Compliant plate seals for turbomachinery

A shaft seal serves to reduce leakage between a rotating shaft and a static shell. The shaft seal includes a plurality of compliant plate members, each having a root and a tip. The compliant plate members are secured to the static shell at their root in facing relation. The tips of the compliant plate members define a sealing ring between the static shell and the rotating shaft. An axial flow resistance member is disposed within the compliant plate members and serves as a barrier to axial leakage flow between the compliant plate members.

BACKGROUND OF THE INVENTION

The invention relates to a sealing structure between a rotating component and a static component and, more particularly, to a compliant plate seal arrangement that additionally utilizes features of a labyrinth seal.

Dynamic sealing between a rotor (e.g., rotating shaft) and a stator (e.g., static shell or casing) is an important concern in turbomachinery. Several methods of sealing have been proposed in the past. In particular, sealing based on flexible members has been utilized including seals described as leaf seals, brush seals, finger seals, shim seals, etc.

A brush seal is comprised of tightly packed generally cylindrical bristles that are effective in preventing leakage because of their staggered arrangement. The bristles have a low radial stiffness that allows them to move out of the way in the event of a rotor excursion while maintaining a tight clearance during steady state operation. Brush seals, however, are effective only up to a certain pressure differential across the seal. Because of the generally cylindrical geometry of the bristles, the brush seals tend to have a low stiffness in the axial direction, which limits the maximum operable pressure differential to generally less than 1000 psi.

To overcome this problem, leaf seals have been proposed that include a plate-like geometry with higher axial stiffness and therefore the capability of handling large pressure differentials (an exemplary conventional leaf seal is illustrated on the left side ofFIG. 1). Axial leakage, however, remains a problem due to the leaf seal geometry. That is, if the leaves are packaged tightly close to the rotor, there will be gaps at the leaf roots since the seal is curved, which gaps potentially cause leakage and in turn offset the benefits of the seal.

BRIEF DESCRIPTION OF THE INVENTION

In an exemplary embodiment of the invention, a shaft seal reduces leakage between a rotating shaft and a static shell. The shaft seal includes a plurality of compliant plate members attached in facing relation to the static shell, where the compliant plate members define a sealing ring between the static shell and the rotating shaft. Each of the compliant plate members includes at least one slot therein. The shaft seal also includes at least one static ring attached to the static shell and extending radially into the at least one slot in the compliant plate members.

In another exemplary embodiment of the invention, the shaft seal includes a plurality of compliant plate members, each having a root and a tip, where the compliant plate members are secured at their roots in facing relation to the static shell. The tips of the compliant plate members are arranged circumferentially about the rotating shaft. An axial flow resistance member is disposed within the compliant plate members and serves as a barrier to axial leakage flow between the compliant plate members.

In still another exemplary embodiment of the invention, the shaft seal includes a plurality of compliant plate members attached in facing relation to the stator, the compliant plate members defining a sealing ring between the stator and the rotor. Each of the compliant plate members includes a plurality of varying length slots therein. A corresponding plurality of static rings are attached to the stator and extend radially into the plurality of slots in the compliant plate members, respectively. The plurality of static rings have varying radial lengths corresponding to the varying length slots and serve as a barrier to axial leakage flow between the compliant plate members.

DETAILED DESCRIPTION OF THE INVENTION

The improved compliant plate seal described herein achieves a structure that curtails the above-mentioned axial leakage seen in a conventional leaf seal by employing a geometry that includes features of a labyrinth seal. As noted, in a conventional leaf seal, because the leaves are packed tightly at the tips and loosely at the roots, axial leakage entering the leaf pack tends to flow/expand radially outward, then axially and finally converges as it exits the leaf pack.

With reference toFIGS. 1 and 2, a shaft seal10serves to reduce axial leakage between a rotor12, such as a rotating shaft, and a housing14, attached to the turbine static shell15. The shaft seal10is provided with a plurality of compliant plate members16secured at their roots in facing relation (i.e., face-to-face) to the housing14. The compliant plate members16define a sealing ring between the housing14and the rotating shaft12.

An axial flow resistance member17is disposed within the compliant plate members16and serves as a barrier to axial leakage flow between the compliant plate members16. In a preferred arrangement, the axial flow resistance member17includes at least one ring18attached to the housing14and extending radially into a corresponding at least one circumferential slot20in the compliant plate members16. As depicted inFIGS. 1 and 2, each of the compliant plate members16preferably includes multiple slots20that can be of varying radial lengths and axial widths, and the axial flow resistance member17includes a corresponding multiple rings18of corresponding radial lengths, respectively. Although three rings and slots are shown inFIG. 1, this depiction is exemplary, and the invention is not meant to be limited. In a broader sense, in some applications, adjacent ones of the circumferential slots20comprise different radial lengths with the static rings18extending radially into the circumferential slots20.

Although the illustrated slots20are shown having a rectangular shape, those of ordinary skill in the art will appreciate that other shaped slots may be utilized. The slots20may also be formed in different widths and varying lengths. Other compliant plate shapes may also be suitable, such as T-shaped, trapezoidal, and the like as shown inFIGS. 3 and 4.

The rings18force the leakage flow to follow a more tortuous path, thereby increasing the resistance to leakage flow. The configuration thus mimics a labyrinth seal within a compliant plate seal. The leaves retain their bending flexibility and axial stiffness, which are important for the seal functionality.

Since the compliant plates are packed more tightly at the tips (adjacent the rotor) than at the roots (adjacent the stator), the rings18need not extend radially from the plate root all the way to the plate tip. Rather, the rings18need only extend into a portion of the compliant plates16as shown.

An important advantage of compliant plate seals is a pressure build-up effect that is generated upon rotor rotation. The effect causes the compliant plates16to lift during rotor rotation. In response to this lift, any other pressure forces, and compliant plate material elasticity, an equilibrium state is attained for each compliant plate that leaves a very small clearance between the plate tips and the rotor12. This small clearance between the plate tips and the rotor reduces frictional heat generation by minimizing or eliminating physical contact.

Although a housing14is shown in the figures and described above, compliant plates16and rings18may directly be integrated with the stator15, depending on the application. An intermediate housing may be necessary only for practical purposes such as assembly and fabrication and not for functionality. Additionally, although front and back rings22,24are shown in the figures, the proposed configuration with the rings disposed within the compliant plates should achieve the desired objective without a need for front and back rings. In addition to the described features, the front and back rings can also be added to the seal. Typically, the front and back rings22,24will be part of the housing as shown inFIG. 1.

The figures show a stator that is external to a rotor. In another arrangement, the rotor can be external to the stator.

The compliant plates may be coated with special materials to achieve one or more of the following objectives (without limitation): minimize friction, wear and heat generation in case of relative sliding, act as diffusion barrier, and allow high temperature operation. The surface of the rotor, which is in close proximity to the compliant plate tips may also be coated for the above or other reasons. Common coating methods include Physical Vapor Deposition, thermal spray and galvanic deposition, to name a few. Coating materials include, but are not limited to, Titanium Nitride, Zirconium Nitride, NickelChrome—ChromeCarbide along with solid lubricants, Nickel, etc.

The shaft seal described herein provides a high-pressure dynamic seal between a rotating component and a static component. The seal includes multiple compliant plates that comply in the event of rotor excursion but are very stiff along the direction of pressure drop. Incorporation of an axial flow resistance member forces the axial flow to follow a tortuous path at the seal root. A combination of tightly packed seal tips and flow obstructing features at the seal root results in a significantly reduced axial leakage.