Accordingly, it is a general objective of the present invention to provide a rotor crack detection method which is useful for detecting rotor cracks while the turbine is on line operating under normal load and early enough to permit the rotor to be repaired without extensive down time.
Fracture of Polymers, Composites and Adhesives II, Volume 32
Further objectives and advantages will be apparent from the ensuing description of the invention, its principles, and operation. According to the invention, to detect incipient cracks in the rotor of a fluid powered turbine, very small cracks are made to be active and to manifest themselves as larger cracks which are normally open during parts of each revolution of the rotor during a period following a transient perturbation of the rotor. In essence, a perturbation is used to produce a new and different vibration response mode. In a preferred form of the invention, signature analysis of the normal background vibration pattern is first obtained to establish the spectral content of the normal vibration signal.
The turbine rotor is then transitorily perturbed, preferably by changing the temperature of the motive fluid steam temperature is changed for example in a steam driven turbine , and the signature analysis is again performed to determine changes in the vibration pattern. In particular, the increase in amplitude of the fundamental frequency and the appearance and increase in amplitude of higher harmonics following rotor perturbation is indicative of the presence of a crack.
While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter regarded as the invention, the invention will be better understood from the following description taken in connection with the accompanying drawing in which:. The presence of a crack in the rotor of a fluid powered turbine e.
In addition, a crack introduces stiffness asymmetry in the vertical and horizontal directions. These effects produce vibration patterns in the turbine rotor which are different from the vibration patterns produced by a normally operating turbine having an uncracked rotor. In FIG. It will be recognized, for example, that the rotor 10 is encased within one or more outer shells and further includes a plurality of radially extending buckets or blades assembled in axially spaced rings which, with associated stationary nozzle rings, form the different turbine stages.
Rotor 10, as illustrated, is comprised of two tandemly coupled rotor wheels, 12 and 14, which carry the turbine buckets as described above. The two wheels 12 and 14 are fastened together through coupling 16 and rotate as a unit while supported by journal bearings 18, 20, 22, and 24 and shaft For explanation purposes, an illustrative crack 25 is included on the surface of rotor wheel Vibrations in the turbine rotor 10 are detected by vibration sensors , proximately located in pairs near each journal bearing 18, 20, 22, and The vibration sensors , electrically connected to signal conditioner unit 36, provide electronic signals corresponding to the mechanical vibrations of rotor 10 and may, for example, be displacement or accelerometer type devices such as are well known in the art.
The signal conditioning unit 36 provides excitation to the vibration sensors and receives the various vibration signals from these devices while providing amplification and filtration of the received signals as necessary. The use of multiple sensors as illustrated insures a sensitive response to small cracks and aids in determining their location.
The conditioned vibration signals are presented to a microprocessor-based signal analyzer 38 which processes the signals to obtain an analysis of the spectral content of each signal. The technique of spectrum analysis is well known in the signal processing field and is also often referred to as "signature analysis" or "Fourier analysis. The signal analyzer 38 is capable of handling each vibration signal separately, although with high-speed signal processing techniques currently available, each signal is analyzed essentially concurrently and on a virtual real-time basis.
The signature analysis results are displayed on a cathode ray tube CRT readout 40 and may be provided in permanent, hard-copy format by graphic recorder A crack alarm 44 which may be an audio-visual device is provided to announce the existence of a crack in the rotor 10 should the signature analysis process indicate that one has occurred. Thus, while the foregoing method is demonstrably useful for detecting cracks it is, as mentioned above, highly desirable that any incipient crack be detected at the earliest possible moment.
In many cases, the contribution from various harmonics to the vibration signal is relatively small at normal speed in a steady state condition and the unbalance effects are predominant so that any existing crack is either kept closed or is only opened a relatively small amount. Therefore, it may be difficult to detect the presence of a crack at an early stage when the turbine is running at normal speed under load.
Under the present invention, to promote a detectable vibration response at the earliest possible point in the creation of the crack, a perturbation is imposed on the crack to cause the incipient crack to appear momentarily larger than it actually is and to force the crack to behave in such a manner that an entirely different vibration response is imposed upon the turbine rotor. Preferably, in the case of actual or suspected surface cracks, the perturbation is imposed by lowering the temperature of the motive fluid, e.
In the case of a steam turbine, steam temperature is readily lowered via boiler superheat controls or by decreasing the power output of the boiler. A temperature drop introduces transient thermal stresses in the rotor. A crack, if present in the region of the rotor which is exposed to steam, for example, causes an unbalance in the resultant axial forces over the cross section containing the crack. In addition, transient tensile stresses extending inward from the surface of the rotor open the crack, which was previously closed.
As a consequence of this, for a short time two distinct but important changes take place in the rotor. This permits detection of relatively shallow cracks since they are more likely to remain closed than deeper cracks but are nevertheless opened by the imposed temperature gradient.
More specifically, the preferred technique is to a apply and maintain small temperature reductions to force open cracks that otherwise remain closed, then, b detect that the cracks exist via an analysis of the changes in the vibration signature, i. Finally, c the approximate size of the crack can be determined by relating the amplitude of the transient vibration response of the rotor to the transient temperatures in the rotor.
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In maintaining the small temperature reduction, it is only necessary to do so until the maximum vibratory response is obtained. It is notable that this technique discriminates against other temperature induced vibratory responses so that the existence of a crack can be confidently called. The advantage of using a small temperature drop is that the rotor is not subjected to high stresses that can cause a rapid growth of the crack which would appreciably shorten the subsequent life of the rotor. For example, the smaller the temperature drop, the lower the stresses, and the lower the rate of crack growth, until a threshold is reached below which no crack growth takes place.
An additional advantage to using a small temperature drop is that the increase in vibration caused by opening the crack and stressing the rotor is small, so that damage to other components of the turbine does not occur. The signal analysis, confirming that a crack causes the increase in vibration, is provided by decomposing at least one of the vibration signals into the turbine running speed fundamental and harmonics thereof preferably up to about ten times the running speed. The characteristics of this signal analysis readily identifies whether a rotor crack is present.
The essential characteristics are due to the harmonics contained in the rotor vibration produced by the crack as described previously. If the turbine rotor bearing system normally contains harmonics from causes unrelated to a crack, an additional step is to determine the rotor's particular vibration signature prior to lowering the temperature. This gives the existent, normal, or background harmonic content. The difference between the background signature and the signature taken after the temperature drop removes the existent harmonics and provides an indication that a crack is or is not present.
As the perturbation is removed and the turbine returned to steady state operation, the vibration response returns to that of FIG. The foregoing procedure wherein the turbine rotor is perturbed by transiently lowering the temperature is effective for the detection of outer surface cracks. In many turbines the rotor will include a bore such as bore 52 of FIG. It is generally very difficult to detect bore cracks using vibration signature analysis since the majority of bore cracks are axial in orientation and have very little tendency to open and close during the rotation period.
Thus the stiffness of the rotor remains uneffected with no significant effect on the vibration pattern. However, according to the present invention the rotor may be perturbed by imposing a momentary increase in the temperature of the motive fluid in a contact with the rotor outer surface to effect a corresponding change in the vibration pattern. By increasing the temperature, the surface expands, but the bore remains uneffected since it is not immediately responsive to changes in surface temperature, not being in direct contact with the motive fluid.
The expansion of the surface pulls the cracked region apart thus opening the crack. An asymmetric stiffness of the rotor results which then produces a vibration signature indicative that a crack is present in the bore. In another form of the invention, applicable to the detection of both outer surface cracks and bore cracks, the turbine rotor is perturbated by the application of a mechanical force applied externally to the turbine whose rotor is being tested.
This is most conveniently carried out by using a mechanical shaker operating at different frequencies and applying the vibratory force to the bearing pedestals. For example, in FIG. The shaker, momentarily applies a vibratory force to the machine, and thereby creates a transient response in the turbine while it is running under load.
The signature analysis in the transient phase contains the contributions of the various harmonics indicative of a crack in the manner described above wherein the rotor is perturbed by a temperature transient.
USA - On-line rotor crack detection - Google Patents
With further reference to FIG. This additional information is provided by acoustic emission sensors 46 and 48 located in proximity to, respectively, turbine rotor wheels 12 and 14; and by signal conditioner 50 which provides excitation, filtering, and amplification for the acoustic emission signals. Although shown near rotor wheels 12 and 14 for illustration purposes, it is preferable to monitor the entire rotor that the acoustic emission sensors be located immediately adjacent the rotor bearings.
This has the advantage of not subjecting the sensors to the hostile environment within the turbine.
Acoustic emissions are simply pressure wave emissions produced by a growing crack in any portion of rotor 10 and, in conjunction with the vibration signals analyzed as described above, confirm that a crack exists and is increasing in size. The acoustic emission signals are processed by signal analyzer 38 and may be displayed on recorder 42 and CRT display 40 concurrently with the vibration information or as an alternative thereto. While the foregoing has described certain preferred forms of the invention, it will become apparent to those of ordinary skill in the art that variations may be made in the invention without departing from its true spirit and scope.
It is intended to claim all such variations by the appended claims which follow. Effective date : Year of fee payment : 4. Year of fee payment : 8. Composite Energy Storage Flywheel Design for Fatigue Crack Resistance Abstract: Composite flywheels can be a high density energy storage device because of the very high specific strength and strength per unit weight. The rotors are fiber reinforced in the circumferential direction to resist centripetal loads resulting from high speed rotation. A press-fit process is also used to induce pre-compression in the radial direction that improves mechanical strength by preventing radial separation of rotors.
Modified Disk-Shaped Compact Tension Test for Measuring Concrete Fracture Properties | SpringerLink
This design and fabrication process however leaves the radial and axial directions of rotor vulnerable to propagation of fatigue crack growth in the rotor. A semi-empirical approach is proposed to enhance the "crack growth" resistance of the rotor.
- Open Physics.
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Axial glass plies are incorporated to confine the crack growth through the radius and along the circumferential direction of rotor. The fracture properties of specific hybrid laminates are then measured to provide information required for an optimal rotor design. Published in: 14th Symposium on Electromagnetic Launch Technology. Article :.