Abstract:
A method for gas turbine start-up can include placing a static starter in a torque control mode, sending a torque reference to the static starter to establish a startup torque for the gas turbine, setting current set points for the static starter and modulating a current output to achieve the startup torque.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to gas turbines, and more particularly to a torque control system and method for gas turbine start-up. 
         [0002]    During a gas turbine startup, there are typically two sources of torque to accelerate the gas turbine to full speed with no load. One source is from the gas turbine itself after it is fired and the other source is from a starter, external to the gas turbine, typically a static starter. Conventionally, an average time taken by the gas turbine to reach full speed with no load can be reduced by changing the initial firing of the gas turbine. However, by changing the firing of the gas turbine, operating temperatures of the gas turbine can be increased, which can damage various hot gas path components. As such, it is typically desirable to increase starting torque of the gas turbine by the external static starter, which can be an electric motor or power converter, for example. However, many static starters have set operational points corresponding to set RPM settings, which do not take into account changing operational parameters of the gas turbine, which can increase the amount of time for the gas turbine to start up. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    According to one aspect of the invention, a method for gas turbine start-up is described. The method includes placing a static starter in a torque control mode, sending a torque reference to the static starter to establish a startup torque for the gas turbine, setting current set points for the static starter and modulating a current output to achieve the startup torque. 
         [0004]    According to another aspect of the invention, a computer readable storage medium for gas turbine start-up is described. The computer readable storage medium can include instructions for causing a computer to implement a method, the method including placing a static starter in a torque control mode, sending a torque reference to the static starter to establish a startup torque for the gas turbine, setting current set points for the static starter and modulating a current output to achieve the startup torque. 
         [0005]    According to yet another aspect of the invention, a gas turbine system for gas turbine start-up is described. The system can include a gas turbine, a gas turbine controller operatively coupled to the gas turbine, a static starter operatively coupled to the gas turbine and the gas turbine controller and a synchronous generator disposed between and operatively coupled to the gas turbine and the static starter, wherein the turbine controller is configured to place the static starter into a torque control mode, the static starter having a startup torque profile having associated current set points. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  illustrates an exemplary system for gas turbine start-up torque control. 
           [0009]      FIG. 2  illustrates an exemplary embodiment of a control system for torque control for gas turbine startup. 
           [0010]      FIG. 3  illustrates a plot of speed versus time illustrating a start profile for a gas turbine in exemplary embodiments. 
           [0011]      FIG. 4  illustrates a plot of current limit versus percentage of rated speed for current set points in accordance with exemplary embodiments. 
           [0012]      FIG. 5  illustrates a flow chart of a torque control method in accordance with exemplary embodiments. 
           [0013]      FIG. 6  illustrates a process flow for a torque control implementation in accordance with exemplary embodiments. 
       
    
    
       [0014]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  illustrates an exemplary system  100  for gas turbine start-up torque control. In exemplary embodiments, the system  100  can include a gas turbine  105  operatively controlled to a synchronous generator  110 . As described herein, upon start-up, the synchronization generator  110  can be electrically excited to produce rotational torque to initiate rotation in the gas turbine without the high temperatures of gas paths typically required to start-up the gas turbine. A turbine controller  115  can be operatively coupled to the gas turbine. In exemplary embodiments, the turbine controller  115  provides the control and monitoring of the gas turbine  105 . The system  100  can further include an exciter module  120  operatively coupled to the synchronous generator  110 . In exemplary embodiments, the exciter module  120  produces a field excitation current to control generator ac terminal voltage and/or the reactive volt-amperes for the synchronous generator  110 . The system  100  can further include a human-machine interface  125 , which can be part of an overall control system (e.g., a computer)  200  as described further with respect to  FIG. 2 . In exemplary embodiments, the control system  200  can be operatively coupled to the system  100  via an Ethernet  130 , which can be part of a larger network as further described with respect to  FIG. 2 . 
         [0016]    Referring still to  FIG. 1 , the system  100  can further include a static starter  135 , which is an adjustable speed ac drive system to start a gas turbine-generator set via the synchronous generator  110 . By operating the synchronous generator  100  as a synchronous motor, the static starter  135  accelerates the gas turbine  105  according to a specific speed profile that provides starting conditions for the gas turbine as described herein. As described further herein, static starter control constants decide current set points at given speeds. In exemplary embodiments, control constants are not altered when the synchronous generator  110  is running. The static starter  135  eliminates the need for separate starting hardware, such as an electric motor or diesel engine, torque converters, and associated auxiliary equipment. In exemplary embodiments, the turbine controller  115  sends run and torque commands, and speed reference set points to the static starter  135 . 
         [0017]    In exemplary embodiments, power magnetics are implemented in the system  100  to provide isolation, voltage transformation, and impedance. For example, an isolation transformer  140  feeds 3-phase ac input power to static starter power converters, a source bridge  145  and a load bridge  150 , forming an input bridge. The isolation transformer  140  provides isolation from an ac system bus (not shown) and provides the correct voltage and phasing to the bridges  145 ,  150 . The static starter  135  can be a current controlled device, and the input bridge provides controlled current to feed a DC link reactor  155 . The DC link reactor can be an air core inductor that provides inductance to smooth the current delivered by the bridges  145 ,  150 , and keeps the current continuous over the system  100  operating range. The system  100  can further include a control supply  160  for the static starter  135 . 
         [0018]    Various circuit breakers and motor operated disconnect switches are implemented in the system  100  to make the appropriate power connections for a static start operation. A circuit breaker  165  is implemented to connect a primary side of the isolation transformer  140  to a system auxiliary bus (not shown). The static starter  135  can control the breaker  165  and is closed during starting. The breaker  165  can optionally be left closed after the start is complete. A motor  170  is an operated disconnect switch implemented to connect the load bridge  150  output bus to the synchronous generator  110  (e.g., a generator stator). The load bridge  150  can be commutated by load where as the source bridge  145  can be line commutated. In exemplary embodiments, the turbine controller  115  controls the motor  170 , which can be powered down during starting and powered up after the start is complete. A circuit breaker  175  is implemented to connect the synchronous generator  110  to the system bus via a set up transformer  180 . The turbine controller  115  controls the circuit breaker  175  and it can be open during startup. In exemplary embodiments, the system  100  controls the torque supplied at the output of the torque supplied by the load bridge  150 , which can be considered a load commutated inverter. 
         [0019]      FIG. 2  illustrates an exemplary embodiment of a control system  200  for torque control for gas turbine startup. The methods described herein can be implemented in software (e.g., firmware), hardware, or a combination thereof. In exemplary embodiments, the methods described herein are implemented in software, as an executable program, and is executed by a special or general-purpose digital computer, such as a personal computer, workstation, minicomputer, or mainframe computer. The system  200  therefore includes general-purpose computer  201 . 
         [0020]    In exemplary embodiments, in terms of hardware architecture, as shown in  FIG. 2 , the computer  201  includes a processor  205 , memory  210  coupled to a memory controller  215 , and one or more input and/or output (I/O) devices  240 ,  245  (or peripherals) that are communicatively coupled via a local input/output controller  235 . The input/output controller  235  can be, but is not limited to, one or more buses or other wired or wireless connections, as is known in the art. The input/output controller  235  may have additional elements, which are omitted for simplicity, such as controllers, buffers (caches), drivers, repeaters, and receivers, to enable communications. Further, the local interface may include address, control, and/or data connections to enable appropriate communications among the aforementioned components. 
         [0021]    The processor  205  is a hardware device for executing software, particularly that stored in memory  210 . The processor  205  can be any custom made or commercially available processor, a central processing unit (CPU), an auxiliary processor among several processors associated with the computer  201 , a semiconductor based microprocessor (in the form of a microchip or chip set), a macroprocessor, or generally any device for executing software instructions. 
         [0022]    The memory  210  can include any one or combination of volatile memory elements (e.g., random access memory (RAM, such as DRAM, SRAM, SDRAM, etc.)) and nonvolatile memory elements (e.g., ROM, erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), programmable read only memory (PROM), tape, compact disc read only memory (CD-ROM), disk, diskette, cartridge, cassette or the like, etc.). Moreover, the memory  210  may incorporate electronic, magnetic, optical, and/or other types of storage media. Note that the memory  210  can have a distributed architecture, where various components are situated remote from one another, but can be accessed by the processor  205 . 
         [0023]    The software in memory  210  may include one or more separate programs, each of which comprises an ordered listing of executable instructions for implementing logical functions. In the example of  FIG. 2 , the software in the memory  210  includes the torque control methods described herein in accordance with exemplary embodiments and a suitable operating system (OS)  211 . The operating system  211  essentially controls the execution of other computer programs, such as the torque control systems and methods as described herein, and provides scheduling, input-output control, file and data management, memory management, and communication control and related services. 
         [0024]    The torque control methods described herein may be in the form of a source program, executable program (object code), script, or any other entity comprising a set of instructions to be performed. When a source program, then the program needs to be translated via a compiler, assembler, interpreter, or the like, which may or may not be included within the memory  210 , so as to operate properly in connection with the OS  211 . Furthermore, the torque control methods can be written as an object oriented programming language, which has classes of data and methods, or a procedure programming language, which has routines, subroutines, and/or functions. 
         [0025]    In exemplary embodiments, a conventional keyboard  250  and mouse  255  can be coupled to the input/output controller  235 . Other output devices such as the I/O devices  240 ,  245  may include input devices, for example but not limited to a printer, a scanner, microphone, and the like. Finally, the I/O devices  240 ,  245  may further include devices that communicate both inputs and outputs, for instance but not limited to, a network interface card (NIC) or modulator/demodulator (for accessing other files, devices, systems, or a network), a radio frequency (RF) or other transceiver, a telephonic interface, a bridge, a router, and the like. The system  200  can further include a display controller  225  coupled to a display  230 . In exemplary embodiments, the system  200  can further include a network interface  260  for coupling to a network  265 , which can include the Ethernet  130 . The network  265  can be an IP-based network for communication between the computer  201  and any external server, client and the like via a broadband connection. The network  265  transmits and receives data between the computer  201  and external systems. In exemplary embodiments, network  265  can be a managed IP network administered by a service provider. The network  265  may be implemented in a wireless fashion, e.g., using wireless protocols and technologies, such as WiFi, WiMax, etc. The network  265  can also be a packet-switched network such as a local area network, wide area network, metropolitan area network, Internet network, or other similar type of network environment. The network  265  may be a fixed wireless network, a wireless local area network (LAN), a wireless wide area network (WAN) a personal area network (PAN), a virtual private network (VPN), intranet or other suitable network system and includes equipment for receiving and transmitting signals. 
         [0026]    If the computer  201  is a PC, workstation, intelligent device or the like, the software in the memory  210  may further include a basic input output system (BIOS) (omitted for simplicity). The BIOS is a set of essential software routines that initialize and test hardware at startup, start the OS  211 , and support the transfer of data among the hardware devices. The BIOS is stored in ROM so that the BIOS can be executed when the computer  201  is activated. 
         [0027]    When the computer  201  is in operation, the processor  205  is configured to execute software stored within the memory  210 , to communicate data to and from the memory  210 , and to generally control operations of the computer  201  pursuant to the software. The torque control methods described herein and the OS  211 , in whole or in part, but typically the latter, are read by the processor  205 , perhaps buffered within the processor  205 , and then executed. 
         [0028]    When the systems and methods described herein are implemented in software, as is shown in  FIG. 2 , the methods can be stored on any computer readable medium, such as storage  220 , for use by or in connection with any computer related system or method. 
         [0029]    As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
         [0030]    Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. 
         [0031]    A computer readable signal medium may include a propagated data signal with computer readable program code embodied therein, for example, in baseband or as part of a carrier wave. Such a propagated signal may take any of a variety of forms, including, but not limited to, electro-magnetic, optical, or any suitable combination thereof. A computer readable signal medium may be any computer readable medium that is not a computer readable storage medium and that can communicate, propagate, or transport a program for use by or in connection with an instruction execution system, apparatus, or device. 
         [0032]    Program code embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing. 
         [0033]    Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
         [0034]    Aspects of the present invention are described below with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
         [0035]    These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
         [0036]    The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
         [0037]    The flowchart and block diagrams in the FIGS. illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
         [0038]    In exemplary embodiments, where the torque control methods are implemented in hardware, the torque control methods described herein can implemented with any or a combination of the following technologies, which are each well known in the art: a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate combinational logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc. 
         [0039]      FIG. 3  illustrates a plot  300  of speed versus time illustrating a start profile for a gas turbine in exemplary embodiments. At time  0 , the turbine  105  is powered down and the static starter  135  has not initiated torque. In exemplary embodiments, permissive checks can be performed prior to start-up. Between time  0  and approximately time  250 , the static starter  135  starts at an approximate turning gear speed of 3-6 RPMS, which ramps up to about 30% of the rated speed of the gas turbine  105 . Between about time  0  and about time  250 , there is less impact on the hot gas path parts. At about time  250 , the gas turbine  105  is fired, which now has an initial rotational torque as provided by the static starter  135 . Between times  250  and  650 , the gas turbine  105  continues to start up as it ramps toward the full 100% of the rated speed, during which there is increased impact on the hot gas path parts. In exemplary embodiments, at about 85-91% of the rated speed, the static starter  135  is disengaged and the start cycle is complete. During start-up, the turbine control  115  sends run and torque commands as well as speed reference set points to the static starter  135 . Static starter control constants determine current set points at a given speed as further described herein. Control constants are not altered when the static starter  135  is miming. The start profile illustrated is an example only, and in other exemplary embodiments, other % speed rates and times are contemplated. 
         [0040]      FIG. 4  illustrates a plot  400  of current limit versus percentage of rated speed for current set points in accordance with exemplary embodiments. As described above, the speed reference points can be set up, which correspond to current set points for the static starter. Once set, the current set points can vary the speed of the static starter  135  during start up. In exemplary embodiments, current set points on the Y-axis are set for different % of rated speeds on the x-axis. Based on what the static starter  135  reads, its current output, and thus speed varies based on the pre-determined y-axis set points. In exemplary embodiments, the plot  400  illustrates both step and ramped current set points, until the static starter  135  remains steady between approximately 48% and 81% of rated speed. Between 81% and 90% of rated speed, the static starter  135  is disengaged as described above. The current set points illustrated are just examples of how the speed reference points can be varied during start-up. In other exemplary embodiments, other current set points at % of rated speeds are contemplated. 
         [0041]      FIG. 5  illustrates a flow chart  500  of a torque control method in accordance with exemplary embodiments. At block  520 , when conditions that the static starter  135  is ready to start at  505 , is not running at block  510  and a fast start enable signal is sent by the turbine controller  115  at block  515 , are all true, then the torque control is enabled. At block  525 , the static starter  135  remains in torque control mode when the speed reference remains above 95% at block  530  and the torque control remains enabled from block  520 . The torque control mode then enables the method blocks as now described. 
         [0042]    Referring still to  FIG. 5 , at block  535  torque reference points are received at the static starter  135  from the turbine controller  115  as described above. At block  540 , the turbine controller  115  calculates electrical output required from the static starter  135 , and therefore the current set point(s). At block  545 , the method  500  determines a difference between a calculated current and actual current in the static starter  135 . If the difference between a calculated current and actual current in the static starter  135  is greater than a ramp rate set at 0.05 per unit (PU), then at block  550 , the turbine controller  115  increments the current at a ramp rate of 0.05 PU. For purposes of calculation, 1 PU of current=756 Amps=5000 digital counts, and 0.05 PU=37.6 A=250 digital counts. At block  555 , the static starter  135  modulates current to achieve the desired torque via a current regulator defined by the bridges  145 ,  150  and the DC link reactor  155 . At block  560 , the turbine controller  115  recalculates the static starter output and torque at the actual current produced. At block  565  the turbine controller  115  compares the gas turbine  105  torque request with the output generated by the static starter  135 . In addition, if at block  545  the difference between the calculated current and actual current in the static starter  135  is not greater than a ramp rate set at 0.05 PU, then at block  565  the turbine controller  115  compares the gas turbine  105  torque request with the output generated by the static starter  135 . At block  570 , the turbine controller  115  generates an alarm if there is a mismatch in torque difference of more than about 5% for five seconds. Blocks  535 ,  540 ,  545 ,  550 ,  555 ,  560 ,  565 ,  570  repeat while the static starter  135  is in torque control mode from block  525 . 
         [0043]    Conventionally, the current output from static starters such as the static starter  135  is calculated from speed-current profiles such as the one illustrated in  FIG. 4 . The output of the Speed-current profile is then input to the current regulator defined by the bridges  145 ,  150  and the DC link reactor  155 . 
         [0044]    In exemplary embodiments, the systems and methods described herein compute the required current based on the torque requirement coming from turbine controller  115 , by passing the speed-current profile. 
         [0045]      FIG. 6  illustrates a process flow  600  for a torque control implementation in accordance with exemplary embodiments. In exemplary embodiments, when the static starter  135  is in torque control mode, a variable, uc_crls generated by the turbine controller  115  is generated as a multiplication factor, which can have an upper value of 1.5 PU and a lower value of 0.2 PU. The uc_crls variable can be rate limited to 0.01 PU/sec, and then multiplied with a constant, C, which can be set at 1.45503. The static starter  135  in torque control mode can then be input to and control the speed-current profile, which passes its output to MN and MAX functions. In exemplary embodiments, UC_Trq_min, UC_Trq_max, are input into the MN and MAX functions respectively. In exemplary embodiments, the output of the speed-current profile passes from the MN BLOCK, which allows only the minimum of the two inputs to pass through. When the system  100  is placed into torque control mode, uc_crls can be increased from 1 to 3, thereby maxing out the variable output from speed-current profile to MIN BLOCK. In exemplary embodiments, the constant UC_Trq_max, coming from the algorithm, to the MIN block is ensured to always stay less and is passed through, thus controlling the current input to the current regulator. In exemplary embodiments, current is calculated by the static starter  135  firmware, for which conventional static starters can be over ridden with new current set point calculated from application code, with out modifying the firmware 
         [0046]    In exemplary embodiments, based on the ambient environmental conditions of the gas turbine  105  during a start-up, the gas turbine controller  115  determines the amount of torque required from the static starter  135  to accelerate the gas turbine  105  to full speed no load. The static starter  135  calculates equivalent electrical power and derives various set points. The static starter  135  starts generating the required additional torque that would aid the gas turbine torque, which helps in reducing the start time. Conventionally, if the same amount of additional torque is generated by the increasing the firing/fuel of the gas turbine  105 , it results in high operating temperatures and hence results in reduction of hot gas path parts life. The systems and methods described herein achieve desirable reduced start-up profiles (e.g., ˜six minutes). 
         [0047]    In exemplary embodiments, the systems and methods described herein can calculate the torque as now described. Mechanical Power, P is given by: P=T*ω, where T is torque and ω is the rotational velocity. The rotational velocity is given by: ω=2Π*f, where f is the number of rotations per second, and given by f=N (rpm)/60, which is equivalent to 1.732*Volts*Amps*Power factor. As such, 
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         [0000]    where P in HP=P (in watt)/745.7. Therefore, 
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         [0048]    In solving Equation (1) above, the required current set point is derived. Similarly, the Torque request in Lbf-ft can be obtained as well, as follows: 
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                   lbf 
                    
                   
                     - 
                   
                    
                   ft 
                 
                 ) 
               
             
              
             
               P 
                
               
                 ( 
                 HP 
                 ) 
               
             
             * 
             
               5251 
               / 
               
                 N 
                  
                 
                   ( 
                   rpm 
                   ) 
                 
               
             
           
         
       
     
         [0049]    Technical effects include achieving a reduced start up profile and generating a start up torque with a reduced exposure of turbine components to the hot gas path during start up. 
         [0050]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.