Resolving a Resonant Fatigue Problem on a Large Radial Inflow Expander

A series of failures of a radial inflow hot gas expander driving a centrifugal compressor in a nitric acid plant plagued the second author’s former company over a 24 year period. The 19,000 hp single-stage radial inflow expander inlet and discharge blade tips would fail in service, destroying the inlet nozzle vanes and expander wheel, and causing extensive damage to the rotor, bearings, seals, and shrouds. To finally resolve this severe reliability problem, an engineering team consisting of both authors’ company’s personnel was formed to pinpoint the root cause of failure and develop a corrective action plan. A parametric solid model of the expander wheel was created, and forced response stress and modal analyses were conducted on various design configurations using finite element techniques. Impact resonance testing of several expander wheels was performed to validate the results of the finite element modal analyses. Design changes were implemented based on the analyses and a new expander wheel was manufactured. The startup procedures of the machine were also modified and heat treatments on the expander material were optimized. The new expander wheel has operated successfully with the geometric and operational startup procedure changes without a failure, resulting in record service life to date. This paper chronicles the history of these failures, expander design optimization studies and changes, operating procedure changes, and heat treatment changes for the A286 wheel material. The paper also documents the operational reliability attained in current operations, providing a validation of the engineering. Though the application of the engineering is unique for this case, the methods used in this investigation and problem resolution can be readily applied to numerous root cause failure analyses of rotating machinery.