Abstract:
Apparatus and method of balancing thrust bearing load on a rotor thrust bearing employ a first thrust balance piston cavity for exerting an aft directed thrust balancing force and a second balance piston cavity for exerting an independently controlled forward directed thrust balancing force to allow flexible and wide range balancing of thrust load on the rotor thrust bearing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to a system for balancing loads on a thrust bearing of a gas turbine engine, and more particularly, to a system for increasing the range of control for rotor thrust balancing.  
         [0002]     Gas turbine engines include a rotor assembly which is rotatable relative to stationary engine structures, including rotor mounting structure The rotor assembly includes a number of rotatable components, such as a central shaft, shaft cones, compressor blades and disks, turbine buckets and wheels, and dynamic air seals. Each component is reacted upon by static and/or dynamic axial pressure forces. The vector sum of these forces is a net axial force or thrust in either the forward or aft direction. This net thrust places axial loads on the stationary mounting structure, and a thrust bearing is employed in order to absorb this load without interfering with the free rotation of the rotor assembly. Typically, a rotor thrust bearing is a ball bearing encased within a thrust bearing housing. The load on a thrust bearing varies as the pressures on the various rotor parts change. If net axial thrust is excessive, considerable wear and premature failure of the thrust bearing may occur. A gas turbine engine rotor generates a high thrust, and a rotor thrust bearing must be able to sustain the axial thrust load. In order to limit the amount of net axial force imposed on a rotor thrust bearing and allow for an appropriate safety factor, a thrust balance system is utilized to limit thrust loads on the bearing.  
         [0003]     Under certain operating conditions net rotor axial thrust may change direction, a condition known as “cross-over”. If cross-over occurs, unloaded ball bearings may allow radial movement of the rotor which may adversely affect seal clearances resulting in deterioration of engine operating characteristics.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     In an exemplary embodiment, an apparatus for balancing thrust bearing load on a rotor thrust bearing employs a first thrust balance piston cavity for exerting an aft directed thrust balancing force and a second balance piston cavity for exerting an independently controlled forward directed thrust balancing force.  
         [0005]     In an exemplary method, balancing thrust bearing load on a rotor thrust bearing is accomplished by controlling air pressure in a first balance piston cavity configured to provide an aft directed thrust balancing force to an engine rotor component; and independently controlling air pressure in a second balance piston cavity configured to provide a forward directed thrust balancing force. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a schematic, longitudinal cross-sectional view of a gas turbine engine including a rotor thrust balancing system;  
         [0007]      FIG. 2  is an enlarged, schematic, partial cross-sectional view of a prior art thrust control system in a gas turbine engine;  
         [0008]      FIG. 3  is a schematic, partial cross-sectional illustration of a system for balancing loads on a gas turbine engine thrust bearing including a pair of thrust balance piston cavities;  
         [0009]      FIG. 4  is a schematic, partial cross-sectional illustration of a system for regulating air pressure within one of the balance piston cavities in a thrust control system shown in  FIG. 3 ;  
         [0010]      FIG. 5  is a schematic, block diagram illustration of a control system for controlling air flow to the balance piston cavities for balancing loads on the rotor thrust bearing depicted in  FIGS. 3 and 4 ; and  
         [0011]      FIG. 6  is graphical representation of one technique for pressure regulation in a system as shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Referring now to the drawings in detail, wherein identical numerals indicate the same elements throughout the figures, and in which “upstream” indicates a direction toward the engine air intake, and “downstream” indicates a direction toward the engine exhaust,  FIG. 1  schematically depicts an aeroderivative gas turbine engine  10 . Engine  10  comprises in axial alignment in serial flow sequence a low pressure compressor  12 , a high pressure compressor  14 , a combustor  16 , a high pressure turbine  18 , and a low pressure turbine  20 , disposed concentrically about longitudinal axis  22 . The standard configuration for engines of this type is a dual concentric shaft arrangement, in which low pressure turbine  20  is drivingly connected to low pressure compressor  12  by low pressure drive shaft  24  and is typically connected to a load (not shown) at its downstream end  29 . High pressure turbine  18  is similarly drivingly connected to high pressure compressor  14  by high pressure drive shaft  26  disposed external to low pressure drive shaft  24  and concentrically with longitudinal axis  22  and supported from the stator  25 .  
         [0013]     In operation, air is drawn into engine inlet  28 , and compressed through low pressure compressor  12  and high pressure compressor  14 . Compressed air is delivered to combustor  16  where it is mixed with fuel and ignited to produce air flow through high pressure turbine  18  and low pressure turbine  20 , and exits through exhaust nozzle  30 . As discussed above, thrust forces are produced within gas turbine engine  10  which act axially at different points or portions in the engine. While a compressor driven by a turbine can compensate to some degree for a net force directed axially downstream in the turbine, at least one rotor thrust bearing is normally used to react the net rotor thrust.  
         [0014]     To regulate the bearing load, at least some known gas turbine engines, such as that shown in  FIG. 2 , use compressor bleed air, to pressurize a forward balance piston cavity  32  and an aft balance piston cavity  34 , each of which is a defined volume pressurized by air bled from a selected compressor stage. A crossover tube  36  connects the balance piston cavities  32 ,  34  to equalize the pressure in the interiors of balance piston cavities  32 ,  34  in flow communication to limit the thrust load on thrust bearing  38 . During thrust balance operation essentially equal pressure is maintained inside balance piston cavities  32  and  34 , so that the pressure applied to piston area having radius  37  of rotating surface  33  of forward balance piston cavity  32  is essentially identical to the pressure applied to piston area having radius  39  of rotating surface  35  of aft balance piston cavity  34 . The respective radii  37 ,  39  of surfaces  33  and  35  are selected as a design feature to establish respective fixed design surface areas to establish a fixed ratio of forces to be applied to the rotor, which have been determined by engine design to be optimal for protecting the thrust bearing  38  during engine operation under anticipated operating conditions. The air pressure in the respective balance piston cavities  32 ,  34  acts to maintain the force ratio to counter anticipated loads acting on the thrust bearing  38  during engine operation.  
         [0015]     In an exemplary embodiment of a rotor thrust balancing system, shown in  FIG. 3 , thrust load on rotor thrust bearing  40  is controlled by a thrust balance system which includes a forward balance piston cavity  42  located upstream of the rotor thrust bearing  40  of the engine  10  and an aft balance piston cavity  46  located downstream of the rotor thrust bearing  40 . Balance piston cavity  46  is a closed volume defined at its aft end by rotatable member  43  connected to the high pressure drive shaft  26  of the rotor and defined at the axially opposite forward end by stationary surface  45  connected to the stator  25 , and is pressurized by high pressure compressor discharge air flow via air pressurized volume  72  through seal  41 . High pressure compressor discharge air from a compressor stage selected to provide the required air pressure and flow rate pressurizes air pressurized volume  72  and air flow through seal  41  flows into and pressurizes the interior of balance piston cavity  46 . The magnitude of force acting in the axially aft direction represented by arrow  74  which can be supplied by balance piston cavity  46  is determined by the air pressure inside balance piston cavity  46  applied to an annular surface area having radius  51  of rotatable member  43 . Air pressure relief tube  47  connects the interior of balance piston cavity  46  in flow communication to a control system shown in  FIG. 5  and described hereinafter.  
         [0016]     As shown in  FIG. 4 , balance piston cavity  42  is defined at its axially forward end by the annular plate  53  connected to high pressure drive shaft  26  of the rotor and at its axially aft end by annular plate  55  connected to the stator  25 . High pressure compressor discharge air from a compressor stage selected to provide the required air pressure and flow rate pressurizes air pressurized volume  70  and air flow through seal  52  flows into and pressurizes the interior of balance piston cavity  42 . The magnitude of force acting in the axially forward direction represented by arrow  76  which can be supplied by balance piston cavity  42  is determined by the air pressure inside balance piston cavity  42  applied to annular surface area having radius  54  of plate  53 . Air pressure relief tube  56  connects the balance piston cavity  42  in flow communication to a control system shown in  FIG. 5  and described hereinafter.  
         [0017]     A thrust balance control system is shown in block diagram form in  FIG. 5 . Balance piston cavity  42 , pressurized by discharge from high pressure compressor  69  via air pressurized volume  70  through seal  52 , is connected via air pressure relief tube  56  to air flow control valve  58  and air exhaust tube  62 . Balance piston cavity  46 , pressurized by discharge from high pressure compressor  69  via air pressurized volume  72  through seal  41 , is connected via air pressure relief tube  47  and air flow control valve  59  to air exhaust tube  64 . Exhaust tubes  62 ,  64  direct air to a downstream area  66  of the engine, which allows the air to contribute to overall engine efficiency, or alternatively may be exhausted to ambient. Control unit  60  receives pressure measurements from respective sensors  82  and  84  in balance piston cavities  42  and  46 , respectively. Control unit  60  is operatively connected to provide independent control of air flow control valves  58  and  59 . Control unit  60  may be a manually operated system providing readout of the pressure measurements from sensors  82  and  84 , so that an operator may manually activate air flow control valves  58  or  59 . Alternatively, control unit  60  may be an automatically controlled unit which adjusts settings of air flow control valves  58  or  59  in accordance with a numerical control system In either manual or automatic configuration air flow control valves  58  and  59  may be independently activated by control unit  60  to raise or lower the pressure in each or both of balance piston cavities  42  or  46 . The pressure in one balance piston cavity may be raised by a certain pressure, while the pressure in the other may be raised an identical amount, a lesser amount, a greater amount or may be lowered by any amount within the pressure range capability of the system.  
         [0018]     During engine operation control unit  60  may operate air flow control valves  58  and  59  independently to control pressure levels in the respective balance piston cavities  42  and  46 . Air pressure inside balance piston cavity  46  is controlled via air pressure relief tube  47  by activation of air flow control valve  59 . When air flow control valve  59  is closed, the pressure inside balance piston cavity  46  is held at essentially the maximum pressure available from the high pressure compressor source  72  output to apply proportional pressure to the annular plate  43  as a generally downstream force, represented by arrow  74  in  FIG. 3 , to balance the load on thrust bearing  40 . When air flow control valve  59  is fully opened, the pressure inside balance piston cavity  46  drops to approximately the low pressure of downstream area  66  or ambient, and little pressure is applied to annular plate  43 . Intermediate valve settings for air flow control valve  59  allow the pressure inside balance piston cavity  46  to be maintained at an intermediate pressure level below the maximum pressure available from the high pressure compressor source  72 . Air pressure inside balance piston cavity  42  is controlled via air pressure relief tube  56  by controlling air flow control valve  58 . When air flow control valve  58  is closed, the pressure inside balance piston cavity  42  is held at essentially the maximum pressure available from the high pressure compressor source  70  output and applies that pressure to the annular plate  53  as a generally upstream force, represented by arrow  76  in  FIG. 3 , to balance the load on thrust bearing  40 . When air flow control valve  58  is opened the pressure inside balance piston cavity  42  drops to the low pressure of downstream area  66  or ambient, and little pressure is applied to annular plate  53 . Intermediate valve settings for air flow control valve  58  allow the pressure inside balance piston cavity  46  to be maintained at an intermediate level below the maximum pressure available from high pressure compressor source  70 .  
         [0019]     Control unit  60  selectively activates air flow control valve  58  to maintain, raise or lower the pressure inside balance piston cavity  42  and/or selectively activates air flow control valve  59  to maintain, raise or lower the pressure inside balance piston cavity  46 . Air flow from balance piston cavity  42  via air pressure relief tube  56  is adjustable independently from air flow from balance piston cavity  46  via air pressure relief tube  47 , so that the pressure inside one of balance piston cavities  42 ,  46  may be lowered while pressure inside the other is maintained or raised. This independent pressure control allows the ratio of aft and forward forces to be adjusted during engine operation, in contrast to the fixed ratio of the prior art system. The separate control of pressure in balance piston cavity  42  from that of balance piston cavity  46  provides considerably higher thrust pressure level control and flexibility in controlling thrust loads on rotor thrust bearing  40 . If control unit  60  is automatically operated, the pressure control will be determined by an algorithm designed to predict rotor thrust levels during engine operation on the basis of measurements of pressure within the balance piston cavities. The control system will selectively adjust the pressure within the respective balance pressure cavities to maintain the proper loads on the thrust bearing  40 . As a result, the useful life of a bearing assembly is extended in a highly reliable and cost-effective manner. By separately controlling the pressure supplied to each of the balance piston cavities, i.e. lowering air pressure in one balance piston cavity while raising the air pressure in the other balance piston cavity by the same amount, the piston areas may be effectively added, so that the thrust balance may handle higher total thrust loads.  
         [0020]      FIG. 6  graphically illustrates the bearing load versus horsepower relationship and balance piston cavity pressure versus horsepower in the upper and lower graphs, respectively, for a gas turbine engine of the type shown in  FIG. 3 . The top graph of  FIG. 6  illustrates a typical relationship, but many factors affecting the bearing load, i.e. net rotor thrust, can cause the bearing load versus horsepower relationship to vary so that the curves may have many shapes. The gas turbine engine is designed so that during normal operation the thrust bearing  40  is operating within an acceptable thrust loading range of a nominal design thrust load level relative to horsepower shown at  106  between the maximum acceptable bearing load, shown at  108 , and the minimum acceptable load, shown at  110 . The difference between maximum load  108  and minimum load  110  at any horsepower setting determines the available range of thrust balancing control. As engine size and horsepower rise, the anticipated thrust bearing load increases and balance piston cavities including air relief tubes and valves are sized to accommodate the anticipated load balancing requirement. The engine is designed so that nominal valve settings for the air flow control valves  58  and  59  maintain the air pressures within the respective balance piston cavities  42 ,  46  approximately equal along a curve shown at  100  in the lower graph of  FIG. 6 , within a range between maximum operating pressure, approximately the compressor bleed for a particular horsepower, shown by curve  102 , and a minimum pressure of ambient shown by curve  104 . The difference between the maximum balance piston cavity pressure  102  and minimum balance piston cavity pressure  104  represents the available range of pressure adjustment at a particular engine horsepower level. If pressure sensors in the respective balance piston cavities indicate a pressure rise, a corresponding rise in thrust load in one axial direction on bearing  40  is indicated. The control system is activated to change the pressure inside one or both of the balance piston cavities to limit the axial thrust load on rotor thrust bearing  40  to the acceptable levels. Air flow control valve  58  may be activated to lower pressure on balance piston cavity  42  while the pressure in balance piston cavity  46  is maintained to effectively lower the force level applied to piston area  54  increasing net bearing load in the aft direction. If further adjustment is needed, the pressure in balance piston cavity  46  can also be raised by closing air flow control valve  59 , hence providing larger adjustment. The total force level adjustment available is the difference between the pressure  102  and the pressure  104  on piston area  51  and  54 . If the pressure sensors indicate an excess load in the aft direction, either or both of air flow control valves  58  and  59  may be activated to lower pressure in balance piston cavity  46  and/or raise the pressure in balance piston cavity  42  to apply forward axial pressure on balance piston area  54  and/or less force on piston area  51  to relieve the load on rotor thrust bearing  40 , depending on level of adjustment needed.  
         [0021]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.