Patent Application
20080296847
This present application relates generally to multi-stage turbine engines. More specifically, but not by way of limitation, the present application relates to the packing rings installed between the respective stages to minimize fluid leakage from one stage to the next.
In turbines the efficiency of the turbine is, in part, affected by the ability to prevent the working fluid from leaking from one stage to the next. For this purpose, one or more packing rings may be installed at the interface between stages, and between the outermost stages of the turbine and the atmosphere. The rings provide both a longitudinal seal between stages so the working fluid is properly confined within a stage. The packing ring (or rings) forming the seal comprises a plurality of ring segments (typically 4-12). These are curved or arcuate in shape, and assembled in an end-to-end abutment with one another to form the ring. Each ring segment includes sets of teeth that extend across an inner face of the segment. These teeth interface with the shaft to form a path or labyrinth sufficiently convoluted that leakage from one stage to the next, or to the atmosphere, is minimized. An effective seal is thereby created.
It will be understood by those skilled in the art that packing rings function in an elevated temperature environment and therefore the ring segments experience thermal expansion and contraction. These effects must be taken into account when the packing ring is assembled so a butt gap, which is created between adjacent ring segments, is sufficiently large to accommodate changes caused by the thermal characteristics. However, if the gap is too large, then a residual space will remain between segments after they thermally expand, and a leakage path will be created. On the other hand, if the gap is too small, then when the segments expand, they will butt together and expand to a larger diameter thus increasing the radial clearance between the packing ring and the turbine rotor shaft. Again, an increased leakage path will be created.
Predicting the appropriate butt gap, therefore, becomes essential. However, the varying environment of the turbine as well as other inaccuracies associated with predicting thermal expansion and contraction of packing rings generally means that butt gap allowances are rarely accurate. This condition leads to increased leakage between stages. Further, remedial measures to correct butt gap inaccuracies lead to greater downtime for the turbine. It will be appreciated that there is a need for an improved ring segment design that alleviates this condition.
The present application thus describes a packing ring in a multi-staged turbine that includes two ring segments joined by a dovetail feature. In some embodiments, the dovetail feature may include a male/female configuration. In other embodiments, the dovetail feature may include an insert/groove configuration. The insert of the insert/groove configuration may include a protrusion that extends from an approximately flat surface of an end of one of the ring segments. The groove of the insert/groove configuration may include a notch in an approximate flat surface of an end of one of the ring segments. The insert of the insert/groove configuration and the groove of the insert/groove configuration may be of a similar size such that the insert closely fits within the groove.
In some embodiments, the insert of the insert/groove configuration may include a rectangular protrusion that is positioned at the end of ring segment. The length of the insert may traverse the radial thickness of the ring segment.
In some embodiments, the groove of the insert/groove configuration may include a rectangular depression that is positioned in the end of ring segment. The length of the groove traverses the radial thickness of the ring segment.
The present application may further describe a packing ring in a multi-staged turbine that includes a ring segment that includes an insert at one end and a ring segment that includes a groove at one end. The insert and the groove may be sized so that the insert may engage the groove.
The outer radial face of each of the ring segments may include an engagement feature so that the ring segments may be engaged by an inner radial face of a turbine casing, and the inner radial face of each of the ring segments may include teeth.
The insert may include a protrusion that extends from an approximately flat surface of an end of one of the ring segments. The groove may include a notch in an approximate flat surface of an end of one of the ring segments. The insert and the groove may be of a similar size such that the insert closely fits within the groove.
In some embodiments, the insert of the insert/groove configuration may include a rectangular protrusion that is positioned at the end of ring segment. The length of the insert may traverse the radial thickness of the ring segment. In some embodiments, the groove may include a rectangular depression that is positioned in the end of ring segment. The length of the groove may traverse the radial thickness of the ring segment.
These and other features of the present application will become apparent upon review of the following detailed description of the preferred embodiments when taken in conjunction with the drawings and the appended claims.
Referring now to the figures, where the various numbers represent like parts throughout the several views,
It will be understood that the ring segments 4, 6 may include several teeth 10 that extend across an inner radial face of the ring segment 4, 6. These teeth 10 may interface with the shaft (not shown) to form a path or labyrinth sufficiently convoluted that leakage from one stage to the next, or to the atmosphere, is minimized. Typically, the ring segments 4, 6 may be made from bronze, steel or other similar materials.
As shown, between the ring segment 4 and the ring segment 6 a butt gap 12 may be maintained. The butt gap 12 is a gap between ring segments that allows for the thermal expansion when the ring segments 4, 6 are exposed to the elevated temperatures of the operating turbine. Ideally, the butt gap 4, 6 is sized such that it allows expansion while maintaining a proper seal between stages. If the butt gap is too large, then a residual space will remain between ring segments 4,6 after they thermally expand. Arrows 16 generally show the direction of the flow of the working fluid through the turbine in relation to the ring segments 4, 6. Given this flow, if residual space remains between the segment 4 and the segment 6 after thermal expansion, then a leakage path is formed, as working fluid will flow therebetween.
On the other hand, if the butt gap 12 is too small, then when the ring segments 4, 6 expand, they will butt together and expand to a larger diameter. This will increase the radial clearance between the teeth 10 of the ring segment 4, 6 and the turbine rotor shaft. Again, a leakage path will be formed and the overall efficiency of the turbine reduced.
It will be understood by those of ordinary skill in the art that the insert 24 and the groove 26 are shaped and sized such that the insert 24 may fit closely within the groove 26. Thus, the width of the groove 26 may be slightly larger than the width of the insert 24.
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From the above description of preferred embodiments of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims. Further, it should be apparent that the foregoing relates only to the described embodiments of the present application and that numerous changes and modifications may be made herein without departing from the spirit and scope of the application as defined by the following claims and the equivalents thereof.
