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Turbine Varnishing
How to identify Turbine varnishing
By 6bturbinetech on 27 April, 2017 - 9:27 am

We have 2 6B GE dual fuel Peaking units that went in service in 2001. We do not run that often, have roughly 3000 fired hours on the units. We do quarterly oil sample/testing on each unit with no issues from the testing facility in regards to oil quality. We change lube oil filters once a year even if we don't run.

Question 1: On a filter change we have noticed "blue-ing" on the filters. I have heard of oil varnishing, is this the color that would indicate varnishing? I realize this is a broad general question but thought I would start the conversation. We do not treat the oil at all with off base equipment. We are cycling through our servos, shipping them out to be cleaned and noticed this "blue-ing" on the servo filters. This is the first time they have been removed except for a couple of failures thru the years.

Question 2: Would you recommend a yearly change out of all the servos as a PM?

Varnishing....

I have looked into this and studied some of the main causes, and coming from an excommunicated specialist, much credence might not be given.

But this I have found: As the oil temperature gets to a point of misting/boiling the varnish that is left behind sticks to the surrounding surfaces forming a thin film.

Now..when this happens inside a moog servo, the passages and needle are of such close tolerances, the film formed is enough to restrict movement, causing erratic operation.

In a constant operational unit, noticeable trouble might not be evident until a restart after cool-down, and on cyclic units its even more unpredictable because of a lack of understanding.

On layup units... what's happening is, the lube oil heaters are constantly on and cycling between high and low temps and the needle (moog Servo) is stationary thus allowing the formation of thin film varnish which causes sticking.

There are equipment manufactures that claims to remove varnish...but I wonder about that, because varnish is a characteristic of oil.

On lay up units, I would keep the oil temp down to avoid misting.

The long term solution......well.... I have a couple Ideas.

c2

6bturbinetech,

1) Some types of varnish can cause a discoloration on some types of filters. (But read on for more information.) And, by "failures" are you saying the filter ruptured and dirt and dirty oil went down stream?

2) This topic has been covered many times before on control.com. Around 2000 oil refiners changed turbine lube oil formulations to greatly improve lubricity--which was great for turbines that don't use the same fluid for lubricating oil and hydraulic oil. The type of "fouling" that occurs with the new formulations doesn't harm or reduce lubricity or harm bearings/journals. But, it does have an adverse effect on servo-valves. It's as clear as that. BP-Castrol and GE collaborated on a study in the UK at a site that was repeatedly and prematurely "failing" servos, and BP-Castrol developed an formulation that reduced the issue so that the servo "failures" stopped.

But, the OEM is loathe to tell people they have to buy a particular oil because they use the same fluid for two purposes, and have taken heat (grief) for decades when there was no issue or problem with doing so. But, through no fault of the OEM now that the oil formulations have changed the servo manufacturer gets gobs of undeserved grief for a problem that's NOT of their doing and NOT related to the quality of their products, and the OEM is taking grief for using the same fluid for two purposes when it wasn't an issue for decades.

No; unless your spreadsheets tell you it saves downtime to change servos on some periodic basis instead of waiting for a "failure" it shouldn't be necessary to periodically change servos. That's stictly a management/bean-counter call--but to my mind, getting the proper lube oil is solving the root cause of the problem. And would ultimately save money in the long run. (Of course, most plants and bean-counters don't think about the long run.)

I also believe that the "new" formulations also have different effects on the colorization of oil filter elements--but that's just a SWAG (Scientific Wild-Arsed Guess) on my part.

Finally, I also believe that the oil testing companies are only really testing lube oil for lubricating oil qualities, and are neglecting any other usage/application/purpose. That's my own personal belief, but until I have proof otherwise, it's going to remain my stance.

The last two observations are mine, and reflect personal experience over the last decade-and-a-half, and the decade-and-a-half prior to that. I know of sites that had been operating for more than 20 years with the same lube oil (purchased before 2000) and never had any servo issues until they changed lube oil after 2000. Proper maintenance and proper testing are key, as is not waiting until filters rupture before changing them (that has happened all too frequently, because people ignored high filter differential pressure alarms until the filters ruptured and the dp went to zero and they (mistakenly) believed it was an instrumentation issue--until the servos got plugged and had to be changed on a weekly basis).