Steam turbines represent a large capital expenditure both in terms of the turbine itself and the operative apparatus which the turbine drives. Thus, it is almost always more efficient and cost effective to retrofit or otherwise improve the performance of existing turbines through modification, rather than replacement. As pointed out in U.S. Pat. No. 4,900,223--Groenendaal, Jr., which is incorporated herein by reference, modernizing older steam turbines frequently includes installing longer last row blades to increase the annulus area or flow area at the turbine exhaust. The retrofit of such longer blades results in both improved efficiency and increased power output.
It has been found, however, that the increased blade length causes interference with existing structural elements of the steam turbine. As disclosed by the above-referenced patent, the blade carrier ring of the inner housing of the turbine must be machined to establish a new profile on the ring of the stationary blade assembly and inner housing. An assembly groove for the stationary blade assembly is also machined into the new profile. In this type of retrofit, however, the flow guide is attached to the inner housing by a series of spacers and bosses, using both bolted and welded connections. As disclosed in the above-referenced patent, a drilled circular boss is welded to the end wall of the inner housing and further secured by bolts, the machined cuts described above are made, and a spacer ring is added. The flow guide is then attached with the spacer ring using a series of additional bolts.
Although the structure described in the above-referenced patent permits longer blade rows to be added to existing turbines, numerous difficulties have been found to exist. The above-described retrofit is complex and time-consuming, requiring a number of machining steps and the addition of further structural elements to the turbine. Additionally, after the turbine is fitted with longer last row blades, problems which result from the flow guide being attached to the turbine housing still exist. For example, the mass of the flow guide acts independently of the mass of the blade assembly, thereby creating a limitation in mass distribution that makes it difficult to adjust the natural frequency of the structure. If the natural frequency of the flow guide or blade assembly is at or near the turbine running speed, undesirable harmonic vibrations occur, possibly resulting in excessive stresses and, ultimately, in the cracking of the blade assemblies or flow guide.