GE recommends either Electrostatic or Balanced Charge Agglomeration oil purifiers for use on their machines to control varnish and sludge. They sell BCA themselves, because it works. No oil yet developed is 100% for sludge and varnish control. There are several on the market that can go quite a while without having problems, but none can go indefinately.
Our machines are installed on more than 800 turbines around the world. 7 of them on GE frame 7FA turbines are now in their 5 th year on oil that is almost 9 years old. The oil analysis on the seven turbines oils indicate that QSA averages 4, the gravimetric patch is much cleaner than new oil and RPVOT is 82% remaing useful life. The results of using these BCA machines have been so good that the company doing the analysis ran the samples a second time to be sure that what they were seeing, was true.
This power company just sent their servo valves out for the first time since the plant started, not because there were problems, but because it was nine years of faultless operation. They felt that, maybe, it was time to do some maintenance.
If you remove submicron contaminants, the oil could last the life of the machine. This plant found after 3 years the varnish levels had coated their tanks orange. Oil alone will go bad if just filtered by conventional means. Filters are there just to keep the big chunks of contamination out of critical parts. Active filtration using an electrostatic process, like BCA, cleans oil and protects the machine from oil degradation, sludge and varnish.