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.

Figure 1

Figure 2

The estimated fatigue life for the fan, following convertion to variable speed operation was calculated to be only a few months. This hence confirmed that this fan was not suitable for variable speed operation.

The analysis was repeated for a new impeller design, This incorporated several modifications, including changes to weld specifications and to the cutaway dimensions and location.

Changes were also introduced to minimise the impeller inertia, thus allowing it to run on the existing supports and with the previously used motor.

The results confimed the following for the new design:-

• Static Stress Levels were acceptable.
• A fatigue life could be achieved to comply with the customer’s variable speed operating requirements. This data was presented to the customer in the form of a Miner’s summation, detailing combinations of start/stops and various speed change cycles.
• The impeller natural frequency analysis complied with design rules.
• The rotordynamics gave acceptable results over the full running speed range and complied with critical speed assessement criteria.