Turbine fires are one of the most severe threats facing turbine owners and operators every day, and one we explored in detail at our 2018 Wind Turbine Technical Symposium, held in Golden, Colorado last month. The damage caused by turbine fires is threefold, not only risking the safety of technicians but also incurring significant costs and negative publicity. As an industry, it’s vital that we do all we can to minimise this risk wherever possible – and that starts with correct maintenance of turbine components.
Some of the causes of turbine fires, such as electrical system failures and lightning strikes, are well known. However, mechanical component failures are another leading cause of fires and are seldom discussed; at one point or another faulty main bearings, brakes and gearboxes, and the frictional heat generation these produce, have all been identified as the ignition source in multiple turbine fires.
While wind farm owners and operators typically expect the turbine controller to monitor its mechanical components and shut the turbine down in the event of a problem, in some cases the monitoring system is unable to detect the fault. Alternatively, the sensors needed to detect the problem before a fire develops may not be in place.
We recently saw an example of this with an internal gearbox bolt failure on a sun shaft retaining plate, part of a gearbox’s planetary stage. The purpose of the plate is to resist axial loads on the sun shaft and keep it in position, and it is secured with a total of 16 bolts. However, in this example (pictured), these bolts have failed due to a combination of fatigue and backing out. This would undoubtedly have led to contact between the rotating sun shaft and retaining plate and the stationary low speed cover – with this contact generating significant heat and debris, and the potential for a large-scale fire.
But it’s not just high-speed rotating components that cause turbine fires: high torque and slow speed components are just as much a threat if operating in a failed condition. For instance, damage to a main bearing, if not identified early through condition monitoring and controller supervision, can lead to large pieces of metal accumulating in the grease. In some cases, the rolling elements then cease rolling, and instead begin smearing or skidding along the raceway. This smearing generates significant heat and, if allowed to continue operating, this heat can cause the great and other surrounding components to catch fire.
Older turbine models are most prone to this type of fault, as they do not possess main bearing temperature supervision. Owners of these turbines will need to particularly aware of this threat of fire, especially as they continue to age.
But adopting best practices and implementing a predictive maintenance programme can help prevent bearings reaching this stage of damage in the first place. The first step will be to adopt early-stage monitoring of vibration and grease contamination. Next, ensure a thorough flushing of the grease from the main bearings, and repack it with fresh grease – preferably one of the newer technology greases available. Doing so will help extend the lifetime of assets, prevent any deterioration in bearing condition and, ultimately, avoid instances of turbine fires and their catastrophic results.