the stage  2  piping  18 . Mixing tees  16 ,  20  may be provided in the stage  1  and stage  2  piping, respectively. Alternatively, the blower  12  may be replaced with a vacuum to draw air out of the turbine  2 . 
     The blower  12  is connected to the gas turbine  2  by an externally fed bore (EFB) circuit  10  which may be, for example, a bucket supply system. For existing gas turbines, the blower may be retrofitted to the gas turbine  2  by retrofitting a bore plug under the aft shaft  26 . The blower piping  14 ,  18  can be connected to the inner diameter of the aft shaft  26  and used in conjunction with the check valves  15 ,  17 ,  19 ,  21 . During normal operation, i.e., non-shutdown conditions, the blower  12  is off and the blower check valves  15 ,  21  are closed and the piping check valves  17 ,  19  are open. 
     During operation at any speed, which may include shutdowns, between trips, while purging, etc., of the gas turbine  2 , the blower  12  is operated to cool down the rotor of the turbine section  8  and the blower  12  is sized and timed such that it forces the cooling rate of the rotor to the same speed as or faster than the cooling rate of the casing of the gas turbine  2 . This allows the gas turbine  2  to be restarted at any time and have the rotor equal to or cooler than the stator temperatures. The operation of the blower  12  may be controlled by a controller  48 . The controller  48  may be a specially programmed general purpose computer, or a microprocessor. The controller  48  may also be an ASIC. The controller  48  may control the operation of the blower  12  based on signals from temperature sensors in the turbine section, e.g. the rotor, and the casing that are sent to the controller  12 . The blower  12  may be used for cooling other plant hardware during FSFL operation, such as exhaust frames/casings. 
     The first blower check valve  15  and the second blower check valve  21  are configured to open when a predetermined gaseous flow is generated by the blower  12 . Concurrently, the first piping check valve  17 , and the second piping check valve  19 , are configured to close such that all blower flow be directed to the turbine section  8 . It should be appreciated that the first check valve set  15 ,  17  and the second check valve set  19 ,  21  may be configured to open at the same, or different, gaseous flows. For example, the first check valve set  15 ,  17  may be configured to open at a first gaseous flow, and the second check valve set  19 ,  21  may be configured to open at a second gaseous flow that is higher than the first gaseous flow. It should be appreciated that other valves than check valves may be used. It should be further appreciated that the controller  48  may be configured to control the operation of the valves. 
     As shown in  FIG. 1 , the cooling flow  22  of stage  1  is shown in solid lines, the cooling flow  24  of stage  2  is shown in dashed lines, and a turbine purge  54  flow is shown in dotted lines. 
     The use of the EFB circuit  10  and the blower  12  provides the gas turbine  2  with sufficient clearance as the mechanical growth and out of roundness allow at a lower cost relative to the active clearance control options of prior art systems. The gas turbine  2  provided with the blower  12  and the EFB circuit  10  is able to run with tighter clearances and does not require an expensive system that continuously runs to achieve the required clearances. The blower  12  is run at non-FSFL conditions when the rotor is hotter than the casing. It can also be used to perform other plant functions, such as exhaust frame cooling during FSFL. 
     Heat transfer analysis may be performed that simulates the blower cooling the rotor of the turbine section  8  during a shutdown to determine how much air flow is required to match the stator time constant to match the cooling rate of the rotor to the cooling rate of the casing of the gas turbine  2 . The clearances are thus controlled by matching the shutdown time constants with rotor augmentation. Unlike prior art systems, which use clearance control systems that deal with moving the stator during either startup or FSFL, the gas turbine  2  provided with the blower  12  and EFB circuit  10  has advantages in that it is operates on the rotor during non-design points so is relatively inexpensive in terms of product cost and does not represent a drain on the performance of the gas turbine  2  during FSFL. 
     Although the embodiment described above is in the context of a gas turbine, it should be appreciated that the cooling apparatus and method described above are also applicable to steam turbines. 
     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.

Technology Category: 2