Turbine oils

An oil that has to last years, not hours

Turbine oil works in apparently gentle conditions — light loads on hydrodynamic bearings, moderate temperatures — but stays in the machine for tens of thousands of hours, sometimes more than ten years. That moves the criterion from load capacity to stability: oxidation resistance, measured by RPVOT and TOST, is the central property. The second is demulsibility, because in a steam turbine water gets in constantly past the seals and has to separate in the reservoir before reaching the bearings. The third is air handling: an oil that foams or retains air reduces the stiffness of the hydrodynamic film and affects the governing system. And the sector's modern problem is not wear but varnish — deposits from degradation products that settle on control valves and stick precisely what has to move fast.

Classes and specifications in common use

The ISO 6743-5 classification gives the usual classes: TSA for steam and gas turbines in normal service, TGA and TGB where thermal demands are more severe, TSE and TGE for versions with extreme-pressure additives, used where the turbine drives through a gearbox. Viscosity grades are almost always ISO VG 32 or 46. The specifications actually asked for at purchase are DIN 51515 parts 1 and 2, ASTM D4304 and, most often, the turbine manufacturer's own approval — Siemens, GE and Alstom keep their own approved-product lists, and those are the ones that count. Hydro turbines fall into the same classes, with more weight on corrosion protection, since water is permanently present.

What to watch in a monitoring programme

Turbine oil is not changed on an interval, it is changed on analysis — and the analysis has to watch the right things:

• RPVOT falling below roughly 25 % of the new-oil value: the antioxidant reserve is nearly gone, even if every other parameter still looks normal.
• MPC (Membrane Patch Colorimetry) for varnish tendency — it moves well before valves start to stick.
• Demulsibility, checked periodically: if the oil no longer separates water within the standard time, the reservoir has stopped doing what it was designed to do.
• Particle count against the ISO 4406 class, especially on systems with fine hydraulic governing.
• Compatibility when topping up — a turbine oil from another supplier can meet the same specification and still accelerate varnish formation in the mix.