Patent Application
20140140841
The subject matter disclosed herein relates to turbine systems, and more particularly to turbine bucket shroud arrangements, as well as a method of controlling turbine bucket interaction with an adjacent turbine bucket.
Turbine systems employ a number of rotating components or assemblies, such as compressor stages and turbine stages that rotate at high speed when the turbine is in operation, for example. In general, a stage includes a plurality of free-floating blades that extend radially outward from a central hub. Some blades include a shroud that limits vibration within a stage and provides sealing to increase efficiency of the overall system. The shroud is typically positioned at a tip portion of the blade, a mid-portion of the blade or at both the mid portion and the tip portion of the blade. The shrouds are designed such that the free-floating blades interlock to form an integral rotating member during operation.
Prior to rotation of the free-floating blades, a gap between contact surfaces of the shrouds is present. The distance of the gap determines how early an interlock of the shrouds occurs upon startup of the turbine system. Too large of a gap inefficiently delays the locking speed, which may result in resonance, for example. Too small of a gap results in undesirable effects at high speed operation of the turbine system. Such effects include lower damping effectiveness and flutter margin, as well as high stresses imposed on the turbine bucket due to increased transfer of forces between the contacting shrouds, for example. Therefore, current efforts to beneficially reduce the gap to provide an early interlock to address potential low speed aeromechanics issues are mitigated by the detrimental effects on tip shroud life that occur at steady state operating conditions.
According to one aspect of the invention, a turbine bucket shroud arrangement for a turbine system includes a contact region of a tip shroud, wherein the contact region is in close proximity to an adjacent tip shroud. Also included is a negative thermal expansion material disposed proximate the contact region, the contact region comprising a first volume during a startup condition and a shutdown condition of the turbine system and a second volume during a steady state condition of the turbine system, wherein the second volume is less than the first volume.
According to another aspect of the invention, a method of controlling turbine bucket interaction with an adjacent turbine bucket is provided. The method includes reducing a gap disposed between a contact region of a tip shroud and an adjacent tip shroud by depositing a negative thermal expansion material proximate the contact region. Also included is engaging the contact region of the tip shroud with the adjacent tip shroud during a startup operating condition and a shutdown operating condition. Further included is decreasing a volume of the contact region during increased temperature operating conditions upon contraction of the negative thermal expansion material, wherein decreasing the volume reduces tip shroud contact forces and stresses during a steady state operating condition.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.
The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
Referring to
In operation, air flows into the compressor 12 and is compressed into a high pressure gas. The high pressure gas is supplied to the combustor assembly 14 and mixed with fuel, for example process gas and/or synthetic gas (syngas), in the combustion chamber 18. The fuel/air or combustible mixture ignites to form a high pressure, high temperature combustion gas stream. Alternatively, the combustor assembly 14 can combust fuels that include, but are not limited to, natural gas and/or fuel oil. In any event, the combustor assembly 14 channels the combustion gas stream to the turbine 24 which converts thermal energy to mechanical, rotational energy.
At this point, it should be understood that each of the plurality of stages 26-28 is similarly formed, thus reference will be made to
A tip shroud 50 covers the tip portion 38 of the airfoil 32. The tip shroud 50 is designed to receive, or nest with, tip shrouds on adjacent rotating members in order to form a continuous ring that extends circumferentially about the stage 26. The continuous ring creates an outer flow path boundary that reduces gas path air leakage over top portions (not separately labeled) of the stage 26, so as to increase stage efficiency and overall turbine performance. In the exemplary embodiment shown, during high or operational speeds, adjacent airfoils interlock through the tip shroud 50 of each respective airfoil by virtue of centrifugal forces and thermal loads created by the operation of the turbine 24.
Referring now to
To alleviate the stresses imposed by potential expansion of already contacted components, at least one of the contact region 54 and the adjacent contact region 56, but typically both the contact region 54 and the adjacent contact region 56, include a negative thermal expansion material 60. The negative thermal expansion material 60 is defined by having a negative coefficient of thermal expansion, such that the material contracts in response to increased temperature exposure, rather than expanding. It is to be appreciated that any material having a negative coefficient of thermal expansion may be suitable for inclusion with the contact region 54 and the adjacent contact region 56. Examples of such materials include zircon, zirconium tungstate and A2(MO4)3 compounds. Forming at least a portion of the contact region 54 and the adjacent contact region 56 with the negative thermal expansion material 60 advantageously allows for the gap 58 to be dimensionally reduced to facilitate an early interlock between the tip shroud 50 and the adjacent tip shroud 52, while also reducing the contact forces associated with interaction between the tip shroud 50 and the adjacent tip shroud 52, thereby reducing stresses imposed on various portions of the tip shroud 50, the adjacent tip shroud 52 and the airfoil 32 attached thereto. The stress reduction is achieved by maintaining an interlock, but contracting the negative thermal expansion material 60. In other words, the contact region 54 comprises a first volume during a startup condition of the turbine system 10 and a smaller, second volume during a steady state operating condition of the turbine system 10.
Referring now to
The contact region 54, whether a single layer or the plurality of composition layers 72, may be deposited or integrated with the tip shroud 50 in a number of application processes. Examples of such processes include brazing, welding, laser cladding, cold spraying and a plasma transferred arc (PTA) process. The preceding list is merely illustrative and is not intended to be limiting of numerous other suitable application procedures.
As illustrated in the flow diagram of
While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.
