Does the condition occur at one bearing housing or both?
Do you have a drawing available to post? If not it would be helpful to understand the lub method..
* Is there an oil ring? (if so, is it possible to observe oil ring operation to see if it is smooth or erratic.)
* any other oil pumping/moving device in the reservoir?
* What is oil level relative to the rolling elements?

If you are suspecting contamination (sometimes it is difficult to flush), you can easily try a home-made foaming test comparing characteristics of oil drained from the reservoir to new oil like in the following link:
http://maintenanceforums.com/e...251042004#9251042004

quote:

One thing to mention, due to leakage in discharge line after 1.6 km, we installed an orifice at discharge of pump line after operation team recommendation.

This is the process fluid, not the oil system, correct? Ordinarily I'd assume it is not related to the oil system. But since you mention it, what could be the relationship to the oil system? Operating far to left of curve can increase vib, but if vib is extreme enough to affect the oil system I'd think you will easily see it in your routine vib monitoring. If you really abuse a pump by putting an high-resistance orifice downstream of a lower-resistance recirc line, you could generate high fluid temperatures. Just thinking out loud what could be the relationship. Personally I'd focus more on the other aspects (oil ring and oil level) first.

you are talking about aeration. aeration to me means bubbles in the oil, whereas bubbles on top of the oil as you describe them is called "foam" in my book. you can compare foaming yo the head on a good glass beer, where aeration is more like the bubbles in a glass of coke.

although air is involved in both situations, what actually happens is quite different. air in the oil is just what is says - air bubbles that travel upward in the oil with a certain speed, depending on the viscosity of the oil. foam on the other hand is a air bubble that is "caught", it is covered with a very thin layer of oil that is reluctant "to let the air go free into the atmosphere". foam may be reduced by a anti foaming agent, usually a silicon based additive that is added to the oil in very small amounts (only a few ppm). unfortunately when doing so, also the speed with which the air bubbles travel to the top of the fluid is impaired, so the danger that the oil contains lots of air in situations where it needs to lubricate is quite clear. for that reason anti foaming agents are used with care and usually only when absolutely necessary. in this particular case we are talking about a rather simple type of turbine oil that should have good de-aeration characteristics from itself and based on its viscosity also should release foam quite fast.

if it is indeed foam you are talking about, it not necessarily is a problem, but it can be when it is excessive and when foam enters into situations where lubrication is needed.

the oil used in a fresh state both has good de-aeration properties and foam when formed should also break quite fast. aeration may be increased by low operating temperatures - if the viscosity increases, air bubbles included in the reservoir may take quite a bit longer to rise to the top. foaming will increase when the oil gets contaminated. both air release and foam brake down may be influenced by the way the return oil is fed into the reservoir, when it drops freely from a certain height into the fluid reservoir some air will be entrained into the fluid and also foam may be generated on the top level of the fluid.

apart from noticing foam on top of the fluid, have you noticed any other things that actually indicate some form of malfunctioning (excessive noise, wear etc). the most likely cause of foam is contamination - which can be all kinds of stuff: dirt, packing material, rubber from seals etc. just flushing will not necessarily resolve the problem, when generation of "dirt" is still present after flushing the system.