Abstract:
A method and apparatus for sealing between an inner casing and an outer casing is provided. The method includes positioning a first sealing member in a leakage path defined between an inner casing and an outer casing such that leakage flow in a first direction activates the first sealing member, and positioning a second sealing member in the leakage path such that leakage flow in the first direction bypasses the second sealing member, and such that leakage flow in an opposite second direction activates the second sealing member. The apparatus includes a pair of circumferential grooves in a channel, a divider positioned in the channel that defines a leakage path, a first sealing member positioned to seal against a flow in the leakage path in a first direction, and a second sealing member positioned to seal against a flow in the leakage path in a second direction.

Description:
BACKGROUND OF INVENTION 
   This invention relates generally to steam turbines, and more particularly, to controlling steam leakage paths in the turbine. 
   A steam turbine may include a high-pressure (HP) turbine section, an intermediate-pressure (IP) turbine section, and a low-pressure (LP) turbine section that each include rotatable steam-turbine blades fixedly attached to, and radially extending from, a steam-turbine shaft that is rotatably supported by bearings. The bearings may be located longitudinally outwardly from the high and intermediate-pressure turbine sections. A steam pressure drop through at least some known high-pressure and/or intermediate-pressure turbine sections is at least about 2,000 kPa (kiloPascals), and a difference in pressure of the steam entering the high and intermediate-pressure turbine sections is at least about 600 kPa. In some known steam turbines, steam exiting the HP turbine section is reheated by a boiler before entering the IP turbine section. 
   A steam turbine has a defined steam path which includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. Steam leakage, either out of the steam path, or into the steam path, from an area of higher pressure to an area of lower pressure, may adversely affect an operating efficiency of the turbine. For example, steam-path leakage in the turbine between a rotating rotor shaft of the turbine and a circumferentially surrounding turbine casing, may lower the efficiency of the turbine leading to increased fuel costs. Additionally, steam-path leakage between a shell and the portion of the casing extending between adjacent turbines, for example, a high pressure turbine section to an adjacent intermediate turbine section, may lower the operating efficiency of the steam turbine and over time, may lead to increased fuel costs. 
   To facilitate minimizing steam-path leakage between the HP turbine section and a longitudinally-outward bearing, and/or between the IP turbine section and a longitudinally-outward bearing, at least some known steam turbines use a plurality of labyrinth seals. Such labyrinth seals include longitudinally spaced-apart rows of labyrinth seal teeth. Many rows of teeth are used to seal against the high-pressure differentials that may be in a steam turbine. Brush seals may also be used to minimize leakage through a gap defined between two components, such as leakage that is flowing from a higher pressure area to a lower pressure area. Brush seals provide a more efficient seal than labyrinth seals, however, at least some known steam turbines, which rely on a brush seal assembly between turbine sections and/or between a turbine section and a bearing, also use at least one standard labyrinth seal as a redundant backup seal for the brush seal assembly. 
   Other areas of steam path leakage within a turbine may affect adversely turbine efficiency. One such area is a casing fit between the HP turbine section and the IP section where labyrinth and brush seals are impractical. 
   SUMMARY OF INVENTION 
   In one aspect, a method of assembling a steam turbine is provided. The method includes positioning a first sealing member in a leakage path defined between an inner casing and an outer casing such that leakage flow in a first direction activates the first sealing member, and positioning a second sealing member in the leakage path such that leakage flow in the first direction bypasses the second sealing member, and such that leakage flow in an opposite second direction activates the second sealing member. 
   In another aspect, a seal assembly for sealing a leakage path is provided. The seal assembly includes a first groove defined in a channel, a second groove defined in the channel and substantially parallel to the first groove wherein the second groove is defined radially outward from the first groove, a divider positioned in the channel such that a gap defined between the divider and the channel defines a leakage path, a first sealing member that extends at least partially within the first groove and positioned to substantially prevent a flow within the leakage path in a first direction, and a second sealing member that extends at least partially within the second groove and positioned to substantially prevent a flow within the leakage path in a second direction, the second direction being opposite to the first direction. 
   In yet another aspect, a rotary machine is provided. The rotary machine includes a rotor rotatable about a longitudinal axis and including an outer annular surface, an annular outer casing including an inner surface wherein the outer casing is spaced radially outwardly from the rotor, the casing inner surface includes a first extension extending radially inwardly towards the rotor, and the first extension extends circumferentially about the casing inner surface. The rotary machine also includes a cylindrical inner casing includes an outer surface wherein the outer surface includes a second extension extending radially towards the outer casing, and the second extension extends circumferentially about the outer surface, and the second extension includes a channel in an outer extension surface for receiving the first extension when the outer casing and the inner casing are assembled, a first groove formed in said channel sized to receive a sealing member, and a sealing member positioned at least partially within the first groove for sealing a leakage path. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic illustration of an exemplary opposed flow HP/IP steam turbine; 
       FIG. 2  is an enlarged schematic illustration of a section divider and mating channel that may be included in the steam turbine shown in FIG.  1 . 
       FIG. 3  is an enlarged view of the section divider shown in FIG.  1  and taken along area  3 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic illustration of an exemplary opposed-flow steam turbine  10  including a high pressure (HP) section  12  and an intermediate pressure (IP) section  14 . A single outer shell or casing  16  is divided axially into upper and lower half sections  13  and  15 , respectively, and spans both HP section  12  and IP section  14 . A central section  18  of shell  16  includes a high pressure steam inlet  20  and an intermediate pressure steam inlet  22 . Within outer shell or casing  16 , HP section  12  and IP section  14  are arranged in a single bearing span supported by journal bearings  26  and  28 . A steam seal unit  30  and  32  is located inboard each journal bearing  26  and  28 , respectively. 
   An annular section divider  42  extends radially inwardly from central section  18  and towards a rotor shaft  44  extending between HP section  12  and IP section  14 . More specifically, divider  42  extends circumferentially around a portion of shaft  44  extending between first HP section nozzle  46  and a first IP section nozzle  48 . Section divider  42  is received in a channel  50  formed in packing casing  52 . Channel  50  is a C-shaped channel that extend radially into packing casing  52  and around an outer circumference of packing casing  52 , such that a center opening of channel  50  faces radially outwardly. Channel  50  includes a pair of seal grooves  54  and  56  positioned in a radially extending surface  57  of channel  50 . Seal grooves  54  and  56  are co-axial about a longitudinal axis  58  of turbine  10 . In an alternative embodiment, section divider  42  includes a pair of seal grooves  54  and  56  positioned in a radially extending surface  59  of section divider  42 . 
   In operation, high pressure steam inlet  20  receives high pressure/high temperature steam from a source, for example, a power boiler (not shown). The steam is routed through HP section  12  wherein work is extracted from the steam to rotate rotor shaft  44 . The steam exits HP section  12  and returns to the boiler where it is reheated. The reheated steam is then routed to intermediate pressure steam inlet  22  and returned to IP section  14  at a reduced pressure than steam entering HP section  12 , but at a temperature that is substantially similar to the steam entering HP section  12 . Accordingly, an operating pressure within HP section  12  is higher than an operating pressure in IP section  14 . Therefore, steam within HP section  12  tends to flow towards IP section  14  through leakage paths that may develop between HP section  12  and IP section  14 . One such leakage path may be defined along a rotor  44  extending through packing casing  52 . Accordingly, packing casing  52  includes a plurality of labyrinth and/or brush seals to facilitate reducing leakage from HP section  12  to IP section  14  along a shaft  60 . Another leakage path between HP section  12  and IP section  14  is through a gap between section divider  42  and packing casing  52  in channel  50 . 
     FIG. 2  is an enlarged schematic illustration of a section divider  42  and channel  50  that may be included in steam turbine  10 . Section divider  42  includes a first side  102 , a sealing side  104 , and a joining side  106 . Channel  50  includes a first side  112 , a sealing side  114 , and a joining side  116 . First sides  102  and  112  of section divider  42  and channel  50 , respectively, correspond with each other in a mating fashion when section divider  42  and channel  50  are coupled. Sealing sides  104  and  114 , and joining sides  106  and  116 , similarly mate together when section divider  42  and channel  50  are coupled. Since sides  102 ,  104 , and  106  do not mate exactly to sides  112 ,  114 , and  116 , a plurality of gaps  117 ,  118 , and  119  are formed between corresponding sides,  102  and  112 ,  106  and  116 , and  104  and  114 , respectively. More specifically, each gap  117 ,  118 , and  119  form a potential steam flow leakage path  120  from HP section  12  towards IP section  14 . During some known conditions, such as a trip of turbine  10 , an operating pressure in IP section  14  may exceed the pressure HP section  12  and in such a condition, the flow in leakage path  120  would tend to reverse and flow from IP section  14  towards HP section  12 . To facilitate reducing leakage flow through leakage path  120 , a dual opposing seal assembly  122  is provided in seal side  114 . In an alternative embodiment, the dual opposing seal may be provided in surface  59  of divider  42 . 
   Two parallel grooves  54  and  56  are formed in seal side  114  and grooves  54  and  56  are each sized to receive a sealing member  154  and  156 , respectively, therein. More specifically, seal assembly  122  includes members  154  and  156 , and is a pressure activated sealing member that is configured such that a pressure being sealed provides a motive force to cause the sealing member to seal tighter as pressure applied to the sealing member increases. In the exemplary embodiment, sealing members  154  and  156  are V-seals, such that each has a V-shaped cross-sectional profile. In other embodiments, sealing members  154  and  156  are known C-seals, E-seals, or W-seals. 
   In operation, steam at higher pressure in HP section  12  tends to leak through steam path  120  to IP section  14 , which is at a lower steam pressure. Sealing members  154  and  156  seated in grooves  54  and  56  respectively, activate to facilitate limiting or stopping steam leakage flow through leakage path  120 . 
     FIG. 3  is an enlarged view of section divider  42  taken along area  3 . More specifically,  FIG. 3  is an enlarged view of seal assembly  122 . Section divider  42  is coupled to packing casing  52  such that corresponding sides  106  and  116  are proximate each other, and corresponding sides  104  and  114  are proximate each other. Gaps  119  and  118  are defined between sides  104  and  114 , and between sides  106  and  116 , respectively. Gaps  119  and  118  permit steam from HP section  12  to leak toward IP section  14  through leakage path  120  during operation of turbine  10 . A second leakage path  200  is a reverse flow path that may occur during some turbine operations, such as, for example, a turbine trip. To facilitate reducing or eliminating steam leakage through paths  120  and  200 , sealing members  154  and  156  are positioned in grooves  54  and  56  in side  114 . Each seal groove  54  and  56  is defined by a groove depth  201  and a groove width  202 . In the exemplary embodiment, each groove depth  201  and groove width  202  are between approximately 0.2 inches and approximately 0.5 inches. In the exemplary embodiment, sealing members  154  and  156  are V-seals. More specifically, each sealing member  154  and  156  has a cross-sectional profile including an apex  204  and a pair of opposed legs  206  and  208  that diverge from apex  204 . Legs  206  and  208  form an interior surface  210  and an exterior surface  212 . Sealing members  154  and  156  are sized such that at least a portion of leg  208  extends past side  114  into leakage paths  120  and  200  such that when section divider  42  and channel  50  are coupled, leg  208  at least partially engages side  104 . 
   Sealing members  154  and  156  are fabricated from a material that provides flexibility at apex  204  and rigidity of legs  206  and  208  to withstand a pressure differential across legs  206  and  208 . In the exemplary embodiment, members  154  and  156  withstand a pressure differential of at least approximately 600 kPa. In the exemplary embodiment, sealing members  154  and  156  are fabricated from rolled sheet metal having a thickness of between about 0.005 inches and 0.030 inches. In other embodiments, sealing members  154  and  156  are fabricated from materials such as, for example, Hastelloy ®, Cres 304, and Incoloy 909 ®. Sealing members  154  and  156  are positioned in their respective grooves  54  and  56  such that apexes  204  point toward each other, giving sealing members  154  and  156  an opposed configuration with respect to each other. In another embodiment, sealing members  154  and  156  are E, W, or C seals wherein the open side of each E, W, or C face away from each other. In one embodiment, sealing members  154  and  156  are commercially available from Jetseal, Inc. of Spokane, Wash. In the exemplary embodiment, sealing members  154  and  156  are identical to each other. In another embodiment, sealing members  154  and  156  are different. 
   In operation, steam from HP section  12  attempts to flow to lower pressure IP section  12  during normal operation of turbine  10 . As steam flows through leakage path  120 , the steam contacts sealing member interior surface  210 . Leg exterior surface  212  contacts side  104  due to the flexibility of apex  204  and thus provides a bias to leg  208 . A distal end  214  of leg  208  blocks steam flow from leakage path  120  and directs the steam towards an area  220  defined within interior surface  210  of sealing member  154 . A differential pressure builds up across sealing member  154  due to steam from HP section  12  becoming trapped in area  220  and leakage path  120  downstream of sealing member  154  still being in communication with IP section  14 . The differential pressure across sealing member  154  causes legs  206  and  208  to expand outwardly further tightening the contact between exterior surface  212  of sealing member  154  and side  104 . 
   During operations when the differential pressure tends to reverse, for example during a turbine trip event, sealing member  156  will activate to block leakage path  200  in a manner similar to that of sealing member  154  blocks leakage flow through path  120  during normal turbine operations. Thus, a double seal arrangement in an area of the steam turbine where surface irregularities may provide a leakage path from HP section  12  to IP section  14  facilitates reducing leakage through path  120  during normal operation of turbine  10  and during upsets when steam flow may reverse. 
   The above-described turbine casing seal arrangement is cost effective and highly reliable. The double seal arrangement includes a first sealing member to facilitate reducing steam leakage through an internal leakage path in the turbine during normal operations and a second sealing member in an opposed arrangement from the first sealing member to facilitate reducing steam leakage in an opposite direction through an internal leakage path in the turbine during other than normal operations. As a result, the turbine casing seal arrangement facilitates reducing steam leakage in a turbine during a plurality of modes of operation in a cost effective and reliable manner. 
   Exemplary embodiments of turbine casing seal arrangements are described above in detail. The arrangements are not limited to the specific embodiments described herein, but rather, components of the system may be utilized independently and separately from other components described herein. Each turbine casing seal arrangement component can also be used in combination with other turbine casing seal arrangement components. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.