How to recover corroded retaining rings

When Quartzelec has been contracted to perform major generator overhauls, including the removal of rotor retaining rings, it has been found that under certain conditions, the industry-standard stainless 18/18 material will corrode. This, in turn, can lead to fatigue cracking and increase the risk of stress corrosion cracking (SCC), even in 18/18 steel. SCC (in particular) has in the past been known to cause catastrophic generator failure or, at a minimum, has extended planned shutdown times to enable repair work.

THEORY

Most generator designs will have a separate damper winding with copper ‘fingers’ placed underneath the retaining rings. Very often, damper cage contact points are silver-plated to improve the conductivity. Such a design allows the large circulating currents seen on gas turbine (GT) motor starting (whereby the generator is used to start the machine’s rotation) to flow safely around the damper cage. Similarly it prevents damage during undesirable generator transient conditions.

Most retaining rings are secured to the rotor using an interference f it between the nose and the rotor body. The ring deflection from the asymmetric load from the end winding copper is homogenised with a centring ring at the outboard end, which is also held with an interference fit and located axially with a spigot. During assembly, the rings are heated in a controlled manner to expand them and then f itted over the rotor body shrink fit area.

On one particular occasion – during the routine outage of a 200MW generator – a simple visual and (subsequent) dye penetrant inspection located a high level of pitting, mapping the position of mating components in the retaining ring bore, inboard of the centring ring and at the nose in the axial locking groove and around the damper segments. Local polishing appeared to make the defects more prominent, indicating occluded pitting.

More detailed non-destructive testing, including replica and ultraviolet dye penetrant tests, was performed, and the report confirmed that the indications were formed from corrosion, rather than electrical arcing.

CORROSION MECHANISM

Corrosion develops where surface passivity films have been destroyed, and the corrosion products from a pit attack are often found to create a lid on top of the pit, with only a very small opening; very similar to the occluded pits found by Quartzelec.

Moisture, if present, is drawn into crevices, often by being centrifuged outward by the rotation of the shaft and the moisture becomes entrapped or drawn into gaps under capillary action. When deprived of oxygen, stainless steel lacks the ability to re-form a passivating film of chromium oxide, and crevice corrosion occurs.

Does such corrosion automatically render the rings as end-of-life? In many cases, that may well be the outcome. However, Quartzelec has been able to offer alternative approaches, backed up by rigorous engineering.

RETAINING RINGS STRESS ANALYSIS

For the 200MW generator project, the existing retaining ring stresses and interference fit were calculated at standstill and overspeed, using thin cylinder theory and finite element analysis. The calculations were repeated to simulate the machining needed to radially remove material from the retaining ring to rotor interference fit diameter, bore diameter and inboard groove to remove the defects found.

On this particular occasion, Quartzelec concluded that the rings would be safe after machining, and the machinist was instructed to remove the corroded material carefully.

CONCLUSION

For the 200MW generator project, Quartzelec was able to successfully rehabilitate the rings, and they were returned to successful operation. Due to a lack of available replacement rings this saved the customer circa nine months of lost generation.

Defects in a retaining ring can lead to in-service failure. If this happens, the consequences are catastrophic, and the potential for loss of life is real. As a result, Quartzelec will always take a safety-first approach when considering any repair strategy.

18/18 stainless steel is not fully resistant to corrosion and, if not corrected, can lead to failure, and early-stage pitting is difficult to detect without removing the retaining rings.

In Quartzelec’s experience, such pitting is more likely to appear on generators coupled with gas turbines if they use frequency converters and the main generator during the run-up process. This does not mean that similar conditions could not be created on generators connected to steam turbines, and controlling the environment within the generator is vital – such as ensuring anti condensation heaters are operational and cooling water is off when the generator is shut down.

RECOMMENDATION

Quartzelec’s recent engineering studies have confirmed that machining away a certain amount of material radially, even from the shrink f it diameter, may not adversely affect the mechanical integrity and function of the retaining ring. Depending on the design and safety factor applied by the generator manufacturer, some reduction in interference fit could still maintain the shrink fit up to 120% overspeed.

To allow a thorough, non destructive inspection of the inner diameter of the retaining rings, it is prudent to perform a 'rings off' inspection every time a major generator overhaul takes place.

During periods of shutdown, generator temperatures should always be kept above the dew point to prevent condensation on cold metal surfaces.

If corrosion or pitting is found on the shrink-fit diameters, it might still be possible to rehabilitate the retaining rings by careful modification guided by an appropriate engineering study.

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This article appeared in Renew magazine. To read more or request your personal digital or print edition of Renew, click here.