Abstract:
A combined cycle power plant ( 100 ) utilizing a compressor air bleed ( 62 ) as a source of heat for generating steam in a steam generator ( 78 ) for supplying the gland seals of the steam turbine ( 3 ) and as a motive force for an air ejector ( 90 ) for evacuating the condenser ( 8 ) during plant start-up. The compressor air bleed avoids delay awaiting the availability of steam from the heat recovery steam generator ( 4 ) without the need for an auxiliary boiler.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to the field of combined cycle power plants.  
       BACKGROUND OF THE INVENTION  
       [0002]     Combined cycle power plants are known to include a gas portion and a steam portion. The gas portion includes a gas turbine engine powered by the combustion of a fuel such as natural gas or fuel oil. A steam turbine of the steam portion is powered by steam that is generated by the cooling of the gas turbine exhaust in a heat recovery steam generator (HRSG). The gas turbine and the steam turbine typically provide shaft power for one or more electrical generators.  
         [0003]     The steam portion includes a condenser for converting expanded steam received from the outlet of the steam turbine into condensate for delivery to the heat recovery steam generator. A vacuum is maintained in the condenser during operation by the condensation of steam. During start-up of the plant, the condenser vacuum is established by operating a vacuum pump, such as a mechanical pump or a jet pump. U.S. Pat. No. 6,755,023, incorporated by reference herein, describes the use of a steam jet air evacuation pump for evacuating a power plant condenser. Steam is also needed for supply to the steam turbine shaft gland seals. U.S. Pat. No. 5,388,411, incorporated by reference herein, illustrates a power plant wherein gland seal steam is provided from the heat recovery steam generator.  
         [0004]     Steam is available from the heat recovery steam generator of a combined cycle plant only after a considerable delay due to thermal lag and thermal stress limitations inherent in the system. In order to avoid delaying the start-up of the plant while awaiting steam delivery from the HRSG, an auxiliary boiler may be used to provide steam. Auxiliary boiler steam may be provided to power a steam jet pump for evacuation of the condenser, and it may be provided to the gland seals of the steam turbine. While the use of auxiliary boiler steam provides a benefit by reducing the start-up time for a combined cycle power plant, the use of an auxiliary boiler increases installation, operation and maintenance costs. Because an auxiliary boiler produces airborne emissions, there may also be licensing/permit implications resulting from the use of an auxiliary boiler steam source. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The invention is explained in following description in view of the drawings that show:  
         [0006]      FIG. 1  is a schematic illustration of a combined cycle power plant utilizing compressed air bled from the gas turbine engine compressor to produce gland seal steam and to power a condenser evacuation jet pump.  
         [0007]      FIG. 2  is a schematic illustration of a gland steam and condenser evacuation equipment skid utilized in the power plant of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]     Referring to the drawings, there is shown in  FIG. 1 a  schematic diagram of a combined cycle power plant  100  including a gas portion  98  and a steam portion  96 . The major components of the power plant include a gas turbine engine  2 , a heat recovery steam generator (HRSG)  4 , a steam turbine  6 , and a condenser  8 . The gas turbine engine  2  includes a compressor  10 , a gas turbine section  16  having a rotor shaft  12  connected to the compressor  10  and to an electrical generator  30 , and a combustor  14 . The HRSG  4  includes a superheater  18 , an evaporator  20 , a steam drum  24 , and an economizer  22 . The steam turbine  6  includes a rotor  40  mounted for rotation within a casing  34  so as to form a flow path for the steam there between. Gland seals  68  prevent the working fluid steam from escaping from the steam flow path. As is conventional, a plurality of the rotating blades  36  and stationary vanes  38  project into the flow path.  
         [0009]     In operation, the compressor  10  inducts ambient air  42  and compresses it, thereby producing compressed air  44 . The temperature and pressure of the compressed air  44  produced by the compressor  10  will typically be in excess of 260 degrees C. (500 degrees F.) and 700 kPa (100 psi), respectively, when the gas turbine rotor  7  is at steady state operating speed, typically 3600 RPM.  
         [0010]     A portion (not shown) of the compressed air  44  produced by the compressor  10  may be directed to the turbine section  16  for cooling therein. During steady state operation of the power plant, the remainder  48  of the compressed air  44  produced by the compressor  10  is directed to the combustor  14 , along with a fuel  46 . According to one aspect of the current invention a portion  62  of the compressed air  44  produced by the compressor  10  is used during start-up of the plant to eliminate the need for an auxiliary boiler, as discussed further below.  
         [0011]     In the combustor  14 , the fuel  46 , which is typically natural gas or distillate oil, is introduced into the compressed air  48  via a fuel nozzle (not shown). The fuel  46  burns in the compressed air  48 , thereby producing a hot combustion gas  50 . The hot gas  50  is then directed to the turbine section  16 , where it is expanded, thereby producing power in the rotor shaft  12  that drives both the compressor  10  and the electrical generator  30 . As a result of having been expanded in the turbine section  16 , the temperature of the expanded gas  52  exhausting from the turbine section  16  is considerably less than the temperature of the hot combustion gas  50  entering the turbine section  16 . Nevertheless, in a modern gas turbine operating at full load, the temperature of the expanded gas  52  is still relatively hot, typically in the range of 450-600 degrees C. (850-1100 degrees F.).  
         [0012]     From the turbine section  16 , the expanded gas  52  is directed to the HRSG  4 . In the HRSG  4 , the expanded gas  52  is directed by ductwork so that it flows successively over the superheater  18 , the evaporator  20  and the economizer  22 . After flowing through the HRSG  4 , the cooled, expanded gas  54  is then discharged to atmosphere via a stack  19 . As is conventional, the superheater  18 , the evaporator  20  and the economizer  22  may have heat transfer surfaces comprised of finned tubes. The expanded gas  52  flows over these finned tubes and the feed water/steam flows within the tubes. In the HRSG  4 , the expanded gas  52  transfers a considerable portion of its heat to the feed water/steam, thereby cooling the gas and transforming the feed water into steam.  
         [0013]     In addition to the expanded gas  52  from the gas turbine  2 , the HRSG  4  receives a flow of feed water (condensate)  56  from the condenser  4  that has been pressurized by pump  26 . As is conventional, the feed water first flows through the heat transfer tubes of the economizer  22 , where its temperature is raised to close to saturation temperature. The heated feed water from the economizer  22  is then directed to the steam drum  24 . From the steam drum  24 , the water is circulated through the heat transfer tubes of the evaporator  20 . Such circulation may be by natural means or by forced circulation. The evaporator  20  converts the feed water into saturated steam  58 . From the evaporator  20 , the saturated steam  58  is directed to the superheater  18 , wherein its temperature is raised into the superheat region.  
         [0014]     From the superheater  18 , the superheated steam  60  is directed to a steam chest  28  that distributes the steam to the inlet of steam turbine  6 . In the steam turbine  6 , the steam  60  flows through the flow path formed within the casing  34  and over the rows of rotating blades  36  and stationary vanes  38 , only a few of which are shown in  FIG. 1 . In so doing, the steam  60  expands and generates shaft power that drives the rotor  40 , which, in turn, drives a second electrical generator  32 . Alternatively, the steam turbine rotor  40  and the gas turbine rotor  7  could be coupled to a common shaft that drives a single electrical generator. The expanded steam  66  exhausted from the steam turbine  6  is then directed to the condenser  8  and eventually returned to the HRSG  4 .  
         [0015]      FIG. 1  identifies an equipment skid  70  that is used in lieu of an auxiliary boiler for at least one of two functions during plant start-up prior to the availability of steam from the HRSG  4 : for providing steam to the turbine gland seals  68  and/or for powering a jet pump for evacuating the condenser  8 . The skid  70  may be assembled off-site and shipped to the plant site, where it is then connected to the appropriate systems of an existing power plant, either permanently or at least for a period of the start-up of the power plant. It should be appreciated that the equipment and functions embodied by skid  70  are provided as one illustration of the present invention, since back-fit of this invention on existing power plants is contemplated and is simplified by the equipment skid concept. Other embodiments of the invention may include discreet equipment fully integrated with the systems of the power plant. The fluid interconnections between the skid  70  and the remainder of power plant  100  are schematically illustrated in  FIG. 1 , and details of the specific equipment and interconnections of the skid  70  are illustrated in  FIG. 2 . The following description should be read with reference to both of these drawings.  
         [0016]     Compressed air bleed  62  from the compressor  10  is provided to the skid  70 , with flow control valves  72 ,  74  used to regulate the flow rate to the skid  70  and the flow rate bypassing the skid  70 . The compressed air  62  may be bled at its highest temperature from the outlet of the compressor  10 , or it may be bled from one of the intermediate stages of the compressor  10  at a somewhat lower temperature. Heat energy is transferred from the compressed air  62  into a flow of condensate  76  within a steam generator  78 . Steam generator  78  may be any type of heat exchanger/boiler known in the art; preferably having a low thermal inertia to facilitate the rapid production of steam following the availability of compressed air bleed  62 . A moisture separator  79  may be desired, particularly with a once-through steam generator  78 . The moisture separator  79  may be a separate component disposed downstream of the steam generator  78 , as illustrated, or it may be formed to be integral with the steam generator  78 . The flow of condensate may be regulated by flow control valve  80 , and the flow of steam to the turbine gland seals  68  through steam line  82  may be regulated by flow control valve  84 .  
         [0017]     The cooled compressed air  64  leaving steam generator  78 , as well as any compressed air bypassed through valve  74 , may be provided to another location within the plant, such as to turbine  6 , through vent line  86 . Alternatively, flow control valves  87 ,  88  may direct the cooled compressed air to air ejector  90 . The air ejector  90  is also connected to the condenser  8  via evacuation line  92  and flow control valve  94  so that the cooled compressed air  64  passing through the air ejector  90  will draw off fluids such as non-condensable gasses from the condenser  8  in order to establish a vacuum (i.e. a lowered pressure, not necessarily an absolute vacuum) in the condenser  8 . The combined flow may then be vented to atmosphere or otherwise processed via vent line  95 . One skilled in the art will appreciate that flow control valves, flow sensors, temperature sensors, power and control systems, safety equipment, etc. may be included on equipment skid  70  as necessary to accomplish the desired functioning of the system or as required by applicable design specifications. The specific flow paths, equipment and interconnections illustrated in  FIGS. 1 and 2  are provided by way of example and are not intended to be limiting to the claimed invention.  
         [0018]     Heat energy may be removed from the compressor bleed air  62  by other heat exchange devices or methods, with the removed heat being used in any desired manner or being dumped to the environment. The cooled compressed air retains its pressure/flow characteristics and may therefore be used as the driving force in any type of jet pump device, such as air ejector  90 . Other embodiments may utilize the heat from the compressed air bleed for other purposes, such as for heating other portions of the plant  100 , for example. Alternatively, hot compressed air from the compressor  10  may be provided directly to the air ejector  90 , and heat may or may not be removed from the airflow downstream of the air ejector  90 .  
         [0019]     During start-up of a combined cycle power plant, the compressed air discharged from the compressor  10  will very quickly achieve a temperature high enough to create steam in steam generator  78 . For example, in one embodiment, the temperature of the compressed air bleed  62  may be in excess of 200° C. (360° F.) in as little as 10 minutes after the initial rolling of the gas turbine shaft  12 . Furthermore, the flow of bleed air  62  may be used almost immediately to begin evacuating the condenser  8  via air ejector  90 . Accordingly, the prior art delays associated with the warming of the heat recovery steam generator  4  and costs associated with an auxiliary boiler may be avoided while achieving rapid start-up of the plant  100 .  
         [0020]     While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.