Investigation of varnish-induced bearing temperature fluctuations in a 1750 MW nuclear steam turbine through oil, resin and deposit characterisation

Purpose Field cases linking a specific bearing temperature event to deposit composition are limited for large and nuclear units. The purpose of this study is to use a recurring fluctuation on a 1750 MW nuclear turbine-generator unit to follow how sulphur-, sodium- and phosphorus-bearing additives are degraded and redistributed during varnish formation. Design/methodology/approach Intermittent fluctuations recurred at the thrust and generator radial bearings, with varnish found at overhaul. Five samples – varnish, fresh and used oil after five years, spent adsorption resin and laboratory-aged oil – were analysed. Findings Additive-derived sodium, phosphorus and sulphur were the most abundant in the varnish; although limited in amount, their distribution reflected additive degradation. Methanol-extractable sulphur fell from 2970.0 to 120.9 mg/kg, the 140°C ageing solid contained 5.8 wt% sulphur, and the same species were retained on the spent resin. The element order changed from S > Na > P in the bulk solid to Na > P > S in the varnish, indicating preferential accumulation of sodium- and phosphorus-bearing species at the Babbitt surface. Practical implications On-site purification cut the Varnish Potential Index from 21.6 to 8.8 and stabilised bearing temperatures, but removed functional additives. Originality/value By tracing sulphur-, sodium- and phosphorus-bearing additives from oil through spent resin and ageing products to the deposit, the study shows that additive degradation and redistribution play an important role in varnish formation, providing a basis for varnish monitoring and additive replenishment in large turbine sets.