Flexible housing for steam turbine exhaust hood   

Abstract: A flexible housing is arranged along an axial centerline of a rotor shaft of a steam turbine. The flexible housing extends between an exhaust hood endcover and a seal end packing housing on a separate supporting standard. The flexible housing includes a top cover portion and a bottom cover portion with a flanged horizontal joint that permits removal and replacement without need to remove the exhaust hood cover and the rotor shaft. The flexible housing encloses the rotor shaft with a first cylindrical element and a second cylindrical element of a different radius joined with an thin member that flexes to absorb expansion and contraction of the turbine exhaust hood.

BACKGROUND OF THE INVENTION

The invention relates generally to steam turbines and more specifically to a flexible end housing for the rotor of a steam turbine.

The outer shell of a steam turbine low-pressure section is generally called the exhaust hood. The primary function of an exhaust hood is to divert the steam from the last stage bucket of an inner shell to the condenser with minimal pressure loss. Usually the lower half of the exhaust hood supports an inner casing of the steam turbine and also acts as a supporting structure for the rotor. The upper exhaust hood is usually a cover to guide the steam to the lower half of the hood. The exhaust steam outlet from the turbine is generally conically-shaped and the steam exhaust is redirected from a generally axial extending flow direction to a flow direction 90 degrees relative to the axial flow direction. This 90-degree flow direction may be in any plane, downwardly, upwardly or transversely.

The lower exhaust hood typically supports the inner casing of the turbine and the associated steam path parts such as diaphragms and the like. The lower exhaust hood is further loaded by an external pressure gradient between atmospheric pressure on the outside and near-vacuum conditions internally. A vacuum is, of course, required in the operation of a low-pressure steam turbine to extract maximum work from the unit. However, in a conventional exhaust hood, the bearings are located in the cone areas and the inner casing supports are located inside the lower hood. When the exhaust hood is under vacuum, the inner walls and end cones deflect causing misalignment of the steam path rotor parts, seal end packing and bearing movements/tilt. The extended walls of the lower exhaust hood also support the inner exhaust casing in the conventional arrangement. Temperature and pressure changes in the hood will alter the position of the inner casing being supported by the hood wall, thereby impacting clearances of the rotor relative to the end bearings and the leakage labyrinths.

Some steam turbines may support the inner casing of the turbine on an external curb foundation. A distinct advantage is the reduction of the adverse affects of vacuum within the exhaust hood on leakage through seal end packing. When the inner casing is supported directly by a curb foundation, the effects of temperature and pressure changes of the exhaust hood are reduced relative to the positioning of the inner casing and the rotor within it. The rotor end packing and shaft bearings for the low-pressure turbine may be located outside the exhaust hood in a standard, which is supported directly on the foundation. Easier maintenance is facilitated because the shaft bearings are not tucked under the exhaust hood and the seal end packing can be removed without removal of a large section of the exhaust hood. This may allow use of tighter clearances resulting in a better performing turbine due to less leakage.

In either arrangement of the bearings and packing seals, the pressure boundary of a steam turbine exhaust hood is penetrated by a rotating turbine shaft in order to transmit power outside of the steam environment. Consequently, the shaft must be sealed at the points of penetration to prevent the escape of steam from the casing at locations where the casing pressure exceeds ambient pressure and to prevent leakage of air into the casing at locations where the casing pressure is below ambient.

Many sealing systems have been employed in the past for this purpose. For example, labyrinth-type seals have been employed about the shaft. Because the teeth of the meshing but non-contacting labyrinth seals inherently form leakage paths past the seal, labyrinth-sealing systems require steam seal and vent piping subsystems normally including a steam seal regulator, a gland exhauster and a gland condenser. Typically, labyrinth seals provided at the high-pressure end of the turbine prevent high-pressure steam from escaping the turbine. At the opposite end of the turbine, labyrinth seals prevent the entry of air into the turbine low pressure or vacuum region. At turbine start-up, the labyrinth seal at the steam inlet end of the turbine functions as a vacuum seal in view of the vacuum, which exists throughout the turbine. The labyrinth seals are thus used to restrict the flow of steam and/or air along the shaft.

Even with the small clearances between the teeth of the labyrinth seals, it is necessary to control the pressure differential across the labyrinth at the high-pressure end of the turbine with a stream seal regulator and gland exhauster. Thus, a seal header is conventionally maintained at a predetermined positive pressure and will either supply steam to the annulus between the seals or dump steam from the annulus, depending the pressure differential across the labyrinth seals defining the annulus. Similarly, steam under positive pressure must be supplied the shaft seals adjacent the flow outlet of the turbine, such that the labyrinth seals can effectively preclude entry of air into the turbine. Further, there is some difficulty in matching the steam\'s temperature supplied to the labyrinth seals to the needs of the metal at those locations.

In instances where the seal end packing is placed on a separate standard, the exhaust hood steam environment must be extended to the seal end packing on the in a manner that contains the turbine exhaust conditions. In some applications, a bellows has been applied around the rotor shaft and mounted to the exhaust hood at one end and at the other end to the seal end packing on the standard. The bellows accommodates some degree of expansion and contraction of the exhaust hood, thereby limiting impact on the positioning of the labyrinth packing in the seal end housing. However, bellows-type units fully encircle the rotor shaft. To install or remove the bellows, the upper exhaust hood and upper half of the standard must be removed, as well as the rotor shaft. Such an operation is complex, time consuming and undesirable.

Accordingly, it would be desirable to provide an arrangement for housing the exhaust hood to a seal end-packing box on a separate standard in a manner that accommodates expansion/contraction of the exhaust hood, while at the same time facilitating maintenance operations.

According to a first aspect of the present invention, a flexible housing is arranged along an axial centerline of a rotor shaft of a steam turbine. The flexible housing includes a first cylindrical element with a first radius centered on an axial centerline of a rotor of a steam turbine and a second cylindrical element with a second radius centered on the axial centerline, where the length of the second radius is different from the length of the first radius. The second cylindrical element is disposed adjacent to the first cylindrical element along the axial centerline. A thin annular member includes an inner radius corresponding to the first radius and an outer radius corresponding to the second radius, where the inner radius is fastened to an end of the first cylindrical element and the outer radius is fastened to an adjacent end of the second cylindrical element. A first flange is fastened to a non-adacent end of the first cylindrical element and adapted to mount to an exhaust hood of the steam turbine. A second flange is fastened to a non-adjacent end of the second cylindrical element and adapted to mount to a seal end packing box external to the exhaust hood of the steam turbine.

Another aspect of the present invention provides a steam turbine including an inner casing forming a working steam path through steam turbine stages; an exhaust hood surrounding the inner casing forming an path for exhaust steam from the inner casing; a rotor shaft driven from the inner casing and extending through at least one axial end of the exhaust hood; a standard mounting a bearing support and a seal end packing box for the rotor shaft, wherein the standard is mounted separate from the exhaust hood for at least one axial end of the steam turbine; and a flexible housing mounted between an axial end of the exhaust hood and the seal end packing box. The flexible housing encloses the rotor shaft between the axial end of the exhaust hood and the seal-packing box and fluidly connects exhaust steam from at least one axial end of the exhaust hood to the seal-packing box.

A further aspect of the present invention provides a flexible housing arrangement sealing a rotor shaft for a steam turbine between an exhaust hood and a seal end packing box mounted on a separate standard. The flexible housing arrangement includes an exhaust hood with an endcover for the steam turbine; a seal end packing box housed on a standard separate from the exhaust hood; a rotor shaft extending between the endcover of the exhaust hood and the seal end packing box; and opposing cover portions of the flexible housing. Each cover portion includes a first semi-cylindrical element with a first radius and a second semi-cylindrical element with a second radius, both radii taken from the centerline of the flexible housing. The second radius has a different length than the first radius. The second cylindrical element is arranged adjacent to the first cylindrical element along a centerline of the flexible housing. A thin semi-annular plate includes an inner radius corresponding to the first radius and an outer radius corresponding to the second radius. The inner radius of the semi-annular plate is fastened to an end of the first cylindrical element and the outer radius of the semi-annular plate is fastened to an adjacent end of the second cylindrical element.

A horizontal flange is attached to each of the opposing cover portions along an equator of the first semi-cylindrical element, the second semi-cylindrical element and the semi-annular element. The horizontal flanges are adapted for bolting the opposing cover portions around the rotor shaft. A semi-cylindrical end flange is attached at each opposing end for each cover portion. The semi-cylindrical end flange attached to the first semi-cylindrical element is mounted to the exhaust hood endcover. The semi-cylindrical end flange attached to the second semi-cylindrical element is mounted to a seal end packing box on a standard.

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 provides a perspective view for an embodiment of the inventive flexible housing;

FIG. 2 illustrates a top sectional view of the flexible house interposed between an exhaust hood and a seal-packing box;

FIG. 3 illustrates an expanded sectional view of an embodiment of the flexible housing mounted between the end cover of exhaust hood and the seal end packing box; and

FIG. 4 illustrates a further aspect of the present invention including a condensing steam turbine with a flexible housing interposed between the exhaust hood of the turbine and a standard for housing a seal end packing box and end bearings for the rotor shaft.

DETAILED DESCRIPTION

The following embodiments of the present invention have many advantages, including coupling the exhaust hood of a steam turbine to a seal packing box for the rotor shaft, where the box is mounted separately on a standard and where the housing accommodates relative expansion and contraction of the exhaust hood with respect to the standard. The arrangement advantageously permits maintenance and removal of the housing feature without the need for complex and time-consuming maintenance requiring lifting of the exhaust hood and the rotor.

FIG. 1 provides a perspective view for an embodiment of the inventive flexible housing. FIG. 2 illustrates a top sectional view of the flexible house interposed between an exhaust hood and a seal-packing box. The flexible housing 100 may be arranged around an axial centerline 101 of a rotor shaft 175 of a steam turbine. The flexible housing 100 may include a top cover portion 110 and a bottom cover portion 120, each portion being symmetrical about a horizontal plane centered on the axial centerline. The cover portions may each include a first semi-cylindrical element 130 with a first radius 131 centered on the axial centerline and a second semi-cylindrical element 140 with a second radius 141 centered on the axial centerline. The second radius 141 has a different length from the first radius 131. As shown, the first radius 131 for the first semi-cylindrical element 130 is greater than the second radius 141 for the second cylindrical element. The second semi-cylindrical element 140 is arranged adjacent to the first semi-cylindrical element 130 along the axial centerline. A thin semi-annular element 150 includes an outer radius 132 corresponding to the first radius 131 and an inner radius 142 corresponding to the second radius 141. The outer radius 132 of the semi-annular element 150 may be fastened to an adjacent axial end 134 of the first semi-cylindrical element 130 and the inner radius 142 may be fastened to an adjacent end 144 of the second semi-cylindrical element 140. Fastening may be provided by welding or other known methods.

A horizontal flange 145, 155 may be fixed along the horizontal joint 135 of between the top cover portion 110 and bottom cover portion 120. The horizontal flanges may be used to seal the top cover portion and the bottom cover portions around the rotor shaft 175, which would run therethrough between endcover 171 of the exhaust hood 170 of the steam turbine and the seal end packing box 180 on the standard 190. The horizontal flanges 145, 155 may include vertical-mounted bolts 146 and boltholes 147 for fastening. Each opposed flanged joint 145, 155 may include vertical expansion splits 148 on an outer edge 149. The vertical expansion slits 148 on the opposed flanged joints 145, 155 are preferentially disposed axially proximate the thin semi-annular member 150. An expanded view of the horizontal flange and end mounting details is shown in FIG. 3.

A first upper semi-cylindrical flange element 160 may be attached to a non-adjacent end of the first semi-cylindrical element 130 of the top cover 110. A first lower semi-cylindrical flange element 161 may be attached to a non-adjacent end of the first semi-cylindrical element 130 of the bottom cover 120. A second upper semi-cylindrical flange element 165 may be attached to a non-adjacent end of the second semi-cylindrical element 166 of the top cover 110. A second lower semi-cylindrical flange element 166 may be attached to a non-adjacent end of the second semi-cylindrical element 140 of the bottom cover 120. Together the first upper semi-cylindrical flange element 160 and the first lower semi-cylindrical flange element 161 are configured to mate with a complimentary flange on the endcover 171 of exhaust hood 170. Together the second upper semi-cylindrical flange element 165 and the second lower semi-cylindrical flange element 166 are configured to mate with a complimentary flange on the endcover 181 of seal packing box 180.

Together, the top cover 110 and the bottom cover 120 form a sealed enclosure 115 around the rotor shaft 175 from the exhaust hood 170 of the turbine to the seal-packing box 180. The exhaust hood end of flexible housing 100 extends from the conical recess 176, bounded by exhaust hood wall 178 and exhaust hood flanges 179. The seal end packing box 180 may be mounted in pedestal 190 together with bearing housing 185.

When the exhaust hood 170 flexes relative to the rotor shaft 175 for the reasons previously described, the flexible housing 100 absorbs the relative motion. The thin semi-annular members 150 flex in response to the axial expansion and axial contraction of the exhaust hood 170 when the flexible housing 100 is installed between the exhaust hood 170 and a separately mounted seal-packing box 180 for a steam turbine. The thickness 153 of the semi-annular members 150 may be selected to provide sufficient deflection to accommodate the expected axial motion of the exhaust endcover 171 under exhaust hood temperature and pressure variations without itself incurring permanent deformation. Also, the vertical expansion slits 148 on the opposed horizontal flanges 145, 155 of the flexible housing 100 flex in response to the axial expansion and axial contraction of the exhaust hood 170.

FIG. 3 illustrates an expanded sectional view of an embodiment of the flexible housing 100 mounted between the end cover 171 of exhaust hood 170 and the seal end-packing box 180. The first semi-cylindrical element 130, the second semi-cylindrical element 140 and the semi-annular element 150 are fixed to horizontal flanges 145, 155. End flanges 160 (161) of the flexible housing 100 bolt to endcover 171 of the exhaust hood 170. End flanges 165 (166) bolt to end flanges 181 of seal end packing box 180. Horizontal flanges 145, 155 include a plurality of boltholes for attaching the top cover 110 to the bottom cover 120. Some of the boltholes 147, particularly the boltholes in proximity to the semi-annular element 150, include vertical expansion slits 148. Rotor shaft 175 is sealed at seal end packing box 180 by seal end packing housing 183 maintaining seal end packing 184 against the rotor shaft.

FIG. 4 illustrates a further aspect of the present invention including a condensing steam turbine 200 with a flexible housing 100 interposed between the exhaust hood 170 of the turbine and a standard 190 for housing a seal end packing box and end bearings for the rotor shaft 175. The steam turbine 200 may be a double flow, low-pressure steam turbine. Rotor shaft 175 carries rotating blades (not shown) and turbine inner casing 195 supports the stator vanes (not shown), thereby defining a working steam path through the turbine. The rotor shaft 175 is driven from the turbine inner casing 195 and extends through at least one axial end 174 of the exhaust hood 170. A standard 190 provides support for a bearing housing 185 and a seal end-packing box 180 with housings 183 for labryrinth packing 184 for the rotor shaft 175. The standard 190 may be mounted directly to a foundation 192, separate from the exhaust hood 170 for at least one axial end 174 of the exhaust hood 170. A flexible housing 100 is mounted between the axial end 174 of the exhaust hood 170 and the seal end-packing box 180 (FIG. 2). The flexible housing 100 encloses the rotor shaft 175 between the exhaust hood endcover 171 and the seal-packing box 180 and encloses the exhaust steam in the pressurized state or vacuum state existing in the exhaust hood to the seal end packing box 180. End flanges 160, 165 on opposing ends of the flexible housing 100 mate with a complimentary flange 172 on the endcover 171 of the exhaust hood 170 and the complimentary endcover flange 181 of the seal-packing box 180, respectively.

Horizontal joint 135 on flexible housing 100 includes bolted upper horizontal flange 145 and lower horizontal flange 155. Conical recess 176 at end of exhaust hood 170 provides space to access flexible housing 100 for maintenance. Top cover portion 110 of flexible housing may be removed for inspection or maintenance by unbolting upper horizontal flange 145 from lower horizontal flange 155 and unbolting end flange 160 to exhaust hood endcover flange 172 and end flange 165 to seal packing box flange 181 (FIG. 3). Lower cover portion 120 may similarly be removed by unbolting the horizontal joint 135 and unbolting end flange 161 to exhaust hood endcover flange 172 and end flange 166 to seal packing box flange 181 (FIG. 3). With this flexible housing arrangement, the necessity to remove the rotor shaft 175 for such maintenance is eliminated, resulting in a savings of significant time and expense.

While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made, and are within the scope of the invention.