Abstract:
A seal for a valve stem in a steam turbine valve includes a ring carrier sleeve surrounding at least a portion of a valve stem mounted for axial reciprocating movement in a valve housing. At least one inner ring groove is formed in an interior surface of the ring carrier sleeve, and a radially in-sprung ring is seated in the at least one annular ring groove and engaged with the valve stem to thereby substantially block leakage of steam along the valve stem.

Description:
[0001]     This invention relates generally to steam turbine valves and more specifically, to a sealing arrangement for a valve stem.  
       BACKGROUND OF THE INVENTION  
       [0002]     Parasitic steam losses due to poor valve stem sealing in steam turbine applications results in reduced efficiency and lost power output. The steam leakage to atmosphere can also be dangerous and leaves a negative visual image of the valve.  
         [0003]     Prior valve configurations have incorporated tight valve stem clearances in a single or a multi-stage leak-off arrangement. This method results in a loss of useful energy and is therefore not completely satisfactory. Other valve stem arrangements allow a small percentage of steam leakage to atmosphere, especially when the valve is not wide open and back-seated.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0004]     The present invention involves the application of a single or combination of in-sprung and/or out-sprung piston rings to create a seal against steam leakage along the valve stem. In addition, the piston rings can be placed in an integrated bushing-ring carrier that allows for appropriate thermal expansions, maintaining the tight seal. The piston rings can be used exclusively if the required pressure drop is low, or in combination with a single or multi-stage leak-off from the valve stem if the required pressure drop is high.  
         [0005]     In one exemplary embodiment, a single in-sprung piston ring is located in an integrated bushing-ring carrier surrounding a valve stem. In another embodiment, a pair of in-sprung rings are employed, with dual leak-off passages, one of the latter located axially between the pair of in-sprung rings. In still another embodiment, a pair of in-sprung rings are employed in combination with leak-off passages as described above, with the further addition of a pair of out-sprung rings between the bushing-ring carrier and the valve head or cover plate wall, with the out-sprung rings located on either side of a leak-off passage and between the pair of in-sprung rings.  
         [0006]     In still another embodiment, three in-sprung rings are utilized in combination with a pair of leak-off passages in an alternating arrangement, with the intermediate ring located between the leak-off passages, and the remaining pair of leak-off passages located between the outer pair of in-sprung rings.  
         [0007]     The utilization of multiple rings in combination with leak-off passages permits a staged pressure drop along the axial extent of the valve stem.  
         [0008]     It is also contemplated that the integrated bushing-ring carrier can be constructed of multiple axial segments to facilitate assembly of the rings, particularly in field applications.  
         [0009]     Accordingly, in one aspect, the present invention relates to a seal for a valve stem in a steam turbine valve comprising a ring carrier surrounding at least a portion of a valve stem mounted for axial reciprocating movement in a valve component; at least one inner ring groove formed in an interior surface of the carrier; and a radially in-sprung ring mounted in the at least one ring groove and engaged with the valve stem to thereby inhibit leakage of steam along the valve stem.  
         [0010]     In another aspect, the present invention relates to a seal for a valve stem in a steam turbine valve head comprising a ring carrier surrounding at least a portion of a valve stem mounted for axial reciprocating movement in a valve component, the carrier having a radial flange to facilitate seating of the carrier in the valve head; at least one inner ring groove formed in an interior surface of the carrier; and a radially in-sprung ring mounted in at least one ring groove and engaged with the valve stem to thereby inhibit leakage of steam along the valve stem, wherein the radially in-sprung ring comprises a pair of back-to-back ring elements.  
         [0011]     In still another aspect, the present invention relates to a seal for a valve stem in a steam turbine valve head comprising a ring carrier surrounding at least a portion of a valve stem mounted for axial reciprocating movement in a valve component, the carrier having a radial flange to facilitate seating of the carrier in the valve head; a pair of axially-spaced inner ring grooves formed in an interior surface of the carrier; a radially in-sprung ring mounted in each of the grooves and engaged with the valve stem to thereby inhibit leakage of steam along the valve stem, wherein the radially in-sprung ring comprises a pair of back-to-back ring elements; a first leak-off groove formed in an inner face of the carrier and located axially between the pair of axially-spaced inner ring grooves, the leak-off groove communicating with a leak-off port extending through the carrier; and a second leak-off groove formed on the inner face of the carrier, below the pair of axially spaced inner ring grooves and above the flange.  
         [0012]     The invention will now be described in connection with the drawings identified below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a partial cross section illustrating an integrated bushing-ring carrier and valve stem in accordance with a first exemplary embodiment of the invention;  
         [0014]      FIG. 2  is a partial cross section illustrating an integrated bushing-ring carrier and valve stem in accordance with a second exemplary embodiment of the invention;  
         [0015]      FIG. 3  is a partial cross section illustrating an integrated bushing-ring carrier and valve stem in accordance with a third exemplary embodiment of the invention; and  
         [0016]      FIG. 4  is a partial cross section illustrating an integrated bushing-ring carrier and valve stem in accordance with a fourth exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Referring to  FIG. 1 , a valve stem and ring carrier assembly  10  in accordance with an exemplary embodiment of the invention includes a valve stem  12  movable in opposite axial directions within a circular, integrated bushing-ring carrier  14  that is, in turn, seated in a valve head or valve cover plate (or other valve component)  16 . The bushing-ring carrier (or simply, ring carrier)  14  is provided with a radial shoulder  18  by which the carrier is properly located within the valve head, typically in a press-fit arrangement. An annular ring groove  20  is formed in the carrier, with the open end of the groove facing the valve stem  12 . Seated within the groove  20  is a “piston ring”  22  that, in the illustrated embodiment, is sprung inwardly, i.e., the ring is biased radially inwardly to engage the valve stem  12 . The ring  22 , like conventional piston rings is composed of a pair of ring elements  24 ,  26  but will be referred to herein simply as the ring  22 . It will be appreciated that the incorporation of the in-sprung ring  22  within the valve stem and bushing assembly establishes a seal that eliminates or at least substantially minimizes steam leakage along the valve stem. While the utilization of a discrete bushing-ring carrier  14  for carrying the ring  22  is preferable in most situations, it is, of course, possible to seat the ring  22  in a groove formed in other components, for example, in the valve head or cover plate directly.  
         [0018]      FIG. 2  illustrates an alternative embodiment of the invention including a generally similar valve stem and bushing-ring carrier assembly  28 . The assembly incorporates a valve stem  30  movable in opposite axial directions within an integrated bushing-ring carrier  32 . Like the earlier described carrier  14 , a radial shoulder  34  is provided to accurately seat the carrier within the valve head or cover plate  36 . In this embodiment, first and second ring grooves  38 ,  40  are formed in the bushing-ring carrier  32  and are adapted to receive annular, radially in-sprung rings  42 ,  44 , respectively. A first annular leak-off groove or area  46  is formed on the interior surface or inner face of the bushing-ring carrier  32  below the first ring  38  while a second and larger annular leak-off area or groove  48  is formed on the same inner face of the carrier, axially between the first and second rings  42 ,  44 . The leak-off areas  46  and  48  communicate with one or more leak-off ports  50 ,  52 , respectively, that communicate with one or more passages  54 ,  56  in the valve head  36 .  
         [0019]     With this arrangement, some of the leakage steam flowing along the valve stem  30  is diverted through the leak-off port  50  while steam that passes the first ring  42  can be removed via the leak-off port  52 . Any residual steam leakage is substantially minimized if not prevented from escaping along the valve stem by the upper or second ring  44 .  
         [0020]     This seal arrangement provides a staged pressure drop along the valve stem  30  that is particularly desirable when the pressure drop is high.  
         [0021]      FIG. 3  illustrates yet another exemplary embodiment of the invention. Here, the valve stem and integrated bushing-ring carrier assembly  58  includes a valve stem  60  movable in opposite linear directions within the bushing-ring carrier  62 . Carrier  62  is provided with an annular, radially outwardly extending shoulder  64  to locate the carrier within the valve head or cover plate  66 . Inner ring grooves  68  and  70  are formed on the interior surface of the carrier  62  while outer ring grooves  72  and  74  are formed on the outer surface of the carrier. Annular in-sprung rings  76  and  78  are seated in the inner ring grooves  68  and  70 , while annular out-sprung rings  80 ,  82  are seated in the outer ring grooves  72 ,  74 , respectively. Note that rings  76  and  78  are biased inwardly to engage the valve stem  60  while rings  80 ,  82  are biased outwardly to engage the valve head or cover plate wall  84 . A first leak-off area or annular groove  86  is formed on the inner face of the carrier below the lower or first in-sprung ring  76 , while a second leak-off area or annular groove  88  is formed on the same inner face between the in-sprung rings  76 ,  78  and generally centered between the out-sprung rings  80 ,  82 . The annular leak-off grooves  86 ,  88  communicate with one or more leak-off ports  90 ,  92 , respectively, which in turn, communicate with one or more passages  94 ,  96  in the valve head or cover plate  66 .  
         [0022]     The use of out-sprung rings is especially useful if there is a thermal mismatch between the bushing-ring carrier and the valve head or cover plate. In the event, however, an interference fit is obtained between the ring carrier  62  and valve head or cover plate wall  66  at temperature (see  FIG. 2 ), the out-sprung rings  80 ,  82  may be eliminated. Note also, that in this embodiment, the bushing-ring carrier  62  is made up of three individual sleeve sections or segments  98 ,  100  and  102 , with the sections joined at the ring grooves  68 ,  70 . This arrangement facilitates insertion of the in-sprung rings and is particularly advantageous in field-replacement applications.  
         [0023]     With this seal arrangement, leakage steam is minimized if not eliminated in stages along the inner side of the carrier  62  as well as along the outer periphery of the carrier. In the latter instance, it is possible that leakage steam passing through ports  90  and  96  may escape along the wall  84  if not prevented by use of the out-sprung rings  80 ,  82 . As noted above, if an interference fit is obtained at operating temperature between the carrier  62  and valve head or cover plate wall  84 , the out-sprung rings may be eliminated.  
         [0024]     In  FIG. 4 , a fourth exemplary embodiment of the invention includes a valve stem and bushing-ring carrier assembly  104 . The valve stem  106  is movable in opposite linear directions within the bushing-ring carrier  108 . A radially outwardly directed shoulder  110  provides accurate seating of the carrier within the valve head or valve cover plate  112 .  
         [0025]     First, second and third inner ring grooves  114 ,  116  and  118  are formed on the interior surface of the bushing-ring carrier  108  such that the grooves open toward the valve stem  106 . Annular in-sprung rings  120 ,  122  and  124 , respectively, are seated in the grooves and are biased inwardly into engagement with the valve stem  106 . Here again, the integrated bushing-ring carrier  108  may, if desired, be composed of individual sections or segments  126 ,  128 ,  130  and  132  that are joined at the interfaces with grooves  114 ,  116  and  118 , respectively, to facilitate assembly. Note that with this segmented approach, the in-sprung ring  122  may be comprised of a pair of rings to improve the sealing capacity.  
         [0026]     A first annular leak-off groove or area  134  is formed on the inner face of the carrier  108  between the ring grooves  114  and  116  while a second annular leak-off groove or area  136  is formed on the same inner face of the carrier between rings  122  and  124 . One or more leak-off ports  138  and  140  carry leakage steam away from the valve stem carrier via one or more passages  142 ,  144  in the valve head or cover plate  112 . This is yet another seal arrangement where steam leakage pressure drop can be staged along the valve stem. It will be appreciated that still other variations of in-sprung and out-sprung rings with or without leak-off arrangements lie within the scope of this invention.  
         [0027]     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.