Abstract: The vertical coal mill of a coal gasification project malfunctioned during commissioning. According to the experience of installation and debugging, the fault point of the vertical coal mill is inferred to be between the connecting ring of the seal of the machine frame and the driving disc of the coal mill. In order to determine the actual wear of the fault location, it is necessary to remove or jack up the driven disc of the coal mill to see the fault location. However, the clearance here is relatively small, and the labor and machinery required for such removal is costly. After investigation and analysis of the structure of the coal mill, it is found that after the grinding roller is raised, enough operating room can be left for the thickness measurement by ultrasonic testing instrument and for the personnel to operate. Therefore, the thickness of the connecting ring and the local wear between the driving disc can be quantitatively determined by ultrasonic scanning. Based on the comprehensive analysis of problems occurred on-site, the online repair method for the coal mill was proposed. The coal mill ran smoothly after repair, and similar failures did not occur again. Practice proves that the online repair is safe, feasible, time saving and reliable. It can save the cost investment of repair and is of high application value.

Key words: coal mills, fault, clearance, online repair