GAS TURBINE ASSEMBLY

Abstract

A gas turbine assembly having a first shaft, a first compressor and a first turbine mounted on the first shaft, a combustor between the first compressor and the first turbine and a second shaft and a second turbine mounted on the second shaft, the second turbine having an inlet connected to an outlet of said first turbine. The gas turbine assembly further includes a third shaft on which an upstream compressor is mounted, the upstream compressor having an outlet which is connectable to an inlet of the first compressor.

Claims

1. A gas turbine assembly comprising: a first shaft, a first compressor and a first turbine mounted on the first shaft, a combustor between the first compressor and the first turbine, and a second turbine having an inlet connected to an outlet of the first turbine, a third shaft on which a controllable motor and an upstream compressor are mounted, the upstream compressor having an outlet which is connected to an inlet of the first compressor, wherein the controllable motor varies the speed of the third shaft and upstream compressor independently from an air flow passing through the upstream compressor, wherein the air flow is ducted through the upstream compressor and the first compressor in series.

2. The gas turbine assembly of claim 1, further comprising: a second drive shaft connected to the second turbine providing a torque to drive a mechanical load or a generator.

3. The gas turbine assembly of claim 1, wherein the controllable motor is an electrical motor.

4. The gas turbine assembly of claim 1, wherein the upstream compressor has a pressure ratio of 1/3 or lower of the pressure ratio for the first compressor, and the pressure ratio for the first compressor is the pressure ratio at full load.

5. The gas turbine assembly of claim 1, wherein the upstream compressor has a pressure ratio of 1/5 or lower of the pressure ratio of the first compressor.

6. The gas turbine assembly of claim 1, wherein the upstream compressor has a pressure ratio of 1/10 or lower of the pressure ratio of the first compressor.

7. The gas turbine assembly of claim 1, further comprising: an intercooler between the first compressor and the upstream compressor.

8. The gas turbine assembly of claim 1, further comprising: a common bearing housing between the inlet bearing of the first compressor and the outlet bearing of the upstream compressor.

9. The gas turbine assembly of claim 1, further comprising: a brake associated to the third shaft for preventing the rotation thereof and a plurality of variable guide vanes at the inlet of the upstream compressor.

10. The gas turbine assembly of claim 1, further comprising: an air filtration system arranged to filter the air flow entering the upstream compressor.

11. A method for operating the gas turbine assembly of claim 1, the method comprising: varying the speed of the upstream compressor by varying the speed of the third shaft in order to control the mass flow rate through the first compressor and the first turbine.

12. The method of claim 11, further comprising: holding the pressure ratio of the first compressor approximately constant and wherein the pressure ratio of the upstream compressor is greater than 0.9.

13. The method of claim 11, further comprising: changing the positions of the plurality of variable guide vanes at the inlet of the upstream compressor in order to control the mass flow rate through the first compressor and the first turbine.

14. The method of claim 11, further comprising: increasing the speed of the upstream compressor to increase the speed of the first shaft until ignition conditions in the combustor are reached, continuing to control the speed of the upstream compressor in order to raise the pressure ratio over the first turbine and achieve a steady acceleration of the first shaft, reducing the speed of the upstream compressor until the second turbine starts to generate mechanical power.

15. The method of claim 11, further comprising: closing the plurality of variable guide vanes at the inlet of the upstream compressor.

16. The method of claim 11, further comprising: operating the brake associated to the third shaft for achieving standstill of the third shaft.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited.

[0038] FIG. 1 illustrates a schematical view of an exemplary embodiment of a gas turbine according to the present invention,

[0039] FIGS. 2 and 3 illustrate two schematical views of two respective conventional two-shaft gas turbines.

DETAILED DESCRIPTION

[0040] The illustrations in the attached drawing are schematic. It is noted that in different figures similar or identical elements are provided with the same reference signs.

[0041] FIG. 1 shows a twin shaft gas turbine assembly 100 according to the present invention. The gas turbine assembly 100 may be manufactured directly or may be obtained as an upgrade for the existing gas turbine assembly 200 as shown in FIG. 2 (or FIG. 3) above. The gas turbine assembly 100 includes a gas generator 110 (or, as an alternative embodiment, a gas generator 121) and a second power turbine 106, connected to an electrical generator 112 by means of a second shaft 105. The second power turbine 106 includes an inlet 106a which is connected to the outlet 103b of the turbine 103 of gas generator 110. The second power turbine 106 and electrical generator 112 are analogue in function, respectively, to the second power turbine 206 and electrical generator 212 of the conventional twin shaft gas turbine assembly 200. The second power turbine 106 further comprises an outlet 106b.

[0042] The gas turbine assembly 100 further comprises a third shaft 108 on which an upstream compressor 109 is mounted. The upstream compressor 109 includes an inlet 109a through which a mass flow of air, e.g. atmospheric air, enters the upstream compressor 109 and an outlet 109b which is connectable to an inlet 102a of the first compressor 102 of the gas generator 110.

[0043] The gas turbine assembly 100 further comprises an air flow 122 and an air filtration system 124. The air flow 122 can be an ambient or atmospheric air source. The air flow 122 is ducted into and through the air filtration system 124 to ensure suitable filtered air is supplied to the upstream or upstream compressor 109 before being ducted and supplied to the first compressor 102. Advantageously, the two compressors 109 and 102 are in series and which allows the use of a common air filtration system for both compressors 109, 102.

[0044] According to a different embodiment (not shown) the outlet 109b of the upstream compressor 109 is connected to an inlet of one of the compressors 112, 114, 116 of the gas generator 121.

[0045] The upstream compressor 109 comprises a plurality of variable guide vanes (not shown in the schematic views of the attached figures) at the inlet 109a. A brake128 is provided on the third shaft 108 for preventing the rotation of the shaft 108 and, in particular, for achieving standstill. The brake 128 can be a standalone system as shown in dashed lines in FIG. 1 or the brake can be part of or integral to the electric motor 111. Indeed the electric motor or other controlled motor 111 can be operated as a braking mechanism for the shaft 108 and upstream compressor 109.

[0046] The upstream compressor 109 is designed for high efficiency and over a wide operating range. The number of compressor stages depends on the pressure ratio of the upstream compressor. The efficiency of the compressor stage is dependent on the pressure ratio for the stage as well as the inlet Mach number. A low number of stages in the upstream compressor are advantageous to reduce cost and complexity.

[0047] In operation, the outlet 109b of the upstream compressor 109 can be either connected or disconnected to the inlet 102a of the first compressor 102, for example by opening or completely closing the VGVs of the upstream compressor 109. When connected to the first compressor 102, the upstream compressor 109 permits an increase in the mass flow rate through the gas turbine 100, even if a high specific flow compressor is used as the first compressor 102. A further way of disconnecting the upstream compressor 109 from the first compressor 102 is that of applying the brake to third shaft 108 of the upstream compressor 108, alone or in cooperation with the VGVs closure.

[0048] In case of upgrading from the existing gas turbine 200 shown in FIG. 2 or 3, the mass flow rate and power output are increased proportionally to the pressure ratio of the upstream compressor 109. To accommodate the new mass flow and increased power output, the power turbine 106 and generator 112 need to be correspondingly greater in size, with respect to the original power turbine 206 and generator 212. This will leave the original gas generator 110 unchanged in size and cost.

[0049] According to a possible embodiment of the present invention, to reduce the overall length and cost, providing a compact design of the upgraded gas turbine 200 the inlet bearing of the first compressor and the outlet bearing, or the only bearing if in overhung configuration, of the upstream compressor may share a common bearing housing. The spokes providing services to the bearing (e.g. lubrication oil, air and instrumentation) may reach through the gas path ducting air from the upstream to the first compressor.

[0050] When connected to the upstream compressor 109, the working of the gas turbine 100 at partial load can be managed without changing the pressure ratios in the gas generator 110 or 121. Maintaining the pressure ratios in the gas generator 110 at their design optimum values allows component efficiencies to remain high but also the emissions to be kept low, in particular CO.

[0051] The gas turbine 100 further comprises a variable speed electrical motor 111 connected to the third shaft 108 for driving the upstream compressor 109 independently from the gas generator 110 and the second power turbine 106. By controlling the electric motor 111, as a driver, the user can control the speed of the upstream or second compressor 109. Therefore, the speed of the upstream compressor 109 can be controlled independently of the air flow passing through it. Thus the electric motor 111 is one of a number of controlled motors 111 that can be used to independently vary the speed of the shaft 108 and upstream compressor 109. Other controlled motors 111 can include a hydraulic motor and a reciprocating motor for example. Furthermore, the electric motor 111 can operate as a brake

[0052] The upstream compressor 109 has a pressure ratio selected to achieve an optimal overall pressure ratio between the inlet 109a of the upstream compressor 109 and the outlet of the first compressor 102, in order to have high cycle efficiency, high power output and low emissions.

[0053] According to a different embodiment the gas turbine assembly 100 comprises an intercooler 106 between the first compressor 102 and the upstream compressor 109, in order not to exceed the material limits of the centre section of the gas turbine, in case high overall pressure are reached, or in order to increase the power output from the gas turbine.

[0054] As will be appreciated another aspect of the present invention is a method of operating the gas turbine assembly 100 as described above. The method of operation comprises controlling the controllable motor 111 such that it controls of varies the speed of the upstream compressor 109 by varying the speed of the third shaft 108. This control of the shaft speed is in order to control the mass flow rate through the first compressor 102 and the first turbine 103. The method can include the step of holding the pressure ratio of the first compressor 102 approximately constant so that the compressor operates at its optimum design capability and therefore efficiently. At this stage it is advantageous for the pressure ratio of the upstream compressor 109 to be greater than 0.9.

[0055] Further steps in the operational method and in order to control the mass flow through the engine to an optimum design level include changing the positions of the plurality of variable guide vanes situated at the inlet 109a of the upstream compressor 109. Altering the angle of the guide vanes alters the amount of work done on the mass flow of the air by the compressor and controls the mass flow rate through the first compressor 102 and the first turbine 103.

[0056] With the arrangement of the gas turbine assembly it is also possible to improve and control engine start up and ignition. By increasing the speed of the upstream compressor 109, via the controllable motor 111, the speed of the first shaft 101 increases until ignition conditions in the combustor 104 are reached. During and after ignition, continuing to control the speed of the upstream compressor 109, via the controllable motor 111, advantageously raises the pressure ratio over the first turbine 103 and achieves a steady acceleration of the first shaft 101. Once ignition and combustion is established and is self-sustaining, reducing the speed of the upstream compressor 109 until the second turbine starts to generate mechanical power.

[0057] A further step of operation includes closing the plurality of variable guide vanes at the inlet 109a of the upstream compressor 109. This reduces the pressure ratio across the engine which enables operation of the gas turbine engine at a lower load where the mass flow is reduced yet the volume of air flow remains constant and therefore the turbomachinery operates at or near its optimal design point and thus the engine remains at a high efficiency.

[0058] Finally, the method of operating the gas turbine assembly 100 can include braking and shutting down of the upstream shaft 108 and compressor 109 via the brake 128. An air flow can still pass through the stationary upstream compressor 109 while the first compressor 102 and turbine continue to operate. Nonetheless, this step can also be employed to shut down the entire engine.

[0059] Closing the variable guide vane during braking can be useful to ensure a positive pressure ratio through the engine is maintained and to prevent surging or reverse flow which can otherwise damage components of the engine and its ancillaries.