Steam turbines include a shell that functions to contain the high pressure, high temperature steam and to support the nozzles and casings that direct steam in the most efficient manner possible through rotating airfoils to produce maximum torque on the shaft. The shell includes support arms that extend outward from the shell. Shell arms rest on a support structure that is integral to a structure that also provides support for the turbine rotor, and which serves to house other turbine related components and instrumentation. This structure is often referred to as a “standard”. The elements of the stator portion of the steam turbine are coupled to the shell, thus, the stator portion of the steam turbine is supported by the shell arm support structure.
The rotor of the steam turbine is typically supported by bearings. The bearings are typically mounted within a bearing housing that is supported by a bearing support structure. The bearing support structure can be a part of the “standard” mentioned above. The bearing support structure, while it may be integral with the standard, is not integral with the shell arm support structure. The shell arm support structures and the bearing support structures are exposed to different environmental conditions during various stages of operation of the steam turbine. During transient operations, including but not necessarily limited to startup, load changes, shutdown, and cool downs while on turning gear, different portions of the steam turbine and the supporting elements experience changes in temperature. These changes in temperature may occur at different rates within the different parts of the steam turbine and its support structure, which leads to differential thermal growth of the steam turbine elements, and the support elements.
During startup operations, the bearings and the bearing support structure which supports the bearings and the rotor of the steam turbine tend to increase in temperature more quickly than the shell arm support structure. In part, this occurs because the bearings supporting the rotor rapidly heat up during startup because they are being driven by the changing oil temperature which they are in constant contact with, and the heat generated in the bearings is transferred to the bearing support structure. In contrast, the shell and the shell arm support structure, which are not in constant contact with the oil, tend to warm up more slowly.
Similarly, the bearings typically cool more rapidly upon shutdown because oil supply temperature is lowered as the steam turbine moves from full speed operation to operation at turning gear speeds. In contrast, the support structure beneath the shell arms generally cools very slowly because its temperature is driven more by shell temperature and conduction of that heat from the shell arms into the structure. Shell temperatures decay very slowly. And this can cause the bearing support structure to cool more quickly than the shell arm support structure upon shutdown.
When there are differences in the rate at which the temperature of the bearing support structure increases or decreases, as compared to the rate at which the temperature of the shell arm support structure increases or decreases, the temperature differences can lead to differences in the rate or amount of thermal expansion and contraction of these two elements. And differences in the rate or amount of thermal expansion or contraction as between the bearing support structure and the shell arm support structure can cause changes in the amount of radial clearance available between rotating and stationary parts.