Coordinated approach

The repair project was coordinated by Sulzer’s Birmingham Service Center, with assistance being provided by facilities in Daventry, Aberdeen and Southampton. The allocation of work across different service centers ensures each designated task is carried out by the service center that specializes in each required discipline. This also allows several tasks to be carried out at the same time, which is necessary to achieve the short turn-around expected by the customer.

A team of engineers from Aberdeen built the platform, stripped the unit, installed the custom rotor removal gear then unthreaded the rotor which was sent back to the Birmingham Service Center. The stator was cleaned before a full life-assessment investigation was carried out to determine the integrity of the component. The tests carried out included offline partial discharge, tangent-delta, wedge tightness, core lamination imperfection and end-winding resonance.

Unfortunately, the recommended maintenance schedule for the generator had not been followed and this contributed to a number of more serious issues that were discovered during the routine maintenance.

Uncovering hidden issues

In the Birmingham Service Center, the initial visual condition of the generator had already highlighted some detrimental issues. The complete rotor body was covered in rust and the single-length aluminum wedge was corroded and flaking away across the entire length. As-received electrical tests were carried out before the retaining rings were removed.

A cross connection coil was found to have migrated with bare copper only millimeters away from the earthed shaft. A catastrophic failure would have occurred if there had been contact while in service. The overall insulation system on the rotor was also found to be very poor. The retaining rings were corroded and both failed ultraviolet dye penetrant testing. Ultrasonic testing on the single sleeve bearing also highlighted extensive de-bonding of white metal from its parent backing material.

The Sulzer engineers recommended that the rotor be completely refurbished. This was to include a full strip down to the main rotor forging, removing all rust, cleaning all copper coils, machining new retaining rings, machining a set of new rotor wedges, re-metaling the sleeve bearing and rewinding the unit using existing copper. The original work scope was to be turned around in 25 days, with this variation in the order, Sulzer promised that the complete refurbishment and installation would be completed in 50 days.

At the onset, the challenge was to remove the individual slot wedge which was approximately 2.5 m long. The corrosion and bonding of dis-similar metals between the wedge and rotor forging did not allow easy removal. All wedges had to be split in half using a custom-made milling head.

Once the rotor was fully stripped and cleaned, the individual parts were detailed for reverse engineering. Due to the destructive removal of the rotor wedges, the new wedges were detailed off the rotor forging wedge slot.

Creating exact replica parts, fast

As with any reverse engineered part replacement, it is essential to use the same or superior material while maintaining the same tolerance, as per the original part, to ensure that the rotor performs and meets the original equipment manufacturer’s (OEM’s) design criteria. Material analysis on the wedges matched an old French standard for aluminum alloy which took some considerable time to source the required quantities and lengths. All machining works were carried out and quality controlled in-house in the Birmingham Service Center.

The re-metaling of the bearing was tasked to the Southampton Service Center, which has over 45 years’ experience in re-metaling Babbitt alloy bearings. Bearing clearances for this specific application were calculated, specified by the engineers and supplied to Southampton to include into the quality work pack.

The biggest challenge was to source the specialized high tensile, stainless-steel forgings in the correct size, which are not readily available. Only a handful of forge masters and manufactures have the experience and expertise to produce these items. A typical lead-time for procuring retaining ring forgings and machining is around six months; this could be longer when procured from the OEM.

Using Sulzer’s extensive global network of suppliers, the projects team was able to procure a couple of forgings which met the engineers’ specification from the USA, which arrived within five days. In order to minimize the time required to complete the machining, the task was split between the Birmingham and Avonmouth Service Centers.

Working round the clock, the new retaining rings were finish-machined and inspected within a 15-day period, with the time from point of ordering included. This extraordinary effort went a long way to keeping the overall project schedule below the 50-day target.

Ensuring perfect balance

The complete rotor was rewound with new insulation and the cross-over connection was redesigned to eliminate any future migration. Once completely rebuilt, the rotor was electrically tested and dynamically balanced in Sulzer’s in-house high-speed balance pit.

As this unit was of a single bearing design, a stub-shaft with a journal was designed and manufactured to complete the balancing and over-speed exercise. This substantial stub shaft was fabricated and machined at Daventry to the engineers’ drawings and specifications.
As the unit was practically a brand-new rotor, a 120% overspeed test was carried out for two minutes on this rotor and witnessed by the customer. This specification is in line with international standards for brand new turbo-generator rotors.

With the project almost complete, Sulzer engineers returned to the site to help with reassembly of the generator and to conduct the onsite load testing and recommissioning before the project was successfully concluded. In this way the power station made a significant improvement in the reliability of its generators and also made sure that every household can continue to rely on a steady supply of electricity.