Attention to detail critical for preventing collector-ring fires

Collector ring fires are not a new phenomenon. They’ve been occurring for decades, and the reasons for ring fires, and how to avoid them, have been discussed extensively at user-group meetings and in technical papers and articles. Yet they continue to happen, primarily because of inattention to detail. No finger-pointing here to assess blame, the goal is awareness. Plus, plant budgets must provide for quality consumables, inspections, equipment upgrades as necessary, and proper training of personnel.

What follows is the case history of an inspection and root-cause analysis of a recent ring fire conducted by engineers and technicians from Cutsforth Inc. It is a valuable learning experience that emphasizes key points made in the article referenced above.

Cutsforth’s inspection and analysis team arrived onsite a week after the fire, which was put out by plant personnel with a fire extinguisher. A station technician reported seeing sparks/flames around the ring as it was rotating. Observations made during the inspection included the following:

Carbon deposits were found in each brush holder on the edges closest to, and farthest from, the ring (Fig 1). Location of the deposits was in a pattern consistent with unit rotation. There also were divots in at least one of the brush holders near the inside diameter of the yoke/holder. This contributed to the formation of carbon deposits.

• Chipped brushes indicated high running vibration, a contributor to carbon deposition (Fig 2).

• Photographing on the inboard ring indicated poor brush-to-ring contact (Fig 3).

• Yoke stud and associated insulation damage was caused by direct flame/heat/arcing/sparking on the side opposite the main lead connections (Fig 4).

• Shape of the melted metal on the inside diameter of the outboard yoke brush holder appeared to mirror the thread groove on the outboard ring surface (Fig 5).

• White dust found inside the dog house area likely was residue from the fire-extinguisher discharge.

Click figures to enlarge
Ring Fire 1 Ring Fire 2 Ring Fire 3 
Ring Fire 4 Ring Fire 5

Sequence of events. The experts believed that over time carbon deposited on the interior surfaces of the brush boxes. This, combined with the effects of spring pressure and unit vibration, probably compromised the brush-to-ring connection and caused arcing across the gap between the stuck brushes and the ring surface. The intense heat produced by arcing likely started the ring fire and disfigured the insulation covering one of the two support studs. The exposed stud created a direct short between positive and negative polarities and subsequent arcing caused the unit trip experienced.

Corrective action. The Cutsforth team suggested the following steps be taken to prevent a repeat of this incident:

• Install a state-of-the-art brush holder system capable of reducing carbon deposit formation and restore the collector-ring surface and helical grooves.

• Review plant maintenance routines to ensure brush wear and vibration are monitored regularly. Also, specify that rings be strobed regularly to identify possible photographing or unusual wear.

• Instruct plant personnel on where to look for carbon deposition and how to properly clean brush holders if it occurs.

• Megger the existing insulation between the collector rings and shaft to be sure it was not adversely affected by the heat generated by the event.

 

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