Witbank - Efficient engineering solves impeller failures

One of the ventilation stations at a mine in Witbank coalfield, South Africa, was experiencing repeated impeller failures in fans installed by a third party. Howden engineers were invited to investigate and propose a remedy.

One of the mine's main ventilation stations uses three 4.5m centrifugal fans in a trifurcated arrangement, VSD driven, with two fans running and one on standby at any one time. The fans were installed between 2008 and 2010 by a South African supplier and Howden competitor. However, between the time of commissioning and early 2012, seven impeller failures occurred in the new fans. The owners invited Howden Fan Equipment to carry out a preliminary site investigation, and subsequently commissioned us to follow this up with a full surface fan design review.

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Centrifugal fan case study

Our full review included:

  • Vibration analysis of the rotating assemblies.
  • Finite element analysis of the rotating assemblies.
  • Ground condition analysis.
  • Civil design review.
  • Hydrological analysis of water table conditions.
  • Up-cast shaft visual inspection.

The solution

These investigations yielded some significant results. Most importantly, mechanical run-down testing demonstrated that the installed fans were running at twice the critical speed of the impeller. This dramatically reduced the expected life of the impeller to as little as a few hours. However, we also found that the scantlings were below Howden specifications, and serious miscalculation at the civil engineering and construction stages meant that the fan bases were built higher than originally designed. At that height, they were not robust enough to absorb the vibrations from the fans, and this contributed to the shortened impeller life. In addition, the ground around the collar had not been properly compacted, allowing the collar to move slightly. Fortunately, a visual inspection of the up-cast shaft showed no damage had yet been caused by the collar movement.

Rather than suggesting complete fan replacement, we replaced the entire rotating assemblies comprising the impeller, shaft, bearings and bearing bases with custom engineered Howden assemblies designed to take all the extraneous factors into account. After rectifying the problems with the fan bases by casting a concrete mass around them to increase stiffness, the new rotating assemblies were retrofitted into the existing casings.

The client was pleased not only to have the problems identified and solved, but to have the solution managed in the most cost effective and least time consuming way possible.

 

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