BACKGROUND OF THE INVENTION
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.
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.
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
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.
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
FIG. 1 is a schematic, longitudinal cross-sectional view of a gas turbine engine including a rotor thrust balancing system;
FIG. 2 is an enlarged, schematic, partial cross-sectional view of a prior art thrust control system in a gas turbine engine;
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;
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;
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
FIG. 6 is graphical representation of one technique for pressure regulation in a system as shown in FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Patent Application
20070122265
