Variable speed drives were fitted to Waste Gas Sinter Fans in a steelworks and the operating regime changed. Instead of running at constant speed these fans were now running with several speed changes per day.
Soon after the change to variable speed, a routine inspection of the fan flagged numerous cracks at weld toe positions, some up to 50mm in length.
In addition full thickness cracks were noted at the centreplate cutaways, as shown in the photograph and image in figure 1.
The depth and extent of the cracking observed resulted in the fan being deemed unsafe to use and immediately taken out of service.
Howden were asked to carry out a Finite Element Analysis of the original fan and to design a replacement which would be suitable for variable speed operation.
The analysis of the original impeller showed it had acceptable structural integrity for constant speed operation. This had been demonstarted by its actual service life to date (over 20 years of start/stops).
However, an area of relatively high stress at the root of the centreplate cutaway was noted, as shown in the following plots. This was consistent with the location of the cracks found during inspection.