US2702688A - Gas turbine nozzle casing - Google Patents

Description

1955 F. R. ERICSON GAS TURBINE NOZZLE CASING Filed Dec. 26

. Inventor": Franklin FF. Ericson,

y f His At tor ey.

United States Patent M GAS TURBmE NOZZLE CASING Franklin R. Ericson, Leominster, Mass, assignor to General Electric Company, a corporation of New York Application December 26, 1951, Serial No. 263,185

1 Claim. (Cl. 25378) This invention relates to high temperature gas turbines, particularly to the structure of the nozzle casing for directing hot motive fluid to the bucket-wheel.

The object of the present invention is to provide an improved high temperature gas turbine nozzle arrange ment including a liquid'cooled casing having a fabricated nozzle ring in which the artificial cooling for the casing helps to preserve its dimensional stabihty and the fabncated nozzle ring is specially arranged to permit free differential thermal expansion of the comparatively thin nozzle blades without imposing excessive stresses on the blades or the attachment means which secure them in the casing.

Other objects and advantages will become apparent from the following description taken in connection with the accompanying drawings in which Fig. 1 IS a longitudinal section through a gas turbine having a nozzle casing incorporating the invention, Fig. 2 is an end view of the bucket shroud structure and nozzle ring assembly, taken on the irregular plane 2-2 in Fig. 1, Fig. 3 is a partial end elevation view of the inlet casing, 1 1g. 4 1s a sectional view taken along lines 44 of Fig. 1, Fig. 5 is a detailed view of the longitudinal section on an enlarged scale of the fabricated nozzle ring assembly of Fig. l, and Fig. 6 is a detail of a modification ot tne inner nozzle band structure. I

Generally, the invention is practiced by providing a cast nozzle casing forming water-cooling passages to keep the temperature of the casing within allowable limits, and to prevent excessive differential thermal expansion relative to the main frame of the turbine. The nozzle r ng proper is separately fabricated by welding, after which the ring assembly is welded into the casing. The fabricated ring is specially arranged to permit free differential thermal expansion of the comparatively thin blades which define the nozzle passages.

Referring now more particularly to Fig. 1, the invention is shown as applied to the gas turbine of a turbosupercharger, indicated generally at 1, for a diesel engine,

shown diagrammatically at 2 as being a 6-c yl inder engine having a fuel injection pump 2a and a divided exhaust manifold arrangement including separate manifolds 2b, 20, three cylinders being connected to each manifold.

The gas turbine bucket-wheel 3 has a shaft supported in suitable bearings (not shown) and a circumferential row of open-ended or shroudless buckets 4. The main frame of the turbosupercharger comprises a cast casing only a portion of which is shown at 5. The details or the turbosupercharger compressor, bearing arrangement, etc., are not material to an understanding of the present invention. It may be noted that the casing 5 defines an exhaust passage 5a and is provided with cored passages 5b through which cooling water is circulated from inlet conduit 50 and to an outlet 5d.

Since the buckets 4 are of the shroudless type, it is necessary to provide special means for reducing the leakage of motive fluid around the tips thereof. To this end, a stationary shroud arrangement is employed, consist ng of a plurality of arcuate segments 6, the shape of which will be seen from a comparison of Figs. 1 and 2. Each segment is secured to the water-cooled casing 5 by a single threaded fastening 7. The segments are so dimensioned that, when in the cold condition, they define a clearance space indicated at 8 in Fig. 2, with the end of the next adjacent segments. With this arrangement, the segments are free to expand circumferentially when going from the cold to hot condition without any change in the radial dimensions of the shroud ring. This type of segmental shroud arrangement for turbine buckets of the open-ended type is disclosed more particularly in the 2,702,688 Patented Feb. 22, 1955 United States Patent to Alan Howard, 2,606,741 issued August 12, 1952, and assigned to the same assignee as the present application.

in the present case, the shroud segments 6 are located radially by a circumferential rabbet portion 6a, which seats in a circumferential groove in the casing 5, as will be apparent from Fig. l. The area of metal-to-metal contact between segments 6 and easing 5 is reduced by the clearance spaces 6b, the function of which is to reduce heat transfer from the segments to the casing. The inner circumferential surface of the segments defines a rather small clearance space 9 with the tips of buckets 4, and has a recessed portion defining a plurality of circumferentially extending ridges 10 forming small clearances with the outer surface of the fabricated nozzle ring assembly indicated generally at 11.

The cast inlet casing 12 is provided with cored coolant passages 13 through which water is circulated from the inlet conduit 14 to the drain conduit 1-5. Casing 12 also defines motive fluid inlet passages as shown at 16, 17 in Fig. l. These are separate circular openings, as will be seen in Fig. 3. Around the inlets 16, 17 casing 12 defines a rectangular surface 18 having a plurality of threaded fastenings 19 for securing the attachment flanges of the exhaust conduits (not shown). It will be apparent from Fig. 1 that one engine exhaust manifold 2b is connected to the turbosupercharger inlet port 15, while the other manifold 2c is connected to port 17. It will be appreciated by those familiar with the diesel engine turbosupercharger art that the ducting of the engine exhaust ports to the respective manifolds 2b, 2c is so arranged, giving due consideration to the engine valve timing schedule, that the average pressure maintained in the manifolds 2b, 2c will be as high as possible. Such arrangements provide a significant improvement in the average pressure of the motive fluid supplied to the gas turbine. This is important in a diesel engine turbosupercharger adapted for automotive or railway use, Where the load changes materially and frequently, requiring the turbosupercharger to accelerate rapidly in order to follow the changes in speed of the engine. It will be appreciated that the two separate circular inlet ports 16, 17 transition smoothly to an arcuate shape, as indicated, partly in dotted lines, at 16a, and 16b in Fig. 3. The two arcuate nozzle casing portions Ida, 16b of course cooperate to define a substantially continuous annular nozzle supply passage.

The fabricated nozzle ring proper comprises a pair of annular radially spaced bands 2!), 21, with a plurality of c rcumferentially spaced airfoil shaped nozzle blades 22 disposed therebetween. The cross-section shape of the blades 22 may be seen in Fig. 4. As will be apparent from Figs. 1 and 4, the bands 29, 21 are provided with circumferentially spaced openings of a shape similar to and of a size to fit the respective ends of the blades 22. The outer end of each blade passes through an opening in band 21 and is welded thereto as indicated at 23 in Figs. 1 and 5. The inner end of the blade is not welded to the inner band 20 but projects therethrough with a sl ght clearance between, as also may be seen at 24 in Figs. 4 and 5. The actual size of the clearance 24 will be smaller than shown in the drawing, where it has been exaggeratedfor purposes of illustration. In practice, this clearance will be as small as practicable, consistent with manufacturing tolerances and the provision of the necessary freedom of the blade ends to expand relative to the band.

With this arrangement, the blades are fixed at their outer ends to the band 21, and are free to expand inwardly as their temperature increases from the cold to the hot condition. This prevents the setting up of excessive differential thermal expansion forces in the bands 29, 21 and the blades 22. The engagement of the inner blade ends with the band 2% will of course maintain it concentric with the outer band 21.

In order to prevent leakage of motive fluid through the clearance spaces 24 between the inner ends of blades 22 and the respective openings in band 20, the legs of a channel shaped ring 25 is secured to the innei circumference of band 20. It will be apparent that inner band 20 pansion betwen the casings.

and the channel member 25 cooperate to define an annular chamber 29 into which project the inner ends of blades 22.

When this fabricated nozzle ring structure is used in connection with a turbosupercharger supplied by divided manifolds in which the pressure varies cyclically, the schedule of pressures in one manifold being out of phase with the pressure changes in the other manifold, there will tend to be a certain loss in efliciency due to motive fluid leaking inwardly through the clearance space 24, traveling circumferentially through the annular chamber defined betweenband 20 and ring 25, and re-entering the nozzle passages in another portion which happens to be at a lower instantaneous pressure than that in the are from which the fluid leaked. In other words, there will be an erratic cross flow of motive fluid through this annular chamber between the arcuate nozzle casing portions 16a, 16b unless special means are provided to prevent this. In the present case, such cross-flow is prevented by two or more baflle or dam members indicated in dotted lines at 26 in Fig. 4. It will be seen that the dam is disposed at an angle to the axis of the machine between a pair of adjacent blade ends. One of these dams is shown at 26 in Figs. 1 and 5. It will be apparent that the number of these dams must be at least equal to the number of inlet ports 16, 17, and one will be located between each pair of adjacent arcuate nozzle casing portions.

In assembly, the dams will be secured in channel ring 25, and the legs of ring 25 will then be secured by circumferential welds to the opposite edges of inner band 20, after which the blades 22 will be disposed radially through the respective openings in the outer band 21 and the inner band 20 and secured to the outer band by welding at 23. The fabricated nozzle ring assembly is then secured to the nozzle casing 12 by a circumferential weld at 27. The outer band 21 is secured to the casing 12 by a separate ring 28, which may be secured to the outer band by the weld 23, and sealed to the casing by another circumferential weld as shown at 28a in Fig. 5. It is to be noted that the comparatively thin connecting ring 28 introduces some flexibility so that outer band 21 can expand relative to the cooled casing 12.

The inlet casing 12 is secured to the main frame casing 5 by a plurality of threaded fastenings 12a. Interposed between is a shim 12b, the thickness of which is selected in accordance with the clearance desired between the nozzle ring 11 and the adjacent bucket-wheel 3. This shim provides a very convenient means for adjusting the nozzle-to-bucket clearance. Casing 12 is maintained concentric with casing 5 by a circumferential shoulder shown at 12c.

It will be apparent that, after the exhaust pipes have been disconnected from pad 18, the entire nozzle casing may readily be removed by disassembling the threaded fastenings 12a. When this is done, the fabricated nozzle ring 11 is completely exposed for inspection, cleaning, and other servicing, while the segmental shroud 6 and the bucket-wheel 3 are exposed in the casing 5 for ready inspection. This is a great convenience to the maintenance mechanic, since the condition of the bucket-wheel, the clearance between the bucket tips and stationary shroud segments, and the condition of the nozzle blades, are factors which must be very carefully watched. Because the inlet casing and main frame members are both watercooled, they may be expected to have a long trouble-free life, with no difficulties due to diflerential thermal ex- Since the cooling system limits the temperature rise of these castings, they will have good dimensional stability, with the result that nozzle ring 11 will be maintained accurately concentric with bucketwheel 3 and the segmental shroud 6. Because each nozzle blade 22 is completely free to expand and contract lengthwise. the stresses therein will be kept to a minimum.

The invention is of course susceptible to numerous modifications, one of which is shown in Fig. 6. Here the nozzle ring assembly 11' comprises a comparatively thick band member 20 having airfoil shaped holes punched therethrough as described above in connection with Figs. 1 and 5. Because of the thickness of band 20', the thin exit edges of blades 22 are cut away as shown at 20a, thus avoiding the necessity for punching an extremely narrow opening in the comparatively thick band 20. Since the problem is not the same in connec- 4 tion with the thin outer band 21, the hole therein may be punched to fit the full cross-section outline of the blade. Leakage of motive fluid through the clearance spaces 24 defined between the inner ends of the blades 22 and the thick band 20' is prevented by an inner band 25a secured to the band 20 by circumferential welds 25b. With this arrangement it willbe seen that each of the holes punched in the thick band'20' defines a recess 20b into which each blade 22 freely projects. Since this' recess 20b is substantially closed, except for the clearance spaces 24, the problem of cross-flow of motive fluid between nozzle segments' is avoided. To put it another way, the portions of the thick band 20' between the blades provide a dam between each pair of adjacent blades.

It will be seen that the invention provides a high temperature turbine nozzle ring and easing structure which is comparatively simple to fabricate and disassemble for servicing, yet may be expected to have long life and the high degree of reliability required of a turbosupercharged power plant used for locomotive or marine service.

It will be apparent to those acquainted with turbine design that many minor alterations and substitutions of mechanical equivalents may be made. Specifically, while in the present case, the engine 2 has been described as having only two groups of cylinders connected to separate manifolds, 2b, 2c, in larger engines there may be four or more separate manifolds communicating with separate inlets in the nozzle casing. Each inlet will of course communicate with a separate nozzle arc, and there will be baffle members like those shown at 26 in Fig. 4 between each pair of adjacent nozzle arcs. It is of course intended to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire Patent of the United States is:

In a high temperature turbine having a bucket wheel receiving motive fluid from at least two separate nozzle inlet passages supplied with motive fluid at varying pressures, the combination of a fabricated nozzle ring assembly disposed coaxially with and adjacent the bucket wheel and including at least two adjacent nozzle arc portions communicating with said respective nozzle inlet passages and comprising radially spaced concentric annular band members each having a plurality of circumferentially spaced radially aligned elongated openings, a plurality of circumferentially spaced radially extending nozzle blades each having end portions disposed through said aligned elongated openings in the inner and outer band respectively, the outer end of each blade being'secured to secure by Letters to the outer band while the inner blade end portion pro-' jects freely through the elongated opening in the inner band member and defines a small clearance space therewith whereby each blade is free to expand radially, a ring member of channel shaped cross section having opposite side walls secured to the inner circumference of the inner band and a bottom wall radially spaced from said inner band member to define therewith an annular chamber intowhich the free inner ends of the respective blades project, and a plurality of circumferentially spaced dam members located across said annular chamber between said adjacent nozzle arc portions and secured to at least one Wall of said ring member with edge portions of the dam members sealingly engaging the inner band member and the walls of said ring member between pairs of adjacent blade ends to prevent cross-flow of motive fluid leaking through said clearance spaces from one portion of the nozzle ring to another by reason-of the pressure differentials existing between the separate nozzle inlet passages.

References Cited in the file of this patent UNITED STATES PATENTS V Losel Dec. 21, 1926

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