US3588278A - Blade structure for an axial flow elastic fluid utilizing machine - Google Patents

Abstract

THE INVENTION PROVIDES AN IMPROVED BLADED ROTOR STRUCTURE FOR AN AXIAL FLOW TURBINE, OR THE OPERABLE AT CONSTANT OR VARIABLE SPEED WITHIN A SPEED RANGE EXTENDING TO MAXIMUM RATED SPEED, WHEREIN THE VIBRATORY STRESS AT RESONANT SPEEDS IS SUBSTANTIALLY REDUCED BY CONNECTING THE BLADES TO EACH OTHER, AS BY SHROUD OR LASHING STRUCTURE IN GROUPS EQUAL IN NUMBER TO THE RESONANT HARMONIC FREQUENCY OF THE BLADES DIVIDED BY THE RESONANT SPEED.

Description

United States Patent Inventors Ralph L Ortolano Saratoga; William P. Welch, Sunnyvale; Harold W. Sennr, Los Altos, Calif.

Appl. No. 809,242

Filed Mar. 21, 1969 Patented June 28, 1971 Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

BLADE STRUCTURE FOR AN AXIAL FLOW ELASTIC FLUID UTILIZING MACHINE 4 Claims, 7 Drawing Figs.

U.S. Cl 416/190, 4l 6/l9l,4l6/195 Int.Cl. F01d 5/10, FOld 5/22 Field of Search 416/190,

[56] References Cited UNlTED STATES PATENTS 1,366,667 1/1921 Junggren 416/191 1,378,464 5/1921 Junggren 416/190 1,639,247 8/1927 Zoelly et a1. 416/190 2,454,115 11/1948 Allen 416/189(X) 2,970,808 2/1961 Coppa 416/190 3,048,365 8/1962 Foster et a1. 416/190 3,306,577 2/1967 Sagara 416/191 Primary Examiner-Everette A. Powell, .1 r. Attorneys-A. T. Stratton, F. P. Lyle and F. Cristiano, Jr.

ABSTRACT: The invention provides an improved bladed rotor structure for an axial flow turbine, or the like, operable at constant or variable speed within a speed range extending to maximum rated speed, wherein the vibratory stress at resonant speeds is substantially reduced by connecting the blades to each other, as by shroud or lashing structure in groups equal in number to the resonant harmonic frequency of tee blades divided by the resonant speed.

PATENTED JUH28 197i SHEET 1 OF 2 FIGS.

woDt iq RELATIVE AMPLITUDE PATENTEUJUNZBIQH 3588278 sum 2 0r 2 TQIZI sm FIG.6.

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INVENTORS y Rolph J. OrtolonqWilliom Ff Welch and Harold W. Semor BLADE STRUCTURE FOR AN AXIAL FLOW ELASTIC FLUID UTILIZING MACHINE BACKGROUND OF THE INVENTION Because of thermal differences between a turbine rotor and the shroud rings for the rotor blades, it has been the practice of turbine manufacturers for many years to divide the shroud rings into a plurality of arcuate ring segments, thereby to provide expansion gaps between segments. The number of expansion gaps required is thus a function of the degree of thermal difference expected and the arcuate length of each shroud segment. In high temperature blading more gaps are necessary because the blades are short and rigid, and quite high stresses can be developed by small distortions. However, on long low temperature bladin'g, fewer gaps are necessary due to greater blade flexibility and less thermal distortion. Thus; linear expansion of the shroud ring segments is permitted to occur with minimal imposition of stress on the shroud ring, blades and/or the rotor.

However, with thermal expansion as the only criterion, vibration at one or more of the blade frequency harmonics has become a problem and, to overcome this vibration problem, the blades have been tuned by laborious methods to minimize vibratory stress. At least one such method involves filing or otherwise removing metal from the blades to increase their vibrational frequency to a value higher than encountered within the speed range of the rotor.

SUMMARY OF THE INVENTION In accordance with the teachings of this invention, the vibratory stresses on rotor blades for turbines or the like may be substantially minimized in the tangential inphase mode by connecting the blades to each other in groups by lashing structure or shrouding structure divided into arcuate segments of substantially equal arcuate extent and equal in number to the lowest harmonic frequency of the blades that it is desirable to suppress. Stated another way, the blades are connected into groups by shroud ring structure or lashing structure equal in length to the length of one cycle of tangential vibration of the harmonic frequency that is to be suppressed.

Thus, in general, rotor blades operating at the fourth harmonic frequency would be provided with four 90 shroud ring segments or four 90 lashing structures, while rotor blades operating at the sixth harmonic frequency would be provided with six 60 shroud ring segments or lashing structures, etc.

Two of the most common causes of rotating blades vibration are:

l. Excitation induced as they sweep past the Wakes of the stationary nozzle vanes.

2. Excitation induced as they sweep past a local nonuniformity in an adjacent stationary component such as nonuniform nozzle vane spacing, or change in clearances due to thermal growth or manufacturing distortion.

The first type of excitation of a high frequency excitation and therefore resonance questions are primarily considered on the small or initial blades of the turbine which typically have a high vibrating frequency.

The second type of excitation is most severe when the blades have a vibrating frequency that is a low multiple of the rotor speed in cycles per second. This is the type of blade vibration with which the invention is concerned.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary view ofa turbine rotor having blades provided with shroud segments in accordance with the inven tion;

FIG. 2 is a radial sectional view, on a larger scale, taken along line ll-Il of FIG. 1;

FIG. 3 is a diagrammatic view of an unshrouded bladed rotor structure, showing the blades in fourth harmonic vibration;

FIG. 4 is a diagram showing the fourth harmonic frequency occurrence in the rotor of FIG. 3;

FIG. 5 is a chart showing relative amplitude of vibration of the blades plotted against a ratio of harmonic frequency of rotor running speed to number of shroud segments in a blade row;

FIG. 6 is a view similar to FIG. 1, but showing a lashed rotor blade structure; and

FIG. 7 is a radial sectional view, on a larger scale, taken along line VII-VII of FIG. 6.

Referring to the drawings in detail, in FIG. 1 there is shown a portion of a turbine rotor 10 comprising a rotor spindle 11 having an array of radially extending blades 12 supported therein and connected to each other in arcuate groups by arcuate shroud members or segments 13. Although the entire rotor 10 is not shown, it will be understood that the spindle 11 is of circular cross section and the blades 12 are arranged in an annular circumferential array about the rim 14 of the spindle.

The blades 12 may be connected to the rotor rim 14 in any desirable manner. However, as best illustrated in FIG. 2, the blades are formed with bulbous l-shaped roots 15 received in a mating peripheral groove 16 formed in the spindle rim 14.

The blades are provided with air foil vane portions 17 extending radially outwardly from the roots l5 and are further provided with tenons 18 extending through uniformly spaced holes 19 and secured to the shroud segments 13 by deformation of the tenons as by riveting.

As thus far described, the structure is substantially conventional, and, in operation as steam or other motive fluid is directed past the turbine rotor blades 12 by the stationary nozzle vanes (not shown) the rotor 10 is rotated, as well known in the art. However, during rotation, the blades are susceptible to serious and damaging vibration at frequencies which are harmonies of a resonant running speed of the turbine rotor. Two of the most common causes ofblade vibration excitation are:

l. Excitation induced as the blades sweep past the wakes induced by the stationary steam directing nozzles.

2. Excitation induced as the blades pass a local variation in any stationary part associated therewith, such as, for example, nonuniformly spaced nozzles.

The first type of excitation is a high frequency excitation and therefore resonant frequencies occurring as high multiples of rotor speed are primarily considered in short blades, typically formed in the early stages of expansion in the turbine.

The second type of excitation is a lower frequency excita' tion and therefore resonant frequencies occurring as low multiples of rotor speed are primarily considered in the longer blades typically found in the later stages of expansion in the turbine. The invention is concerned with this type ofvibration.

Both causes of excitation above provide force variations on the rotor blades which are continuously repeated during rotation and when the blade frequency is resonant with the frequency of the force variation, the vibration amplitude of the blades will increase to dangerous levels. When blade resonance occurs, the blade frequency will be a multiple of the force variation frequency. This multiple is called the harmonic frequency and is calculated as follows:

F F1X60 EL H "RPM RPS where:

F the harmonic frequency in cycles per second F the blade fundamental frequency in cycles per second R.P.M. turbine rotor speed in revolutions per minute RPS= turbine rotor speed in revolutions per second.

At any instant, the pattern of deflection of the row of blades 12, without the shroud segments 13, will be a function of some multiple of turbine rotor speed. For example, if the blades have a fundamental vibration frequency of 400 c.p.s. and the rotor is rotating at 6000 rpm. or I00 r.p.s., by substitution in the above formula,

F =400/l00 =4; it will be seen that as illustrated in FIG. 4, the blades are operating in the fourth harmonic F ofthcir fun damental frequency F,i.e., 400 e.p.s. They will also be vibrating at a frequency equal to four times the rotational speed of the rotor.

The above phenomenon manifests itself on the unshrouded blades 12 as illustrated in FIG. 3, as four complete cycles of vibration, each extending across the blades disposed in the arcuate group extending across a central angle of 90. At any in stant, one halfof the blades in each group are deflecting in one tangential direction and the other half are deflecting in the opposite tangential direction. That is, one half of the blades are going through half of the fourth harmonic vibration cycle (-HQF, while the other half of the blades are going through the other halfof the fourth vibration cycle /2F Therefore, if the blades are isolated into four separate arcuate systems or groups, the excitation forces supplied to the blades in each group, above, have a cancelling effect.

in accordance with the above, the blades 12 are formed into such groups by connecting them to each other by shroud segments 13 extending peripherally to the same extent. Thus, the tangential deflecting forces on each of the blades are trans mitted to the other blades through the shroud segments with a cancelling or vibration dampening effect. To dampen the fourth harmonic F four shroud segments of 90 arcuate extent are employed.

If the blades have a fundamental vibration frequency such that at a running speed of the rotor they are operable in another harmonic, for example the sixth harmonic, then there will be six vibration waves at any one instant occurring in the blades, each vibration extending across the blades included in 1/6 of 360 or 60 and six 60 shroud segments would be employed to dampen such blade vibrations.

in FIG. 5 there .is shown a chart plotting relative vibrational amplitude in thousandths of an inch against F /N where F,,= Harmonic frequency of rotor running speed N Number of shroud ring segments.

In the example given, if: F =4, and N,:4, F,,/N ,=l, and all of the blade vibrations are suppressed, as indicated on the chart.

If the harmonic frequency of rotor running speed is increased, for example, so that F,,=6, then F IN I .5 and the blades will vibrate at an amplitude of about 2.3 thousandths of an inch, considerably lower than the unsuppressed amplitude of l0 thousandths of an inch. Therefore, any plurality of shroud ring segments, in accordance with the invention, equal to, or lower than, the expected harmonic frequency of rotor running speed will result in a ratio of F /N' greater than one, so that the blades are never operable in the high amplitude range (below unity).

Although in the first embodiment described above, the blades 12 are connected into groups by shroud ring segments 13, they may be connected into similar groups by other connecting means. FIGS. 6 and 7 show another embodiment of the invention wherein a rotor structure 50 is provided with an annular row of unshrouded blades 52 that are connected to each other in arcuate groups by suitable lashing structure 54 and 55. The lashing structure 54 and 55 connects the blades to each other at two points along their radiallength in the embodiment shown. However, the number of lashing structures may be modified as desired. For example only one lashing structure may be employed, or alternately, more than two, depending on the length and size of the blades, for example. The arcuate lengthof the lashing structure 54 and/or 55 is selected in the same manner as the arcuate shroud ring segments to suppress the same resonant harmonic frequencies of the blades. That is, for the fourth harmonic, four lashing structures of angular extend are employed, etc.

The lashing structure 54 and 55 may be of any type, as well known in the prior art, and in a similar manner, the shroud ring structure 13 may be of any type, as well known in the art.

We claim:

1. A rotor structure for an axial flow elastic fluid utilizing machine, comprising a rotor spindle,

an annular row of radially extending blades carried by said rotorspindle, and means connecting said blades to each other in arcuate groups of substantially equal central angular extent in degrees,

said blades being susceptible to vibrate in a tangential inphase mode having a resonant frequency at least twice that of the rated maximum running speed of said rotor, and

said connecting means dividing the blade groups into a number equal to said resonant frequency divided by said rotor running speed.

2. The structure recited in claim 1, wherein the connecting means comprises arcuate shroud ring structure attached to the radially outermost tips of the blades.

3. The structure recited in claim 1, wherein the connecting means comprises lashing structure connecting the blades to each other intermediate their inner and outer ends. 7

4. The structure recited in claim 1, wherein the machine is a steam turbine and is operable at variable speed up to the rated maximum running speed.

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