GB2076480A - Shaft - Google Patents

Abstract

A shaft seal for preventing escape of hydrogen gas in a large dynamoelectric machine comprises a pair of annular rings (10a, 10b) divided into semicircular portions having a layer (12) of babbitt metal bonded along their radially inner circumferences for mating with the journal surface of a rotating shaft (22). The seal further comprises a spring (13) exerting a radially inward force against the rings (10a, 10b) and also an axial force urging the rings (10a, 10b) against corresponding annular surfaces of a two piece annular seal casing (11a, 11b) which is fixed with respect to the rotating shaft (22). Sealing fluid supplied at (14) flows between the rings (10a, 10b) to seal between the layer (12) and the shaft (22), and also flows through passages (20, 21) for sealing at the axial faces. <IMAGE>

Description

SPECIFICATION Generator hydrogen seal This invention relates to seals and in particular to hydrogen seals for use in large dynamoelectric generators.

In the recent past, the increased demand for electrical energy has produced a corresponding increase in the power ratings for generators used by electric utilities to supply the desired load demand. As a consequence of this increase in generator power rating, it has become necessary to provide cooling not only for the generator stator but also for the generator rotor. The cooling fluid which is preferred for this purpose is hydrogen because of its ability to absorb and carry away large amounts of thermal energy. However, cooling the rotor with hydrogen or other fluid coolant necessitates the employment of seals to keep the coolant within the confines of the generator casing so that it may circulate around, about and through the rotor to perform the cooling function.In particular, it is necessary to provide a seal to prevent the flow of coolant through the opening for the rotating generator shaft. Oil is typically used as a sealing fluid. However, there is a particular tendency for hydrogen to dissolve in oil, and, over a period of time, an unacceptably large amount of coolant loss could occur. It is thus important that the seal structure minimize the flow of sealing fluid.

Some hydrogen seals in the past have employed a structure comprising four 90-degree arc segments of solid leaded bronze known as BTH metal (British Thomson Houston). These four quadrants were not fastened together but rather were held in position around the rotating shaft with a helical coil spring. While such a seal structure is acceptable, it has been found that more oil than necessary flows past such a seal carrying dissolved hydrogen with it. This flow occurs because of expansions occurring as a result of uneven, radially directed, temperature gradients and the high coefficient of thermal expansion of the BTH metal. The non-uniform expansion produces non-uniform clearances between the shaft and the BTH metal surface, thereby permitting sealing fluid to flow through those arc portions with increased clearance.Moreover, the use of BTH metal is not ideal with respect to protecting the journal surface of the rotor shaft from friction damage.

Accordingly, hydrogen seals for large dynamoelectric generators are required to exhibit several important properties. In particular the seal structure must not be susceptible to damage because of conventional shaft vibration nor should this vibration produce unacceptable shaft clearances. The flow of sealing fluid should be minimized so as not to carry significant quantities of dissolved hydrogen. Additionally, the seal must operate to substantially maintain air on one side of the seal and hydrogen on the other side.

Moreover, the seal should not warp nor be excessively sensitive to temperature gradients.

Lastly, because of the relatively large size of such seals used in a dynamoelectric generator, the seals of necessity have a relatively large weight and the seal structure must be such that significant distortion of the seal does not occur because of its own weight.

In accordance with a preferred embodiment of the present invention, a fluid seal for separating two gases along a rotating shaft comprises a pair of annular discs each divided into two semicircular portions which are fastened together about the shaft, each of the four semi-circular portions having a layer of a suitable bearing alloy, e.g.

babbitt metal bonded along the radially inner circumference. The pair of annular discs is disposed coaxially in a spaced apart relationship defining a fluid channel between them for supplying sealing fluid to the space between the babbitt metal and the journal surface of the shaft.

The seal further comprises an annular two-piece seal casing fixedly mounted with respect to the rotating shaft and abutting the annular discs along their outwardly facing annular surfaces. The seal casing is spaced apart from the shaft and has at least one fluid passage therein in communication with the fluid channel formed by said annular discs. Lastly, the seal comprises a bias means which exerts a radially inward force against the pair of annular discs and also exerts an axial force tending to urge the annular discs against corresponding annular portions of the seal casing.

Accordingly, it is an object of the present invention to provide hydrogen seals for large dynamoelectric generating machines, said seals exhibiting a reduced tendency to damage the generator rotor journal surface, exhibiting reduced oil flow requirements and further exhibiting a reduced tendency to distort because of thermal gradients.

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which: FIGURE 1 is a cross-sectional schematic diagram illustrating the occurrence of nonuniformities in seals employing four solid leaded bronze quadrants.

FIGURE 2 is a partial cross-section diagram of the seal of the present invention showing a side elevation view at the juncture of the rotating shaft and the seal.

FIGURE 3 is a cross-sectional schematic diagram illustrating an axial view of the present shaft seal.

Figure 1 illustrates some of the problems that may arise with respect to seals fashioned in four quadrants. Each quadrant 30 of the seal is disposed about the rotating shaft 22. The four quadrants are typically held in place around the shaft by a helical metal spring and such seals generally comprise a material such as BTH metal.

Figure 1 deliberately exaggerates gap clearances so that the problem of gap non-uniformity may be better appreciated. This non-uniformity is due at least in part to radially directed temperature gradients. These gradients can cause distortions in such a seal.

In contrast, Figure 3 shows a view similar to that of Figure 1 in which the seal of the present invention is employed. The seal of Figure 3 is simpler, generally employing two semicircular annular discs 1 Oa or 1 Ob. These annular discs typically comprise a material such as steel.

However, the seals of the preferred embodiments of the invention have a layer of babbitt metal 12 bonded to their input circumferential surfaces so as to form a tighter seal. Additionally, it is to be noted that the semicircular portions are featured together such as with bolts 24.

A more detailed understanding of the structure and operation of the seal of the present invention may be had from an inspection of Figure 2. Like Figure 3, Figure 2 shows shaft 22 and seal portions 1 Oa and 1 Ob, referred to collectively by reference numeral 10. Additionally, there is shown seal casing 11 having portions 1 a and 11 b as shown.

The pair of annular discs or rings 1 Oa and 1 Ob are described first. Each ring 1 0a and 1 Ob is divided into two semicircular annular portions as shown in Figure 3 so that a total of four pieces are assembled in constructing anular discs 1 Oa and 1 Ob. Also as shown in Figure 3 and discussed above, bolts or other fastening means are provided to hold the semicircular portions together to form circular rings. Of additional significance is layer 12 of babbitt metal bonded to the radially inner circumferential surfaces of the semiannular rings.

Because of the need to consider vibration of the shaft 22, the seal structure is configured so that portions 1 Oa and 1 Ob are at least somewhat slidably movable within the seal casing 11.

Because of this motion, annular grooves 19 are provided in the axially exterior (that is, outer) faces of the discs 1 Oa and 1 Ob as shown. Sealing fluid is provided to this annular groove either through channel 20 or through channel 21, both of which are shown in phantom view. Channel 20 may be formed, for instance, by beveling the edges where the two semiannular rings are fastened together.

Channel 21 is provided simply by drilling a hole through the semiannular ring as shown.

Disc portions 1 0a and 1 Ob are positioned coaxially in a spaced apart relationship. Bias means such as spring 13 acts to urge the annular faces of the discs against corresponding annular portions of the seal. The bias means 13 also acts to separate the ring portions 1 Oa and 1 Ob so that a sealing fluid channel is defined therebetween. It is this channel with which channel 21 (if provided) is in communication.

The bias means operates to exert a radially inward force on rings 1 Oa and 1 Ob and also acts thereon to exert a force in an axially outward direction. This bias means preferably comprises a spring coiled into a toroid and disposed in an annular groove formed by beveling the axially inner, radially outward, circumferential edges of the rings 1 0a and 1 Ob, as shown. The use of such a helical spring as a biasing means not only provides support for the annular discs 1 Oa and 1 Ob but also provides an unobstructed path for the flow of sealing fluid, such as oil, to the space between babbitt metal 12 and rotating shaft 22.

In conjunction with the seal casing 11 , the annular rings 11 are preferably provided with lips at the circumferential edge thereof extending in an axially outward direction. Corresponding lips on the seal casing 11 extend in an axially inward direction so as to form a pair of hook-fit faces. The seal casing 11 of the present invention, like the annular rings 10, is divided into semiannular portions so that the seal assembly may be readily attached to the generator frame.Additionally, seal casing 11 has at least one sealing fluid channel 14 therein in communication with the channel defined between the spaced apart annular portions 1 Oa and 1 0b. As described above, the steel casing 11 also preferably possesses lips along its radially inner circumference, these lips extending axially inward to mate with corresponding annular portions on the rings 1 Oa and 1 Ob. These lips also facilitate the assembly of the seal by limiting the motion of the annular rings 1 Oa and 1 Ob within the seal casing itself.

It is to be noted that the annular rings of the present invention are fixedly fastened together, such as by bolts 24 in Figure 3. These bolts are preferably configured to also act as dowel pins so that the semiannular ring portions may be more precisely aligned during assembly and operation.

An annular oil deflector 1 5 is also preferably affixed to the hydrogen side of the seal. This deflector is held in place preferably by slotted pan head screws 1 8 and internal tooth lock washer 17, as seen in Figure 2. The oil deflector 1 5 also preferably has a radially inward surface with a concave groove 1 6 which further assists in preventing oil mist from entering the coolant flow on the hydrogen side of the seal.

One of the advantages of the preferred embodiment of the invention is that the combination of steel annular rings 1 Oa and 1 Ob with babbitt metal 12 forms a structure which is much less susceptible to thermal expansion than BTH metal seals. Another advantage of the present seal structure is that the babbitt lining has a much less deleterious effect on the shaft journal surface under conditions of vibration and oil contamination.

For purposes of clarification and completeness, it is noted here, that as used herein and in the appended claims, the term "axially outward direction" refers to a particular direction directly away from the annular discs and the term "axial inward direction" refers to a direction toward the sealing fluid channel defined by the space between the annular rings 1 Oa and 1 Ob.

From the above, it may be appreciated that the seal of the present invention provides a structure satisfying the requirements listed above for hydrogen seals in large dynamoelectric generating machines. The seal of the present invention is protected against shaft vibration and produces a structure which is not only easily assembled and retrofitted to existing machines but one which also ensures conservation of the cooling gas by providing a tight seal. Moreover, the seal structure of the present invention significantly reduces the chance of damage to the rotating journal surface.

However, it is to be noted that the seal of the present invention does not act as a weightcarrying bearing for the rotor shaft which function is generally performed by separate structures in large generators.

Claims (9)

1. A fluid seal for keeping two gases separate, said seal being for use along a journal surface of a rotating shaft, said seal comprising: a pair of annular rings, each of which is divided into semicircular portions which are fastened together about said shaft, each of the four semicircular portions having a layer of babbitt metal bonded thereto along the radially inner, circumferential surface, said pair of annular rings being disposed coaxially in a spaced apart relationship, thereby forming a sealing fluid channel therebetween for supplying sealing fluid to the space between said babbitt metal and said journal surface of said shaft;; an annular, two-piece seal casing, fixedly mounted with respect to said rotating shaft and abutting said annular rings on their axially exterior annular surfaces, said casings being spaced apart from said shaft and having at least one fluid passage therein in communication with the space between said annular rings; bias means exerting a radially inward force against said pair of annular rings and also an axial force tending to urge said annular discs against corresponding annular portions of said seal casing.

2. The seal of claim 1 in which said bias means comprises a helical spring disposed within a Vgroove formed by a bevel of the radially outward, axially inward, circumferential edges of the annular rings.

3. The seal of claim 1 or 2 in which said annular rings possess annular sealing fluid grooves disposed along the axially exterior surfaces of the annular rings.

4. The seal of any preceding claim further comprises an annular oil deflector mounted on said steel casing, said deflector being spaced apart from said shaft.

5. The seal of any preceding claim in which said annular rings possess lips along their radially outward surfaces, the said lips extending in exterior axial directions, said lips being for mating with corresponding axially inward extending lips on said seal casing.

6. The seal of any preceding claim in which each semicircular portion of each ring is fastened by bolts to the other semicircular portion of that ring.

7. The seal of claim 6 in which said bolts are also configured as dowels.

8. The seal of any preceding claim, in which said annular rings are of steel.

9. A fluid seal substantially as herein described with reference to and as shown in Figures 2 and 3 of the accompanying drawing.