Patent Publication Number: US-2015082801-A1

Title: Gas turbine and method to operate the gas turbine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European application 13185968.8 filed Sep. 25, 2013, the contents of which are hereby incorporated in its entirety. 
     TECHNICAL FIELD 
     The claimed invention is related to a gas turbine and a method to operate the gas turbine. 
     BACKGROUND 
     Gas turbine systems are known to comprise different stages of fuel injection. Each stage comprises a corresponding control valve. 
     DE 10 2006 015 529 A1 relates to a burner system with different stages of fuel injection. Several valves are each connected to a corresponding fuel pipe. The fuel pipe is connected to a corresponding fuel injection stage. The valves are controlled by means of control equipment. 
     SUMMARY 
     In the view of the prior art, it is solved the problem to provide the gas turbine system and a method to operate the gas turbine system with a stable control during transient changes in the system. 
     According to the present disclosure, this problem is solved by providing a first control valve controlling a fluid mass flow in a first fluid path, and a second control valve controlling a ratio of mass flows in a second and third path. Especially fast changes of the total flow in the first fluid path can be operated while achieving a reduced impact of the fast change of the total flow on the ratio. Particularly, cross-coupling effects of the mass flow in the first path and the ratio can be reduced by the ratio control by means of the second control valve. It can be achieved a better accuracy while applying less complex control methods. This allows further improvements in reducing the emissions and/or stability to operate the gas turbine. On the other hand, transient situations can be controlled better. For example, during transients regarding the fluid mass flow through the first fluid path the ratio of the second and third flow in the respective paths can be kept constant providing improved accuracy regarding the control of the fluid mass flow in the first fluid path. 
     As a third hydraulic diameter of the third fluid path is fixed compared to the hydraulic diameter of the first and second paths, which are adjustable, it is provided more accuracy for the actual fluid mass flows in the second and third fluid paths. Furthermore, the fixed third hydraulic diameter allows the provision of fewer control valves, especially fewer control valves with shut off capability, as a control valve in the third fluid path is omitted. Moreover, the control methods can be less complex therefore providing more stable control while concurrently providing a more precise and better fluid supply. Also the valve size of the second control valve, especially the size of the effective diameter, can be reduced. Especially the reduction of the valve size and the provision of a fewer number of control valves reduce overall system costs. A higher degree of total accuracy can be achieved by smaller valves with the same or lower degree of accuracy. 
     According to an embodiment is provided a shut off valve arranged in series with the first control valve, and the first control valve providing a shut off capability. 
     Advantageously further shut off valves can be omitted, therefore reducing overall system costs while providing a high degree of safety, especially complying with cipher 5.10.5.4 of ISO 21789:2009. 
     In an advantageous embodiment it is provided a fourth, fifth and sixth fluid path. A third and a fourth control valve allow the control of a fluid mass flow in the fourth fluid path and a control of a ratio of fluid mass flows in the fifth and sixth fluid paths, respectively. This embodiment allows controlling highly transient operations of the gas turbine by means of the first and third control valve, by for example leaving the respective ratios essentially stable. Highly transient operations or processes of the gas turbine comprise for example load shedding or the start of the gas turbine. 
     According to a further advantageous embodiment the first and the fourth fluid paths are connected to the same fuel source. The advantages and benefits of the control of the gas turbine in this embodiment allows to reduce costs as only one fuel supply has to be provided for the first and fourth fluid paths. 
     According to a further advantageous embodiment the second path is connected to a fewer number of fluid outlets, especially burners than the third path. This embodiment allows an operation of the gas turbine with all stages connected to the second and third path to be operated with the same amount of fuel. 
     In a further advantageous embodiment the maximum of the second hydraulic diameter is smaller than the third hydraulic diameter. This embodiment allows improvement in accuracy regarding the ratio. The smaller the second hydraulic diameter, the more accuracy of the ratio can be achieved. Consequently, in practice it has to be achieved a trade-off between the requirements of fuel supplied with the second fluid path and the accuracy of the ratio to be achieved, wherein the accuracy of the ratio is essentially influenced by the second control valve which defines the maximum of the second hydraulic diameter. 
     According to an advantageous embodiment the first hydraulic diameter is adjusted in a first time interval and the second hydraulic diameter is adjusted in a second time interval according to a logic exclusive or (XOR) fashion referring to the time intervals. This enables a more precise fuel supply with less complex control methods therefore providing a stable control of the fuel supply. 
     According to a further advantageous embodiment a response time of the second controller is greater than a response time of the first controller which is an advantageous adaption to operate the gas turbine especially in states between a transient state and a steady state. 
     In a further embodiment the second control valve is operated in a transient state s by a feed forward controller reducing or avoiding cross-coupling effects which may occur between two control loops being active and controlling the first and second control valve in the transient state. 
     According to one embodiment of the method a process variable of the first controller is a pressure difference between a pressure before the first control valve and a pressure behind the first control valve and/or a fluid temperature of fluid in the first path and/or a total fluid mass rate through the first path. Alternatively or in combination a process variable of the second controller is a pressure difference between a pressure before the second control valve and a pressure behind the second control valve. Further, alternatively or in combination an emission of the gas turbine, especially a ratio of nitrogen oxides, of carbon monoxide or of unburned hydrocarbons in the emissions,) and/or a flame stability criteria can be used as variable of the first and/or the second controller. A the flame stability criteria can for example be the determination of pressure fluctuation in the combustion chamber (e.g. combustor pulsations) or radiation (e.g. OH-radicals) or temperature measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above described and other features are exemplified by the following figures and detailed description. Referring now to the figures, which are exemplary embodiments, and wherein like elements are numbered alike: 
         FIG. 1  shows a gas turbine with sequential combustion; 
         FIG. 2  shows a section through a combustor of the gas turbine; 
         FIGS. 3 and 4  show a schematic block diagram of a part of a fluid supply, respectively; 
         FIGS. 5 to 7  show schematic block diagrams of control methods, respectively; 
         FIG. 8  shows the schematic state diagram; and 
         FIGS. 9 to 15  show a schematic valve and fluid path configuration, respectively. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows in a schematic way a gas turbine  2  with sequential combustion in an exemplary embodiment. The gas turbine  2  includes a compressor  4 , a first combustor  6 , a first turbine  8 , a second combustor  10  and a second turbine  12 . Typically, the gas turbine  2  includes a generator  14  which, arranged at the cold end of the gas turbine  2 , that is to say at the compressor  4 , is coupled to a shaft  16  of the gas turbine  2 . A fuel, gas or oil, is introduced via a fuel feed  18  to a burner  20  of the first combustor  6 , mixed with air which is compressed in the compressor  1 , and combusted. Hot gases are partially expanded in the subsequent first turbine  8 . 
     A fuel feed  22  supplies combustion fluid to a burner  24  of the second combustor  10 . The burners  20  of the first combustor  6  are referred to belong to a group. The combustors  24  of the second combustor  10  are referred to belonging to a second group. 
     The reference signs are consequently used in a more general fashion to refer to groups  6  and  10 . Of course, the grouping can be arranged differently and the present disclosure may be applied to other types of gas turbines other than the gas turbine  2  shown in  FIG. 1 . Of course, the teachings described herein can be applied to any fluid which is supplied to the gas turbine. The fluid can be or include fuel, like gas or oil, water, and/or air. 
       FIG. 2  shows a schematic section through the first combustor  6  or the second combustor  10 , respectively in an exemplary fashion. Of course, the respective combustors  6  and  10  can be embodied differently. The shown combustors  6 ,  10  in  FIG. 2  shows also a fuel distribution system which supplies to sub-groups of the burners  20 ,  24 . A sub-group of burners  20 ,  24  is also referred to as a stage. It is provided a first fuel ring for a main stage or first stage  26 . A further fuel ring is provided for a second stage  28  of burners. The respective stages  26  and  28  are supplied with combustion fluid, fuel, by the connection points  30  and  32 , respectively. Consequently the stages  26  and  28  can be operated differently to realize a staging and therefore to stabilize the combustion of the respective burner  6 ,  10 . 
     According to an embodiment of the gas turbine not shown, there can be a plurality of fuel outlets attached to at least one of the burners  20 ,  24 . Therefore, also different stages can be assigned to a plurality of the burners  20 ,  24 . For example, the stages  26  and  28  can be connected to each of the burners  20 ,  24  of  FIG. 2 , resulting in each of the burners  20 ,  24  comprising two fluid outlets. Therefore a group of outlets can be assigned to at least one of the burners  20 ,  24 . 
     Furthermore, according to an embodiment not shown, there can be a first plurality of first fuel outlets of a first stage attached to a single fuel outlet of a first burner, a second plurality of second fuel outlets of a second stage attached to a first fuel outlet of a second burner, and a third plurality of third fuel outlets of a third stage attached to a second fuel outlet of the second burner. 
       FIG. 3  shows in a schematic way a block diagram of a block  34  with a connection point  36  connected to a fluid supply not shown and with the connection points  30  and 32 to be connected to a respective stage  26 ,  28  of a group  6 ,  10  of fluid outlets. The connection point  36  as an input leads to a first fluid path  40 , which is separated into a second fluid path  42  and a third fluid path  44  by means of a separation unit  46 . Of course, the fluid paths  42  and  44  can comprise further elements like fixed restrictors or fixed orifices which are not shown. 
     The first fluid path  40  is characterized by a first hydraulic diameter  50 . The second fluid path  42  is characterized by a second hydraulic diameter  52 . The third fluid path  44  is characterized by a third hydraulic diameter  54 . The hydraulic diameter d_h is determined for a respective path by the following equation 1, wherein A is a cross sectional area, and wherein U is the wetted perimeter of the cross-section. In contrast to an effective cross sectional area or a flow coefficient the hydraulic diameter can be determined independently of a volume flow of the fluid. 
     
       
         
           
             
               
                 
                   
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                     U 
                   
                 
               
               
                 
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     The block  34  includes a first control valve  60  and a second control valve  62 . The first control valve  60  controls a fluid mass flow in the first fluid path which includes an adjustment of the first hydraulic diameter  50 . The second control valve  62  controls a ratio  64  of fluid mass flows in the second and third paths  42  and  44  which either can include an adjustment of the second and third flows through the respective paths by adjusting the second hydraulic diameter  52  only. The first and second control valve  60  and  62  can be implemented as a variable orifice, respectively. The term control valve does not necessarily involve a closed-loop control of the respective valve but may also relate to an open-loop control of the respective valve. 
       FIG. 4  shows an embodiment of block  34  in a schematic way. The connection point  36  is supplied with fluid in a flow direction  66  of the fluid. In an embodiment the first fluid path  40  begins at a connection point  68  and ends on the separation unit  46 . 
     In front of the connection  68  it is arranged a shut off valve  70  arranged in sequence with the first control valve  60 . The control valve  60  can comprise a shut off function. A further drain unit not shown is arranged between the valves  60  and  70 , especially for double block situations where both valves  60  and  70  are shut off and therefore closed, and furthermore for bleed set up situations in which the drain unit is opened between the valves  60  and  70  to relieve the first fluid path  40 . 
     A control valve like the valves  60  and  62  comprises an open position and a closed position and further positions in between the opened position and the closed position. A shut off valve like the shut off valve  70  on the other hand does only include an opened position and a closed position. 
     Furthermore, in  FIG. 4  it is shown a control unit  72  which applies control signals  74 ,  76  and  78  to the valves  70 ,  60  and  62 , respectively. Furthermore, a pressure sensor  80  is arranged ahead of the first control valve  60  to measure a pressure  82  which is supplied to the control unit  72 . A pressure sensor  84  is arranged in flow direction  66  behind the first control valve  60  to measure the pressure  86  and supply the pressure  86  to the control unit  72 . A further pressure sensor  88  is arranged behind the second control valve  62  to supply a pressure  90  to the control unit  72 . Of course, a measurement of the flow can be accomplished by means of other suitable sensors too, or the flow is estimated. 
     Especially a fluid flow through the first control valve  60  it can be determined with the pressures  82  and  86 . The fluid flow through the second path  42  and especially through the second control valve  62  can be determined by the pressures  86  and  90 . By the fluid mass flow through the first path  40  and the second path  42  the fluid mass flow through the third path  44  can be determined, for example by determining the difference between the two fluid flows. 
     In an embodiment not shown the second control valve  62  is arranged at the separation unit  46  in the form of a 3-way-valve. 
       FIG. 5  shows a schematic block diagram  92  of a feed-back control loop. A controller  94  is adapted to produce the signal  76  to operate the first control valve  60 . 
       FIG. 6  shows a schematic block diagram  96 . A controller  98  is adapted to produce the signal  78  to control the second control valve  62 . 
     According to an embodiment a bandwidth of the control loop  92  is greater than a bandwidth of the control loop  96 . This means particularly that a time constant of the first controller  94  is at least the half of a time constant of the second controller  98 . 
       FIG. 7  shows a schematic block diagram  100  with a feed forward controller  102  which produces the signal  78  to control the second control valve  62 . 
       FIG. 8  shows a schematic state diagram  104  with a steady state  106  and a transient state  108 . The steady state  106  of the gas turbine  2  is characterized by and is determined by a constant load or a constant fluid mass flow through the first path  40  for example. The ratios of the respective variable or variables analyzed to determine the steady state  106  are tolerated to vary within limits for a defined period of time. The transient state  108  on the other hand is determined based on the respective variable or variables exceed the previously mentioned limits for a further defined period of time. The transient state  108  may relate to situations like up-loading, de-loading, power load shedding or the startup of the gas turbine  2 . 
     In an embodiment the first coefficient  50  is adjusted and the second coefficient  52  is maintained at a fixed value in the transient state  108 . In the steady state  106  the first hydraulic diameter  50  is maintained at a fixed value and the second hydraulic diameter  52  is adjusted. 
     According to an embodiment in the steady state  106  the signal  76  is maintained fixed and the control loop according to the block diagram  96  is active. In the transient state  108  the signal  78  is maintained fixed and the control loop according to the block diagram  92  is active. 
     According to an embodiment in the steady state  106  control loops according to the block diagrams  92  and  96  are active. In the transient state  108  a control loop according to the block diagram  92  is active and the signal  78  is hold at a fixed value. 
     According to an embodiment in the steady state  106  the control loop according to the block diagram  92  is active and a control loop according to the block diagram  96  is active. In the transient state  108  a control loop according to the block diagram  92  is active and a control according to the block diagram  100  is active. 
     In  FIG. 9  it is shown a valve/path configuration for two groups  6 ,  10  burners. With reference to  FIGS. 1 and 2  instead of the burners  20  and  24  it can be referenced to in a more general fashion to two inlets. 
     Each of the groups  6  and  10  comprises a configuration according to a configuration between the connection points  68  and  32  and  30  of the block  34  in  FIGS. 3 and 4   s.  Therefore reference can be made to the first control valve  60   a  and the second control valve  62   a.  A fourth path  40   b  comprises a third control valve  60   b.  The fourth path 40b is divided into a fifth path  42   b  which comprises a fourth control valve  62   b,  and a sixth path  44   b.    
     The first and fourth fluid paths  40   a  and  40   b  are connected to the same fluid source and/or the same fluid path via a further fluid path  110  which comprises the shut off valve  70 . 
     In  FIG. 10  it is shown a further valve/path configuration for two groups  6  and  10 . In comparison to the configuration shown in  FIG. 9 ,  FIG. 10  comprises two different connection points  36   a  and  36   b  for connecting it with a different fluid source. The shut off valves  70   a  and  70   b  are connected to the first and third valves  60   a  and  60   b,  respectively. 
       FIG. 11  shows a further embodiment of a valve/path configuration for different groups  6  and  10 . For the group  6  an arrangement according to the block  34  is provided. For the group  10  it is provided a connection point  112  and a further control valve  60   b.    
       FIG. 12  shows schematically a valve/path configuration for groups  6  and  10 . With difference to  FIG. 11  the configuration of  FIG. 12  provides a common fluid source by connection point  36 . 
       FIG. 13  shows schematically a valve/path configuration for groups  6 ,  10  and  114 . With difference to  FIG. 9  is provided a further configuration of paths and control valves according to the configuration between the connection points  68 ,  30  and  32  of block  34  in  FIG. 4 , wherein this configuration is attached to the shut off valve. 
       FIG. 14  shows schematically a further embodiment of a valve/path configuration for groups  6 ,  10  and  114 . With difference to  FIG. 9  it is provided a further path separation on connection point  30   b  of  FIG. 9  for the group  114 . 
       FIG. 15  shows a further valve/path configuration for groups  6  and  10 . With difference to  FIG. 9  the configuration of  FIG. 15  provides connection points  30   a   1  and  30   a   2  for two different stages wherein the corresponding path of connection points  30   a   1  and  30   a   2  comprises and/or includes a respective control valve.