Steam Turbine N2 Packing Casing Repairs
Mechanical Dynamics & Analysis (MD&A) received a contract from a customer in Japan to inspect and repair the N2 Packing Casing of a GE® A-13 frame, 131 MW reheat steam turbine. This unit is a single-shell design with an opposed-flow turbine section that includes 12 HP impulse stages, 9 reheat IP impulse stages, and an N2 packing casing in between the HP and IP flows. The turbine operates at 3000 rpm, 1000F inlet and 1000F reheat.
During full-load operation, the steam pressure differential between the HP and IP turbine section forces the packing casing towards the IP section. The outer perimeter main axial locating fit of the packing casing resists this unbalanced steam force.
At full operating temperature, the casing material is vulnerable to cumulative creep rupture damage. Radial circumferential cracking has occurred within the industry and has led to loss of axial positioning of the N2 packing casing, potentially causing internal damage to the turbine rotor and diaphragms.
Inspection and Engineering
Incoming disassembly, blast cleaning and detailed inspections were performed by component specialists at MD&A’s Turbine Generator Repairs Division facility in St. Louis, MO.
Following incoming inspections, the original geometries of the casing main axial fits were reverse engineered, a key activity by MD&A’s experienced and precision-oriented professionals. This process measured the main axial fit widths, main axial fit fillet radii and chamfers, and fit faces critical axial positions. MD&A experts found evidence of distortions, galling and fretting damage.
A complete and clear listing of recommended repairs and hardware replacements was then prepared and reviewed with the owner/operator.
Findings and Resolutions
Initial recommendations focused on the lower half and upper half N2 packing casings.
For the lower half casing, inspection revealed rough surface and erosion on the outside diameter main axial locating fit steam seal face. MD&A experts would restore the steam seal face by welding and machining. For the upper half casing, findings included rough surface and erosion on the OD main axial locating fit steam seal face. Also, the fillet radius-to-steam seal face showed visible intermittent liner crack indications. They removed the crack indications and restored the original geometry by welding and machining.
Further investigations revealed additional requirements. For the upper half casing, OD left and right-side dowel pin though holes would need repair due to excessive clearance.
Horizontal zero-clearance joint packing groove key pocket threaded holes, both left and right sides, showed thread damage and excessive tap wobble. This appeared on both the turbine and generator ends. MD&A actions included removal of the damaged threaded hole, weld restoration and machining for all four locations, followed by post-weld heat treatment.
Horizontal joint studs, softened during the original work scope, were removed by destructive machining and replaced with new hardware. Selected new hardware would also be required for horizontal joint steam dam springs and screws; left and right-side elevation block bolts, clearance keys and key screws; horizontal joint nuts; and horizontal joint studs.
Restoration
The packing casing halves were welded and post-weld heat treated. The main axial locating fit was weld restored, as was the zero-clearance groove keeper block screw holes.
Post-weld heat treatment was performed, utilizing distortion control bars, which were removed , followed by blast cleaning of both casing halves.
Final machining and grinding were then performed. This included the main axial fit geometry, and final grinding and blending to clear all NDE indications. In addition, the back-side boss surface of the main axial fit steam seal face was welded to increase thickness and improve stiffness.
Inspections, Assembly and shipment
Final inspections, and final NDE and dimensional checks were then performed by MD&A Quality personnel.
Full 100 percent magnetic particle inspections revealed no findings. Casing upper and lower halves were laser mapped to confirm flatness. Final diameters and vertical radial drops were also completed.
MD&A then completed final assembly of the upper and lower halves in preparation for return shipment.
MD&A successfully completed the inspections, engineering, repairs, restoration and improvement of the N2 packing casing at its St. Louis facility. This project enters the MD&A portfolio as another example of the company’s global response as a premier, non-OEM service provider to the worldwide steam and gas turbine generator industry.
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