Once the generator rotor was removed, visual inspections confirmed melted copper turns, damaged insulation, and arcing damage to the rotor forging.

Inspections after disassembly (unwinding) of the generator rotor revealed arcing damage to the inboard side of the collector end retaining ring and to the slot wedge above the ground fault area. The rotor forging was then blast cleaned, clearly revealing the severity of arcing damage to the generator rotor forging slot tooth sidewall.

The unit owner/operator asked MD&A to evaluate and perform an analysis of the damage in support of providing a recommendation for running/repairing/replacing the generator rotor forging.

The slot tooth sidewall arc-damaged and heat-affected material was removed, the forging NDE inspected, material composition confirmed, and the geometry of the rotor forging as well as the excavated tooth sidewall obtained. A detailed stress analysis was performed consisting of both hand calculations and finite element analysis for the damaged geometry. The analysis calculated the damaged tooth stresses under centrifugal load for both normal operational speed as well as over-speed. The results of the analysis revealed the current condition was not acceptable for safe and reliable operation.

Solution: MD&A recommended and performed a local weld repair of the damaged tooth section utilizing a weld filler material that met the base material chemistry, strength, and magnetic permeability of the generator rotor forging.

The welding testing for this repair option not only included the typical base material and weld filler material qualifications, but also actual base material HAZ (Heat Affected Zone) testing utilizing a bore ring sample removed from the coupling end of the rotor forging. The results of these tests were utilized in a final weld repair acceptance and remaining life analysis.

Repair of the rotor included: Initial dimensional inspections, nondestructive testing, and rotor runout checks; Weld prep machining to remove a section of the slot tooth; Preheat and weld buildup of the removed slot tooth section; Post weld heat treatment; Rough machining along with both surface and subsurface nondestructive testing; Final machining, final dimensional checks, final nondestructive testing, and final rotor runout checks. The rotor was then released for winding, final assembly, and balance.

A detailed acceptance/life expectancy analysis was performed on the rotor weld repair. The analysis consisted of a 3D Finite Element stress analysis, fracture mechanics calculations, as well as low cycle fatigue calculations. The results of the analysis offered positive results and increased confidence in the repair performed.

Based on the past and expected future operational parameters of the unit, it was calculated that the original rotor material, the deposited weld material, and the HAZ material would conservatively last for 36, 41, and 16 years, respectively.

The repaired rotor was returned to service without any operational restrictions and it was recommended the rotor forging be inspected after ten years of operation or 150 stop/starts, whichever comes first.

This generator rotor tooth local weld repair allowed the unit owner/operator to return to service much quicker than a full rotor forging replacement option.

MD&A experts are there when you need us! Providing comprehensive refurbishment and repair of generator stators and fields for many different OEMs across the world.

Call MD&A today for any generator repair at +1 (314) 880-3000 or use our Contact form.

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Take the generating unit whose valve stand suffered an exceptionally deep crack in its inner bore caused by an improperly installed bushing. MD&A received the valve stand at its St. Louis repair facility not knowing just how extensive the crack was. Working with the utility customer, a repair program was designed that included excavating the crack, weld-repairing the stand, and installing new bushings. The repair was completed in three weeks and saved the utility from buying an entirely new valve stand.

At MD&A’s repair facility, a full set of readings were taken on the valve stand. Inspection revealed heavy galling on the bushing fit ODs, and measurements were taken to determine if there was enough stock for the proper interference fits.

NDE inspection revealed a crack extending some 14” into the valve stand bore. It began on one side of the bore, moved toward the valve’s outboard side, wrapped around the bore, and came back toward the inner bore.

The repair plan included machining the bushing bore to determine how deep the crack was, and removing an OEM modification that held the inner bushing fits. After removing ¾” on a side from the bore, MD&A determined that the crack persisted under the stud flange, so the repair was modified to hand grind the crack.

With the crack removed, the valve stand was pre-heated and MD&A technicians hand-welded deep pockets that were present on the valve stand to return those areas to the proper bore diameter.

Once the hand-ground area was welded and hand-worked, a second NDE was done to ensure no new cracks had formed. Seven layers of weld were added to the valve stand using an automated welder. After completing the post-weld heat treatment (PWHT), the stand was set up in a horizontal boring mill for rough machining to within .030” final size.

A final NDE was done to ensure no further cracking had occurred, and then the bore was machined to the final size. All bushings were machined, installed and the valve underwent a try bar check. As a final step, the valve was set between centers, the casing fit OD, and the gasket face was cut to design size.

For this critical valve stand repair, MD&A tapped its deep and experienced team of experts to complete the repair in a time frame that quickly returned the unit to full service versus replacing the valve stand which would have taken months.

Check out our video: Full-Service Valve Capabilities Video

Have MD&A inspect or repair your valves today, call our Turbine-Generator Repair Facility at (314) 880-3000, or use our Contact form.

The post Quick Valve Stand Repair appeared first on MDA Turbines.

MD&A welded and repaired the HPIP outer shell, HP & IP inner shells, and nozzle box on a 677MW large steam unit. Inspections revealed axial distortion and mechanical damage of the critical positional fits on the outer shell, inner shells, and nozzle box caused by galling during an overhaul disassembly, and out-of-roundness of the HP and IP inner shells due to operational thermal stresses.

With over 40 years of experience, MD&A has the technical expertise in the power industry to repair your turbine. Our philosophy involves intelligent, technically sound, and proven repair options.

The HPIP outer shell upper half galling damage on the axial positional fits to the inner shells was weld repaired on-site. The repair involved weld preparation, NDE, preheating, welding, heat treatment, final machining, runout checks, dimensional checks, and final NDE and hardness checks.

Due to the welding and machining operations performed on the outer shell to repair the damage to the axial fit face surfaces, this slightly changed the axial position of the sealing face and reduced the overall width of the tongue for the tongue and groove fit in the outer shell. In order to restore the correct axial position of both the HP and IP inner shells within the outer shell, and to ensure the tongue and groove fits had the correct designed clearances, the entire 360° seal faces and clearance faces of the HP and IP inner shell fits were weld restored and machined.

Weld repairs and a full stress relief re-rounding operation of the HP and IP inner shells was performed by MD&A’s experts in St. Louis Turbine-Generator Repair shop. The HP and IP inner shells were re-rounded to within 0.036” of original design diameters after the stress relief re-rounding operation was completed.

Also, due to the amount of as-found horizontal joint gaps and diameter out of roundness with all studs tensioned, it was decided to perform horizontal joint welding and heat treatment of both the HP and IP inner shells. The repair process involved weld preparation, NDE, preheating, welding, heat treatment, final machining, and checks.

The upper half nozzle box damage was weld repaired, the snout ring bores were honed, and new snout rings were installed. The raised material and galling damage on the axial fits of the nozzle box were ground out and the fits were prepped for welding. The areas were preheated, welded, and locally post-weld heat treated. A GTAW welding process was used for this operation. Once the work was completed, a final NDE inspection was performed on the welded and dressed areas and no indications were noted.

Because of our vast knowledge and experience with different types of OEMs, MD&A experts are capable of performing any welding repairs. We are your full-service turbine-generator service provider. No matter what the problem, MD&A’s experts have the solution!

Have MD&A look at your casings/shells today, call our Turbine-Generator Repair Facility at (314) 880-3000, or use our Contact form.

The post Casing/ Shell Weld Repairs & Re-Rounding appeared first on MDA Turbines.

The HP valves were the first to be inspected in-shop. It took very little effort to liberate the stellite from these valves. In some cases, the entire inlay popped completely out of the valve seat, which indicated very little, if any, bonding to the parent metal. It is our valves project manager’s expert opinion that a butter pass of either Inco or 309L was not inlayed between the parent metal and the stellite inlay. It has also been his experience that the butter pass is a must to ensure proper bonding takes place.

This is the stellite inlay from one of the HP valves. Obviously there was a bonding issue, specifically a lack thereof. The lack of bonding was found to be worse on the HP valves than it was on the IPs. MD&A has experienced this issue on several occasions, and our preferred method of repair is to remove all of the stellite, along with an amount of parent metal, to excavate any of the material that may have migrated.

We then pre-heat and weld build up using for 410SS filler material. After a PWHT, the valve seats are then machined to the as found contour, and final lapped to achieve 100% blue contact with the associated disc. The method of welding is semi-automatic gas tungsten arc welding (GTAW). This is performed with a machine that was developed by MD&A and built in conjunction with an industry leader in the manufacture of semi-automatic welding equipment. Below is a picture of that machine mounted to the end of a valve body.

All valve seats were overlaid with 410SS, except the LS HP CV. The parent metal was full of porosity/trash and a solid overlay could not be produced. MD&A machined out enough parent metal to allow for the installation of a shrunk in seat insert that was made from 410SS forging.

It was then seal welded both in the bore and in the front face of the valve seat. That was done with a 500°F pre-heat, and no PWHT was performed. After machining, all seats were lapped and 100% blue check was achieved.

Below is a picture of a completely refurbished valve seat, showing the blue contacts from the disc to seat check.

MD&A also performs valve seat repair in-place or on-site, saving you time and money! A fabricated seat can be installed in the valve body and seal welded in lieu of a complete weld repair solution for the seating surface.

Have MD&A look at your valve seats today, call our Turbine-Generator Repair Facility at (314) 880-3000 or use our Contact form.

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MD&A’s Turbine-Generator Repair Facility performed a diaphragm dishing repair to the stage 15 diaphragm by cutting out the old weld, re-positioning, re-welding, stress relief, and final machining. Incoming inspections and evaluations consisted of blast cleaning, NDE, visual inspections, and dimensional checks. The stage 15 drop checks were taken as a baseline for the as-received diaphragm. Based on the opening clearance measurements, it was found that the stage 15 diaphragm was dished downstream by approximately 0.215”.

Due to the unacceptable amount of dishing found on the stage 15 diaphragm, the stage 15 diaphragm was repaired utilizing a welding and post-weld heat-treating process.

The repair process included machining to remove the spacer bands to inner and out ring welds, re-positioning the diaphragm back into the correct axial position, restoring the welds to provide a 100% full penetration weld, and then post-weld heat treatment.

After post-weld heat treatment, the diaphragm halves were disassembled from the jacking and backing plates, blast cleaned, and dimensional checks were performed to verify and ensure the diaphragm halves inner web rings were re-positioned back into the correct axial position.

The dimensional checks confirmed that the diaphragm halves’ inner web rings were positioned back upstream by between 0.240” and 0.260”. Once confirmed, each diaphragm half HJ was set up on a horizontal boring mill to machine and restore the HJ surface as well as the dowel holes and keyways. The diaphragm halves were assembled and the assembled diaphragm was set up on a vertical boring mill to the final turn machine. Once all machining was completed, a final NDE was performed on all welded and machined surfaces. Lastly, the stage 15 diaphragm was assembled to the stage 16 diaphragm.

See our handout: Dished Diaphragm Repair Options

Let MD&A perform your next Diaphragm Dishing Repair today, call our Turbine-Generator Repair Facility at (314) 880-3000, or use our Contact form.

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