Forced Response in Axial Turbines Under the Influence of Partial Admission

Abstract

High cycle fatigue (HCF) due to unforeseen excitation frequencies, underestimated force magnitudes, or a combination of both causes controlstage failures for steam turbine stakeholders. This paper provides an extended design criteria toolbox, as well as validation data, for controlstage design based on experimental data to reduce HCF incidents in partialadmission turbines. The upstream rotor in a twostage air test turbine is instrumented with pressure transducers and strain gauges. Admission degrees extend from 28.6% to 100%, as one or two admission arcs are simulated by blocking segmental arcs immediately upstream of the first stator vanes with aerodynamically shaped filling blocks. Sweeps across a speed range of 50%–105% of design speed are performed at a constant turbine pressure ratio during simultaneous highspeed acquisition. A forcedresponse analysis is performed and results presented in Campbell diagrams. Partial admission creates a large number of lowengineorder forced responses because of the blockage, pumping, loading, and unloading processes. Combinations of the number of rotor blades and lowengineorder excitations are the principal sources of forcedresponse vibrations for the turbine studied here. Altering the stator and/or rotor pitches changes the excitation pattern. We observed that a relationship between the circumferential lengths of the admitted and nonadmitted arcs dictates the excitation forces and may serve as a design parameter.