The corrosion/erosion induced premature failure of some draught fan blades in a 250 MW boiler

The present paper attempts to elucidate the cause of failure of induced fan blades during service in a gaseous environment at temperatures around 150 C. In an effort to assess the stresses involved in the initiation of the failure event, which occurred midway along the blade length, two separate approaches were adopted, viz., a Linear Elastic Fracture Mechanic based fractographic survey and a purely mechanical engineering approach. The former method considered such parameters as surface pitting characteristics, cyclic stress amplitudes and the materials level of threshold stress intensity range. Δ Kth, which is the ΔK level below which fatigue crack propagation cannot occur to any measurable degree while the latter method concentrated on stresses which arose from blade shape and other dynamic factors. It was established that initial failure occurred by a fatigue crack extension process which was triggered off through blade thinning by corrosion/erosion. Both methods of assessment exhibited good agreement in as much that they predicted that the stress range Δσ, of the fatigue cycle was about 50 MPa. Finally it was suggested that fan blades, which have a life of only some 16,000 hours, be coated with two selected coating materials and subjected to normal working conditions in an effort to significantly increase the service lifetime of ID fans blades. (orig.)