A portable vibration monitor must be highly reliable, configured for wind turbine, easy for wind farm technicians to use, and rugged enough for use in the nacelle.
Ashley Crowther, Tugan Eritenel, John Coultate, and Mike Hornemann, Romax Technology, www.romaxtech.com
The development of vibration diagnostics for machinery ramped up in the 1960’s after a researcher published a modern method of Fast Fourier Transform. This allowed converting the time histories of acceleration into frequencies with reasonable computing speed. High quality, economical vibration transducers were introduced in the 1970s and 80s along with improvements to various signal-processing techniques such as envelope analysis, cepstrum (a math operation to a signal spectrum), and time synchronous averaging ¬ basically everything needed for an efficient portable vibration data logger to tell what is failing inside industrial machinery.
Vibration analysis is a mature tool routinely use by mechanical engineers for many industries with rotating machinery. The wind-turbine application presents particular challenges such as variable speed and load, slow-speed rotation at the drivetrain input, equipment located 60m and more up a tower, and stringent requirements for health and safety.
In response, companies have developed portable vibration equipment, powerful tools for many aspects of wind turbine operation and maintenance, tools that can trim considerable time and cost from maintenance tasks.
Modern wind turbines, even those with proven drivetrains, often exhibit failure issues similar to ground based equipment. Tooth cracking and eventually, separation occurs when gears have inclusions in their steel, or teeth are ground with an improper temper. Bearings, mostly the through hardened variety and mounted on parallel stage shafts, can develop axial cracks, which eventually lead to the race spinning on the shaft after through-cracking. Planet bearings (rotating elements) suffer from rolling contact fatigue manifested as spalling of the raceways and typically later in life near wear out. Bearings throughout a gearbox, occasionally develop micropitting with the damaged region eventually progressing into macropitting.
Planet ring (annulus) gears, when through hardened, are softer than debris generated from other gears and bearings, and the debris damage sometimes leads to tooth fracture. Main bearings that transmit thrust loads through spherical rollers often suffer micropitting. The debris leads to race macropitting and eventually edge cracking near the bottom-dead-center location. All this typical damage is detectable with good portable vibration equipment and effective analysis. The table Detectable bearing and gear damage provides a list of exactly that.
Equipment requirements and data processing
Portable vibration data-acquisition equipment fundamentally consists of an industrial computer with data acquisition cards, some on-board processing, an intuitive user interface, and various sensors. In principle, this type of equipment is nothing new. Industrial computers with data acquisition have been used for decades. However, the modern version must be highly reliable, configured specifically for wind turbines, easy for wind farm technicians to use, and rugged enough for use in the nacelle. Meeting all these requirements is far from trivial. Example equipment features to 8 channels of acceleration, a current transformer for power measurement, speed sensor, 3G connectivity (third-generation mobile phone connectivity that allows accessing data remotely), and a user interface for various aspects of configuration and turbine input. Key requirements for the wind turbine application include measuring power only when the turbine is producing power (you actually don’t want too much data) and a speed signal so that the measurements can be time-synchronized (resampled to the same shaft speed).
In one project, the portable system was shipped to the site where technicians installed it on three successive turbines. Examples of recorded data provide comparisons. The time-versus-power chart shows data from three machines, and how the power and speed varied. Note that the equipment is setup to record intelligently, not continuously.
It is generally good practice to analyse data recorded at rated power (T2 and T3 reached rated power), but sometimes one must make do with lower power data (T1 in this case only reached 700 kW) or leave the equipment uptower for longer periods. The 3G connectivity allows accessing the equipment by a Virtual Private Network (VPN), one that uses a public telecommunications to provide remote techs secure access to their company network. The network allows assessing the data before climbing to retrieve the equipment, or for medium-term running, for example when a known issue is being monitored. After collecting the data, each sample point can be analysed to provide a frequency spectrum for each acceleration transducer. The example in Frequency-versus-acceleration is for a sensor on the planet stage. The analyst is looking for the gear frequencies to line up with those calculated from kinematic principles. With this check in place, the fun begins.
Help! Serious noise from the gearbox, but we can’t find damage
Wind techs often ask for support after they identify a noisy gearbox but cannot pinpoint the problem. At this stage it is time to ship out the portable vibration equipment and get a reading. Vibration from a cracked bearing shows a spectrum from such a bearing after specific signal processing removes the background vibration and brings forward the vibrations due to the crack. The frequency at approximately 198 Hz lines up with the bearing inner race fault frequency. Many other telltale signs of the bearing damage appear around this frequency due to the physics of the vibrating system. The cause: the bearing was cracked, but the end caps must be removed for access to confirm the damage. After doing so and replacing the bearing, the vibration returned to normal levels. In this instance and others, the portable vibration-monitoring system was a useful tool for the site manager to determine where the damage is and to order repairs.
Monitoring progression of known damage and vibration assessment after gearbox repair
Another common issue arises after the identifying the damage. For example, suppose planet-bearing spalling is detected by a filter
bypass alarm and confirmed by borescope inspection. The asset manager must decide to either run the turbine until a crane and parts are available or shut the machine down to avoid catastrophic failure (and lose revenue). By installing a portable monitoring system, a gearbox can be monitored daily over a 3G network so its vibration increases become inputs to decision making. Monitoring bearing damage provides an example of a failing planet bearing that was detected and monitored up to gearbox replacement. As the vibration ramped up, the gearbox was borescoped to make sure it was still operable. For several months the vibration measurement provided all the monitoring that was necessary.
After repairing a gearbox, it is good practice to check for misalignment, gear runout, and other issues. Monitoring bearing damage shows what you hope to see: vibration levels returned to normal. Occasionally after a repair, the gearbox can be noisy or the turbine can trip out due to high vibration – in this case portable vibration measurement will help to diagnose the issue.
Cost effective machinery health assessment
The real value and intelligence in portable vibration monitoring is in the analysis method. The signal processing that works effectively is different from machine to machine and quite specific. General
approaches such as RMS values, basic enveloping, and looking at the frequency spectrum are generally reliable only in cases of severe damage such as missing teeth. Component damage, such as bearing cracks, planet-bearing spalling, and main bearing spalling, need more rigorous treatment. With this in mind, conducting an effective vibration sweep requires a world-class approach to data analysis alongside careful measurement using equipment designed specifically for use in wind turbines.
A few considerations when planning on portable vibration equipment include:
- Eliminating third party installers can save significant cost. So, can the portable equipment be shipped directly to your local site staff and installed according to clear instructions?
- Make sure it is windy. There is little value in measuring vibration at times of insufficient wind.
- Plan carefully into the end-of-warranty process. Use vibration to look for serial issues that are hard to find by borescoping. Consider running a portable vibration campaign with local site staff several months before the borescope inspections. The vibration will then help guide inspections for better overall results.
- For a full wind farm measurement campaign, measurements can be made on up to six turbines per day using two systems and a team of three people (depending on site-specific rules and requirements).
- Review the site failure history and make sure the analysis method is proven for detection of the site serial issues as well as the known problems (at other sites) with the component make and model. WPE
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