Uniform Clearance, Temperature Responsive, Variable Packing Ring

The present invention relates to seals between rotating and stationary components of a machine, for example, a steam turbine, and particularly relates to apparatus for controlling the movement of packing ring segments between large clearance start-up and small clearance steady-state operating positions about a rotary component.

In many machines, seals are provided between rotating and stationary components. For example, in steam turbines, it is customary to employ a plurality of arcuate seal ring segments to form a labyrinth seal about and between the stationary and rotating components. Typically, the arcuate seal ring segments are disposed in an annular groove in the stationary component designed to be concentric about 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 and which teeth 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. In any event, 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. As a further alternative, a combined labyrinth/brush seal configuration may be used as part of the sealing faces of the segments. The primary seal is therefore accomplished by the brush seal, with the labyrinth teeth serving as back-up seals in the event of a failure of the brush seal.

In a typical installation, the annular groove of the stationary component is dovetail-shaped, having locating flanges directed axially toward one another and defining a slot therebetween. The stationary component is split lengthwise such that the semi-annular dovetail grooves may receive correspondingly shaped arcuate seal ring segments. More particularly, the arcuate 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 joining the seal face and the flanges of the segment and passing through the slot defined by the locating flanges of the groove. The neck carries the arcuate seal face radially inwardly of the groove when installed.

In this type of seal, the ability to maintain a tight uniform clearance without physical contact between the rotating and stationary components is critical to the formation of an effective seal. If this radial clearance between the seal faces of the segments and the opposing seal surface of the rotating component becomes too large, less turbulence is produced and the sealing action is compromised. Conversely, if the clearance is too tight, the sealing teeth may contact the rotating element, with the result that the teeth lose their profile and tight clearance and thereafter create less turbulence, likewise compromising the sealing action.

Current variable clearance packing rings often use high pressure steam to overcome a radially outwardly directed spring force and move the packing ring segments radially inwardly toward the rotor after start-up. That is, at start-up, the springs locate the packing ring segments in a radially outermost position, affording a large clearance between the sealing faces of the segments and the rotor to accommodate relative movement of the rotor and stationary component, particularly radial excursions of the rotor during start-up. At steady-state, the steam pressure overcomes the spring force and displaces the segments radially inwardly to a small clearance position relative to the rotor. These seals, however, may possibly hang up or bind due to uneven friction forces and potentially open up large leakage gaps in comparison with the more conventional fixed or springbacked packing ring segments. Moreover, existing packing rings that are fitted to the turbine with no inter-segment interference can be subject to considerable bias leakage between segments due to a mismatch in thermal expansion coefficients.

In accordance with the present invention, there is provided a variable clearance packing ring seal between stationary and rotating components that 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. Particularly, the present invention provides a packing ring segment having a lower coefficient of thermal expansion than a seal holder, i.e., the stationary component, typically a turbine diaphragm or packing casing. Also provided is a centering ring having a greater coefficient of thermal expansion than the seal holder. In a preferred form, a pair of centering rings are provided on axially opposite sides of the neck of the sealing 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. Preferably, the packing ring segments are 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, in accordance with the present invention, 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 centering 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 centering rings, thus assuring concentricity of the packing ring segment sealing surfaces relative to the rotary component, enabling a small 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.

In a preferred embodiment according to the present invention, there is provided, for use in a machine having a component rotatable about an axis, a stationary component about the axis, and supports carried by the stationary component about the axis, a variable clearance packing ring comprising a packing ring segment about the axis, the segment having an arcuate seal face in opposition to the rotatable component and at least a first arcuate flange extending in an axial direction, an element engageable by one of the supports and engaging the first flange at a first location therealong for maintaining the segment seal face in a first clearance position relative to the rotating component, the element having a coefficient of thermal expansion greater than a coefficient of thermal expansion of the segment, the element engaging the first flange at a second location therealong in response to relative thermal expansion of the segment and the element, enabling the segment for displacement radially inwardly relative to the element to locate the seal face in a second clearance position relative to the rotating component and radially inwardly of the first clearance position.

In a further preferred embodiment according to the present invention, there is provided, for use in a machine having a component rotatable about an axis, a stationary component about the axis, and a support carried by the stationary component about the axis, a variable clearance packing ring comprising a packing ring segment about the axis, the segment having an arcuate seal face in opposition to the rotatable component, an element engageable by the support and engaging the segment at a first location therealong for maintaining the segment seal face in a first clearance position relative to the rotating component, the element having a coefficient of thermal expansion greater than a coefficient of thermal expansion of the segment, the element engaging the segment at a second location therealong in response to relative thermal expansion of the segment and the element, enabling the segment for displacement radially inwardly relative to the element to locate the seal face in a second clearance position relative to the rotating component and radially inwardly of the first clearance position.

Accordingly, it is a primary object of the present invention to provide a novel and improved variable clearance temperature-responsive packing ring for sealing between stationary and rotary components and which accommodates large and small clearance positions at start-up and steady-state, respectively.

FIG. 1 is a fragmentary cross-sectional view illustrating a plurality of packing ring seal segments between a stationary and rotary component according to the prior art;

FIG. 2A is a fragmentary axial cross-sectional view illustrating a labyrinth seal packing ring segment in a large clearance position;

FIG. 2B is a view similar to FIG. 2A illustrating the packing ring seal segment in the small clearance position;

FIGS 3A and 3B are enlarged fragmentary cross-sectional views illustrating the ends of the packing ring segments and supports therefor in the large and small clearance positions, respectively; and

FIGS 4A and 4B are fragmentary cross-sectional views taken generally about on lines 4A--4A and 4B--4B in FIGS. 3A and 3B, respectively.

Referring now to FIG. 1, there is illustrated a portion of a steam turbine, generally designated 10, having a turbine shaft 12 disposed in a turbine housing 14 and which shaft 12 is supported for rotation by conventional means, not shown, within housing 14. A multi-stage labyrinth seal 16 includes a plurality of seal rings 18, 20 and 22 disposed about turbine shaft 12 separating high and low pressure regions 28 and 30, respectively. Each seal ring is formed of an annular array of a plurality of arcuate seal segments 32. In general, the labyrinth seal 16 functions by placing a relatively large number of partial barriers to the flow of steam from the high pressure region 28 to the low pressure region 30. Each barrier forces steam attempting to flow parallel to the axis of turbine shaft 12 to follow a tortuous path whereby a pressure drop is created. The sum of all the pressure drops in the labyrinth seal 16 is, by definition, the pressure difference between the high and low pressure regions 28 and 30. Referring to FIGS. 2A and 4A, each arcuate seal ring segment 32 has a sealing face 34 and radially projecting teeth 36, each sealing face 34 being formed by a pair of flanges 38 extending axially away from one another. The radially outer portions of the seal ring segments 32 include first arcuate locating flanges or hooks 40 which similarly extend from segment 32 in axially opposite directions away from one another. The turbine housing 14 includes a generally dovetail-shaped annular groove 42 defined along radially innermost portions by a pair of locating flanges 44 which extend axially toward one another defining a slot 46 therebetween. An axially reduced neck 45 extends between the locating flanges 40 and seal flanges 38 and in the slot 46 between flanges 44. Springs 47 are preferably disposed in the cavity 42 for biasing the packing ring segments 32 radially inwardly.

In accordance with the present invention, a plurality of packing ring segment supports 48 are disposed in the cavity 42 between the locating flanges 38 and 44 of the segments 32 and housing 14, respectively. The supports may comprise dowels 48 at circumferentially spaced positions along the cavity 42. A centering ring or element 50 is provided in the cavity 42 between the dowels of the seal holder and the locating flange 38 on each axial side of the neck. The centering rings 50 have a circumferential extent comparable to the circumferential extent of the packing ring segments and a uniform diameter radially outer surface 52. It will be appreciated that the undersides 53 of the locating flanges 40 also have a uniform diameter radially inner surface for mating with the surface 52.

As illustrated, the opposite ends of each centering ring 50 have radial outward projections 54 with a canted contact surface 56. The canted contact surface 56 may be linear, convexly arcuate or may comprise a contact area or point. Each of the opposing end surfaces of each locating flange 38 has a contact surface 58 engaging the contact surface 56 in the large clearance position of the seal segment relative to the rotor. The radially innermost surface of the centering ring 50 at its opposite ends is inset or recessed such that the centering ring 50 rests on dowels 48 along recessed surfaces 62. The centering ring 50 also has canted surfaces 64 for engaging the dowels to prevent the centering ring from displacement in a circumferential direction relative to the segment. It will be appreciated that, with the springs 47 biasing the packing ring segments-radially inwardly, the contact surfaces 58 on the locating flanges 38 engage the contact surfaces 56 on the centering rings 50. The centering rings, in turn, are supported by the engagement between the dowels 48 and recessed surfaces 62. With that arrangement and with the various machine elements being cold, i.e., at start-up, each packing ring segment about rotor 12 is maintained in a large clearance position relative to rotor 12. Note the radial gap (FIGS. 2A and 3A) between the radial inner surfaces 53 of the locating flanges 40 and the outer surfaces 52 of the centering rings 50.

In accordance with the present invention, the centering rings 50 are formed of material having a coefficient of thermal expansion greater than the coefficient of thermal expansion of the sealing ring segments, the seal holder, i.e., the housing 14, and the rotor 12. Thus, any material which has a higher coefficient than the coefficient of the adjoining elements may be used as the material for the centering ring consistent with the expansion required for operation and at the temperatures of the machine. As a specific example, the packing ring segment may be formed of 409 Stainless Steel or Invar (nickel iron alloy) while the centering rings may be formed of 304 Stainless Steel. Also, as illustrated in FIG. 3A, the packing ring segments at their adjoining ends in the large clearance positions thereof provide a substantial gap therebetween.

After start-up; and as the temperature of the working fluid rises, the various elements react dimensionally differently relative to one another and enable the packing ring segments to be displaced from the large clearance to the small clearance positions, increasing slightly the gap between adjacent ends of the packing seal ring segments. To accomplish that, as the parts are heated by the working fluid, the centering rings 50 will expand in a circumferential direction, i.e., grow thermally in the circumferential direction to a greater extent than the circumferential growth of the packing ring segment. As the centering rings expand circumferentially, the contact surfaces 56 at the ends of the centering rings are displaced circumferentially away from the contact surfaces 58, enabling the springs 47 or steam in the cavity 42 to displace the packing ring segments radially inwardly such that the surfaces 52 and 53 may engage one another. Note that the radial position of each centering ring remains the same during the thermal expansion, i.e., at the cold start and at the steady-state operating condition. With the displacements of the segments radially inwardly, the seal surfaces of the segments are displaced closer to the rotary component to the small clearance position illustrated in FIG. 3B. Concurrently, the end gaps between the seal ring segments and increase slightly to the desired running clearance. With the surfaces 52 and the undersurfaces 53 of the locating flange 40 concentric with one another, a uniform clearance concentrically about the rotor is achieved.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.