Abstract:
A method for warming the rotor of a gas turbine during extended periods of downtime comprising feeding ambient air to an air blower; extracting compressed air from the air blower; feeding a portion of the compressed air to one side of a heat exchanger and steam (typically saturated) from e.g. a gas turbine heat recovery steam generator; passing the resulting heated air stream from the exchanger into and through into defined flow channels formed within the rotor; continuously monitoring the air temperature inside the rotor; and controlling the amount of air and steam fed to the heat exchanger using a feedback control loop that controls the amount of air and steam feeds to the exchanger and/or adjusts the flow rate of heated air stream into the rotor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and apparatus for warming a gas turbine engine rotor and, in particular, to a method for controlling the gas turbine rotor temperature during periods of extended downtime using steam to heat air extracted from a gas turbine enclosure which is then fed directly to the rotor. In an alternative embodiment, the method utilizes auxiliary boiler steam for purposes of heating the air fed to selected rotor passages during extended periods of downtime. 
     Gas turbine engines typically include a compressor section, a combustor section and at least one turbine that rotates in order to generate electrical power. The compressor discharge feeds directly into the combustor section where hydrocarbon fuel is injected, mixed and burned. The combustion gases are then channeled into and through one or more stages of the turbine which extracts rotational energy from the combustion gases. The temperature of gas turbine rotor blades rises very quickly when a gas turbine is started because the blades are exposed to very high-temperature exhaust gases. The temperature of the outer peripheral parts of a turbine also increase very quickly due to heat conduction from the blade as compared to inner peripheral rotor components. The rate of increase in temperature thus tends to be slower on the inner side of the rotor than on the outer side. The difference in conductivity of components can also cause a temperature differential between the inner and outer peripheries of rotor components, creating additional thermal stresses during startup. A separate centrifugal stress also exists during startup due to rotation of the engine. 
     Thus, the combination of thermal and centrifugal stresses on the rotor are much higher when the engine has been sitting idle during, for example, periodic maintenance. As a result, during startup following extended periods of downtime, the rotor disks can undergo significant thermal and mechanical stresses and are vulnerable to premature failure due to the shock occurring during startup, particularly at or near the rotor disks. 
     An example of a conventional rotor warming structure for a combined cycle plant includes a central gas flow passage with gas from a compressor fed into the central passage in the rotor. Normally, a portion of the compressed gas is introduced into the gas turbine blades through branches emanating from a central passage. Another known method for warming the rotor prior to startup relies on an electrical heating system surrounding the rotor. However, such systems can be prohibitively expensive and often do not sufficiently protect against temperature differentials during startup. Both air and electrical systems also do not take advantage of the potential heating and cost-saving benefits using on-site steam available within the same power generating plant. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A primary object of the present invention is to provide a method and apparatus for keeping a gas turbine rotor warm during periods of extended downtime by using a portion of the flow from a steam turbine or, alternatively, from an outside steam source, in order to heat air originating from the gas turbine enclosure. The higher temperature air in turn serves as a more effective primary heat source for the rotor cavity and blades. 
     As detailed below, a new method for warming the rotor of a gas turbine engine comprises the steps of feeding an ambient air stream to an air blower to increase the air pressure; extracting a portion of compressed air from the discharge of the air blower while feeding a partial air stream to one side of a heat exchanger (e.g., shell and tube type); feeding steam (typically saturated) to the other side of the same heat exchanger; passing the resulting heated air stream from the heat exchanger into and through defined flow channels inside the rotor; continuously monitoring the air temperature inside the rotor during the warming operation; and controlling the amount of air and steam fed to the heat exchanger based on the temperature detected inside the rotor using a feedback control loop. The feedback control data can also be used to monitor and adjust the flow rate into the rotor of a heated air stream. 
     The invention also includes a related structure for warming a gas turbine rotor during periods of downtime comprising an air blower, a heat exchanger for heating compressed air from the air blower using heat from an internal steam source, a plurality of air passages into and out of the rotor sufficient in size and number to carry a prescribed amount of heated air through the rotor to heat the turbine blades and rotor cavities to a uniform temperature, steam fluid flow passages into and out of the heat exchanger, and a feedback control loop for controlling the amount of air and steam fed to the heat exchanger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary gas turbine engine adapted to incorporate the steam-based heat exchange system for a rotor during extended periods of downtime; 
         FIG. 2  is a sectional view of a gas turbine rotor depicting a nominal flow profile for use with the steam-based heat exchange system according to the invention; 
         FIG. 3  is a process flow diagram illustrating the flow configuration of an exemplary heat exchange system according to the invention; and 
         FIG. 4  is a process flow diagram depicting the major pieces of process equipment for a gas turbine engine, HRSG and steam turbine for purposes of illustrating candidate sources of steam during periods of downtime for a steam-based heat exchange system according to the invention. 
         FIG. 5  shows Table 1. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The warming structure described herein is particularly useful in cold weather conditions and specifically intended to replace conventional electric heaters used to warm a gas turbine rotor during extended periods of downtime. In an exemplary embodiment, steam from another part of the plant is used as the principal heating medium and results in a more cost effective and reliable heating system without using conventional electric heaters. A portion of sealing flow from the steam turbine or gland leakage steam transfers heat to an inlet air feed using a combination heat exchanger and air blower. An alternative embodiment uses a similar configuration but with auxiliary boiler steam as the primary source of heat. 
     The invention offers particular advantages to multi gas turbine plant configurations where saturated steam is readily available for warming up one or more gas turbine rotors during periods of downtime. As the principal heating source, the steam can be extracted from auxiliary boiler/gland steam/sealing sources. A shell and tube heat exchanger transfers heat from the steam to air taken from a gas turbine enclosure that has been compressed using a blower. The heated air is then fed to the gas turbine inlet plenum through a control valve and piping network and the spent steam is fed back into the gas turbine engine bottoming cycle. As such, the invention provides a much more cost effective method for keeping the gas turbine rotor warm while the system is out of service. 
     Turning to the figures,  FIG. 1  is a schematic diagram of an exemplary gas turbine engine adapted to incorporate the steam-based heat exchange system for a rotor during periods of downtime. Gas turbine engine  10  is coupled to an electric generator  16  and includes a compressor  12 , a turbine  14  and generator  16  arranged with a single monolithic shaft  18 . Compressor  12  supplies compressed air to combustor  20  where the air is mixed with fuel supplied via fuel stream  22 . In operation, air flows through compressor  12  and compressed air is supplied combustor  20 . Combustion gases  28  from combustor  20  propel turbine  14  which in turn rotates shaft  18 , compressor  12  and electric generator  16  about longitudinal axis  30 . Low pressure turbine  20  rotates first shaft  26  and low pressure compressor  12  about the longitudinal axis. 
       FIG. 2  is a sectional view of a gas turbine rotor depicting a nominal flow profile for use with the steam-based heat exchange system of the present invention. See, e.g., expired U.S. Pat. No. 4,880,354. Normally, the rotor includes a plurality of circumferentially-spaced rotor blades coupled to the turbine rotor, where each rotor blade includes a shank, a platform having an upper and lower surface coupled to the shank and a first component coupled to the platform lower surface and the shank. A substantially hollow plenum is defined between the first component, the shank, and the platform lower surface. An airfoil is also coupled to the platform. 
     A typical gas turbine rotor such as that shown in  FIG. 2  and identified at  10  includes a first stage disk  20 , a spacer  21  between first and second stages, a second stage disk  22 , spacer  24  between second and third stages, a third stage disk  25  and front and rear side shafts  26  and  27  joined together as shown. A first stage blade  30 , a second stage blade  31  and third stage blade  32  are fitted to the outer peripheries of the respective disks. Once installed, the disks define a series of central fluid passages  33 ,  34 ,  35 , respectively, sufficient in size to accommodate air flow through tubular member  40  which is common to all disks and extending between the disks to define a common fluid flow channel. Tubular member  40  includes a plurality of smaller fluid passageways  50  opened at positions facing the respective inner walls of the central opening for the disks as shown. 
     Heated air from the heat exchanger system generally described above and in more detail in  FIG. 3  passes through the plurality of small fluid passages and against the inner walls on the sides of each disk. The effect of blowing hot air in this manner allows the disks to be warmed uniformly without significant temperature gradients during a cold startup of the gas turbine engine. After the compressed and heated air from the heat exchanger is blown against the inner wall of the central openings in the disks, the air passes through a channel formed between the tubular member  40  and the inner walls of the central openings in the manner illustrated into the space between the first and second stage disks. 
       FIG. 3  is a process flow diagram depicting an exemplary flow configuration for heat exchange systems effective in warming a rotor during extended periods of downtime. An air feed at ambient temperature passes through inlet air line  101  through air filter  102  into air blower  103  where the increase in pressure results in an air flow sufficient to ensure a constant flow of heated air using heat exchanger  105  as the feed to the rotor. Compressed air stream  104  from air blower passes into and through the heat exchanger using saturated steam as the heating medium (see the exemplary heat exchanger design details in Table 1 below). The resulting heated air stream passes through air filter  106  and exits as shown through outlet damper  107  as the primary air feed to the rotor as described above in connection with  FIG. 2 . The gland leakage steam, sealing steam and/or boiler steam passes through heat exchanger  105  through steam inlet line  120 , with the spent steam exhaust  113  being returned to the gas turbine bottoming cycle  121 . 
     In order to ensure that warming of the gas turbine engine rotor occurs at a prescribed rate without creating potential damage to the rotor disks,  FIG. 3  also shows the use of a feedback control loop which includes an initial temperature control signal  109  from the gas turbine engine control panel used to control the amount of steam being fed to heat exchanger  105  via control valve  112 , along with input signal line  107 , feedback control line  111  and heat exchanger steam feed  114 . Feedback control loop  113  also includes means for controlling damper  107  downstream of heat exchanger  105  and thus operates to control the amount of the heated air stream  108  being fed to the gas turbine rotor. 
       FIG. 4  is a process flow diagram depicting the major pieces of process equipment for a gas turbine engine, HRSG and steam turbine for purposes of illustrating candidate sources of steam during periods of downtime for a steam-based heat exchange system according to the invention. As noted above, one side of an exemplary heat exchanger as used to provide controlled heat to the rotor during periods of downtime relies upon steam obtained from one of three different sources as shown in  FIG. 4 , namely from an auxiliary boiler (taken from boiler feed line S 1 ), or from the available gland steam and seal leakage steam lines (S 2 ) or from the heat recovery steam generator (S 3 ). As  FIGS. 2 and 3  illustrate, one or more candidate steam feeds will be used as the primary heating medium for the rotor on one side of the heat exchanger, with a controlled air feed on the other side. As also noted above, the resulting heated air at a defined elevated temperature feeds into and through the rotor during periods of downtime. The spent steam from the heat exchanger is then returned to the bottoming cycle. 
     An exemplary heat exchanger design useful in achieving the objectives of the invention is summarized below in Table 1. The shell and tube heat exchanger for the rotor uses one of two alternative streams, namely a portion of the sealing flow from a steam turbine or, in the alternative, a portion of auxiliary boiler steam form an outside source. In the example of Table 1, the amount of air to be heated on the tube side and corresponding steam flow requirements are identified for an exchanger having the specific tube sizes, dimensions and pitch configuration as shown. Table 1 also includes exemplary tube bundle design criteria, as well as inlet and outlet design temperatures for the air and steam as they enter and exit the exchanger. The resulting heated air is used in connection with the control system as described above in order to bring the rotor disks to the desired minimum temperature and thereafter maintain the same internal temperature during startup of the engine. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.