In a previous blog post we discussed a gearbox failure that caused a turbine fire, in this post we are going to discuss another mechanical system failure which led to a turbine fire. While most people typically consider rotating high-speed components as a likely source of fires, high torque and slower speed components may also pose a fire risk if allowed to operate in a failed condition.
Wear on rotor flange due to shaft movement from failed main bearing, generating heat as well as potentially sparks.
Fires caused by a failing main bearing typically results from a lack of condition monitoring and controller supervision of the main bearing, letting the failure progress too far. As the damage in the bearing progresses, large pieces of metal accumulate in the grease and in some cases the rolling elements cease rolling and begin to smear/skid along the raceway.
Main bearing roller in a severe damaged state “chicken nugget”. The damage generates heat and the wear results in the main shaft assembly moving axially and potentially causing rubbing on rotor lock flange.
This smearing generates significant heat. If allowed to continue operating, the heat generated can be so high that the grease and other surrounding components may catch fire. This failure mode has led to turbine fires, with the majority occurring on older model turbines without main bearing temperature supervision. Owners with these turbines need to be aware of this; especially as they keep aging.
To help prevent bearings reaching this stage of damage in the first place it is good practice to implement predictive maintenance practices. First, proactively monitor the vibration and grease contamination, identify wearing bearings early. Produce a list which we call the “flushing prioritizer”. Then arrange a thorough flushing of the grease from main bearings with subsequent repacking with fresh grease. If possible source one of the newer technology greases available. This will extend the life and keep you on top of bearing condition.
Main bearings can deteriorate to such a poor state that the internals can be in pieces. These samples were removed from one bearing during grease flushing. The bearing was soon after repaired.
Another mechanical failure mode that may lead to a fire is the contact between a rotating and stationary component. This failure mode has been observed between the rotor flange and turbine housing, defective slip rings and the application of turbine high speed brakes in emergency settings. While the heat and sparks generated from the contact alone may not cause the fire, flammable materials surrounding the component may be ignited and lead to a turbine fire. This will be discussed further in the upcoming symposium.
Interested in understanding the root cause of failures that have caused turbine fires? Attend the Third Annual Wind Turbine Technical Symposium to get into the details.