Abstract:
A method of delivering a purge flow through a gas turbomachine includes generating a purge flow, guiding the purge flow through at least one of a combustor assembly and a turbine portion of the gas turbomachine, determining a cumulative purge volume passing through the one of the combustor assembly and the turbine portion of the gas turbomachine to determine a predetermined purge volume, and discontinuing the purge flow once the predetermined purge volume has passed through the one of the combustor assembly and the turbine portion of the gas turbomachine.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to the art of power plants and, more particularly, to method of meeting a purge flow requirement for a power plant. 
     Prior to light off or ignition, a gas turbomachine (GT) undergoes a purge cycle to expel combustible gases from non-combustion portions of the system. Conventional systems define the purge cycle in terms of purge time at a predetermined purge speed. During a purge cycle, air is introduced into the GT. Once firing speed is achieved, a timer is initiated. The GT continues to accelerate to purge speed and the purge cycle then continues until a predetermined time has passed. Once the predetermined time has passes, the GT typically decelerates to firing speed. Generally, a lower airflow passed through the system while accelerating from firing speed to purge speed is compensated for by the additional airflow after the timer has stopped and the GT decelerates from purge speed to firing speed. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the exemplary embodiment, a method of meeting a purge flow requirement for a gas turbomachine includes generating a purge flow, guiding the purge flow through at least one of a combustor assembly and a turbine portion of the gas turbomachine, determining a cumulative purge volume passing through the one of the combustor assembly and the turbine portion of the gas turbomachine to meet a predetermined purge volume, and discontinuing the purge flow once the predetermined purge volume has passed through the one of the combustor assembly and the turbine portion of the gas turbomachine. 
     According to another aspect of the exemplary embodiment, a method of meeting a purge flow requirement for a power plant includes generating a purge flow in a gas turbomachine, guiding the purge flow from the gas turbomachine into a heat recovery steam generator (HRSG) fluidly coupled to the gas turbomachine, determining a cumulative purge volume passing through the HRSG to meet a predetermined purge volume, and discontinuing the purge flow once the predetermined purge volume has passed through the HRSG. 
     According to yet another aspect of the exemplary embodiment, a turbomachine includes a compressor portion, a turbine portion operatively connected to the compressor portion, a combustor assembly fluidly connected to the compressor portion and the turbine portion, and an adaptive purge control system operatively connected to at least the compressor portion. The purge control system being configured and disposed to adaptively deliver a defined volume of purge flow through the turbine portion. 
     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 
       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: 
         FIG. 1  is a schematic view of a combined cycle power plant including a gas turbomachine fluidly connected to a heat recovery steam generator (HRSG), including a purge control system in accordance with an exemplary embodiment; 
         FIG. 2  is a flow chart illustrating a method of delivering purge flow through the gas turbomachine and HRSG in accordance with an exemplary embodiment. 
     
    
    
     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 
     With reference to  FIG. 1 , a combined cycle power plant (CCPP) in accordance with an exemplary embodiment is indicated generally at  2 . CCPP  2  includes a gas turbomachine  4  fluidly coupled to a heat recovery steam generator (HRSG)  6 . Gas turbomachine  4  includes a compressor portion  10  fluidly connected to a turbine portion  12  through a combustor assembly  14 . Compressor portion  10  is also mechanically linked to turbine portion  12  through a common compressor/turbine shaft  16 . Compressor portion  4  is also mechanically linked to a generator  18  and fluidly coupled to HRSG  6 . At this point it should be understood that while described as being associated with a steam turbine, CCPP  2  may form part of configurations including those that do not employ a steam turbine. It should also be understood that CCPP  2  includes an exhaust system (not shown). 
     Compressor portion  10  delivers compressed air to combustor assembly  14  to be mixed with fuel to form a combustible mixture. The combustible mixture is combusted within combustor assembly  14  to form hot gases that are delivered to turbine portion  12  through a transition piece (not shown). The hot gases expand through turbine portion  12  creating work that is employed to drive, for example, generator  18 . Exhaust gases pass from turbine portion  12  to HRSG  6 . The exhaust gases pass in a heat exchange relationship with fluids in HRSG  6 . For example, the exhaust gases form steam that is used to drive a steam turbomachine (not shown). 
     Prior to combusting the combustible mixture, turbine portion  12  and HRSG  6  are purged of any combustible/gases that may ignite in regions of CCPP  2  not designed for combustion. In accordance with the exemplary embodiment, CCPP  2  includes an adaptive purge control system  40 . Adaptive purge control system  40  includes a central processor unit  42  and a memory  44  that are configured to control a purge cycle in CCPP  2 . In accordance with one aspect of the exemplary embodiment, adaptive purge control system  40  controls the purge cycle such that purge flow ends substantially, simultaneously, with turbine portion  12  reaching firing speed as will be discussed more fully below. 
     Reference will now follow to  FIG. 2  in describing a method  100  of meeting a purge requirement for power plant  2 . Initially, a purge flow speed is defined for turbine portion  12  as indicated in block  102 . Purge flow speed may be held at a single speed, or may include varying turbine speed, wobulating turbine speed, or operating turbine  12  at different speed levels during purge. At this point a purge flow is initiated as indicated in block  104  and guided through turbine portion  12  and toward HRSG  6  as indicated in block  106 . In block  108 , a determination is made whether the purge flow has achieved a predetermined flow rate. The predetermined flow rate may vary from installation to installation and from location to location. If the purge flow rate is not at the predetermined level, the purge flow is allowed to build accordingly. Once the predetermined flow rate has been achieved, adaptive purge flow system  40  monitors a cumulative flow volume passing toward HRSG  6  as indicated in block  120 . Cumulative flow volume is monitored by using temperature and pressure values of the purge flow. In block  130 , a determination is made whether the cumulative flow volume is nearing a predetermined flow volume. The predetermined flow volume may vary from installation to installation. In some cases, the predetermined flow volume represents a number of volume exchanges for HRSG  6 . More specifically, HRSG  6  includes a defined internal volume. The predetermined flow volume would ensure that gases in the defined internal volume would be replaced one or more times. 
     As indicated in block  140 , once the cumulative purge volume nears the predetermined flow volume, adaptive purge control system  40  causes a deceleration of turbine portion  12  from the purge speed to firing speed and determines a purge flow cut off point in block  150 . In accordance with the exemplary embodiment, adaptive purge control system  40  determines a purge flow cut off point that substantially coincide with turbine portion  12  reaching firing speed. In block  160  the purge flow is cut off and a determination is made in block  170  whether the purge flow cut off substantially coincides with turbine portion  12  reaching firing speed. If the purge flow cut off did substantially coincide with turbine portion  12  reaching firing speed, adaptive purge flow control system  40  stores the purge flow cut off point data in memory  44  as a positive result as indicated in block  180 . If the purge flow cut off did not substantially coincide with turbine portion  12  reaching firing speed, the purge flow cut off point is stored in memory  44  as a negative result as indicated in block  190 . Adaptive purge flow control system  40  uses both the positive and negative results to adaptively determine future purge flow cut off points. Alternatively, the purge flow cut off point could be set when a timer, activated when 8% of base load flow is achieved, times out as indicated in block  200 . In general, the cumulative purge flow is based at least in part on a multiple of a total HRSG volume. 
     At this point it should be understood that the exemplary embodiments describe a purge flow control system that “learns” or adapts to turbine speed and purge flow conditions to set a purge flow cut off point that substantially coincides with the turbine reaching firing speed. The purge flow is configured to remove potentially combustible gases from the turbine portion, the HRSG as well as any associated exhaust ducting prior to ignition of the turbomachine system. By substantially matching purge flow cut off with turbine firing speed, the purge flow control system enhances an overall operating efficiency of CCPP  2 . In addition, substantially matching purge flow cut off with firing speed allows for a more open inlet gas vane (IGV) during purge, and for varying purge speeds, so as to reduce thermal stresses on the gas turbomachine and HRSG, lower costs associated with start up and decrease restart times. Finally, it should be understood that the purge volume includes, in addition to a volume of the HRSG, a volume of any associated exhaust system. 
     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.