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No user group serving gas turbine owner/operators covers generators, high-voltage (HV) equipment, and I&C to the degree CTOTF™ does. The day-long Gen-EI&C Roundtable conducted at the 38th annual Spring Turbine Users Conference and Trade Show, chaired by Moh Saleh, Engineering Manager at SRP’s Desert Basin Generating Station, offered four presentations with actionable content.
The opening presentation, “7FH2 Generator Noise,” by Vice Chair for Generators Craig Courter, maintenance manager at Guadalupe Power Partners LP, was the perfect segue for the second: “Generator Belly Bands,” by Bill Dollard, Manager of Contracts and Business Development for AGT Services Inc. The HV portion of the program was anchored by a presentation on “The Use of Ultrasound for Arc Flash and Electrical Failure Detection,” by VP Engineering Mark Goodman of UE Systems Inc.
The final formal presentation, “Early Warning of Stator-Vane Cracking in Combustion Turbines,” by David Sinay, Power Industry Market Manager for Mistras Group Inc, was included in the I&C portion of the proceedings, directed by Vice Chair John-Erik Nelson, Principal Mechanical Engineer for Braintree Electric Light Dept’s Potter 2 and Watson Generating Stations.
In his opening remarks, Courter noted that generators, particularly those installed during the “bubble” years, continue to report key-bar rattle events. Left uncorrected, the condition is conducive to deterioration of the generator stator. The “rattle” can be detected with the Harmonic Noise Index (HNI), a test proprietary to GE that analyses acoustic data. It is a useful tool, the vice chair said, for identifying, prior to disassembly, what may be happening inside a generator.
Unit operating data indicated a slight uptick in vibration on the collector-end bearing. Operating temperature was normal and the low-frequency noise was heard only at base load. The sound was directional and there were no visual indications. Testing proceeded this way, Courter said:
• A generator load test verified that noise attributed to high deck vibration occurred only at base load.
• Onsite vibration analysis, based on a three-point test, identified the exciter end as having higher levels than the opposite end of the unit.
• A third-party vibration analysis confirmed the plant’s findings.
• The harmonic content of the acoustic data was analyzed using HNI to determine the extent to which the 2/rev frequency and its harmonics were present in the overall noise level.
The HNI level calculated was higher than that of a normally operating hydrogen-cooled generator serving a 7FA gas turbine. It also was within an HNI range that suggested significant core/key-bar interaction (Figs 1, 2). The average sound pressure level at base load was the highest of all load points examined. Having accurate diagnostics, Courter said, allowed the plant to run until the next planned outage and to plan and obtain competitive bids for repairs with no exploration costs and no surprises.
Dollard began his presentation by explaining exactly what belly bands—a/k/a core compression bands or belts—are and why they may be needed (Fig 3). Simply put, their function is to control radial vibration between the core and stator frame (key bars, building bolts, etc). As the field rotates, he said, it applies a force to the core that makes it slightly egg-shaped (Fig 4). Depending on the OEM, stator-frame design, size (large units are most prone), the distortion may lead to key-bar vibration.
Dollard asked attendees, “How do you know belly bands are the problem?” He answered that question by offering the following tell-tale signs:
• An increase in stator-frame vibration, usually in the radial direction.
• A step-change or gradual increase in “sound” level.
• Noisier than a sister unit.
• Acoustic survey indicates the unit is more noisy on one end than the other.
• Visual inspection of key bars or building bolts reveal greasing or dusting.
There are three types of belly-band projects, Dollard told the group. They are: (1) inspection and tightening, (2) replacement of existing belly bands, and (3) installation of new belly bands on units that didn’t have them originally, or extra belly bands on units that already have one or more.
It is relatively easy to inspect belly bands installed during frame manufacture, because access doors generally have been provided for this purpose (Fig 5). After removing doors, Dollard said, check bolt torque and verify tightness with a “ring” test of the belly band. If tightening is required, grinding of shims and/or buckles and welding inside the outer wrapper likely will be required (Fig 6) and care must be taken to prevent foreign material from entering the generator (Figs 7, 8).
A user asked, “Why might you replace or add belly bands?” Replacement usually is motivated by improper design, the AGTServices expert said. For example, the band might be of the wrong diameter relative to core OD, or the material might not be quite right for the application. Poor installation or broken bands are other reasons for replacement.
When belly bands must be installed in an operational unit to reduce vibration, it often is necessary to provide one or more access doors. Blisters, where used to facilitate the flow of cooling air, must be removed first (Fig 9). Then doors are cut in the wrapper with grinders (Fig 10), until about 1/32nd of an inch of steel remains. Chisels are used from this point on to help keep debris from getting inside the unit. FME (foreign material exclusion) considerations contribute to the time-consuming process. It normally takes a couple of weeks to add belly bands on a GE 324 generator (Fig 11). Final welding after installation of belly bands is shown in Fig 12.
Testing after completion of work should include standard outage electrical testing and an EL CID test if the field was removed. You might want to check core torqueing as well, if accessible—in particular if data indicate the possibility of core looseness. Validate your efforts with an acoustical survey on restart and a visual check during the next major outage. Finally, does the generator sound quieter? Does the floor not shake as much?
Dollard’s presentation focused on GE generators. Regarding Westinghouse units, he mentioned that they generally should be core-torqued every 10 years or so and that requires removal/replacement of belly bands where installed.
Goodman began by asking the group to consider ultrasound an integrating technology because it can be used with infrared and vibration inspections, or alone, to detect impending failures in a wide variety of mechanical equipment—from gears and bearings, to pumps, to steam traps—as well as to locate leaks and to warn of potential failures of electrical equipment. The technology has the capability to sense high-frequency sounds transmitted either through air or solids and to translate those sounds into lower frequencies, within the range of human hearing.
Airborne ultrasound is effective, Goodman said, because all operating equipment and most leakage issues produce a broad range of sounds; plus, the high-frequency ultrasonic components of these sounds are extremely short wave in nature. Because these short wave signals are fairly directional, Goodman continued, it is relatively easy to detect their exact locations by separating them from other noises. Warnings of degradation in mechanical equipment can be detected early—well before failure—and often before vibration or infrared methods can be of help.
Goodman stressed the value of ultrasound for electrical inspection of closed cabinets, such as motor control centers, and suggested that it be used in conjunction with infrared technology. One “hears,” he said, the other “sees.” Airborne ultrasound can identify the ionization characteristics of corona and arcing, while infrared thermal imaging identifies the heat generated at loose connections and by bad fuse clips, overloaded wiring, bad or worn breakers, and load imbalances.
Goodman focused for a while on the hazards of arc flash, what it is, and how it occurs. Most, if not all attendees, were at least familiar with arc flash from plant safety training. Ultrasound, he said, enables operators to scan cabinets housing energized electrical equipment to be sure no arcing or tracking is present before an access door is opened.
Sound analysis, the next topic in Goodman’s “short course,” may have been the most valuable part of his presentation. Recordings of sounds created by corona, loose components, arcing, tracking, etc, enabled attendees to experience first-hand the capabilities of the technology. Goodman stressed that it always is necessary to examine both the noise-spectra and time-domain images to evaluate the severity of the condition being examined.
Fig 13 illustrates the spectrum for corona with sound in decibels on the vertical axis and frequency on the horizontal axis. The peaks occur at 1x, 2x, . . . nx, where x is 60 Hz. Note that the amplitudes of sounds between the peaks are about half those of the peak amplitudes. The time series in Fig 14 reveals a uniform band of signals in both spacing over time (horizontal axis) and amplitude (vertical axis), with very few peaks extending much above the average “band.”
Sinay introduced users to acoustic-emission monitoring technology capable of detecting cracking of compressor stator blades while the unit is operating. Mistras Group Inc’s Acoustic Combustion Turbine Monitoring System (ACTMS™) has been installed on forty gas turbines to date and is credited with at least one documented “save,” having detected and located an S1 vane crack in an F-class gas turbine at a combined-cycle plant owned and operated by Florida Power & Light Co (Fig 15).
He reminded attendees that vane cracking in some engine models is a recognized industry concern. ACTMS’s non-intrusive sensors, mounted on the turbine case by magnets or waveguides that transfer the cracking energy to the sensor while dissipating heat. A typical installation on a GE 7FA has 12 sensors arranged in a conical array to detect cracking in rows S0 through S5, an area of concern.
The sensors are wired to a monitoring system located outside the turbine enclosure that evaluates sensor signals in real time. Use of multiple sensors enables ACTMS to locate the position of a crack in three dimensions for follow-up verification during a borescope examination. Details on how ACTMS works are in Sinay’s presentation, available through CTOTF’s Presentations Library along with the other presentations summarized here. Access is available to all registered (a process that takes just a few minutes) employees of gas turbine owner/operators.