Precision laser alignment can help end users avoid multiple problems in wind turbine applications.
Mechanical failure in wind turbines presents unique challenges for operations and maintenance. Unlike traditional rotating equipment assets in other power generation plants, wind turbine assets sit on top of a tower 90 meters in the air. We do not have the luxury of performing maintenance tasks on solid ground. Tools and replacement parts must be brought up to the work site—and if you forget a tool or a part, it is not as easy as traveling back to the shop to retrieve it. It is critical to be able to prevent mechanical failure in a wind turbine whenever possible.
With wind turbines, 60 percent of downtime is related to drive train failure—gearbox, generator, main shaft and their associated bearings. We also know from industry studies that misalignment of rotating shafts is responsible for more than half of all bearing failures.
Precision shaft alignment is the process of aligning the center line of the shafts of one or more rotating machines. When the shaft center lines are co-linear, they can turn freely, and the external forces that destroy key components of the system (bearings, seals, couplings, rotors) will be mitigated.
Misalignment will most affect the main bearings in the gearbox and generator. Left unattended for too long, this misalignment will destroy the bearings and then go to work on other components down the line on the shaft—such as seals, rotors, etc. Precision alignment at install and periodic checks can help prevent component failure, up tower repairs and catastrophic failure. A bearing failure up tower can cost $10,000 to $15,000 to repair.
Catastrophic failure can cost an estimated $260,000.00. A good alignment program can prevent these problems before they even start. When performing alignment in a wind turbine, a number of issues must be considered.
Safety
Safety is a first and foremost concern. Laser alignment tools must be mounted in such a way that they will not be able to come in contact with technicians if the turbine brakes give way. Turning the shafts is critical to performing a precision alignment and operation and maintenance (O&M) teams have to be aware under what conditions they can work up tower when the blades will be turned. The requirement to perform a measurement without turning the shaft is becoming more prevalent.
A sudden strong gust of wind can catch the maintenance team off guard and cause the shaft to spin freely as the brakes are being released for the alignment process. Some turbine manufacturers have developed solutions for alignment that do not require that the gearbox turn.
Mounting a Laser Alignment Tool
Mounting a laser alignment tool on a wind turbine is a bit different than in the industrial world. While we can usually use chains and brackets to mount onto the rotating shafts, in a wind turbine, this is not always possible (or feasible). Laser measuring units may have to be mounted on brake discs or coupling flanges. It can be a challenge to find a mounting solution for which the measuring units can rotate 1,800 without contacting other components.
Once the mounting location is determined, check the measurement two or three times to make sure it repeats. On some turbines, we have found that some components will shift when the brake is engaged. Mounting close to the shaft will minimize the effect. If your results are not repeating, look for an alternative spot to mount and re-test. If they do repeat, you have found a good solution.
Dynamic Movement
In a wind turbine generator, everything moves, pretty much all the time. Dynamic movement is caused when the brake is released and the hub and blades turn freely. The weight and turning forces can cause the gearbox to move slightly—even imperceptibly to the human eye—and cause misalignment. Once you know the amount of movement, you can add those calculations to your alignment calculations. When compensating for dynamic movement, you will actually misalign the gearbox and generator at rest so that when it's running, the machines will move into alignment.
Wind Tower Flange Flatness
Lasers are also used in the wind industry as a valuable tool for measuring flanges on wind tower and blade root flanges. Flatness and taper measurements help ensure that these components fit together properly during construction. Flange flatness ensures that each tower section is aligned with the previous section. Towers' flange bolts are tightened with torque or tension. You don't want to close a gap between flanges by using either torque or tension.
Wind Turbine Blade Flanges
Blade flange flatness is another important measurement for wind turbine performance. Wind turbine blades can weigh more than 18 tons. Attachment to the hub must be done with a minimum of stress. Once again, gaps cannot be closed with torque or tension. The challenge with blade flanges is that the blades are made of composite material, not steel, making it difficult to mount lasers and detectors with magnets. Special mounting brackets have been developed to ensure accurate measurement and a good result.
Conclusion
Precision alignment is a vital first step to prevent mechanical failure in wind turbines. Regardless of whether alignment is performed by the OEM, your O&M contractor or your own O&M team, good tools and training are not optional. Remember, an ounce of prevention equals a pound of cure and lessens the chance for catastrophic machine failure up tower.
Pumps & Systems, May 2011