Patent Publication Number: US-2015068213-A1

Title: Method of cooling a gas turbine engine

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to gas turbine engines, and more particularly to a method of cooling a gas turbine engine. 
     The economy of gas turbine engine operation dictates that gas turbines be available to produce power to the maximum extent possible. However, it is known that planned and unplanned outages for gas turbine maintenance and repair are required over the life of the equipment. It is advantageous to be able to expeditiously shutdown the gas turbine engine, establish the conditions required to perform the maintenance, and then return to operation quickly after the maintenance is complete. 
     One portion of the process outlined above specifically relates to a cool down procedure for the gas turbine engine, referred to as a cool down cycle. The cool down cycle is associated with operation of the gas turbine engine during a transition from full operation at full speed-full load (FSFL) to complete or temporary shutdown. Users of the gas turbine engine want this process to be performed as quickly as possible to reduce total down time, whether for scheduled maintenance or for unexpected outages. One consideration related to the cool down cycle relates to component life impacts. Specifically, the speed of the cool down process impacts the stresses imposed on various components of the gas turbine engine and such thermal cycling directly impacts component life. Typically, a single time period is provided to the user, based on a conservative determination of acceptable stresses to be imposed on the components. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a method of cooling a gas turbine engine is provided. The method includes removing a load from the gas turbine engine. The method also includes operating the gas turbine engine at a rated speed of the gas turbine engine. The method further includes modulating an angle of at least one stage of inlet guide vanes disposed proximate an inlet of a compressor section of the gas turbine engine, wherein modulating the angle modifies a flow rate of an inlet flow for reducing a cooling time of the gas turbine engine. 
     According to another aspect of the invention, a method of cooling a gas turbine engine is provided. The method includes operating the gas turbine engine at a rated speed of the gas turbine engine. The method also includes decreasing a rotor speed of the gas turbine engine to a first predetermined cool down rotor speed. The method further includes increasing the rotor speed from the first predetermined cool down rotor speed to a second predetermined cool down rotor speed. The method yet further includes modulating an angle of at least one stage of inlet guide vanes to modify a flow rate of an inlet flow. The method also includes injecting water into a region of the gas turbine engine. The method further includes holding the rotor speed at the second predetermined cool down rotor speed for a period of time determined by ambient conditions. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 illustration of a gas turbine engine; 
         FIG. 2  is a plot of gas turbine speed as a function of time during a method of cooling a gas turbine engine; and 
         FIG. 3  is a flow diagram illustrating the method of cooling a gas turbine engine. 
     
    
    
     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 
     Referring to  FIG. 1 , a turbine system, such as a gas turbine engine, for example, is schematically illustrated with reference numeral  10 . The gas turbine engine  10  includes a compressor section  12 , a combustor section  14 , a turbine section  16 , a rotor  18  and a fuel nozzle  20 . It is to be appreciated that one embodiment of the gas turbine engine  10  may include a plurality of compressors  12 , combustors  14 , turbines  16 , rotors  18  and fuel nozzles  20 . The compressor section  12  and the turbine section  16  are coupled by the rotor  18 . The rotor  18  may be a single shaft or a plurality of shaft segments coupled together to form the rotor  18 . 
     The combustor section  14  uses a combustible liquid and/or gas fuel, such as natural gas or a hydrogen rich synthetic gas, to run the gas turbine engine  10 . For example, fuel nozzles  20  are in fluid communication with an air supply and a fuel supply  22 . The fuel nozzles  20  create an air-fuel mixture, and discharge the air-fuel mixture into the combustor section  14 , thereby causing a combustion that creates a hot pressurized exhaust gas. The combustor section  14  directs the hot pressurized gas through a transition piece into a turbine nozzle (or “stage one nozzle”), and other stages of buckets and nozzles causing rotation of turbine blades within an outer casing  24  of the turbine section  16 . 
     Referring to  FIG. 2 , a method of cooling  30  the gas turbine engine  10  is illustrated. The method of cooling  30  may be employed in response to a number of scenarios. One example is a planned shutdown of the gas turbine engine  10  due to scheduled maintenance. Another example is an unplanned shutdown due to a variety of factors. Regardless of whether the method of cooling  30  is employed as a result of a planned or unplanned shutdown, the method of cooling  30  advantageously reduces the time required to sufficiently cool components of the gas turbine engine  10 . Additionally, the method of cooling  30  provides a user options relating to the cool down time period, as will be described in detail below. 
     The plot in  FIG. 2  illustrates a rotor speed  32  as a function of time during at least a portion of the method of cooling  30  time period. For illustration purposes, the gas turbine engine  10  is shown initially with the rotor speed  32  at 100% and with a load coupled thereto, representing an operating condition of full speed-full load (FSFL) over time period  34 . It is to be appreciated that the gas turbine engine  10  often operates at what is referred to as a rated speed that is typically greater than about 90% of the full speed (i.e., 100%) referenced above. As such, the speeds and relative percentages discussed herein may be related to the full speed or the rated speed. 
     The rotor speed  32  is then decreased  42  over time period  44  to a first predetermined cool down speed  46 . The load is removed from the gas turbine engine  10  at time  38  and the gas turbine engine  10  operates briefly at full speed-no load (FSNL), or the rated speed, over time period  40 . It is to be appreciated that the load may be removed during time period  44  in some cases. The first predetermined cool down speed  46  will vary depending upon the particular application. In one embodiment, the first predetermined cool down speed  46  comprises what is referred to as a “ratchet speed” or a “turning gear speed.” The terms ratchet speed and turning gear speed each correspond to a relatively slow rotor speed, where the rotor  18  is driven by a mechanical device operatively coupled to the rotor  18 . The rotor speed  32  may be defined by an extremely slow constant rotation of the rotor  18  or an intermittent turning. In one embodiment, the first predetermined cool down speed  46  corresponds to about ¼ of a turn of the rotor 18 every 1 to 5 minutes. The precise speed of the first predetermined cool down speed  46  varies depending upon the application. As illustrated, in certain embodiments, the rotor speed  32  may actually decrease to a complete stop, represented by 0% rotor speed, prior to reaching the first predetermined cool down speed  46 . In one embodiment, the first predetermined cool down speed  46  corresponds to the turning gear speed and ranges from about 0.1% to about 10% rotor speed. 
     Upon reaching the first predetermined cool down speed  46 , the rotor speed  32  is held at the first predetermined cool down speed  46  for a selectable time period. In particular, a user is provided options between a plurality of time periods in which the rotor speed  32  is held at the first predetermined cool down speed  46 . Illustrated are three time periods, referred to as a first time period  48 , a second time period  50  and a third time period  52 . These time periods represent the holding time at the first predetermined cool down speed  46  prior to increasing the rotor speed  32  to a second predetermined rotor speed  54 . In one embodiment, the second predetermined rotor speed  54  may correspond to a “crank speed” of the rotor  18 . In such an embodiment, the rotor speed  32  ranges from about 10% to about 40%. 
     The first time period  48  represents a holding time of about 0 minutes at the first predetermined cool down speed  46 . In other words, the rotor speed  32  is increased to the second predetermined cool down speed  54  along line  49  directly past the first predetermined cool down speed  46  or held for a short period of time, such as less than 1 minute. The third time period  52  represents the longest holding time option at the first predetermined cool down speed  46  before increasing the rotor speed  32  to the second predetermined cool down speed  54  along line  53 . The second time period  50  represents an intermediate holding time relative to the first time period  48  and the third time period  52 . After holding for the second time period  50 , the rotor speed  32  is increased along line  51  to the second predetermined cool down speed  46 . Although three time durations have been illustrated and described herein, it is to be appreciated that more or less time duration options may be provided to a user. As will be appreciated from the description below, each of the plurality of time periods is associated with a corresponding maintenance factor impact, with the used determining which time period option based on the maintenance factor impact. 
     Advantageously, the user is able to select from the plurality of time periods based on the specific operation of the gas turbine engine  10 . In particular, some users operate the gas turbine engine  10  predominantly at base load (FSFL) and not in a cyclical manner. Such users are not as concerned with rotor cyclic capability, which is influenced by thermal stresses imposed during thermal cycling, as they are with a reduced outage time. These users benefit the most from the option utilizing the first time period  48 , with little or no holding time at the first predetermined cool down speed  46 . At the opposite end of the spectrum, a user with frequent cycling of the gas turbine engine  10  benefits the most from the third time period  52 , which takes longer to bring the gas turbine engine  10  to FSFL, but conservatively accounts for thermal stresses imposed on the rotor  18 . The second time period  50  is an intermediate option for users between the above-described extremes. As noted, more or less than the three options described may be employed and the three options are not intended to be limiting. 
     Regardless of which option the user selects, the rotor speed  32  is increased to the second predetermined cool down speed  54  and held for a time duration that is determined by ambient conditions detected by various devices associated with the gas turbine engine  10 . It is contemplated that the ambient conditions may also be employed to determine the plurality of time periods corresponding to the first predetermined cool down speed  46 . Such conditions may include temperature, pressure and humidity, for example. The ambient conditions are input automatically or manually into rotor analytical models to determine the time duration for holding at the second predetermined cool down speed  54 . At the conclusion of the holding period, the rotor speed  32  may be increased toward full speed or decreased in a full shutdown. Alternatively, the rotor speed  32  may be increased to a third predetermined cool down speed  68  that corresponds to an elevated crank speed. 
     During operation at the second predetermined cool down speed  54 , the method of cooling  30  includes one or more cooling actions employed to facilitate effective and time reducing cooling of the gas turbine engine  10 . One cooling action includes modulating an angle of at least one inlet guide vane set. Typically, a plurality of inlet guide vanes (IGVs) are disposed proximate an inlet of the compressor section  12 . At least one, but up to all stages of the IGVs may be modulated to alter their respective angles relative to an inlet flow entering the compressor section  12 . The angle relative to the inlet flow may be increased or decreased depending on the particular conditions of the gas turbine engine  10 . In one embodiment, the IGVs are modulated to a “fully open” position, which fully increases the flow rate of the inlet flow entering the compressor section  12 , thereby enhancing the cooling effect on various components of the gas turbine engine  10 . The particular angle that the IGVs are modulated to may be fine-tuned to account for distinct operating and/or ambient conditions. Another cooling action that may be employed is an injection of water into at least one region of the gas turbine engine  10  for heat transfer purposes that reduce the cool down time of the gas turbine engine  10 . The region(s) into which the water is injected may vary. In one embodiment, the water is injected into the compressor section  12 . Such an embodiment cools the air flowing through the compressor section  12 , which allows the air to pick up additional heat from the gas turbine engine  10  and further reduce the cool down time duration. In alternative embodiments, it is contemplated that the water is injected into other regions of the gas turbine engine  10 , such as the turbine section  16 , the combustor section  14 , or a combination of the turbine section  16 , the combustor section  14  and the compressor section  12 . 
     An alternative embodiment is represented with path  60  that decreases the rotor speed  32  from FSNL, or the rated speed, to the second predetermined rotor speed  54 . This embodiment does not require decreasing the rotor speed  32  to a speed corresponding to the first predetermined cool down speed  46  or slower than the first predetermined cool down speed  46 . It is to be appreciated that the second predetermined rotor speed  54  in this embodiment may correspond to the crank speed described above, or alternatively may be the elevated crank speed  68 , with the elevated crank speed  68  greater than the crank speed being greater than about 40% of full speed, or the rated speed. 
     Referring to  FIG. 3 , a flow diagram further illustrates the method of cooling  30 . The method of cooling  30  includes removing a load from the gas turbine engine  70  and operating the gas turbine engine at a rated speed of the gas turbine engine  72 . The method also includes decreasing a rotor speed of the gas turbine engine to a first predetermined cool down rotor speed  74 . The method further includes increasing the rotor speed from the first predetermined cool down rotor speed to a second predetermined cool down rotor speed  76 . The method yet further includes modulating an angle of at least one stage of inlet guide vanes to modify a flow rate of an inlet flow  78 . The method also includes injecting water into a region of the gas turbine engine  80 . The method further includes holding the rotor speed at the second predetermined cool down rotor speed for a period of time determined by ambient conditions  82 . The additional features of the method of cooling  30  are described in detail above with reference to  FIG. 2 . 
     Advantageously, the method of cooling  30  provides significant time savings for the cool down process, thereby helping start outage efforts more rapidly. Additionally, a user may select from the plurality of time periods described above to suit the specific operating needs of the gas turbine engine  10 , with particular emphasis on the maintenance factor impact associated with each of the time periods. 
     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.