Abstract:
Disclosed herein is a method for enabling turn down of a turbine engine, comprising: extracting compressor discharge air from a working fluid path before it enters a combustion zone of the turbine engine; and reintroducing the extracted air to the working fluid path downstream of a combustor exit. Further disclosed herein is an apparatus related to a gas turbine, comprising: a compressor section, one or more combustors downstream from the compressor section, a turbine section downstream from the compressor section; and at least one conduit for extracting compressor discharge air from a working fluid path prior to a combustion zone and reintroducing the extracted air to the working fluid path downstream of a combustor exit in response to the turbine being in a turned down condition.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to rotary machines and, more particularly, to methods for improving the ability to operate at low loads. Many known combustion turbine engines bum a hydrocarbon-air mixture in a combustor assembly and generate a combustion gas stream that is channeled to a turbine assembly. The turbine assembly converts the energy of the combustion gas stream to torque that may be used to power a machine, for example, an electric generator or a pump. In many cases the engine is coupled to a generator who&#39;s rotational speed is a fixed rate that is defined by the electrical frequency of the electric grid. The temperature of the combustion gas stream is referred to as the combustor exit temperature. A common range of combustion gas stream temperatures is approximately 2400° F. to 2600° F. In some of these engines, a lower temperature limit may exist due to the ability of the combustor to completely bum the hydro-carbon fuel at low temperatures. When the combustion process is not completed, high levels of carbon-monoxide (CO) will exist in the turbine exhaust system. High CO emission levels are prohibited by regulatory agencies. Typically, when a turbine is operated at a high load, the combustor exit temperature is high and CO emissions are held to a minimum. As turbine load is decreased, it is necessary, in many gas turbines, to reduce the combustor exit temp, which may result in increased CO emissions. To prevent this increase in CO emissions it is desired to employ a method that can maintain high combustor temperatures while the engine is at low loads.  
         [0002]     In order to maintain the emissions below a desired limit, the combustor exhaust temperature must be maintained within a specific range. Since the structural integrity of turbine hot gas path components such as nozzles and buckets is related to working fluid flow velocity and temperature, and coolant temperature and flow rate, managing the gas turbine generator load reduction can have significant life benefits while meeting the stringent regulatory emissions requirements.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0003]     Disclosed herein is a method for enabling turn down of a turbine engine, comprising: extracting compressor discharge air from a working fluid path before it enters a combustion zone of the turbine engine; and reintroducing the extracted air to the working fluid path downstream of a combustor exit.  
         [0004]     Further disclosed herein is an apparatus related to a gas turbine, comprising: a compressor section, one or more combustors downstream from the compressor section, a turbine section downstream from the compressor section; and at least one conduit for extracting compressor discharge air from a working fluid path prior to a combustion zone and reintroducing the extracted air to the working fluid path downstream of a combustor exit in response to the turbine being in a turned down condition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:  
         [0006]      FIG. 1  depicts a partial cross sectional view of a gas turbine engine in accordance with an embodiment of the invention;  
         [0007]      FIG. 2  depicts the first stage nozzle area and a method of delivering the bypass air to the turbine flowpath of  FIG. 1 ; and  
         [0008]      FIG. 3  depicts an exploded view of a first stage nozzle and inserts in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]     Gas turbines generally include a compressor section, a combustion section and a turbine section. The compressor section is driven by the turbine section typically through a common shaft connection. The combustion section typically includes an array of spaced combustors. A fuel/air mixture is burned in each combustor to produce a hot energetic gas, which flows through a transition piece to the turbine section. For purposes of the present description, only one combustor is discussed and illustrated, it being intended that any number of the other combustors arranged about the turbine can be substantially identical to the first including all combustors being substantially identical to one another.  
         [0010]     It should be appreciated by those skilled in the art that alternate embodiments of the invention may be applied to machines with multiple shaft turbines and to those with single chamber combustor sections, which may be annular or may be positioned non-symmetrically around the machine.  
         [0011]     Referring to  FIG. 1 , a gas turbine engine according to an embodiment of the invention is depicted generally at  10 . Working fluid, illustrated here as compressor discharge air  20 , from a compressor section  14  is contained within the turbine engine  10  by an engine casing  18 . A portion of the compressor discharge air  20 , referred to as combustor air  24 , flows into a combustor  30 . The combustor air flows axially along an outside wall  21  of the combustor liner  22  into a combustor head  26 . Most of head end air then enters fuel injectors  34  where it is mixed with fuel before being combusted in a combustion zone  23  inside a combustor liner  22 . Another portion of the air in the combustor head  26  becomes cooling fluid illustrated here as extracted air  25 . After combustion, combustion gases  98  travel through a transition piece  38  and a section of the combustor known as a combustor exit  46  before passing through a first stage nozzle  42  and into a turbine section  44 .  
         [0012]     The combustion process takes place within the combustor  30 , and the parameters necessary to meet desired emissions limits are substantially controlled within the combustor  30 . It has been determined that the temperature of the combustion process plays a key role in whether or not an engine meets the desired emissions limits. The temperature at the combustor exit  46 , in particular, has a strong correlation to emissions output, in that, if the combustor exit  46  temperature falls below a certain level, the emissions quickly increase. The combustor exit  46  temperature depends on factors such as, air flow and fuel flow, for example. By reducing both the air flow and the fuel flow, the total amount of air and fuel that combust in the combustor  30  is decreased resulting in a decreased level of enthalpy entering the turbine. This reduction in enthalpy causes a reduction in engine output at a constant speed. In this case, since the air fuel ratio is maintained at acceptable levels, the temperature of the combustor exit  46  is also maintained thereby preserving an acceptable level of emissions.  
         [0013]     It should be appreciated by those skilled in the art that embodiments of the invention may be applied to machines that reduce their load with turbine variable vanes configurations, compressor variable guide stator configuration and gas turbine variable rotor speed configuration.  
         [0014]     An embodiment of the invention maintains the air fuel ratio in the combustion zone  23  by varying the amount of extracted air  25  for a given level of fuel delivered to the nozzle  34 . More specifically, the extracted air  25  is removed from somewhere upstream of the combustion zone  23 , by porting it into extraction sleeve  50 . It is then ported through an extraction conduit  54 , which may be insulated, and an optional valve  27  and is combined with extracted air from the other combustor heads  26 ; if more than one combustor head  26  is having air extracted, before being fed to a booster pump  58 . Although this embodiment illustrates the use of a booster pump  58 , it should be understood that embodiments without a booster pump  58  may also be utilized as will be described in more detail below. Additionally, alternate embodiments may use the valve  27  to vary the amount of extracted air  25  without the pump  58 , and still other embodiments may use the valve  27  and the pump  58 , however when both the valve  27  and the pump  58  are used the pump  58  should be of the non-positive displacement type thereby allowing the flow variation to be controlled by the valve  27 . It should be appreciated, by one skilled in the art, that it is not necessary to extract air from all combustor heads  26 , of a turbine engine  10 , however, if balancing of air flow through all combustors  30  is desired, then it is an option. The booster pump  58  is located outside of the engine casing  18  and is driven by a pump driver  62 . The pump driver  62  may be any motive system for example a variable speed electric motor or a steam turbine. If a steam turbine is used then expanding steam from a heat recovery steam generator (HRSG) of a combined cycle power plant, for example, as is shown in  FIG. 1  may be supplied from the HRSG through supply conduit  66  and returned to HSRG through return conduit  68 . The booster pump  58  may operate over a wide range of speeds. By using a Roots pump, which puts out a given volume flow rate based on its rotational speed, as the booster pump  58 , a pump outlet flow  60  can be predictably controlled. It should be appreciated, by one skilled in the art, that a plurality of booster pumps  58  may be used thereby allowing pumping of air to continue during down time of a single booster pump  58 .  
         [0015]     The pressurized outlet flow  60  is then directed back through a return conduit  72  and enters a working fluid path  94  through the first stage nozzle  42 . By reintroducing the outlet flow  60 , downstream of the combustor exit  46 , to a first stage nozzle airfoil  96  and platform  102 , the air enters the working fluid path  94  without having a significant impact on the temperature profile at the axial plane of the nozzle trailing edge. Establishing a proper ratio of airfoil  96  and platform  102  flow will allow the system to minimize the impact to the critical core flow temperature profile. A change to a temperature profile for a hot gas path piece of hardware ( FIGS. 2 and 3 ) will result in a local temperature spike that results in a reduction of the down stream hot gas path part lives.  
         [0016]     An embodiment of the invention introduces the outlet flow  60  into the working fluid path  94  in a way that will reduce the average temperature of the turbine working fluid path  94  while minimizing the impact on the temperature profile. This reduction in the average temperature results from the outlet flow  60  mixing with the combustion gases  98  resulting in a lower average temperature and extending the life of the turbine hardware.  
         [0017]     Referring now to  FIGS. 2 and 3 , the pump outlet flow  60  cools the hot gas path components illustrated in this embodiment as, a first insert  80 , a second insert  82 , the first stage nozzle airfoil  96 , thereby extending their operational life. It should be appreciated that other embodiments may port the pump outlet flow  60  to nozzles later than the first stage while still falling within the scope of the invention. The return conduit  72 , which may be insulated, ports the pump outlet flow  60  through the engine casing  18  and into a manifold  76  that surrounds the turbine engine  10  peripherally outside of the first stage nozzles  42 . A cross over tube  84  fluidly connects the manifold  76  to the first stage nozzles  42 . Pump outlet flow  60  flows into both the first insert  80  and the second insert  82  that are inserted into a first cavity  88  and a second cavity  92 , respectively, of an airfoil  96  of the first stage nozzle  42 . Impingement holes  100  formed in the inserts  80 ,  82  and cooling holes  104  formed in the airfoil  96 , and cooling holes  106  in the platform  102 , allow pump outlet flow  60  to flow therethrough such that it mixes with combustion gases  98  exiting the transition piece  38  of the combustor  30 . The sizing of the cooling holes  104  and  106  can result in a proportioning of the reintroduction of the pump outlet flow  60  in such a way to improve uniformity of cooling of the hot gas path components, thereby extending their operational life. It should also be appreciated, as noted above, that an embodiment of the invention may not use a pump  58  at all and may rely on the differences in pressure from the combustor head  26  to the first stage nozzle  42  to draw compressor discharge air  20  through conduits  54 ,  72  to the first stage nozzle  42 .  
         [0018]     In the exemplary embodiment illustrated, in addition to increasing the cooling of the hot gas path components, recombining all of the extracted air  25  (cooling fluid) with the combustion gases  98 , prior to the first stage nozzle  42 , assures that maximum power production will be achieved since all compressor discharge air  20  (working fluid) will pass through all of the turbine sections  44  of the gas turbine engine  10 .  
         [0019]     A doubling of the pump outlet flow  60  through the first stage nozzle  42  will allow a significant extension of engine turn down. To minimize an increase in pressure inside the first stage nozzle  42  at double the pump outlet flow  60 , diameters of the impingement holes  100 , in the inserts  80 ,  82 , and the cooling holes  104  in the nozzle airfoil  96  and/or platform  102  should be sized to meet the back pressure requirements of the booster pump  58   
         [0020]     Some advantages of some embodiments of the invention include: increase in the range of engine turn down while meeting desire emission limits, improved and uniform cooling of hot gas path components, increased life of hot gas path components, and reduced fuel consumption at low loads.  
         [0021]     While the embodiments of the disclosed method and apparatus have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the embodiments of the disclosed method and apparatus. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiments of the disclosed method and apparatus without departing from the essential scope thereof. Therefore, it is intended that the embodiments of the disclosed method and apparatus not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the embodiments of the disclosed method and apparatus, but that the embodiments of the disclosed method and apparatus will include all embodiments falling within the scope of the appended claims.