Steam Turbine Oil Change Frequency

The ASTM 4378 (in service monitoring of mineral oils for gas and steam turbines) quoted above should be a good start. There is also ASTM D6224 (auxillary equipment for power plant). Speak to your oil analysis service provider about including a varnishing index potential and filter patch tests for the oil.
The achievable service hours on the oil will depend on each system and how it is operated. Only you can determine that through regular oil analysis.

go for condition base. tan, water content, flash point analysis are normal. people talks about varnish nowadays but it is more towards GT, not st where there are a lot of hyraulic controls involved in GT. most equipment manufacturer do recommend 6 years oil change, IF you do not have regular oil analysis. now that you have condition monitoring in place, why not optimise it? oil loss from vent line or leak is normal hence fresh oil top up will always be there. this kind of rejuvenate the oil quality to some extent.

35K litres is enormous. TA normally done every 3 years. 5 years is the figure most industry looking now. the question you should ask yourself is, why would you change your oil after 2.5 years when your oil analysis result, bearing temperature, vibration, turbine control system are all running in good shape?

btw, if foaming is noted, if there is, is it possible to add in anti-foam additives inside the reservoir rather than we change out the oil?

Why dont you use biochemicals which will prolong the sludge & foam forming.

20 years is a good number to shoot for. Condition monitoring is essential, as is avoiding contamination of all kinds. TAN may not be as useful an indicator as it once was, or it needs to be looked at differently than a few decades back.

It used to be that the TAN would creep up very slowly over years and when it got to ~0.5 mg KOH/g, it was getting ready to take off. With some newer turbine oils I've seen them shoot up to ~0.5 in a relative hurry, then hang there. This has caused no small amount of consternation to the operators in question.

Has anyone else seen anything similar? I wonder if this is a phenomenon that relates to Grp II base stocks?

We believe it is related to Grp II oils. If you send me an email I would be happy to forward you a presentation that may be of interest.

A large pharmaceutical company nearby bought our first BCA machine about 17 years ago for a steam turbine. They never changed oil and the purification system kept contamination in a cleaner than new condition for all of these years. Proper purification along with a good program for oil analysis will keep the oil running for a long time. The side benefit is they have had no oil related break downs during those years.