US2451261A - High and low pressure turbine rotor cooling arrangement - Google Patents

Description

Oct. 12, 1948. G. B. WARREN ETA;

HIGH AND LOW PRESSURE TURBINE ROTOR C001 ING ARRANGEMENT Filed Oct. 29. 1946 Md /8 m 20 ,4 l4 /2 26 22a. (6 7/ I SLIPERHEATER -Fi .2.

a9 a g REA/E4 TER JO L ER The'ir Attor ne particularly the rotor.

irritants-d Jct. i2, 184% HIGH AND LOW PRESSURE TURBINE ROTOR, COOLING ARRANGEMENT Glenn B. Warren and Arthur R. Smith, Schenectady, N. Y., assignors to General Electric Company, a corporation of New York Application October 29, 1946, Serial No. 706,310

This invention relates to a method of cooling high temperature elastic fluid turbines, especially the initial stages of a multi-stage turbine. More particularly, it relates to an arrangement for circulating a cooling fluid through a high temperature elastic fluid turbine in order to cool the bucket wheels and shaft of the turbine. The invention constitutes a further improvement of the turbine sealing and cooling arrangement disclosed by U. S. Patent 1,878,731, issued September 20, 1932 in the name of Paul W. Thompson.

As is well known, high thermal efiiciency can be attained in a fluid pressure turbine prime mover by utilizing a high temperature, high pressure fluid operating medium, such as highly superheated steam at temperatures over 1000 F. Special materials, such as the so called "18-8 wrought chrome-nickel steel, which can safely withstand these high temperatures are commercially available, but their cost is such that it is economically impractical to use these materials throughout a turbine, especially in large multistage axial flow turbines, where the shaft and bucket-wheels are ordinarily machined from a single forging. The allowable working stress for ordinary, less expensive materials, such as chrome-moly-vanadium steels when subjected to temperatures of the order of 1000 F. is considerably below the stresses encountered in present day turbines. However, it is found that by decreasing the normal working' temperature of chrome-moly-vanadium steel, for instance from 1100" F. to 900 F., the allowable creep stress may be increased by about five times its former value. Since the permissible working stress increases rapidly as a function of decrease in temperature, readily available low cost materials can be used for the turbine rotor, provided the rotor is maintained at a temperature such that the allowable stress is greater than the maximum working stresses encountered. The more expensive special materials can then be used for the buckets or blades in the first few stages where the temperature conditions are most severe. To obtain eflective cooling action with a reasonable quantity of cooling fluid, the temperature of the coolant should preferably be at least 150 below that of the motive fluid, when the latter is supplied to the turbine at temperatures over 800 F. Our invention is intended to provide a cooling arrangement which will permit the use of ordinary materials in turbines using a high temperature fluid operating medium.

Accordingly, an object of the invention is to provide an improved method of and arrangement for cooling high temperature turbine parts,

Another object is to provide a turbine arrangement which permits increasing the normal op- 3 Claims. (Cl. 60-64) crating temperatures of turbines manufactured from ordinary low cost materials.

A further object is to provide means for keeping the stresses in a steam turbine rotor within allowable safe limits while operating at tempera tures of the order of D F. and above.

Other objects and advantages will be apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is a partial sectional view of an elastic fluid turbine of a type which may advantageously employour invention; and Figs. 2 and 3 are diagrammatic views of elastic fluid turbine powerplants embodyin our invention.

For convenience in explanation, the invention shall be described as applied to a steam turbine powerplant; however, it should be understood that the invention may be also applicable to other elastic fluid turbines, such as those using mercury vapor, products of combustion, or other high temperature operating media.

Referring now to Fig. 1, the turbine comprises a casing 5 defining a steam inlet conduit 6 communicating with a first-stage nozzle ring I having a plurality of circumferentially spaced nozzles 8 and secured by suitable fastening means (not shown) to casing 5. A rotor indicated generally at 9 is disposed within casing 5, supported by suitable bearings (not shown), and comprises a shaft in and a plurality of bucket wheels, II, l2, 13, which may be formed integral with shaft ill from a single forging.- Secured to the circumferences of the bucket wheels are a plurality of spaced buckets or blades M surrounded by shroud bands l5. Stationary diaphragm s I5, I! are supported in casing 5 and associated with wheels i2, 13 for directing operating medium to the respective bucket annuli. These diaphragms are in the form of disc members containing a plurality of circumferentially spaced blades 68 forming nozzle passages. Arranged around the central openings in the diaphragms and cooperating with the shaft l0 are labyrinth packings 39 which limit the flow of steam through the clearance spaces between the shaft IO and the respective diaphragms. Fig. 1 is intended to represent diagrammatically a general type of diaphragm arrangement, many specific forms of which are well known in the steam turbine art.

Circumferential flanges or "spill bands 20 are formed on either side of the first stage wheel II adjacent the root of buckets Ma, and are arranged to form close axial clearances with nozzle ring I on the inlet side and a. ring member 2! on the discharge side, the latter being supported by the diaphragm [6. Ring 2| also forms the inner wall of the steam distribution chamber 22 between the first. stage wheel H and the second stage diaphragm [6. Similar spill bands on subsequent stage wheels l2 and I3 are arranged to cooperate with portionsof the respective adjacent diaphragms "5, l1, etc. The purpose of the spill bands will be noted hereinafter.

In order to prevent excessive axial forces on rotor 9, pressure balancing holes 23 are provided through the web portions of the respective wheels, in a manner well known in the art. Formed in casing 5 are axially spaced labyrinth seal packing ring structures 24 and 28, which form close clearances with shaft l8 for resisting the flow of steam axially along the shaft. Provided in casing 5 is an admission port 28 for sealing and cooling fluid, communicating with an annular chamber 28a defined in the casing between the labyrinth seals 24, 25. Seal 25 communicates between chamber 28a and a chamber 22a defined between casing 5 and the first stage wheel H and within nozzle ring I.

For reasons which will appear hereinafter, a pressure tap port 21 communicating with chamber 22 may be provided in casing 5.

The operation of the turbine of Fig. 1 is as follows. superheated steam enters the, turbine casing from a valve chest (not shown), thence through passage 6 to the first stage nozzles 8 and path of the operating steam at eachstage. It is important, from the standpoint of both thermal and aerodynamic efiiciency, that provision be made to keep this leakage in'the first few stages to a minimum. To accomplish this, the spill band clearances in these stages are made very small, so that most of the entry of cooling steam into the motive fluid occurs in the lower pressure stages, where the temperature of the operating medium is more nearly equal to that of the cooling steam. Furthermore, the static pressure of cooling fluid in chamber-22a is roughly of the axially through the respective buckets 14a, l4 and diaphragm nozzles l8, imparting rotational energy to the rotor 8. In so-called impulse stages" of this general arrangement, substantial pressure drops are experienced by the operating medium in passing through the first stage nozzles 8 and the subsequent diaphragm nozzles l8. Some drop in pressure may occur in the buckets of the several wheels, but the major portions of the pressure drop across any given stage occurs in the stationary nozzle for that stage.

In order to cool the turbine, cooling and sealshaft l8. This portion of the cooling steam which 'flows outwardly may be collected in any one of several well-known ways and led to some lower pressure stage of the turbine or to a suitable heat reclaiming device (not shown). The above-described arrangement is more particularly described and claimed in United States patent to P. W. Thompson 1,878,731. 1

Because of the material drop in pressure across nozzles 8, chambers 22 and 22a will both be at pressures appreciably below that in chamber 26a;

of the superheated steam within the coil consame order of magnitude as that of the motive fluid at the first stage bucket inlet (and in chamber 22) because of the pressure drop in packing 25. Therefore, there is no marked tendency for I the cooling fluid to enter the first stage bucket flow path past sealing rings 28.

The cooling steam may be obtained in various ways, one of which is illustrated in Fig. 2. This represents a so-called tandem compound turbine" powerplant comprising a high pressure steam turbine. 28 and a low pressure turbine .32, which may have rotors connected to a common shaft l8. Each of the turbines 28, 32 are arranged internally as shown in Fig. 1. The high pressure turbine 28 is supplied with superheated steam, which is generated in a'boiler and then passes through supply conduit 38 containing a superheater 29 and a suitable shut-off valve 31, thence to the turbine. From the high pressure turbine 28, exhaust steam passes through the "cross-over conduit 33 to a reheater 34, where the temperature is raised somewhat, thence through conduit 35 and stop-valve 36 to the inlet of the low pressure turbine 32.

Cooling fluid for the high pressure turbine 28 is obtained by taking superheated steam from conduit 38 at a point downstream from the shutoff valve 3| (i. e. between valve 3| and the inlet to turbine 28), and passing it through a conduit' siderably, while the pressure remains substantially constant. From coil 38, the cooled steam passes through conduit 38 to the cooling fluid inlet port 28, thence through the turbine 28 in T the manner described above.

therefore some cooling steam will flow axially' inwardly between the shaft and labyrinth seal 25. This part of the cooling steam which flows in-'- wardly past seal 25 will fill the chamber 22a adjacent the first stage wheel The spill bands 28 resist the mixing of the hot operating fluid from nozzles 8 with the cooling steam in space 22a, therefore the cooling steam, following the path of least resistance, will flow through the respective balance holes 23, and between the labyrinth seals I9 and shaft l8 through subsequent stages in the manner indicated by the arrows in Fig. 1. This circulation of cooling steam through the turbine effectively cools the shaft, bucket wheels, and the central portions of the diaphragms in the high temperature section of the turbine.

It is preferred that the cooling steam be taken from conduit 38 at a point downstream from the shut-off valve 3|, since with this arrangement, if a turbine overspeed condition should occur, the action of the emergency overspeed governor (not shown) will cause shut-off .valve 3| to close automatically, thereby stopping the supplyof both the operating steam and the cooling steam.

The flow of cooling steam to coil 38 may be controlled by a manually controlled valve 48 in conduit 31. ,The flow of the cooled steam to the turbine 28 may be further controlled by a valve 4| in conduit 38. While valve 4| may be manually controlled, it is preferred that a suitable v automatic valve be used. For instance, a type of valve well-known as aficonstant pressure differentlal valve" may be advantageously employed. This is arranged to maintain constant the pressure differential between supply conduit 39 and the chamber 22 of Fig. 1. To this end a conduit 42 connects pressure tap 21 in the tur-v bine casing with a pressure responsive member such as flexible bellows 48 contained in a housing 41a, while another conduit 43 connects housing 4la with conduit 39 at the discharge side of valve 4|. As may be seen in Fig. 2, the stem and disk of valve 4| is attached to bellows 48 so that a pressure decrease in chamber 22 of Fig. 1 will cause bellows 48 to contract thereby moving the valve disk towards the closed position with the result that the fluid pressure in conduit 39 between valve 4l and port 26 decreases proportion ately. Conversely, a pressure increase in chamber 22 is accompanied by a proportionate pres-- sure increase in conduit 39.

Cooling steam for the low pressure turbine 32 is advantageously obtained by extracting low temperature exhaust steam fromcross-over conduit 33, before it enters the reheater 34, and passing it through conduit 44 to the annular chamber 26a of the turbine 32.

It should be noted that, ordinarily, means for controlling the rate of flow and pressure in the conduit 44 will be unnecessary, since the exhaust steam pressure in cross-over conduit33. varies automatically with changes in the load on turbine 32. The pressure differential between the inlet port 26 and the chamber 22a of turbine 32 will be substantially equal to the sum of the pressure drops across the reheater 34, valve 36, noz- Zles 8, and the friction losses in the related conduits. As the load increases the rate of flow of operatin steam increases, with a proportionate increase in this pressure differential. This increase in the pressure differential automatically causes the rate of flow of cooling steam through conduit 44 to increase, thus providing the increased cooling effect needed for the higher load condition. With this arrangement, the pressure tap 21 of Fig. 1 is obviously not needed and would be omitted or suitably blanked off.

Another method of obtaining the cooling steam for the high pressure turbine 28 is shown in Fig. 3. Here again turbine 28 is supplied with high temperature steam from superheater 29 through conduit 30 containing a suitable shut-oil valve 3|, as in Fig. 1. Substantially saturated steam, considerably lower in temperature and slightly higher in pressure than the superheated operating steam supplied to .the turbine, is taken from conduit 30 at the inlet side of superheater 29 and passes through conduit to the cooling steam inlet port 26, thence through the turbine 28 in the manner described above. Control of the cooling steam is here effected by a suitable shut-off valve 46 and a constant pressure differential valve arrangement 41 in conduit 45. The constant pressure difierential valve 41 may be arranged as described above, so as to maintain a constant difierential between the cooling fluid inlet port 26 and the chamber 22 surrounding the first stage wheel ll of turbine 28.

The arrangement illustrated in Fig. 2 is particularly desirable in that the cooling steam can be taken from the supply conduit 30 at a point comparatively near the turbine, whereas the arrangement of Fig. 3 requires a pipe line 45 running all the way back to the boiler (superheater 29 ordinarily being incorporated in the boiler which supplies the operating steam), which in large modern power plants may be several hundred feet from the turbine.

It will be seen that the invention provides an improved arrangement for effectively cooling high temperature steam turbines, permitting the use of ordinary low-cost materials for operation with considerably increased normal operating temperatures, while stresses in the rotor are maintained within safe limits.

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

1. In a powerplant the combination of a high pressure multi-stage elastic fluid turbine for high temperature operation having a casing with a first operating fluid inlet port and a second exhaust port and a third cooling fluid inlet port, a-rotor disposed in the casing and forming a plurality of fluid pressure energy conversion stages in series, said rotor defining also a cooling fluid flow path, nozzle means disposed in the casing for delivering motive fluid from said flrst port to the rotor, said casing defining passage means for conducting cooling fluid from said third port to the rotor cooling fluid flow path, a boiler and a superheater in series flow relation with a first conduit for supplying high temperature motive fluid to the first inlet port, a heat exchanger adapted to be cooled by steam from said turbine exhaust port, and second conduit means for conducting fluid from the first conduit between the superheater and the turbine inlet, through said heat exchanger, and to said cooling fluid inlet port.

2. In a powerplant the combination of a high pressure turbine and a low pressure turbine with first cross-over conduit means including a reheater connecting the high pressure and low pressure turbines in cross compound relation, each of the turbines having a casing with an operating medium inlet port and an exhaust port and a cooling fluid inlet port, a boiler and a superheater in series flow relation with a second conduit for supplying high temperature motive fluid t0 the inlet port of the high pressure turbine, a heat exchanger adapted to be cooled by the exhaust steam in said first cross-over conduit, third conduit means arranged to conduct fluid from said 'second conduit between the superheater and the inlet port of the high pressure turbine, through said heat exchanger and to the cooling fluid inlet port of the highpressure turbine, and fourth conduit means arranged to conduct comparatively low temperature fluid from said cross-over conduit upstream from the reheater to the cooling fluid inlet port of the low pressure turbine.

3. In an'elastic fluid turbine powerplant, the combination of a high pressure turbine and a low pressure turbine each having a casing with an operating medium inlet port and an exhaust port and a cooling fluid'inlet port, a cross-over conduit connecting the exhaust port of the high pressure turbine with the inlet port of the low pressure turbine, a reheater in said cross-over conduit, and second conduit means for supplying comparatively low temperature fluid to the cooling fluid inlet of the low pressure turbine from said cross-over conduit between the reheater and the exhaust port of the high pressure turbine.

GLENN B. WARREN.

ARTHUR R. SMITH.

REFERENCES CITED The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 1,820,725 Bailey Aug, 25, 1931 2,212,471 Hagemann Aug. 20, 1940

Images