HIGH SPEED SUPERSONIC IMPULSE TURBINE

Abstract: The buckets (20) of a turbine wheel (10) are formed as a series of equally spaced, overlapping portions of a cylinder in the grooved rim of a wheel forging. As a result of this construction, the leading edge of the buckets (20) is elimin­ ated and is therefore not subject to 90° erosion. Additionally, the resulting bucket profiles result in very low impingement angles for moisture droplets or particulates which further minimizes low angle erosion.

Description: HIGH SPEED SUPERSONIC IMPULSE TURBINE Background of the Invention The buckets or blades of turbines are subject to wear or erosion due to a number of factors. In a steam turbine prime mover, for example, the kinetic energy that is absorbed from the steam by the moving blades or buckets and delivered as shaft work to the device being driven results from the expansion of the steam into the heat of vaporization region resulting in a lowering in the quality of the steam. As the moisture content rises with the lowering of steam quality, the buckets or blades become more susceptible to erosion. Although wet steam is generally associated with the last stages of a condensing steam turbine, the advent of developing geothermal power, and the use of saturated steam boilers for plant heating, for example, have resulted in the supplying of low quality saturated steam at the turbine inlet. Steam qualities ranging from 20% all the way to 100% are typical. The blade erosion is also a function of the velocity and impingement angle of the moisture particles. Summary of the Invention The present invention is directed to an overhanging turbine of solid wheel construction which is capable of tip speeds on the order of 1000 to 2000 ft./sec., depending upon the material used. An overhung design is used to simplify shaft sealing normally required with a conventional between bearing design. The buckets are machined into the outer diameter of the wheel. The nozzle ring construction is of the tangential inflow type with expanding nozzle inserts and low incidence angles for maximum performance. Because of the bucket geometry, moisture droplets or solid particulates moving slower than the gas flow will impinge upon the buckets at low angles, greatly reducing erosion. The inlet and exhaust casings are simply constructed to enable partial to full admission of motive fluid at very high pressures. Since the turbine wheel has buckets machined directly into it, bucket failures are essentially impossible. The wide rim and profile shape make the disk less prone to vibration. Integral rotor or through bolt construction may be used. With this rugged construction, the present invention is suitable for a wide range of gases, either superheated or saturated. By using a gear unit, any output shaft speed is obtainable at optimum turbine efficiency, and with an electric governor and a centerline location of the governing valve, low casing distortion is possible. It is an object of this invention to provide a solid wheel turbine and the method of making the same. It is an additional object of the invention to provide a turbine wheel having high moisture and particulate erosion resistance, low windage and low thrust capabilities. It is a further object of this invention to provide a turbine having high speed capability to thereby make re-entry unnecessary for normal large heat drop applications. These objects, and others as will become apparent hereinafter, are accomplished by the present invention. Basically, a plurality of uniformly spaced buckets are formed in the rim of a solid wheel. Each of the buckets is nearly tangential with respect to the wheel and has an overlapping relationship with the adjacent buckets in the machining operation such that the wall of each bucket defines an essentially semicircular pocket or bight made up of a portion of the side and base of a cylinder. Motive fluid is supplied in a generally tangential direction, from points axially spaced from the center of the wheel, such that converging flow in the buckets is along one side of the wall defining the bight and the fluid is turned through approximately 150 to 1800 with a transfer of kinetic energy to the wheel before it exits along the other side of the wall defining the bight. Brief Description of the Drawings For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein: Figure 1 is a partially sectioned side view of a solid wheel formed according to the teachings of the present invention; Figure 2 is a sectional view taken along line II - II of Figure 1; Figure 3 is an edge view of the solid wheel of Figure 1; Figure 4 is an enlarged, partial sectional view of the machining operation; Figure 5 is a sectional view of a turbine; Figure 6 is a partially sectioned view of the wheel and nozzle ring; Figure 7 is a sectional view taken along line VII - VII of Figure 6; Figure 8 is a pictorial representation of the flow in a bucket; and Figure 9 is velocity diagram. Description of the Preferred Embodiment In Figures 1-8, the numeral 10 generally designates the solid wheel of a turbine 100. Wheel 10 is initially formed as a blank such as a forging. Prior to the forming of the buckets, the rim of the wheel 10 is preferably profiled by machining an arcuate groove 14 therein, as is best seen in Figure 4. Preferably, the groove 14 forms a segment of an ellipse, since the buckets are wider than they are deep, so as to reduce the amount of material to be removed in a milling operation. Labyrinth seal lands 12 are machined on the downstream face 11 of wheel 10. The buckets 20 are then individually machined into the profiled groove 14 as by an end mill, by transverse milling or cutting. In the illustrated embodiment, the milling tool 30 is of a diameter less than the axial extent of the rim of the wheel 10 and tends to machine out a slot terminating in a semicircular opening or pocket with a flat bottom 22, which is best viewed in Figure 4. Flat bottom 22 forms a portion of the base of a cylinder and wall 24 forms a portion of the side of a cylinder. The milling tool 30 goes from the Figure 1 position to the Figure 4 position for each bucket. However, the axis of the tool 30 is located on a non-diametrical chord preferably such that at point A, which is the top of the rearmost point of the wall 24 of each bucket 20, the wall 24 makes an angle of 100 to 300, with 150 preferred, with a radius of the wheel 10. The exact angle will be a function of the diameter and thickness of the wheel, as well as the number and depth of the buckets which are determined by the design turbine speed and steam pressure. However, as best shown in Figure 4, the location and depth of each bucket cut, taken in isolation, is such that a portion of the tool 30 extends past the rim of wheel 10 at the full depth of the cut. As best shown in Figures 3 and 8 this results in a cut approxi noting the major portion of a circle, when viewed along the axis of the tool 30 at the full cut, and is of a generally crescent shape after the cutting of the adjacent bucket. Each bucket 20 has a local minimum depth or vertical extent at point A which is in the middle of the bight of wall 24. The vertical extent of wall 24 increases from point A to points B which represent the greatest vertical extent and are the locations where the bight of wall 24 meets the outer rim diameter. The arc EAB forms a portion of an ellipse since it is not parallel to bottom 22. The vertical, extent of wall 24 reduces to zero at the ends of the crescent or points C. When formed as described above, each bucket 20 has a considerable thickness of the wheel backing it up so that bucket failure is essentially eliminated. In Figure 5, the numeral 100 generally designates an impulse turbine employing the solid wheel 10 of the present invention. Turbine 100 includes an inlet casing 40 and an exhaust volute casing 44. Inlet casing 40 consists of a short flanged inlet pipe for connection to the steam source on one side and a flanged inlet cone 41 on the other end. Inlet casing 40 serves as the connection between the steam source and the turbine. Exhaust volute casing 44 serves as a collector for the exhaust steam and a housing for other turbine components. Wheel 10 is supported in an overhanging fashion by bearing 46 which supports shaft 48. Nozzle ring 50 is supported by turbine casing 42 radially outward of wheel 10. As best shown in Figures 6 and 7, nozzle ring 50 has a plurality of uniformly spaced, generally tangentially discharging passages 52, each having a nozzle 54 located at the inlet end of the passage. Additionally, nozzle ring 50 supports abradible, replaceable seal 56 which separates the inlet and outlet portions of the buckets 20. A normal running clearance of .030 to .045 inches provides the necessary preferred seal for an impulse stage. As is conventional in impulse turbines, steam is supplied to turbine 100 via inlet casing 40 and is directed by inlet cone 41 and inlet guide vanes (not illustrated) to the nozzle ring 50. The steam then passes through nozzles 54 into passages 52 and tangentially enters the buckets 20. The steam enters each bucket on one side and is turned through 1500 - 1800 by the wall 24 and exits via the other side of the bucket and passes into the exhaust volute casing 44. In passing through the buckets 20 and being turned, the steam imparts kinetic energy to the wheel 10 causing it to rotate together with shaft 48 and any power generating equipment connected thereto (not illustrated). This operation does not significantly differ from that of a conventional impulse turbine. However, the bucket configuration of the present invention provides considerable advantages when used with low quality/wet steam or dirty gas. As described above, the steam impinging upon the walls 24 of the buckets 20 imparts kinetic energy to the wheel 10 causing it to rotate in the same direction which the steam is supplied. At operating speed, the wheel tip speed is about 40 to 60% that of the steam being supplied by the nozzles 54. With wet steam, the velocity of the water droplets is much less than that of the steam as well as that of the wheel 10 so that the wheel overtakes the water droplets. In a conventional bucket configuration, the unloaded upstream leading edge of the buckets would overtake and impinge against the water droplets and be eroded thereby. However, when the buckets 20 are configured in accordance with the teachings of the present invention, the leading edge portion of the conventional bucket does not exist and therefore cannot be eroded by 900 impingement. Referring now to Figure 8, as indicated by arrow 90, steam enters the moving bucket 20 with a relative velocity and flows along the wall 24 and is turned through 1500 - 1800 before exiting from the bucket 20. The water droplets contained in the steam, as indicated by arrow 91, have a negative relative velocity with respect to the bucket 20 and so is overtaken by the rotating wheel 10 such that the water droplets impinge against the flat bottoms 22 of the buckets 20 at a very low angle of incidence which is not conducive to erosion. The droplets impinging upon the bottoms 22 flow over the edge of the bucket and down wall 24 whereupon the droplets are reentrained in the steam, as indicated by the arrow 90, and pass from the bucket 20. Figure 9 is a velocity diagram of the steam (gas) and water droplet flow. In the diagram the wheel velocity, u, the relative gas velocity entering the bucket 20, W1, and the absolute gas velocity leaving the nozzle, C1 form a triangle representing the gas flow. Superimposed upon this triangle is a second velocity diagram wherein the wheel velocity, u, the absolute water droplet velocity, C2, and the relative water droplet velocity impinging upon bucket bottom 22, W2, form a second triangle representing the droplet flow. Although a preferred embodiment of the present invention used in a steam turbine has been described an illustrated, other changes will occur to those skilled in the art. It is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims.

Claims: CLAIMS 1. A method of manufacturing a turbine wheel from a blank having a rim of a predetermined width with a symmetrically located groove formed in the rim comprising the steps of: forming a bucket in the rim of the blank such that the bucket forms a portion of the side and base of a cylinder having a diameter less than the predetermined width of the rim and with at least a portion of the remainder of the side and base of the cylinder extending beyond the blank; and sequentially forming additional, uniformly spaced, overlapping buckets in the same manner.- 2. The method of claim 1 wherein each of the buckets is formed at an angle to a radius of the blank such that the greatest axial extent of the side portion of each cylinder is at the intersection of the side portion and the outer diameter of the blank. 3. The method of claim 1 wherein each of the buckets is formed at an angle to a radius of the blank such that the side portion of each cylinder is of a varying axial extent and the top of the rearmost point of the side portion makes an angle of 100 to 300 with a radius of the blank. 4. A turbine wheel comprising: a solid wheel having a rim of a predetermined width; a plurality of uniformly spaced, overlapping buckets formed in said rim; each of said buckets defining a portion of a cylinder having a diameter less than said predetermined width and including a side portion of varying axial extent that makes an angle of 200 + 100 with a radius of said wheel at the top of the rearmost point of the side portion. 5. In an impulse turbine having an inlet ana an outlet with a flow path therebetween and a nozzle ring and a turbine wheel forming a portion of the flow path, the improvement comprising: said wheel having a rim of a predetermined width with a plurality of uniformly spaced, overlapping buckets formed therein with each of said buckets defining a portion of a cylinder having a diameter less than said predetermined width and including a side portion of varying axial extent that makes an angle of 100 to 300 with a radius of said wheel at the rearmost point of said side portion.

Inventors: ELLIOTT, THOMAS J., MILLER, ARTHUR JAMES
Applicants: ELLIOTT TURBOMACHINERY COMPANY, INC.
Application year: 1983
Main classification: F01D1
IPC codes: B23P15, F01D1