Summary
Owners of Siemens 13.9 m² low-pressure turbine rotors face a well-documented inspection problem. The 1,800 RPM double-flow design is built up from a shaft carrying three shrunk-on wheels per flow, and the high stresses inherent to that shrink fit, combined with steam cycle chemistry, leave the disk bores, keyways, and blade attachments susceptible to intergranular stress corrosion cracking. The damage initiates where it is hardest to see. Short of de-stacking the rotor, ultrasonics are the only viable way to interrogate the disk ID and keyway surfaces, and complex geometry, long sound paths, and rotor-to-rotor variation have historically made complete coverage difficult to demonstrate — leaving life assessments resting on partial data.
Structural Integrity rebuilt its inspection protocol specifically to close that gap. Working from detailed CAD models and CIVA ultrasonic simulation, SIA’s Turbine & Generator NDE team evaluated probe, wedge, and focal law combinations disk by disk until the technique delivered 100 percent disk bore coverage with deliberate overlap and redundancy across every high-stress region. The redeveloped automated phased array technique uses primary probes with a larger near field to increase coverage and reduce the risk of signal attenuation loss, a scan plan of 17 probe locations producing 51 scans per flow and 102 scans per rotor, and proven contingencies for local geometric variation and attenuative material.
That protocol sits inside a complete rotor examination. SIA performs manual phased array ultrasonic examination of the curved, axially mounted L-0 blade roots, where large blade size and rotational speed drive low cycle fatigue, high cycle fatigue, and stress corrosion cracking — calibrated to EPRI blade standards and demonstrated against EDM flaws and EPRI blind trials. Automated 360-degree encoded phased array covers the disk rim blade attachments — an EPRI-demonstrated process for detecting and sizing dovetail cracking. Visual inspection and wet fluorescent magnetic particle testing cover the rotor periphery and stationary components, performed in situ with portable high-amperage equipment and multi-turn cables for rapid results. A six-person, two-shift crew completes setup, inspection, and analysis in roughly five calendar days.
Inspection data earns its keep only when it supports a decision. SIA trends crack growth and applies deterministic and probabilistic fracture mechanics through its advanced engineering group to establish remaining life within a defined operating envelope, giving nuclear and fossil owners a defensible technical basis for run, repair, or replace decisions without de-stacking the rotor. Findings feed directly into metallurgical evaluation and root cause analysis and into online monitoring and diagnostics, so a single outage inspection feeds an integrated asset management program rather than standing as an isolated event. Contact Structural Integrity Associates to scope a Siemens 13.9 m² rotor inspection.