Abstract:
A seal is provided at the intersegments of hydrogen seal rings for preventing oil leakage through this potential gap between the seal ring segments. By minimizing leakage through the gaps between the open rings, hydrogen consumption in hydrogen-cooled generators due to leakage oil is reduced. In a preferred embodiment, the seal is provided by a brush seal secured to one of the seal ring segments and received in a slot defined in the other of the seal ring segments.

Description:
This application is a continuation of application Ser. No. 09/669,452, filed Sep. 25, 2000 now U.S. Pat. No. 6,431,550, the entire content of which is hereby incorporated by reference in this application. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to hydrogen seal rings and, more specifically, to a seal structure for preventing oil leakage through a circumferential gap between the seal ring segments in rotary electric machines. 
     Hydrogen has been widely used as a coolant in a variety of rotary electric machines, due to its desirable thermophysical properties including low density and high specific heat. However, a primary disadvantage of using hydrogen is that the hydrogen purity must be maintained above its explosive limit (74% hydrogen in air). Therefore, a primary consideration for ensuring the safe operation of hydrogen-cooled rotary machines, such as turbogenerators, is designing highly reliable and efficient hydrogen seal systems therefor. 
     In a hydrogen-cooled turbogenerator, hydrogen seals are utilized both to seal high-pressure hydrogen at the interface of the rotating shaft, and to prevent air from entering the casing and developing an explosive mixture with the hydrogen. 
     Before the early 1980s, hydrogen seal systems consisted of a pair of four segmented bronze rings disposed in a seal casing. The newer babbitted steel seal rings  10  are each made in two 180° segments  12 , 14  as illustrated in FIG. 1. A typical hydrogen seal system is schematically shown in FIG.  2 . In that illustration, an annular seal casing is partially shown which is adapted to be mounted to a generator end shield (not shown) in surrounding and sealing relationship with a rotor/shaft  16 . The casing is formed in two main parts, referred to hereinbelow as casing halves, each extending 180° about the shaft. For ease of description, the upper casing half  18  and the seal ring segments  12  disposed therein are illustrated and will be described in detail. It is to be understood, however, in an exemplary embodiment, the lower casing half has a corresponding construction. The upper casing half  18  is of two-part construction, including a seal casing main body  20  and a seal casing cap segment  22 . The seal casing cap segment has a generally h-shaped cross-section, forming a radially inwardly directed chamber or cell  24  opening in a radially inward direction towards the shaft  16  for housing radially inwardly projecting seal rings  12  which in turn engage the shaft. 
     Each seal casing cap segment  22  is formed with an axial portion  26  connecting an upper radial flange portion  28  and lower inner and outer radial portions  30 , 32 . The axial portion  26  thus defines a base for the chamber  24  while radial portions  30  and  32  form two, axially spaced, parallel sides of the chamber  24 . Axially opposed shoulders  34 , 36  define an opening facing the rotor shaft  16 . The seal casing cap segment  22  is fastenable to the seal casing main body  20  by a semi-annular array of bolts  38  passing through holes in the radial portion  28  of the cap and threadably received in the main body  20 . 
     Within the chamber or cell  24 , there are seated a pair of side-by-side seal ring segments  12 , each extending approximately 180° about the casing half  18 . The rings  10  are held together radially and apart axially by two coil springs  40  (only one of which is shown in FIG.  2 ), each extending substantially 180° within the cell or chamber. The spring is seated within an area created by tapered surfaces  42  on the respective ring segments  12 . Opposite ends of the spring are anchored to axially extending pins (not shown) via a hook or the like (not shown) formed at each end thereof. As is conventional, the pin is located within aligned bores in radial wall portions of the casing cap segment. The pin is also used to align and hold a labyrinth type oil seal  46 . The spring biases the seal ring segments  12  radially inwardly and in axially opposite directions, against opposed faces of the inner and outer radial wall portions  30 , 32  of the chamber  24 . 
     In use, seal oil is introduced into the cavity of chamber  24  behind or radially outside the seal rings  12 , at a pressure higher than the hydrogen pressure inside the casing. Then, the high pressure seal oil flows radially between the seal rings toward the rotating shaft, where the sealing oil flow divides and runs with the clearance between the shaft and seal rings. At the hydrogen side  48  of the seal rings, the oil flows evenly between the shaft and the inner seal ring all the way around the seal ring at their interface and thus seals hydrogen from leaking and keeps the seal ring centered on the shaft. Similarly, the oil is uniformly distributed between the shaft  16  and the outer seal ring  10  at the air side  50  of the seal. 
     Hydrogen is absorbed by the oil that passes to the hydrogen side of the seals. The amount of hydrogen that is absorbed is proportional to the oil flow rate and the degree of solubility of hydrogen in oil. In general, a large oil flow rate yields high hydrogen consumption. 
     As illustrated in FIG. 1, hydrogen seal rings  10  are usually made into segments  12 , 14 , split at horizontal joints. The two segments can either be bolted together at the horizontal joint or held by two coil springs suitably attached to the casing. As described hereinbelow, the invention is applicable to non-bolted seal rings. As noted above, the purpose of the hydrogen seal springs  40  is to separate the two sealing rings and keep the sides of the rings against the casing. In normal operation, these rings maintain a uniform clearance and do not allow oil leakage at the ring segment joints. They are free to expand radially but prevented from rotating by either the pins to which the springs are attached or an anti-rotation device. In this way, the rings can float freely with respect to the seal casing cap. However, under certain circumstances, the seal rings may stick at certain positions and stop floating with respect to the casing cap. As a result, leakage could take place through the circumferential gaps formed between the open rings. This may lead to oil flooding inside the generator and high hydrogen consumption. The possible causes for the seal ring sticking or becoming skewed include a high vibrating shaft, rough sliding surfaces of the seal rings and/or the seal casing cap, and weak seal springs. Because the gap provides a short flow path for the sealing oil, it reduces the oil flowing through the annular clearances between the rings and the shaft. In severe cases, it may lead to the failure of the hydrogen seal system. In that case, the generator will be shut down automatically. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention was developed to address the potential problem of oil leakage through the circumferential gaps between the seal ring segments, in a case where a seal ring part stops floating with respect to the casing cap, resulting in flow through the gaps formed between the opened rings. 
     The invention more specifically proposes to provide a seal at the intersegments of the hydrogen seal rings for preventing oil leakage through this potential gap between the seal ring segments. The invention was further developed to minimize leakage through the gaps between the open rings so as to reduce hydrogen consumption in hydrogen-cooled generators due to leakage oil. The invention also has the advantageous result of increasing generator reliability and life cycle time by minimizing the risk of oil flooding and related inefficiencies or downtime. 
     Thus, the invention is embodied in the use of a seal at the hydrogen seal ring intersegment to minimize oil leakage through the intersegment gaps. In a presently preferred embodiment of the invention, the seal provided at the hydrogen seal ring intersegments is a short brush seal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view showing upper and lower hydrogen seal rings; 
     FIG. 2 is a schematic cross-sectional view of a hydrogen seal system; 
     FIG. 3 is a schematic isometric view of a brush seal structure; 
     FIG. 4 is a schematic perspective view, partly in phantom, showing a brush seal assembled to an upper seal ring segment and in opposed facing relation to a seal slot machined in a lower ring segment; 
     FIG. 5 schematically illustrates the assembly of a brush seal to a seal ring using a leaf spring as an embodiment of the invention; 
     FIG. 6 is a schematic illustration showing a brush seal mounted to a seal ring using a rubber strip as a further alternate embodiment of the invention 
     FIG. 7 is a schematic illustration of a brush seal welded to a seal ring as an alternate embodiment of the invention;; and 
     FIG. 8 is a schematic view, partly in cross-section, of a brush seal mounted to a seal ring using a screw or bolt as yet a further alternate embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As summarized above, the invention provides a seal at the intersegments of the hydrogen seal rings for preventing oil leakage through this potential gap between the seal ring segments. The improved seal rings  112  of the invention are adapted to be disposed in a seal casing as described above with reference to FIG.  2 . Accordingly, that figure has been annotated to include reference number  112  to illustrate the disposition of the improved seal rings  112  and to the extent that it corresponds to the conventional assembly, a description of the seal assembly incorporating the improved seal rings  112 , will not be provided here. 
     As shown in FIG. 3, a brush seal  52  is made by sandwiching bristles  54  between two metal plates  56  and orienting the sandwiched bristles at an angle of about 90° to the surface with which it forms a seal. Typically, the bristle diameter ranges from about 0.002 to about 0.01 inch, depending on the bristle material (metal or non-metal), temperature, pressure and operating condition (static or dynamic). The bristles are fabricated either by brazing/welding for metal bristles or gluing for nonmetal bristles. From FIG. 3 it can be observed that at the lower part of the brush, a space  58  is left at each side of the bristles  54 . This allows the bristles to flex and recover during operation. Since this type of brush is capable of sealing positive or negative pressure changes with respect to the axial direction, it is a bi-directional brush. The use of such bi-directional brushes is preferred because it can simplify the manufacturing process and avoid a possible installation error. 
     In the embodiment illustrated in FIG. 4, a brush seal  52  is mounted in a T-shaped slot  60  defined in at least one and preferably each circumferential end of one of the seal ring segments  112 . As illustrated, the slot extends in a direction that is generally radially with respect to a center of the shaft (not shown in FIG. 4) disposed through the seal ring. 
     In the illustrated embodiment, a seal slot  62  is machined in the circumferential end of the seal ring segment facing the brush to receive the same to provide an additional guarantee of oil sealing. Under normal operating conditions, the two seal ring segments  112 , 114  contact each other so that no oil flows between them. However, in the event of sealing malfunction, the brush seal is brought into play to seal oil that would otherwise seep between the open seal ring segments. As illustrated, the brush seal  52  is preferably but is not necessarily mounted to each circumferential end of the upper seal ring segment  112 . 
     The brush seals may be installed to the seal ring segment in any one of a number of ways. For example, with reference to FIG. 5, a gap  64  can be reserved between the metal plate(s)  56  to which the brush  54  is mounted and the opposing wall(s) of the T-shaped slot or cut out in which the brush seal is disposed. A spring mechanism such as a leaf spring  66  can then be disposed between the brush seal and the wall of the slot of the seal ring to urge the brush seal toward the other seal ring segment. Although in this embodiment one leaf spring is preferably disposed at the top of the brush seal, a leaf spring or other spring structure may be disposed in addition or in the alternative between opposed side wall(s) of the brush and groove. 
     Another alternate spring structure for mounting the brush seal is to provide a rubber strip  68  fit into a space at the side (FIG. 6) or top of the brush seal. More specifically, a rubber strip may be disposed in the T-shaped slot or against a surface of the brush seal in a stretched disposition to reduce its thickness. Once the parts are assembled, the strip is released. When the strip is released, it will tend to recover its original thickness and will tighten or wedge the brush seal within the slot or groove of the seal ring. 
     Another alternative is shown in FIG.  7 . In this embodiment, the ends of each brush seal may be spot welded as at 70 to reliably secure the brush seal  52  with respect to the seal ring  112 . It is apparent that spot welding would be the most reliable attachment of the brush seal to the spring ring segment. 
     Yet another mounting alternative, shown in FIG. 8, is to fix the brush seal  52  to the seal ring segment  112  at the side of the seal brush with one or more screws or bolts  74  (only one of which is shown in FIG.  8 ). In this case, to avoid damaging the bristles, the screw or bolt hole(s)  76  in the metal plate(s)  56  should not go through the bristles. 
     Other known mechanical attachments, components and methods could be used to secure the brush seals with respect to seal rings, as will be appreciated by the skilled artisan. While in the illustrated embodiment, the brush seal is illustrated as mounted to each circumferential end of the upper seal ring, since it is desirable for the seal brush to depend downwardly, it is to be understood that the gap seal, such as a brush seal, could in the alternative be mounted to the lower seal ring for extending to and sealingly contacting a seal slot therefor. As a further alternative, each seal ring segment could be provided with a gap seal structure, such as a brush seal, at one circumferential end and a complementary structure, such as a groove, at the other circumferential end. 
     In most brush seal applications, bristle wear is a problem that will affect the sealing operation. In the embodiments of the invention, since the brush does not contact any rotating surfaces, no such wear problem would be expected. 
     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 embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.