Wind Turbine Vibration Monitoring-Mapping a Plan for Success-

While the wind industry is in the midst of adopting more advanced maintenance strategies, it is clear that machine condition monitoring will play a major role in realizing the life cycle cost reductions. It goes without saying that one of the conditions to monitor is the vibration emanating from the rotating components of a wind turbine. The challenge to wind farm operators is establishing a vibration monitoring scheme that will deliver long term value to the organization. The first step for a wind farm operator is determining how to make decisions today that will deliver that value. As with most business decisions, economic factors come into play in the desire to get best return for the investment. When it comes to deciding how vibration will be monitored on a wind turbine, the long term value proposition may not be so clear. The following discussion points are for consideration in establishing a vibration monitoring scheme.

Background

Vibrations are created from all things that rotate. The amplitude and frequencies of the vibrations produced are influenced by such things as the speed of rotation, component interaction, excitation of component resonant frequencies, the amount of load being transmitted, transient loading, component defects (gears, bearings, shafts, generator windings), installation errors (misalignment, mass unbalance), operating anomalies, etc. Maintenance management personnel migrating toward advanced maintenance management strategies are interested in those particular vibrations that indicate that something is in need of corrective or restorative action. Once a direction has been set to use a vibration monitoring scheme, the next step is specifying the hardware, establishing the monitoring locations and acquisition parameters, determining a proper collection interval, and determining the vibration parameters to trend that are indicative of machine health.

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Establishing a Foundation for Hardware Specifications

First in the discussion is the accurate conversion of the mechanical energy in the form of motion emanating from equipment into digital data that is an adequate representation of the energy of interest. It is not sufficient to “slap on any ol’ vibration transducer at any ol’ place” and press the record button on any ol’ data acquisition system”. Careful attention must be paid to the transducer type, its mounting location, and its mounting method relative to the specific machine defect energy sought to be detected. Additionally, the proper digital sampling rate of the analog signal must be considered in relationship to the detection of a defect frequency of interest. A vibration transducer that has a frequency capability of 0-10 Hz will not detect any energy produced at 15,000 Hz. Likewise, a vibration transducer that has a frequency capability of 3-10,000 Hz will not detect energy produced by the mass or aerodynamic unbalance of a wind turbine rotor at 0.3 Hz. A vibration transducer of 3-10,000 Hz whose output signal is sampled digitally at 256 Hz will not detect any energy at 1,000 Hz.

The critical starting point is to know what the range of frequencies will be produced by the component being monitored such that the hardware and data acquisition may be properly specified. If the transducer selection is wrong, the best vibration monitoring system and analyst in the world will miss a bearing fault like the one illustrated below!

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Cracked Bearing Race is shown as discovered from a borescope inspection (photo courtesy of Spin Trends, LLC).

Data Acquisition Intervals

The next discussion point relates to how often the data must be collected in order to realize the intended benefits. The time interval between data collection in industry covers the range from near-never acquiring the data to continuous data acquisition. The proper collection interval is established by the objectives of the maintenance strategy and of course, the risk versus the economics. The goal for establishing the proper data acquisition interval should start with the consideration of the time between the point of detecting a defective component to the point in time where it can be most economically repaired or corrected prior to catastrophic failure. The interval must be sufficiently frequent (or operating conditions at the time of data collection sufficiently equivalent) so that variations in machine loading and other process dynamics are well understood. Having a sufficient number of data points is critical to the ability to establish an actual trend in a parameter of machine health as contrasted by mere changes in the process dynamics. Infrequent data collection has a tendency promote unwarranted excitement and unnecessary expenditure of maintenance resources. The premature “false positive” is

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juxtaposed and contrasted by a catastrophic event that happened that was “undetected” due to an excessive interval between data collection. Both scenarios are bad news for the promoter of vibration monitoring from misapplication of technology and are cause for taking the steps necessary to implement a vibration monitoring program properly.

Wind Turbine Application - Data Acquisition Intervals

In general industry, the most common method of vibration monitoring is to acquire vibration data periodically by route-based collection. This involves one or more people, each with a vibration data collector, collecting preprogrammed points of vibration data on motors, pumps, gearboxes, etc. The data collection is relatively fast and 40-70 pieces of equipment can be surveyed in one day. These data collection intervals are typically on the order of every 30 days for process related industry. The regular data collection interval is established such that a failure mode may be detected as well as trended (as illustrated above) to establish the best economic maintenance plan for machine restoration While occasional “misses” are inevitable for a number of reasons, the number of “saves” resulting from detection and repair of fatigue related failure modes prior to catastrophic failure has produced yields in excess of 20X the investment.

Route-based data collection works equally well for wind turbines. Each drivetrain would require about 7-10 minutes of data collection much like the collection time in general industry. However, technicians would have to make a 25 minute climb to start the 7-10 minute collection process and another 12 minutes to get back down for a conservative total of 47 minutes of time to get data from one turbine. A major obstacle is that, generally, personnel are not allowed to be in the nacelle at the time when it is most beneficial to collect the data (moderate power generation). This obstacle could be

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overcome by setting up a portable vibration data collection unit (8-16 channels) which could be initiated remotely or by having long sensor cables from the monitored equipment down to the data collection unit at the base of the tower. Vibration collection time has now gone from 47 minutes to several hours.

47 minutes X 2 men X 100 turbines = 157 Man Hours – not such a small deal. 2 hours X 2 men X 100 turbines = 400 Man Hours --- a huge deal!

Establishing a reasonable collection interval of 30 days would require 1,880 man hours for the former and 4,800 man hours for the latter.

Obtrusive Wind Farm Math and Compromise

The old adage “if sounds too good to be true, then it probably is” rings loudly when it comes to applying sound principles of vibration condition monitoring to wind farm assets. Obtrusive wind farm math is sufficient to drive a propensity to discover “short cuts” that will supposedly provide “very close to almost the same results” for less than the cost of doing it properly. On the one hand, every turbine could be outfitted with its own condition monitoring system and data could be gathered as frequently as necessary with minimal labor. Unless the vibration monitoring equipment was purchased as a part of the turbine, the $600K-$700K+ per 100 turbine price tag would give most operators sticker shock.

On the other hand, a stealthy approach to stay below the capital investment radar is to implement a vibration monitoring scheme that would purchase a single portable vibration monitoring unit and go for the route based approach. In this scenario, the capital cost might be on the order of $20,000 (hardware and software) with another $75,200/year (1,880 mh X $40/mh – burdened labor cost ) to use the equipment to collect the data -- assuming that personnel can be in the nacelle during operation. If the data collection has to be done remotely, the labor dollars quickly escalate to $192,000/year. The financial comparison between the route-based data collection and permanent equipment data collection after 10 years is certainly worthy of consideration.

Initial Capital Cost

Annual Data Collection Cost

($40/mh)10 Year Data

Collection CostRoute-Based - Down Tower 20,000$ 192,000$ 1,940,000$ Permantly Installed equipment 700,000$ -$ 700,000$

Vibration Data Collection Costs 100 turbine Wind Farm

The problem for the operator who has a large number of wind turbines is that a single portable vibration data collection unit with a two man team may not be able to collect the data at a 30 day interval under perfect conditions. And every operator knows Murphy’s Law of scheduling work on a wind farm; the wind conditions will not be right to do the work at the time that it is scheduled or there will be a myriad of the other environmental factors that can prevent it. This means either an increase in

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the interval time between data collections (increased risk of a “miss”) or allocating more than one team and equipment to make sure there is no compromise in the collection interval.

If the operator has set up the infrastructure to do the route-based vibration monitoring properly (i.e. relatively consistent data collection every 30 days), the next management issue is work priority. Unplanned work will always exist to some degree and it must be addressed by management. In a “lean” work environment, the labor matches the planned requirements. The unplanned work will get addressed by delay of the planned work. Of course, this has the strong potential to head into a downward spiral. And when it comes to the labor used in the collection of vibration data, this labor is the first to get shuffled as the work is often viewed as having low priority. Without vigilance, it does not take much time for the vibration data collection initiative to be on the shelf along with the complete loss of the potential to realize life cycle cost reductions from the pursuit and execution of advanced maintenance strategies.

Conclusion

There is no question that properly executed vibration monitoring program will provide the information necessary to plan maintenance and play an important role in the reduction of overall life cycle costs. There is also no question that having some marginal vibration monitoring program (right hardware, right monitoring location, right acquisition parameters, wrong interval) is better than not having one at all. If you are starting down the path of implementing advanced maintenance strategies involving vibration condition monitoring, start walking with your eyes wide open. Don’t think that you will get the benefits of a proper vibration monitoring scheme for a 1/3 of the cost of a monitoring scheme executed properly….it is indeed “too good to be true”!

Author:

Michael LenzProcess & Asset ManagerAsset Maintenance & Condition MonitoringFrontier Pro Services(262) 436-9592mlenz@FrontierPro.com

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