No abstract

A last stage turbine blades failure was experienced in two units of 660 MW. These units have one high-pressure turbine and two tandem-compound low-pressure turbines with 44-inch last-stage blades. The blades that failed were in a low pressure (LP) turbine connected to the high pressure (HP) turbine (LP1) and in LP turbine connected to the generator (LP2). The failed blades had cracks in their roots initiating at the trailing edge, concave side of the steeple outermost fillet radius. Laboratory evaluation of the cracking indicates the failure mechanism to be high cycle fatigue (HCF). The last-stage blades failure evaluation was carried out. The investigation included a metallographic analysis of the cracked blades, natural frequency test and analysis, blade stress analysis, unit’s operation parameters and history of events analysis, fracture mechanics and crack propagation analysis. This paper provides an overview of this failure investigation, which led to the identification of the blades torsional vibrations near 120 Hz and some operation periods with low load low vacuum as the primary contribution to the observed failure.

This article presents a closed-form solution to the computation of unbalance correction masses for rigid rotors mounted on flexible bearings. The solution requires measurements of the response of the rotor-bearing system to the unbalance that is to be corrected, as well as knowledge of its total mass or other stiffness or damping parameters that are readily deducible from the mounting configuration. No detailed mathematical models or information from previous balancing runs are required. The computation is based on the relationships that exist between some spatial and modal parameters of rotor-bearing systems. Such relationships are derived in this article, and their use in the computation of balancing masses is presented and verified with results from an experimental test rig. This work demonstrates the feasibility of balancing rigid rotors without trial runs, detailed models or historical balancing records.