Non-contacting packing ring labyrinth seals are conventionally used in steam turbines at various axial locations along the turbine rotor to seal against excessive steam leakage between regions of differential pressure. These packing ring seals typically include a plurality of spaced-apart annular teeth extending radially inward from the turbine casing to within close proximity of the rotating surface, leaving only a very small working clearance between each ring and the rotating part. This type of seal is very effective and is utilized both to prevent steam from leaking out around the shaft and to prevent leakage between stages of the turbine where the shaft passes through the diaphragms.
A certain amount of steam continuously enters and exits the packing ring structure with a flow component generally along the shaft in an axial direction. However, the steam flow also has a component in the circumferential direction, in a whirling pattern. This steam whirl results from two principal causes. First of all, steam enters the seal structure with a whirl component imparted by the most adjacent upstream turbine stage; and secondly, the drag effect of the rotating shaft produces a circumferential flow component. Although the latter frictional component is always in the direction of rotor rotation, the entering whirl may be in either direction depending on the operating parameters of the stage of the turbine immediately upstream from the seal. On turbines with double flow first stages, for example, it is known that the turbine stage that supplies steam to the end packing seals produces a forward running whirl (i.e., in the direction of shaft rotation) at high loads.
Steam flow within a seal structure is known to produce lateral forces on the turbine rotor due to asymetrical pressure gradients which arise in the sealing chambers. In some cases, where it is known that forward whirl within the shaft end seals is very strong, the turbine rotor begins to experience rotational instability related to the whirl conditions. In particular, in turbines of the double flow type mentioned above, there is a susceptability to rotational instability at higher load levels associated with forward steam whirl within the seals. In some installations it has been necessary to limit the load on the turbine to avoid destructive levels of vibration. It is generally the case that load related instabilities are discovered only after turbine installation is complete and when full load cannot then be satisfactorily attained. Thus, in seeking methods and apparatus to alleviate these problems, it has been particularly desirable to provide means which can be installed in the field as a "retrofit" without extensive modifications to the turbine and without prolonged turbine downtime.
The cause-effect relationship between fluid whirl in labyrinth seals and rotational instability has been investigated on a theoretical basis by numerous workers in the field, but to little practical effect. One attempt to deal with the problem (although not necessarily from a retrofit viewpoint) is shown by U.S. Pat. No. 4,273,510 to Ambrosch et al which appears to seek reduction of lateral forces in the seals by introducing a second fluid flow (presumably steam) into the seal in such a manner that the lateral forces are negated. While the exact dimensions of the Ambrosch et al disclosure are difficult to determine, it appears that this second flow is in addition to, or is perhaps an alternative to, the use of axial baffles in the seal gap between the rotor and stationary elements. The stated purpose of the baffles is to modify the rotary flow of fluid in the gap to negate the lateral forces. The structure and precise manner in which the apparatus of Ambrosch et al functions appears to be complex and not readily adaptable to be retrofitted to an installed turbine. In particular, if the apparatus of Ambrosch et al requires the introduction of a second steam flow to function properly, implementation after turbine installation would not be without difficulty.
Accordingly, it is a general object of the present invention to provide labyrinth sealing apparatus which is effective to prevent rotational instability in the rotor of a steam turbine wherein such instability is of the type inducible by steam whirl within the labyrinth seals.
Another object of the invention is to provide apparatus by which steam flow within at least a portion of a steam turbine labyrinth seal is caused to flow in a retrograde direction counter to the direction of rotor rotation thereby producing a stabilizing lateral force on the rotor to offset other destabilizing rotor forces which may be present but which cannot be readily eliminated or reduced.
More particularly, it is among the objects of the present invention to provide a labyrinth seal apparatus for a steam turbine which overcomes those problems outlined above, which is simple and easy to install as a retrofit to turbines experiencing such instabilities, and which does not rely on the introduction of a second steam flow to achieve its function.