After a significant overhaul, a turbine will receive a final assembly with two tight tolerances and will be placed back into service. A post-overhaul inspection will verify that the shop did everything correctly, that the machine is safe to run, and that the performance expectations are well-founded. The post-overhaul inspection is a systematic inspection to protect your outage investment and provide valid as-left expectations for service and maintenance.

Why Post-Overhaul Checks Matter

Post-overhaul inspections have the potential to display issues that may not be recognizable until the first fire or roll-up process happens. Fasteners can relax, seals can settle, and clearance can become reduced as components heat and cool. Verification at this point helps to reduce the possibility of significant A-life implications and improve warranty protection in the event of a casualty. The post-overhaul check also provides post-overhaul “as-left” alignment, vibration, and performance metrics that may help with future analysis if a defect or drifts occurs.

Key Steps in Post-Overhaul Inspections

Static checks will begin the post-overhaul inspection sequence, progressing into items that incorporate the run testing that was controlled. The inspection team will look through paperwork and traces the parts back to work. The inspection team will then confirm all measurements taken throughout the adjustment or assembly measures. While gas units would hold hot gas path evaluations and controls logic as crucial to the inspection, steam units would use casing seal integrity and vacuum as critical.

Teams that support aeroderivative inspection services would have the experience with high-speed and fast-starting engine process and tighter balance tolerances. The last check would often include a final visual borescope inspection before the first fire.

  • Verify documentation and parts: Confirmation of confirmation of the work order completion (i.e., the completion of writing torque) and confirming serial numbers back to the OEM drawings and specs.
  • Dimensional checks: Evaluate the important measurements for clearances, axial float, and coupling gaps; verify rotor positioning information and journal condition for bearings.
  • Lubrication and hydraulics: Obtain oil samples to check for cleanliness, water, and additives; verify interlocks and lubrication and control oil pressures.
  • Alignment and run-out: Evaluate shaft alignment, coupling fits, and total indicated run-out; fix soft-foot and perform laser alignment if indicated.
  • Controls and protection: Validate logic, trips and permissive; test overspeed, vibration, and temperature protection.
  • Electrical systems: Check connections on generator, insulation resistance, and grounding; check exciter settings and verifying synchronizing ready.
  • Pre-run baselines: Record initial vibration analysis, bearing temperature, and casing expansion during a short roll.

Common Findings and Adjustments

Even the best refurbishment efforts will uncover small anomalies, such as minor steam, or fuel leak, near flange joints, gasket weeps, or packing adjustments following thermal cycling. Often, crews simply need to correct the shim stack for couplings, re-torque critical fasteners, or adjust valve stroke settings to improve response. Rubs lightly observed on seals or labyrinth tips are usually evident any time during the first heat soak but will clear and allow for operation during controlled run-in. If the rub is persistent, then clearance will have to be adjusted.

Instrument drift is also a common post-refurbishment find, therefore RTDs and pressure transmitters are usually recalibrated. Finally, if weld repairs or crack indications are suspected, NDTs (such as dye penetrant or ultrasonic testing) are accessed, as appropriate, to verify integrity prior to release of the unit. Read here for more information.

How Inspections Verify Quality of Work

Quality confirmation is based on evaluating the “as-found” and “as-left” conditions against acceptance criteria that are predetermined. Then the inspectors will confirm the measured value was within OEM or engineering limits, as well as confirm operational expectation during test runs. This all creates some form of documentation that creates traceable evidence that the work, materials, and set-ups, reflect those expectations.

  • Measured sign off: Alignment, clearances, and run-out measurements are within tolerance and should have supervisor sign-off and QA.
  • Protection proof tests: Overspeed, high vibration, and high temperature trips actuated as expected and timestamped.
  • Performance confirmation: Heat rate, output, and efficiency should be within baseline; look for steam path or compressor cleanliness with the naked eye.
  • Emissions and safety: With gas turbines, confirm combustor tuning with NOx/CO checks; validate safety interlocks and purge sequence.
  • Life-cycle readiness: Archive spares lists, torque charts, and “as-left” vibration spectra for future reliability reviews and to act as a reference.

Best Practices for Documenting Results

Clear and credible documentation is as important as the actual inspection before the equipment is released for use. Utilize a checkbox which is uniform for all inspections and aligned with projected work scope and includes location for measurements, photos and signatures where applicable. Data should include items that allow trends—alignment measuring targets, managing bearing temperatures, peaks in the spectrum— so that the subsequent outage team can normalize and compare like-for-like. Stores records in the plant CMMS (https://en.wikipedia.org/wiki/Computerized_maintenance_management_system) or EAM associated with any digital assets in the asset’s history.

Clearly indicate any photos associated with a borescope inspection and plot any results from vibration analysis, annotated as necessary. Lastly, provision a closure on the process with an expectation of after-action review that will justify lessons learned and provide figures for updating procedures in consideration of the next provided turnaround. Attending to details and purposefully repeating a process turns the effort of one outage into lasting reliability for the gas and steam fleets.