Abstract:
A method is provided of reducing lockout time of a gas turbine engine which includes: an inlet, a compressor, a combustor, a turbine, and an exhaust duct, where the compressor and the turbine are carried on a turbomachinery rotor and each include an array of blades mounted for rotation inside a casing of the engine. The method includes: operating the engine at a first power output; shutting down operation of the engine without substantially reducing the power output beforehand, wherein thermomechanical changes occur in the engine subsequent to shutdown that tend to reduce a radial clearance between at least one of the blades and the casing; and subsequent to shutting down the engine, (1) heating the casing and/or (2) pumping an airflow of ambient air into the inlet and through the casing, past the rotor, and out the exhaust duct, so as to reverse at least partially the thermomechanical changes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to gas turbine engines and more particularly to methods for operating such engines during a shutdown period. 
         [0002]    A gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure or gas generator turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. In a turboshaft engine, another turbine disposed downstream of the core (referred to as a low pressure, “work”, or “power” turbine) extracts energy from the primary flow to drive a shaft or other mechanical load. One common use is to couple the gas turbine engine to an external load such as a pump, compressor, or electrical generator. 
         [0003]    For efficient operation, the turbomachinery in a gas turbine engine depends on maintaining small but definite radial clearances between the tips of the rotating blades and the stationary annular casing that surrounds them. The casing is generally more “thermally responsive” than the rotor, i.e. it generally expands or contracts at a greater rate than the rotor during a change in engine power output, and the associated temperature change. As a result the blade clearances tend to open or close during changes in engine power output. For this reason, gas turbine engines are generally shut down by gradually reducing the output power level, so that the radial clearances can stabilize. 
         [0004]    However, operational reasons can require that the external load be removed and that the engine be shut down immediately, without being able to gradually reduce power. This is referred to as a “hot shutdown”. When a hot shutdown occurs, the engine components cool rapidly. In general the casing cools down faster than the rotor, causing the case to compress against the rotor blades and close the airflow clearances in the compressor. Also, once the engine stops rotating natural convection patterns cause the upper portions of the rotor to heat up and expand more than the lower portions. This causes bending or bowing of the rotor that further reduces radial clearances at specific locations. The combined effect of case shrinkage and rotor bowing cause the rotor to become “locked”, a condition in which the rotor and casing actually contact each other. 
         [0005]    When the engine experiences a hot shutdown, the engine must be restarted or undergo a hot crank within a short time after the shutdown (for example about 10 minutes) in order to prevent the rotor from locking up. If the rotor locks up, the engine cannot be restarted until after the passage of a “lockout period”, in order to avoid rotor and casing damage. This period is undesirable for a number of reasons including the cost and physical inconvenience of not having the engine in service. 
         [0006]    Accordingly, there is a need for a method of operating a gas turbine engine that minimizes or eliminates the lockout period after a hot shutdown. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    This need is addressed by the present invention, which according to one aspect provides methods for heating the casing and/or cooling the rotor of a gas turbine engine in order to reduce the lockout period. 
         [0008]    According to one aspect of the invention, a method is provided of reducing lockout time of a gas turbine engine which includes, in serial flow communication: an inlet, a compressor, a combustor, a turbine, and an exhaust duct, where the compressor and the turbine are carried on a turbomachinery rotor and each include an array of blades mounted for rotation inside a casing of the engine. The method includes: operating the engine at a first power output level; shutting down operation of the engine without substantially reducing the power output level beforehand, wherein thermomechanical changes occur in the rotor and the casing subsequent to the shutdown that tend to reduce a radial clearance between at least one of the blades and the casing; and subsequent to shutting down the engine, heating the casing of the engine so as to expand the casing and increase a radial clearance between the blades of the rotor and the casing. 
         [0009]    According to another aspect of the invention, a method is provided of reducing a lockout time of a gas turbine engine which includes, in serial flow communication: an inlet, a compressor, a combustor, a turbine, and an exhaust duct, where the compressor and the turbine are carried on a turbomachinery rotor and each include an array of blades mounted for rotation inside a stationary casing of the engine. The method includes: operating the engine at a selected power output level; shutting down operation of the engine without substantially reducing the power output level shutdown, wherein thermomechanical changes attributable to cooling occur in the rotor and the casing subsequent to then shutdown that tend to reduce a radial clearance between at least one of the blades and the casing, thereby resulting in a lockout condition; and subsequent to shutting down the engine, pumping an airflow of ambient air into the inlet and allowing the pumped air to flow through the casing, past the rotor, and out the exhaust duct, so as to reverse at least partially, the thermomechanical changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0011]      FIG. 1  is a schematic cross-sectional view of a genset module constructed in accordance with an aspect of the present invention; 
           [0012]      FIG. 2  is a schematic cross-sectional view of a gas turbine engine shown in  FIG. 1 ; 
           [0013]      FIG. 3  is a schematic drawing illustrating bowing of the rotor of the engine shown in  FIG. 2 ; 
           [0014]      FIG. 4  is a half-sectional view of a portion of the engine shown in  FIG. 2 , illustrating heating elements mounted thereto; and 
           [0015]      FIG. 5  is a schematic drawing illustrating a blower coupled to an inlet of the engine of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a genset module  10 . It includes a base  12  upon which are mounted an engine enclosure  14  and an electrical generator  16 . The electrical generator  16  is used as a representative example of an external load device. A gas turbine engine  20  (or simply “engine”) is disposed inside the engine enclosure  14 . The engine enclosure  14  includes a combustion air inlet  22  coupled in flow communication with an inlet  24  of the engine  20 , and an exhaust gas exit  26  coupled in flow communication with an exhaust duct  28  of the engine  20 . The gas turbine engine  20  is coupled by an output shaft  30  to the electrical generator  16 . 
         [0017]    Referring to  FIG. 2 , the engine  20  includes a high pressure compressor (“HPC”)  32  carrying a number of stages of rotating compressor blades  34 , a combustor  36 , and a high pressure turbine (“HPT”)  38  carrying a number of stages of rotating turbine blades  40 . The HPC, combustor, and HPT are all arranged in a serial, axial flow relationship along a central longitudinal axis denoted by line “A”. Collectively these three components are referred to as a “core”. The high pressure compressor  32  provides compressed air that passes into the combustor  36  where fuel is introduced and burned, generating hot combustion gases. The hot combustion gases are discharged to the high pressure turbine  38  where they are expanded to extract energy therefrom. The high pressure turbine  38  drives the compressor  32  through a rotor shaft  42 . Combustion gases exiting from the high pressure turbine  38  are discharged to a downstream power turbine  44  (also sometimes referred to as a “low pressure turbine” or “work turbine”). The power turbine  44  drives the output shaft  30  described above. 
         [0018]    Collectively the high pressure compressor  32 , the rotor shaft  42 , and the high pressure turbine  38  are referred to as a “core rotor” or simply a “rotor”  46 . The rotor  46  rotates within a stationary annular casing  48 , which in this example includes a high pressure compressor case  50  and a compressor rear frame  52 . The radial tips of the compressor blades  34  and the turbine blades  40  have defined radial clearances from the inner surface of the casing  48 . 
         [0019]    During steady-stage engine operation, the fuel flow rate to the combustor and the rotational speed (RPM) of the rotor  46  are approximately constant. As a result, the temperatures of the various components are approximately constant, along with the radial clearances between the blade tips and the casing  48 . Increased engine power output implies increased RPM, fuel flow, and component temperatures, while decreased engine power output implies decreased RPM, fuel flow, and component temperatures. During changes in power output, the physical properties of the casing  48  tend to make it more thermally responsive than the rotor  46 . In other words, the casing grows or shrinks in the radial direction as a faster rate than the rotor  46  in response to a temperature change. This property is of a special concern during the above-mentioned hot shutdown and will often lead to rotor lockup. 
         [0020]    During engine operation, air, gas, and component temperatures within the engine  20  tend to be relatively evenly distributed around the periphery of the rotor  46 . In other words, when viewed forward looking aft, temperatures at the various clock positions are approximately equal or vary within relatively narrow limits. When the engine  20  is shut down, the rotor  46  decelerates and stops rotating in a very short period of time, for example about 2-3 minutes. Once the rotor  46  stops, natural convection currents cause the upper half of the rotor  46  to heat up and expand axially more than the lower portions. As a result the rotor  46  becomes “bowed”.  FIG. 3  schematically illustrates this bowing, where line A represents the location of the central longitudinal axis of the rotor  46 , which is nominally coaxial with the engine&#39;s central longitudinal axis, and the line “A′ ” represents the central axis of the bowed rotor  46 . The degree of bowing is greatly exaggerated for the purposes of illustration. It can be seen that this bowing will tend to reduce the radial clearance between the rotor  46  and the casing  48  at specific axial locations along the rotor  46 . Collectively the bowing and the casing shrinkage constitute thermomechanical changes in the engine  20 . 
         [0021]    The present invention provides apparatus and a method for reducing both the loss of radial clearance and the bowing. According to one aspect of the invention, means may be provided for selectively heating the casing  48  in order to expand it. For example,  FIG. 4  illustrates a series of electrical resistance heating elements  54  of a known type applied to the exterior of the casing  48 . Each heating element  54  is in the form of an annular ring disposed in contact with the outer surface of the casing  48 . The individual heating elements  54  are connected to a controller  56 , which is in turn connected to an electrical power source  58 , such as a battery, generator, or electrical power grid. It is envisioned that other types of heating devices could be used in place of the resistance heating elements  54 . For example, flexible heating blankets could be used instead of rigid elements. As another example, hollow tubes (not shown) could be mounted around the outer surface of the casing  48 , and a heated fluid such as water, bleed air, oil, or steam could be circulated through the tubes. 
         [0022]    The controller  56  is a device capable of selectively supplying electrical power from the power source  58  to the heating elements  54 . The controller  56  could be implemented using, for example, a number of relays, or it could incorporate a programmable logic controller or a microprocessor-based general purpose microcomputer. In the illustrated example, the heating elements  54  are divided into zones that can be individually powered, with some zones containing a single heating element  54  and other zones containing multiple heating elements. The use of zoned control permits the heating rate to be tailored as needed depending on the wall thickness and component configuration at a number of locations along the casing  48 . 
         [0023]    After a hot shutdown occurs, the casing  48  (or selected portions thereof) is heated so that it expands and increases the radial clearance between the compressor blades  34  and the inner surface of the casing  48 . For example, the casing  48  may be heated to a temperature in a range of about 260° C. (500° F.) to about 370° C. (700° F.). Such temperatures are not high enough to damage the materials of the casing  48 , but are effective to increase the clearances substantially and thereby unlock the rotor  46  so the engine  20  can be restarted. Once the engine  20  is started, heating can be terminated. Control of the heating elements  54  could be through a simple timer implementing a fixed-duration heating cycle. Alternatively, the engine  20  could be provided with one or more strain gages  60  or similar sensors operatively coupled to the controller  56  (shown schematically in  FIG. 4 ), and the heating cycle could be terminated when the casing  48  is expanded a predetermined amount. As another alternative, the engine  20  could be provided with one or more thermocouples  62  or similar sensors operatively coupled to the controller  56  (shown schematically in  FIG. 4 ), and the heating cycle could be terminated when the casing  48  is heated to a predetermined temperature. 
         [0024]    According to another aspect of the invention, means may be provided for cooling the rotor  46  after a hot shutdown. In the illustrated example, a blower  64  includes a base  66 , a fan  68  (such as a centrifugal fan), and an electric motor  70  driving the fan  68 . An outlet duct  72  of the fan  68  may be positioned in front of the engine inlet  24 , and a divergent adapter duct  74  may be placed in-between the two. The blower  64  is arranged to discharge air in a substantially axisymmetric pattern through the engine inlet  24 . The blower  64  is connected to a suitable power source such as a battery, generator, or electrical grid (not shown). 
         [0025]    After a hot shutdown occurs, the blower  64  is positioned in front of the inlet  24  of the engine  20 , and the motor  70  is started. The blower  64  forces ambient air at room temperature, for example about 15° C. (59° F.), through the casing  48  and past the rotor  46 . The air flow produced by the blower  64  has a relatively high volume flow rate, for example about 28-51 m 3 /min. (1000-1800 ACFM), at a low pressure, for example about 3-4 kPa (12-16 in. H 2 O). The air flow is effective to cool the rotor  46  and the casing  48  to some degree, and is also effective to break up natural convection patterns around the rotor  46 . This results in the equalization of temperatures around the periphery of the rotor  46 . The equalization of temperatures reverses the bowing of the rotor  46 . The discharge pressure of the blower  64  is not sufficient to turn the rotor  46  in a lockup condition, but is sufficient to turn or “windmill” the rotor  46  if it is not in a lockup condition. Accordingly, the blower  64  can be controlled by monitoring the rotational speed of the rotor  46 . When the rotor  46  begins to turn at a significant speed (for example a few hundred RPM), the blower  64  can be stopped and moved away from the inlet  24 . 
         [0026]    The heating and cooling techniques described above may be used in combination to reduce lockout times substantially, and potentially eliminate lockout times completely. Tests have shown that the heating or cooling techniques described above, or a combination thereof, can reduce the lockout period from hours to minutes. For, example the lockout time may be about 10 to about 30 minutes using a combination of the techniques. 
         [0027]    Control of the combined heating and cooling techniques may be integrated. For example, after a hot shutdown, heating elements  54  and blower  64  may be started simultaneously while rotor speed is monitored. When the rotor  46  reaches a predetermined threshold speed, both heating and cooling may be terminated. 
         [0028]    The foregoing has described a method for reducing lockout in a gas turbine engine. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.