According to the alert, a 23 cubic metres per second fan, operating at 2980rpm, suffered a catastrophic failure.
A failure at the drive end bearing of the flameproof motor resulted in the shaft severely overheating and deforming, causing extreme out-of-balance forces to be generated.
This resulted in the fan impeller being shed and the motor junction box falling off, with resultant extreme damage to the explosion protected properties of the motor.
At the time the incident was noticed by the crew in the crib room who heard a pitch change in the noise of the fan, and then the power tripped off.
The department said the fan failure had been subject to an extensive engineering investigation.
“It has been concluded that the incident was caused by excessive vibration, resulting in the failure of the drive end bearing carrier. This resulted in the motor shaft contacting the flameproof housing with the resultant heat and rotary energy causing the other damage,” the safety alert said.
The department said the fan manufacturer had informed its clients of this failure mode.
DPI said other issues identified during the investigation included:
Many failed fan motors were two-pole high speed (3000 rpm);
The impeller was installed directly on the motor shaft;
Many of the failures rendered the motor non-flameproof in service;
Fan maintenance practices were deficient; and
Vibration monitoring practices were deficient.
The DPI has recommended that all operations must review the suitability of their auxiliary fan design and application and adhere to the following practices:
Complete a design risk assessment for the life cycle of the fan that incorporates a fan and motor failure mode effects analysis;
Incorporate design features to prevent a reoccurrence of this type of event;
Ensure compliance with MDG 3;
Ensure that fan monitoring and interlocks are designed and operated to an appropriate safety integrity level;
Implement a suitable vibration monitoring program utilising correct type of sensors, correctly located sensors, calibrated sensors, correct alarm and trip settings, and review by vibration analysts;
Implement a suitable auxiliary fan maintenance program that ensures correct greasing of bearings, regular cleaning of fan impellers, proof testing of alarms and trip systems;
Consider the use and availability of fan operational data via data-loggers; and
Include an operational risk assessment, with consideration given to proper investigations of fan stoppages, restoration of fan power procedures and provision of alternative ventilation in the event of a failure.