They must have high strength and fracture toughness to withstand the tangential and axial stresses that are developed from the centrifugal load of the copper coils, insulation and blocking, the centrifugal stresses developed due to its own mass, and the shrink-fit stress.
Retaining rings can be manufactured from either magnetic low alloy steel or non-magnetic austenitic steel.
Thirty years ago, alloy non-magnetic 18Mn-5Cr type retaining rings were the norm for generator rotors. But those rings have been known to fail, with devastating results. Current state-of-the-art non-magnetic rings are forged from an 18Mn-18Cr alloy that has very high strength, fracture toughness, and stress corrosion resistance.
“Some of the older retaining rings, including the 18Mn-5Cr type rings, have a tendency to develop stress corrosion cracks if exposed to moisture or water,” says Dr. Donald McCann, a PhD in metallurgy and director of materials and analysis for independent turbine and generator service provider ReGENco.
“If a crack in the ring grows to the critical size during operation, the tangential stress will burst the ring in the axial-radial direction,” says McCann. “The ring usually bursts into two major pieces with minor fragments.”
When that happens, the broken edge of the ring rubs against the stator coil ends, tearing the insulation, copper and stator iron until the stator windings and core iron are gouged so deeply that the ring and rotor stop. During the gouging and tearing process, the rotor can become so unbalanced that it can be destroyed. Fires from insulation resins, oil line breakage or hydrogen, if hydrogen cooled and seals are damaged, are also common.
Although the stator copper coils and the core iron will usually absorb sufficient energy during the failure process so that retaining ring pieces are not liberated through the generator casing, repairing the generator will be very extensive, expensive and time consuming.
McCann says there have been more than 30 instances where 18Mn-5Cr type retaining rings have failed in service. And many such rings have been retired due to severe cracking. “Although the failure rate is low, when it does occur it is catastrophic,” he says.
He notes that most OEMs have experienced retaining ring failures that resulted in wrecks with subsequent long repair times and high costs. Yet some owners choose to keep their 18Mn-5Cr type rings because they feel they can monitor the rings and manage their exposure. “From experience, we have seen that the cost of properly inspecting the rings on a regular basis makes replacement a better alternative,” says McCann.
He says that even with inspection at regular intervals, cracks are unpredictable and can grow rapidly under moisture conditions. He cites a retaining ring failure at a plant in 1990 that occurred just 19 months after a complete inspection with the rings off the rotor. The stress corrosion crack was less than half an inch deep when the ring burst. Subsequently, it was determined that the hydrogen dryer was not operating properly.
His advice: “If you have 18 Mn-5Cr type rings, you should visually inspect all retaining ring external surfaces for cracks. If cracks are found, replace the rigs. Also look for water stains, rust, pitting and fretting debris (dry red powder). If found, eddy current inspect external surfaces and ultrasonic inspect internal surfaces. If indications are found, the rings should be removed for 100 percent inspection by eddy current in order to determine the type and size of flaws. If cracks are found, the rings should be replaced.”
He cautions that retaining rings that are at the greatest risk are on peaking machines. The very nature of the use pattern of the peaking machines (off line for long periods) makes them especially vulnerable to moisture problems – the biggest contributor to cracks. Although the nominal stress on smaller rings is low, stress concentrators, such as fillets, can initiate stress corrosion cracks if moisture is present.
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