Five years ago, San Onofre Nuclear Generating Station’s Unit 3 turbine experienced substantial damage after the supply of oil to its bearings failed. Because the turbine coasted down without oil after it tripped, its bearings, journals, and steam path needed extensive repairs. A follow-up investigation revealed lube oil system vulnerabilities that were subsequently corrected. The lessons learned might also improve your turbine-generators’ lube oil reliability, saving you many millions in lost revenues.
The San Onofre Nuclear Generating Station (SONGS) in northern San Diego County has one of the most beautiful locations of any power plant in the world (Figure 1). The plant, which is 75% owned and operated by Southern California Edison (SCE), consists of two nearly identical pressurized-water reactors rated at about 1,170 MW each. Units 2 and 3 (Unit 1 was mothballed in 1992) went on-line in 1983 and 1984. When the 1,800-rpm turbine-generator (TG) sets fed by the reactors’ steam generators were ordered from GEC (now Alston Power) in the late 1970s, they represented the largest nuclear prime movers ever built by the company.
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The lifeblood of any piece of rotating machinery is lubricating oil (LO) supplied at the right cleanliness, quantity, pressure, and temperature under all anticipated operating conditions. An adequate supply of LO is especially important for turbine-generators as they coast down after a trip. For this reason, GEC’s common design practice in the late ’70s was to provide triple redundancy for LO supply—via a shaft-driven main lube oil pump and AC and DC motor-driven pumps during start-up and shutdown. However, the SONGS units were not equipped with shaft-driven LO pumps, for the following reason.
Under normal operating conditions, one AC motor-driven pump runs while the other remains on standby, ready to automatically start should LO pressure fall below 12 psig. If LO pressure falls below 10 psig, the turbine trip circuits are energized after a short delay, and the TG is tripped. The emergency, DC motor-driven pump, rated at about 65% duty, also is on automatic standby, ready to take over if both AC motor-driven pumps fail. No additional reliability features, such as gravity-feed systems or accumulators, were included in the original design (Figure 2). This double-redundant design served the plant well for many years, until early 2001.
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