Seal for a joint or juncture – Seal between relatively movable parts – Close proximity seal
Reexamination Certificate
2002-01-29
2003-11-25
Gall, Lloyd A. (Department: 3676)
Seal for a joint or juncture
Seal between relatively movable parts
Close proximity seal
C277S416000, C277S546000
Reexamination Certificate
active
06651986
ABSTRACT:
BACKGROUND OF THE INVENTION
In rotary machines such as turbines, seals are provided between rotating and stationary components. For example, in steam turbines it is customary to provide a plurality of arcuate packing ring segments (sometimes referred to as seal ring segments) to form a labyrinth seal between the stationary and rotating components. Typically, the arcuate packing ring segments are disposed in an annular groove in the stationary component concentric to the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. In labyrinth-type seals, the seal faces carry a radially directed array of axially spaced teeth that are radially spaced from an array of axially spaced annular grooves forming the sealing surface of the rotating component. Alternatively, the rotating component may have a smooth surface in radial opposition to the array of teeth on the seal faces. The sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface of the rotating component.
In a typical installation, the annular groove is dovetail-shaped, having locating flanges directed axially toward one another and defining a slot therebetween. The stationary component (e.g., a housing or casing) is typically split lengthwise along a generally horizontal extending midline defining upper and lower halves of the stationary housing. Thus, the semi-annular dovetail grooves receive portions of the arcuate packing ring segments. The packing ring segments are similarly dovetail-shaped, having a pair of flanges directed axially away from one another for disposition within the dovetail groove and a neck which joins the seal face and the flanges of the segment and passes through the slot defined by the locating flanges of the groove. The neck carries the arcuate seal face radially inwardly of the groove when each segment is installed.
In positive pressure, variable clearance type packing rings, the segments are typically spring biased into outer or large clearance positions with the seal faces carried thereby spaced substantially outwardly of the rotary component. Thus, for example, at startup of the machine, the springs displace the segments radially outwardly. After start-up, the working fluid medium, e.g., steam, is inlet to the grooves of the stationary component, biasing the segments to move inwardly against the bias of the springs toward the inner or small clearance positions. It will be appreciated that when the segments are retracted to their large clearance positions during startup and turbine shutdown by the springs, the gaps between the end faces of adjoining segments become increasingly greater. Conversely, when the segments are displaced inwardly, the gaps between the end faces of the segments decrease substantially to a fully-closed position when the segments obtain their smallest diameter. To accommodate the inward and outward movement of the segments, the segments are disposed within the grooves of the stationary component in a sliding or piston-fit arrangement.
In an effort to avoid possible hang-up or binding of the seal segments due to uneven friction forces and potentially open up large leakage gaps in comparison with the more conventional fixed or spring backed positive pressure packing ring segments, another form of a variable clearance packing ring seal between stationary and rotating components has been proposed which uses thermal expansion characteristics of the various elements to enable a large seal clearance between the packing ring segment sealing face and the rotating component at start-up, while ensuring a small clearance and minimal inter-segment leakage therebetween at steady-state operation (See U.S. Pat. No. 6,065,754, the entire disclosure of which is incorporated herein by this reference). According to that patent, there is provided a packing ring segment having a lower coefficient of thermal expansion than a seal holder, i.e., the stationary component, typically a turbine housing or packing casing. Also provided is a centering ring having a greater coefficient of thermal expansion than the seal holder. A pair of centering rings are provided on axially opposite sides of the neck of the packing ring segments and are supported by the stationary component, for example, by reroundable dowels affixed to the seal holder and projecting in an axial direction into the dovetail cavity. The packing ring segment is supported by the centering rings and each segment is biased radially inwardly by springs acting between the segments and the stationary component. The packing ring segments engage contact surfaces on the opposite ends of the centering rings to maintain a large clearance position at start-up between the seal faces of the segments and the rotary component. In that position, the segment ends are closed.
The centering rings have a greater coefficient of thermal expansion than the seal holder, rotor and packing ring segments. After start-up and as the temperature rises, the centering rings thermally expand in a circumferential direction to a greater extent than the packing ring segment. The contact surfaces, forming the support points for the packing ring segment on the center rings, are therefore shifted circumferentially to enable the packing ring segment to be displaced radially inwardly. As the machine reaches steady-state operation, the packing ring segments engage the outer surface of uniform diameter of the center rings, thus assuring concentricity of the packing ring segment sealing uniform clearance between the sealing faces of the segments and the rotary component and opening end gaps between the seal segments. The result is a uniform clearance seal that retains its labyrinth tooth geometry during start-up and a steady-state operation and has minimal bias leakage.
When the rotary machine (e.g., steam turbine) is at rest, for example, prior to startup, the segments, lying in their largest radially outward position, have been observed to slide along the circumferential groove of the stationary component in a downward direction under the action of gravity forces. As a consequence, an accumulation of clearances between the end faces of the segments appears in the upper housing, while the lower segments become closer, i.e., butting end-to-end. For example, when six segments are employed in a rotary machine, three segments are typically disposed in the upper housing of the stationary casing and three segments are disposed in the lower housing, i.e., respectively above and below the horizontal midline or splitline of the machine casing. Prior to startup, the side segments in the upper housing and the side segments of the lower housing may slide under the action of gravity forces such that the side segments of the lower housing engage the lowermost central segment in the lower housing of the machine, with the accumulated clearance between the end faces appearing between the uppermost segment in the upper housing and one or both of the side segments of the upper housing.
Upon startup, the segments are designed to move from their radially outward positions to their radially inward positions. Because of the effect of gravity, the segments tend to move in sequence. First, the top segment moves radially inwardly, followed by the side segments of the upper housing. The side segments of the lower housing then move radially inwardly, followed by the lowermost segment. It has been found, however, that because the segments become displaced circumferentially about the groove of the stationary component by gravity forces, the inward movement of the side segments of the lower housing of the stationary component, essentially in an upward and inward radial direction, can preclude or block radial inward (upward) movement of the lowermost segment. That is, the fluid pressure
Chevrette Richard Jon
Kirby, II George Horner
Skinner David Robert
Wolfe Christopher Edward
Gall Lloyd A.
General Electric Company
Kyle Michael J.
Nixon & Vanderhye PC
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