Fluid reaction surfaces (i.e. – impellers) – With heating – cooling or thermal insulation means
Reexamination Certificate
1998-12-22
2001-02-20
Verdier, Christopher (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
With heating, cooling or thermal insulation means
C416S039000, C416S19800R, C416S20100A, C416S09600A, C416S09600A, C415S175000, C415S176000, C415S178000, C415S148000
Reexamination Certificate
active
06190127
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to turbines and particularly to land-based gas turbines for power generation. More particularly, the present invention relates to tuning the thermal mismatch between rotor parts, for example, a turbine wheel and a spacer or aft shaft by controlling flow of a thermal medium on the exterior surface of one of the turbine parts to substantially eliminate the thermal mismatch or maintain it within a predetermined thermal mismatch.
BACKGROUND OF THE INVENTION
In a typical gas turbine, the turbine rotor is formed by stacking rotor wheels and spacers, the stacked plurality of wheels and spacers being bolted one to the other. Rabbeted joints are typically provided between the spacers and wheels. In more advanced gas turbines, cooling circuits are provided through the rotor for cooling the buckets. For example, cooling steam may be provided through an aft shaft forming part of the rotor assembly for flow along the rim of the rotor to the buckets of one or more of the turbine stages to cool the buckets. Spent cooling steam also flows from the buckets in a return path along the rim of the rotor and through the aft shaft.
BRIEF SUMMARY OF THE INVENTION
With the stack-up of rotor wheels and spacers, and with varying temperatures being applied to various rotor elements at different times during operation of the turbine, i.e., startup, steady-state operation and shutdown, it has been discovered, in accordance with the present invention, that a thermal mismatch between turbine rotor elements may be of sufficient magnitude during particular phases of turbine operation that relative movement of the turbine elements may occur, opening up the rabbeted joints, leading to the below-mentioned deleterious results. This mismatch occurs particularly in the present advanced gas turbine design because steam cooling circuits are provided in the aft shaft which mates with the wheel of the last turbine stage, e.g., the fourth stage. It will be appreciated that during steady-state turbine operation, the thermal mismatch between elements of the turbine rotor and particularly between the aft shaft and the last-stage wheel is within a predetermined acceptable range which substantially precludes relative movement between the wheels and spacers or the aft shaft and last-stage wheel, preventing the rabbeted joints from shifting or opening up. Thus, at steady-state operation, there is no relative movement of the turbine rotor parts which otherwise could cause the rotor to lose balance, possibly leading to high vibrations and a need for rebalancing or rotor replacement at substantial cost.
During turbine shutdown, however, hot gases of combustion no longer flow through the hot gas path and, in a relatively short period of time, approximately one hour, the turbine slows from 3000 rpm to 7 rpm. It will be appreciated that with only marginal flow through the turbine at this low rpm, with the steam cooling circuits shut down, and the relatively large mass of the turbine wheel, the temperature of the turbine wheel decreases at a substantially slower rate than the temperature decrease of the aft shaft, causing a thermal mismatch between those elements. A thermal mismatch of as much as 280° F. between these elements has been demonstrated during turbine shutdown. A large thermal mismatch such as this can unload the rabbeted joints and cause relative movement between the elements. Over time, of course, the thermal mismatch decreases until there is substantial thermal equilibrium between these elements.
Likewise, at startup of the turbine, thermal mismatches occur between various rotor elements. For example, at startup, the hot gas flowing through the hot gas path of the turbine heats up the last-stage turbine wheel very slowly because of its large mass. Conversely, the aft shaft which conveys the cooling medium, initially air and subsequently steam, heats up rather rapidly, causing a thermal mismatch between the aft shaft and last-stage wheel. This again may cause the rabbeted joint between these elements to open, resulting in the potential for an unbalanced rotor.
To solve the aforementioned problem of thermal mismatch, the temperature of at least one of the elements of the thermally mismatched pair of elements is preferentially heated or cooled, depending upon whether the turbine is being shut down or started, respectively. For example, during shutdown, a heated fluid medium, for example, hot air, is provided in a cavity between the aft shaft wheel surface and the forward closure plate. This heated air thus lies in heat transfer relation with the surface of the aft shaft wheel to prevent the aft shaft wheel from rapidly cooling. This flow of heated air reduces the thermal mismatch between the aft shaft wheel and the last-stage turbine wheel to a value within a predetermined acceptable thermal mismatch, for example, on the order of 70 or 80° F. difference. Similarly, during startup, a cooling medium, for example, air, is supplied in that same cavity to maintain the rate of increase in temperature of the aft shaft wheel in substantial correspondence with the increase in temperature of the last-stage wheel such that the thermal mismatch between those wheels at startup is maintained within predetermined acceptable limits. The heating or cooling medium may be provided through suitable piping into the forward closure plate cavity.
In a preferred embodiment according to the present invention, there is provided in a gas turbine having a pair of parts responsive to different applied temperatures creating a transient thermal mismatch, a method of maintaining the thermal mismatch between the parts within a predetermined thermal mismatch, comprising the step of flowing a fluid medium along a surface of one of the parts to either heat or cool one part to a temperature enabling the magnitude of the thermal mismatch of the parts to lie within the predetermined thermal mismatch.
In a further preferred embodiment according to the present invention, there is provided in a turbine having a turbine wheel and an aft wheel secured to and in axial registration with one another and with a rabbeted joint therebetween, the wheels being responsive to different applied temperatures creating a transient thermal mismatch therebetween, a method of preventing relative movement between the wheels consequent of a thermal mismatch between the wheels beyond a predetermined thermal mismatch, comprising the step of flowing a fluid medium along a surface of one of the wheels to either heat or cool the one wheel to a temperature reducing the thermal mismatch to a value within the predetermined thermal mismatch.
Accordingly, it is a primary object of the present invention, to provide apparatus and methods for maintaining the thermal mismatch between turbine rotor elements within a predetermined thermal mismatch by controlling the supply of heated or cooling medium to a surface of one of the elements.
REFERENCES:
patent: 4419044 (1983-12-01), Barry et al.
patent: 4844694 (1989-07-01), Naudet
patent: 5281085 (1994-01-01), Lenahan et al.
patent: 5605437 (1997-02-01), Meylan
patent: 394001 (1933-06-01), None
patent: 635783 (1950-04-01), None
patent: 2-9901 (1990-01-01), None
General Electric Co.
Nixon & Vanderhye
Verdier Christopher
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