DEVELOPMENTS IN HIGHLY STRESSED LARGE BOILER ID FANS

A HISTORY OF INDUCED draft (ID) fan failures in the Invicta Mill 320 t/h No. 3 bagassefired boiler instigated a complete review of the fan installation. These issues have been exacerbated by heavy carry-over from poor performing wet collectors preceding the fan. Traditional materials (e.g. Bisalloy 80) cannot clear heavy carry-over due to the plate surface finish and corrosion of the impeller surfaces. This can be alleviated to some extent by surface coatings, but these degrade, are high maintenance and not as effective as a polished stainless steel surface. The review included: developing a performance specification for the actual boiler fan installation instead of the original manufacturers specification and identifying that the actual duty requirement was different to the original specification; the material specification for a fan impeller behind a wet collector due to recurring maintenance and operational issues which culminated in selecting SAF2205 as the most appropriate compromise for the duty; the impeller design requirements to comply with the fatigue limits of a lower strength material than the material used in the fan impeller that was being replaced. Significant FEA work was required to ensure that this design eliminated further maintenance and expense on this historically troubled item of plant. This work culminated in delivering a 16.5 t fan impeller and shaft, which is the largest SAF2205 fan impeller for the Australian sugar industry. The fan impeller has a novel ‘scythe’ type centre plate to reduce inertia and improve wear performance. The impeller blades were polished to a five micron surface finish to minimise ash build-up and to reduce the need for boiler outages for maintenance cleaning of the impeller due to out-of-balance effects of the build-up.