US3429557A

US3429557A - Steam turbine rotor cooling arrangement

Info

Publication number: US3429557A
Application number: US561982A
Authority: US
Inventors: Ronald E Brandon; Russell J Holman
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1966-06-30
Filing date: 1966-06-30
Publication date: 1969-02-25
Anticipated expiration: 1986-02-25
Also published as: SE348795B; DE1551184B2; CH469185A; GB1174170A; DE1551184A1

Description

Feb. 25, 1969 R. E. BRANDON ET AL 3,429,557

STEAM TURBINE ROTOR COOLING ARRANGEMENT Filed June 50, 1966 v "INVENTORS'.

RONALD E. BRANDON,

RUSSELL J. HOLMAN, BY m flw THEIR ATTORNEY.

United States Patent 3 Claims ABSTRACT OF THE DISCLOSURE A steam turbine in which the rotor parts which are subjected to the hottest steam are cooled by relatively cool steam. A flow path arrangement to recirculate partially expanded steam from a lower stage is provided for this purpose.

This invention relates to elastic fluid turbines generally and, more particularly, it relates to a cooling arrangement for the rotors of steam turbines.

In the turbine art, it is well known that higher operating temperatures can result in more efficient thermodynamic cycles. In the present state of the art, initial steam temperatures of 1000 F. are commonly used. While thermodynamic considerations tend to increase operating temperatures, other considerations such as cost of special material and operating difiiculties with steels of high alloy content make such temperatures diflicult to justify economically. It is thus common to use special materials only in critical areas such as rotor buckets and nozzle partitions, while employing standard materials for such parts as rotor bodies. In order to permit the use of a standard material for a turbine rotor body, it is necessary to provide some arrangement for keeping it at a temperature below the elevated temperatures to which the buckets are subjected. This problem presents itself in the initial stage of any group of turbine stages, where the steam is at its highest temperature.

By way of example, and not of limitation, one environment for the present invention is in a double flow, low pressure steam turbine. Because of rotational stress and long term metallurgical properties of the rotor, rotor temperature at the hottest portion must be limited to approximately 650 F. This hottest portion of the rotor for a double fiow low pressure turbine would normally be in the region of the annular wheel space between its first stage wheels. A combination of leakage, convection, and radiation from the much hotter steam in the low pressure inlet, plus heat generated by fluid friction between rotating and stationary parts, would normally result in temperatures much higher than the 650 F. limit. This particular temperature limitation has not heretofore been a serious problem because steam temperatures approaching the low pressure turbine were generally not higher than about 650 F. Steam temperatures have increased, however, so that it is now essential to provide some method and means of cooling the rotor exposed to this type of duty.

Accordingly, it is an object of the present invention to provide an improved arrangement for cooling the body of a turbine rotor where it is adjacent to the hottest motive fluid.

Another object is to provide a reliable arrangement for turbine rotor cooling which is efiective despite changes, during use, in the clearances of the labyrinth seals between rotor and casing.

Another object is to provide a reliable means to motivate coolant fluid to the critical areas of a turbine rotor.

Other objects, advantages and features of the present invention will become apparent from the following de- 3,429,557 Patented Feb. 25, 1969 scription of one embodiment thereof, when taken in connection with the accompaying drawing in which:

FIG. 1 is a partial longitudinal elevation, partly in section, of a multistage, axial flow steam turbine according to the present invention;

FIG. 2 is a detail view taken along the line II-II FIG. 1.

Briefly stated, the present invention is practiced in one form by a low pressure turbine having holes through its first stage wheel. On the downstream face of the wheel and projecting into the wheel space are scoops assisting the flow of steam from the discharge of the first stage back through the wheel holes and into the wheel space up stream of the first turbine stage. In the turbine casing is a recirculation conduit providing communication from the flow path downstream of the first turbine stage to the upstream wheel space. First stage rotor buckets have negative reaction at their root port-ions causing slightly lower pressure on the upstream side thereof than on the discharge side. The lower pressure at the first stage upstream wheel combined with the scooping action through the wheel holes, causes relatively cool steam to flow from the first stage discharge to the wheel space upstream of the first stage and then to the upstream bucket root portions. Depending on whether fiow from the scoops is more or less than allowed by the labyrinth seals, flow will go through the recirculation conduit either out of or into the upstream wheel space, respectively. Thus, the hottest parts of the turbine rotor are cooled.

Referring now to FIG. 1 in the drawing, a turbine rotor is generally indicated at 1. Axially spaced along rotor 1, are first and subsequent stage rotor wheels 2 and 2a respectively, which carry turbine buckets 3 and 3a respectively which are in turn surrounded by shroud bands 6. The turbine casing, generally indicated at 4, includes nozzle partitions 5, stationary shrouds 7, diaphragm members 8, and labyrinth seals 9. Stationary shrouds 7 cooperate with the sealing strips on shroud bands 6 to restrict steam flow therebetween. Likewise, labyrinth seals 9 cooperate with rotor body 1 to restrict steam flow therebetween. Seals 9 include strips 19 inclined as shown to permit a higher flow coefiicient therearound in the direction of inclination than in the opposite direction. The axially spaced rotor wheels 2 and 2a are separated by wheel spaces shown at 10 and 11. Wheel space 11 is in the center of the rotor, that is to say, adjacent to the steam inlet or bowl 16 and upstream of the first turbine stages. Wheel spaces 10 and 11 are annular chambers, substantially defined by rotor wheels 2, 2a, diaphragm members 8 and other stationary casing members.

First stage rotor Wheel 2 has a plurality of circumfer entially spaced axial holes 13 therethrough such that wheel spaces 10 and 11 communicate with each other through the wheel 2. Extending from these holes 13 and into downstream wheel space 10, are a plurality of scoops 12. On the upstream face of first stage rotor wheel 2 is a root area 15, thus labeled for convenience. A recirculation conduit 14 communicates at one of its ends with wheel space 11. At its other end, recirculation conduit 14 communicates with the interior of the turbine casing 4 in the vicinity of the tips of first stage rotor buckets 3, downstream thereof.

First stage rotor buckets 3 are designed for negative reaction at their roots. That is to say, that in the area 15 adjacent their roots, the upstream steam to the first stage buckets 3 is at a slighlty lower pressure than the discharge steam from first stage buckets 3, because the root portion of the bucket is actually functioning as a diffuser to cause the negative reaction.

In opertion, wheel space 11, since it communicates by way of recirculation conduit 14 and axial holes 13,

with the steam path downstream of first stage rotor buckets 3, is at a steam pressure approximately equal to the steam pressure downstream of first stage rotor buckets 3, and possibly at a slightly higher pressure due to the supercharging effect of scoops 12. In addition, due to the negative reaction of buckets 3, pressure at root areas 15 is less than in space 11. Thus, there is flow of relatively cool steam from scoops 12 to wheel space 11 and then past labyrinth seals 9 to the lower pressure root areas 15 on the upstream sides of first stage rotor wheels 2. This How must be of sufiicient magnitude to absorb heat generated by fiuid rotational friction in the wheel spaces 10 and 11 plus other heat conducted to space 11 from the bowl 16. Such heat entering space 11 may be due to leakage of higher pressure, higher temperature steam through numerous small unavoidable leakage paths or spaces. This heat must be absorbed without permitting the rotor surface temperature to go above 650 F. At this point, there are two distinct possibilities in the mode of operation of the present invention.

In the first, assuming the labyrinth seals 9 to be relatively tight, and permitting relatively small flow from wheel space 11 to first stage root areas 15, the general flow direction starting at scoops 12 is as follows: Steam downstream of first stage rotor buckets 3 is scooped in through axial holes 13 by scoops 12 and enters wheel space 11. From wheel space 11, a small portion flows past labyrinth seals 9 to cool the lower pressure root areas of first stage rotor wheel 2. It is desirable to keep this flow small since large flows at this location are detrimental to turbine efliciency. Due to the fact that scoops 12 provide more steam than can normally flow past labyrinth seals 9, the excess flowing into wheel space 11 is caused to flow out therefrom through recirculation conduit 14 and into the turbine casing at a point downstream of first stage rotor buckets 3, thus maintaining sufiicient flow rate to keep the wheel space 11 cool. Flow through wheel space 11 and conduit 14 is thus motivated by the pumping action of scoops 12.

The second possibility arises where labyrinth seals 9 are relatively worn permitting greater flow therearound. If insufiicient cooling flow were available, that is to say, without recirculation conduit 14, the pressure in wheel space 11 would decrease. This would result because of free communication with one or both of lower pressure root areas 15 (which as a practical matter are never equal in pressure) to the point where hotter steam would be drawn from one root area 15 into wheel space 11 and out to the other root area 15, depending on which area 15 happens to have the lower pressure, thus defeating the cooling system. In the present invention, however, starting with steam within the casing downstream of first stage rotor buckets 3, the flow is as follows: Scoops 12 motivate the steam through axial holes 13 and into wheel space 11 from which the flow continues relatively freely past labyrinth seals 9 to lower pressure root areas 15 on the upstream side of rotor wheels 2. If there is still a capacity for the flow of coolant steam into the relatively low pressure root areas 15, coolant steam flows from the casing downstream of the tips of the first stage rotor buckets 3 and through the recirculation conduit 14, and into wheel space 11 to supplement the flow thereinto through axial holes 13.

Thus, relatively cool steam discharging from the first stage rotor buckets 3 is used to cool the turbine rotor in the vicinities of wheel space 11 and the root areas 15 of first stage rotor wheels 2, regardless of the condition or performance of labyrinth seals 9. The pressure in wheel space 11 is prevented from decreasing to the point where hot steam from areas 15 can be drawn into space 11. In this arrangement, flow through conduit 14 is motivated by the differential between first stage exhaust steam pressure and the lower steam pressure at root areas 15, due to negative reaction at the bucket roots.

It will thus be apparent that an effective arrangement 4 I has herein been described for cooling a turbine rotor in the region where it is adjacent to the hottest motive fluid. The present arrangement is effective despite changes, during use, in seal clearances.

In the foregoing description, of a double flow, low pressure turbine, wheel holes 13 have been provided in the first stage wheels 2 on only one side of the turbine. It is of course within the contemplation of the invention to use them on both sides if required.

Furthermore, it will occur to others of ordinary skill in the art to make other modifications of the present invention which will lie within the concept and scope thereof and will not constitute patentable departure therefrom. For example, it is within the contemplation of this invention that it be useful on steam turbines of all pressure ranges, as well as gas turbines, in addition to the above-described double-flow, low pressure turbine environment. Therefore, it is intended that the invention be not limited by the details in which it has been described but that it encompass all within the purview of the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A steam turbine including a casing and a rotor defining a motive steam path therebetween, said rotor having a row of buckets mounted on a first-stage wheel and disposed in said path, said buckets being shaped over their major outer portions to extract energy from the motive steam with a reduction in pressure across the bucket row, said buckets having root portions also disposed in said path and shaped for negative reaction so as to extract energy as well as to provide a region on the upstream side of the row in the vicinity of the bucket root which is lower in pressure than on the downstream side,

a rotating seal on the upstream side of the first-stage wheel forming close clearances between the casing and the rotor and defining together therewith an upstream wheel space on one side of the seal and providing a restricted flow path from said wheel space across the wheel into said region in the motive steam path,

means for motivating the cooler expanded steam from the downstream side of the bucket row to said wheel space without significant pressure loss when the wheel is rotating.

2. The combination, according to claim 1, wherein said motivating means includes holes through said first-stage wheel and scoops mounted on the wheel and arranged to motivate steam upstream through said holes.

3. A steam turbine including a casing and a rotor defining a motive steam path therebetween, said rotor including buckets mounted on axially spaced rotor wheels,

said casing and said rotor further defining wheel spaces on the upstream and downstream sides of the first stage rotor wheel,

a rotating seal forming close clearances between said casing and rotor on the upstream side of said first stage rotor wheel, said seal restricting communication between said flow path, and said upstream wheel space and being subject to variations in flow there through,

the first stage rotor wheel defining a plurality of holes therethrough and having a scoop mounted thereon on the downstream side thereof adjacent each of said holes,

the first stage rotor buckets shaped to provide a negative reaction at their root portions when the turbine is operating, so that the motive steam is at a higher pressure on the downstream side of said bucket roots than on the upstream side thereof,

a stationary recirculation conduit communicating with the downstream side of said first stage rotor buckets and with said upstream wheel space,

said scoops and the lower pressure on the upstream side of the first stage bucket root portions motivating 5 6 elastic fluid flow through said holes, said upstream References Cited wheel space, and said seal clearances to the upstream UNITED STATES PATENTS side of said first stage rotor wheel, said recirculation conduit carrying excess elastic fluid 3206166 9/1965 Beldecos et 253' 391 2,552,239 5/1951 Warren 60--4O from said upstream wheel space when the flow 5 3 189 3 1 65 B l 1 25 39 15 through the rotor wheel holes is greater than through 20 6/ 9 e decos et a 3 the clearances, and said recirculation conduit carrying elastic fluid into said FOREIGN TE upstream wheel space when said clearances are rela- 6351890 4/1950 Great Bntamtively large and the flow through the rotor wheel 10 holes is adequate to maintain pressure in said up- EVERETTE POWELL Prlmw'y stream wheel space.