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The term "GLITCH" is Bently Nevada Corporation's nomenclature for describing all forms of vibration measurement error from an observed shaft surface. This does not include other possible noise and signal error sources, such as electrical line noise, monitor problems, or problems associated with casing mounted transducers. Casing mounted transducers, such as velocity coils or accelerometers are also subject to signal error sources in the form of cross axis vibrations, improper mountings, line noise as well as several others particular to each type of transducer; however, glitch deals with observed shaft surface anomalies only. In order to clarify the sources of these vibration measurement errors, sometimes referred to as runout, the following discussion shall focus on the two categories of "GLITCH", mechanical and electrical. |
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Sources of Glitch Mechanically Induced Runout: Nonconcentric Surfaces/BowsA shaft surface which has been improperly machined (egg shaped or non concentric) will yield a sinusoidal displacement signal with a frequency coincident with the rotational speed of the rotating element. A second condition on a rotating element, which will yield the same apparent dynamic motion condition, manifests when the rotating element has been physically bent or bowed. Sources of improperly machined surfaces can usually be traced to a worn or defective set of bearings on the machine used for final machining or grinding or a worn-out set of lathe centers on a lathe. Bows in a rotating element are typically introduced due to improper handling of the rotor during its manufacturing cycle. This may be the result of a sudden or jarring load applied or due to long-term storage of the element with improper supports for the rotor. In the case of the latter, an improperly supported rotor may introduce a permanent sag or bow due to gravitational forces. Surface Irregularities or Imperfections The presence of surface imperfections or irregularities will yield a runout condition as observed by the proximity transducer. The surface imperfections discussed herein are in the form of scratches, dents, burrs, etc. In general, surface irregularities are created due to improper handling of the rotor during the manufacturing cycle. Care should be taken to protect the shaft surface to be used for dynamic motion measurements. In essence, these surface areas should be given the same protective measures used to protect a bearing journal surface. Crane lifts should be made with cables attached to shaft areas away from the probe measurement surfaces. Support fixtures for storage of rotors should not introduce surface scratches, dents, etc. Occasionally, surface irregularities are introduced via a machine-cutting tool. If the tool is dull or the feed is too rapid, some tool chatter may occur which can introduce small ripples in the shaft surface. Electrically Induced Runout: Residual MagnetismIn general, proximity transducers will operate satisfactorily in the presence of magnetic field, as long as the field is uniform or symmetrical and not localized to a particular location on the rotor. If a particular area or zone of the shaft surface is highly magnetic, and the remaining surface is nonmagnetic or at a much lower value, an electrical runout condition will manifest. This is due to the resultant change in sensitivity on the shaft surface to the applied field from the proximity transducers. Residual magnetism runout problems are seldom encountered. However, various physical inspection techniques employed during the manufacturing cycle, such as the use of magnetic chucks, can introduce residual magnetism problems. The most common inspection technique, where residual magnetism may be a byproduct, is a Magnetic Particle inspection (Magnaflux®) to check for cracks on castings, or weldments, or after other manufacturing cycles. The magnetic field introduced to the rotor for this inspection should be neutralized after the inspection program is completed. This is done with the Magnaflux machine and involves continuously reversing the polarity and passing a current through the rotor at continuously decreasing amperes. If done properly, this procedure should neutralize the magnetic properties of the rotor. In some cases a proper polarity reversal is not performed, and residual magnetism is produced. Precipitation Hardening 17-4 pH steel nearly always presents an electrical runout problem. Some form of material replacement (shrink a collar, overspray a material) is normally required to eliminate glitch. Other pH steels, such as 15-5 pH, seems less prone to glitch, but any pH steel may cause difficulties. Metallurgical Segregation The scale factor yielded from a proximity transducer is dependent upon several variables. One variable involves the specific metal or metallurgy it is to observe. Typically, steel alloys for shaft materials contain a variety of alloying agents. In general, the final metallurgical composition of these alloys is a homogenous mixture. On some rotating elements microscopic segregation of the steel alloys may occur. Since the proximity transducer responds with different voltage outputs depending upon specific metals, the lack of a homogenous metallurgical composition around the circumference of a shaft may give rise to varying electrical outputs. Residual Stress Concentrations During the normal manufacturing cycle of rotors, a variety of machining and surface treatment processes can introduce small amounts of localized stress concentrations. Although these stress areas do not adversely affect the mechanical properties of the rotor, they may give rise to an electrical runout from the proximity transducers. Since one of the variables affecting voltage output from the transducer is the resistance of the observed shaft surface, any deviation of the resistance around the circumference of the shaft (due to varying stress levels) will produce a voltage change. Measurement of Glitch The observation of mechanical runout at a rotor speed below which dynamic vibration is eliminated (typically less than 10% of the rotor operating speed) on an oscilloscope will yield a sinusoidal dynamic waveform for non-concentric surfaces or shaft bows. Surface irregularities or imperfections will appear on an oscilloscope as sharp voltage spikes superimposed on the dynamic waveform. Nonconcentric surfaces or shaft bows may also be measured by a dial indicator mounted in the probe area of the shaft, which in turn, is mounted at the bearing journals, in vee blocks, or roller bearings. The circumference of the probe area is marked off in 36 positions (10 degree intervals) with zero in line axially with the thrust collar keyway. Dial gauge readings are recorded at each indicated position. Two sets of readings are taken, approximately half an inch apart axially, one on each side of the probe position centerline. The two sets of readings are averaged to give a record of mechanical runout. American Petroleum Institute (API) Standard 670 recommends that "the combined total electrical and mechanical runout does not exceed 25 percent of the maximum allowed peak to peak vibration amplitude or 0.25 mil (6 micrometers), whichever is greater." The shaft surface finish should be from 16 to 32 micro inches (0.4 to 0.8 micrometers) root mean square, also per API 670. The observation of a residual magnetism runout condition on an oscilloscope can yield a sinusoidal motion indication. However, the sine wave will be distorted and to some extent tend towards a square wave. A final check for residual magnetism embraces the use of a small handheld field strength indicator manufactured by Magnaflux Corp. Holding this meter at the shaft surface and hand turning the rotor will confirm the presence or absence of magnetic fields of less than 2 gauss with variations less than 1 gauss. An oscilloscope observation of metallurgical segregation will typically indicate a somewhat sinusoidal waveform with high voltage, high frequency spikes superimposed on the waveform. Observation of residual stress concentrations on an oscilloscope will yield a sinusoidal waveform with high voltage, high frequency spikes superimposed on the waveform. It should be noted that the oscilloscope waveform in all the above cases may also be very irregular, depending on the number of other shaft surface anomalies. If desired, the electrical runout can be determined by subtraction of mechanical runout from the total runout. Glitch Reduction Various methods of reducing glitch are available and have been successfully used by firms. It is not possible to define which method is best because each can achieve the desired result. However, it is possible to narrow the choice of methods when they are considered on a cost and time basis. It is also notable that proper material selection, heat treating, and allowing control can have a large effect on the runout condition of a rotor. If rotors are to be replaced or rebuilt, it is far more cost-effective to detect and correct glitch at the earliest stages of machine assembly.
Protection of Probe Areas The removal of glitch by one or a combination of the treatments discussed above is essential for customer acceptance of levels of recorded vibration. Having treated the probe areas, it is essential to protect these areas to prevent corrosion damage, scratching and indiscriminate "cleaning up". During production and testing, this protection is afforded by the care taken by skilled craftsmen who appreciate the sensitive nature of this surface. Between acceptance testing and startup much less care can be exercised. There have been many field reports of probe areas being damaged by rust, scratches or dents. Cleaning up the damage by stoning or polishing may give the desired appearance, but the original glitch recordings would have been completely altered and undoubtedly taken beyond acceptable limits. Glitch removal then has to be carried out in the field by selective micropeening, and considering the adverse conditions, the results are invariably inferior to those which can be achieved in the shop. It is therefore recommended that after glitch treatment and recordings are taken in the shop, the probe areas be given a coating of non-metallic epoxy resin, which can remains, for the life of the machine. This coating will not affect probe readings but will protect the probe area from corrosion and all minor mechanical damage. Compensation Obviously, shaft treatment to remove the source of runout is the most desirable procedure. If this is done, there is no reason to have to "account for'' runout in subsequent vibration signals. However, the occasional shaft material or forging may not respond well to the standard shaft treatment methods. The shaft may have a "permanent" bow, or it may be impossible or impractical to treat the shaft surface before a time when vibration data is required on a given machine. If it is impractical to treat the shaft surface or remove the shaft bow, an electronic method may be used. The following is an explanation and discussion of this application. Vector Nulling - Digital Vector Filter Also called slow roll compensation; this system is an integral part of the Digital Vector Filter. It provides a means for nulling a slow roll vector. It should be noted that the nulling operation is a true vector subtraction (phase and amplitude) and not merely a voltage suppression circuit. The nulling circuit operates on the filtered vibration waveform (the vector information in the DVF). Since the filter employed in the DVF-3 is tuned to the rotational (1X rpm) frequency of the rotor, the nulling circuit eliminates that portion of shaft runout, which is coincident with the rotational frequency. Typically, it is used to eliminate a 1X component of runout such as a bow in the shaft or a nonconcentric (egg-shaped) shaft condition at the probe measurement plane. All higher orders of runout (non 1X components, such as scratches, metallurgical irregularities, etc.) are eliminated through the filter action of the DVF-3. Once the initial slow-roll vector has been nulled, it is automatically subtracted from all future dynamic signals. This system provides the means for properly examining the mechanical response and impedance of a system, definition of the balance resonances (critical speeds), and amplification factors, over the operating speed range. Vector nulling also allows for the compensation of the residual unbalance vector after a balance resonance, and for observation of a higher balance resonance response. It is possible, and even probable on larger machinery, that nominal axial position changes and differential expansion up to running speed will cause a vibration probe to observe a "new'' lateral location on the shaft. When considering the overall runout pattern, this "new'' shaft location may be significantly different than the overall pattern observed with the machine at slow-roll. Vector nulling does not, however, deal with the overall runout pattern; because of the filter in the system, only the 1X runout vector is considered. The once-per-turn runout vector is not likely to change from slow-roll to operating speed and temperature. In this regard, vector nulling offers a distinct advantage over any other type of digital runout compensation. Vector nulling also offers the capability of nulling the residual vector of shaft motion after passing through a resonant speed region to observe the action of the next higher resonance when Bode plots (amplitude vs. rpm and phase vs. rpm) are made. Recommendations The above considerations lead to the following conclusions and recommendations by Bently Nevada:
"New Techniques in overcoming Electrical Runout" by Dale W. Beebe, Turbodyne Corporation, Hydrocarbon Processing, August 1976. Electrical Runout and Eddy Current Displacement Proximity Transducers, by Biggs, David H., ASME Paper, September 1975. (Bently Nevada Literature No. L0360) Elliot-Wiedeke paper API 670, Second edition, Section 4.1.2: Machine Shaft Requirements for Electrical and Mechanical Runout. |
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