Torsional vibration and fatigue evaluation in repairing the worn shafting of the steam turbine

Defects may be induced by the degradation in long-term service of the equipment, where its components experience complicated conditions such as alternating loads, wearing or fretting conditions and creep. To repair the defects, cutting is the traditional approach to smooth the local profile, reduce the stress concentration and elongate the service life. This paper compared two plans to repair the shafting abrasion defects as the long-term wearing effect of foreign objects, observed in a 660 MW steam turbine. The first plan introduces relatively smaller cutting, compared with the second plan which takes a larger cutting volume with smoother profile. The analysis is following: (I) the local stress concentration at the abrasion location has been presented in contrast of the variation of the stress; (II) torsional vibration features are studied in detail with the shafting model, to determine the influence of the abrasion defects and to analyze the variation caused by the repairing cutting; the torsional vibration responses under four types of fault conditions are compared through the static, transient and steady responses; (III) local stress responses in weak locations of the shafting are acquired, and then the fatigue damage in these locations are further evaluated. The results indicate that, the second plan presents lower stress concentration, on the contrary, the first plan brings higher stress concentration, however it introduces less variation in the torsional vibration corresponding to smaller stress amplitude, and further causes less fatigue damage at the spot of fault. The first plan with relatively smaller cutting volume is therefore suggested as the primary repair plan.