Innovation Allows Efficient and Successful HP Rotor In-Place Machining

Mechanical Dynamics & Analysis (MD&A) has successfully utilized specially designed equipment for HP Rotor in-place machining to effectively repair a damaged steam turbine rotor, resulting in substantial savings in both cost and time.

The high-pressure (HP) section of a 320 MW Alstom® STF30 steam turbine was severely damaged. Over time, the rotor’s shaft end at the oil deflector area developed a groove approximately 0.58 in. (14.7 mm) deep. Either repair or rotor replacement was recommended by the OEM.

Shaft End Close Up View – Damaged Area

Shaft End Close-Up View – Damaged Area

Full unit disassembly and rotor removal would be costly and time-consuming.

Experienced professionals at MD&A’s Machining Repairs Division devised the most efficient and robust long-term solution. The rotor would remain in place within the machine.

MD&A technicians would machine the shaft end to a smaller diameter, removing all damage while restoring the turbine end and generator end transition fillet radii. MD&A would also machine to restore the depth of the oil slinger groove.

MD&A’s scope included:

  • Disassembly and reassembly of the unit front and mid standards.
  • Machining repairs to the HP rotor at the mid standard.
  • Borescope inspection of the HP section.
  • Visual inspection of the LP section L-0 buckets.
  • Repacking and seal replacement of the right-side control/stop valve.
  • Design and supply of a modified oil deflector to match the new machined shaft.

MD&A mobilized to site with equipment, tooling, and dedicated experts to perform all work. The equipment was specifically designed and produced to perform work within the tight quarters of the mid-standard between the HP and IP turbine sections.

The Process

This Alstom® turbine is designed with a separate HP section, IP section, and double-flow LP section.

The specific shaft area of damage was at the mid-standard between the shaft end steam packing gland and the inboard thrust collar, on the turbine-end side of the oil seal housing.

To perform this work, the upper half of the generator exciter bearing was removed for manual lubrication of the lower half bearing. The mid-standard upper half housing and turning gear assemblies were removed for direct access and machining equipment installation. The front standard cover and upper half oil deflector were removed to install a temporary shaft train rolling device to support machining of the damage shaft in the bearings. FME measures, blanks, and filter screens were also installed in the mid-standard supply and oil return lines to eliminate possible contamination.

The main lubrication oil tank was drained for installation of lift oil system jumpers. This allowed the unit to be rolled on lift oil during the machining operation without the main lubricating oil supply. Not having lift oil capabilities, the HP turbine T1 bearing and the generator exciter bearing had their upper halves removed to allow manual oiling during rotating of the shaft train for the work.

Casted molds were made of the damaged area, reverse engineering of the original geometry was performed, and all required measurements were taken. The area was prepared for machining.

Shaft End – Setup for Machining

Shaft End – Setup for Machining

The shaft end was then machined to remove all damage. Shaft diameter rough machining was completed, followed by both fillet transition radii. Final machining and shaft polishing to achieve the required surface finish followed. Shaft diameter was reduced by ~0.6165” per side to a diameter of 8.610 in. (218.69 mm). The distance between axial faces and the transition radii geometry remained the same.

The new final machined geometry was measured, final non-destructive testing performed, and final runout taken of the shaft showing a maximum TIR of only 0.5 mil (0.0005 in.).

HP Rotor Shaft End – Final Machined

HP Rotor Shaft End – Final Machined

At the completion of repairs, the jumper lines were removed, the main oil tank was cleaned, and the lubricating oil system restored.

During the site work, MD&A experts dimensionally reverse-engineered and manufactured a new oil deflector/oil seal housing, installed new seal strips, and precisely machined to achieve the required clearance to the new shaft diameter.

New Oil Deflector – Installed in Upper Half Mid Standard Cover

New Oil Deflector – Installed in Upper Half Mid Standard Cover

Borescope and Visual

Right side stop and control valves were removed for borescope inspection of the HP section by MDA’s Steam Path Division. MD&A also performed a visual through-the-hood inspection of the L-0 buckets.

Additional visual inspections were performed to look for any signs of damage to the exhaust hood struts, hood sprays, and ID casing wall above the bucket tips.

No significant issues were found in the inspections.

With the valves disassembled, new stem packing was installed, and MD&A specialists performed a visual inspection of the seats and discs. New gaskets, seals, and lock plates were installed at reassembly. Also, stop and control valve seal head lock plates were installed and all stem packing and seal bolting was retightened to OEM specifications.

MD&A’s Outage Services Division also provided support for the setup and breakdown of equipment necessary for the post machining cleaning/flushing of the main oil tank and lube oil system.

All components and systems were reassembled, and the steam turbine returned to service.

Full-Scope Repair

The on-site work of the HP Rotor In-Place Machining by the dedicated MD&A experts completed all tasks on time. All inspection findings were positive, and there were no reportable issues during startup.

Specific and comprehensive recommendations to the customer from MD&A should ensure that this type of damage does not occur again.

MD&A routinely rallies its best people from multiple divisions for critical on-site work to save both cost and time for its customers. Creative solutions like in-place machining are key examples of our commitment to our customers, the power generation industry, and its many systems regardless of the original equipment manufacturer.

For all service and repair questions, and to discuss creative resourceful solutions, call MD&A today at +1 (518) 399-3616 or use our Contact form.

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