7FA ring fire suggests shift from peaking to base-load duty is not as simple as just running longer

The day after the CTOTF meeting ended, and the morning after NEC’s W Howard Moudy spoke to attendees about the risks associated with ignoring OEM-suggested maintenance of exciter collector rings and brush systems, Cutsforth Inc’s Senior Application Specialist Steve Thompson called to report a damaging ring fire that required immediate attention. Plant personnel told Thompson the fire was so intense that material literally blew out of the housing, as evidenced by the holes shown in the photos. Cutsforth normally is able to repair/refurbish/upgrade brush rigging onsite, but the severity of the damage here was such that the entire housing required a shop visit for a complete rebuild.

Thompson said Cutsforth is seeing an increasing amount of work from the 7F fleet. One reason, he believes, is because low gas prices have owners running units base-load that had been in daily-start and peaking service. Here’s what Thompson sees as the root causes of the recent damage:

Brushes never have to be changed online when turbine/generators are in daily-start or peaking service. Apparently, some deck-plates personnel were not trained in online inspection and replacement of brushes before the shift to base-load operation and this critical work was not being done, thereby allowing the opportunity for exciter brush systems to spark and arc their way to destruction.

Weak brush springs are a big issue, Thompson continued. Many have not been changed—ever. Most units in the 7F fleet were installed during the bubble and typically are from eight to 12 years old. Springs lose their “spring” whether you run or not. Combine carbon deposits from increased run time and weak springs and you have the perfect recipe for selective action, which is conducive to overheating and fires.

Mixing of brushes is contributing to or directly causing some ring fires. Having a matched set of brushes is important: Do not mix brushes manufactured to different specifications. Proper brush composition and dimensions, and lead current-carrying capacity, are critical to success. Allowing purchasing agents to ignore your directions and buy from irresponsible offshore suppliers creates safety issues.

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