Synthetic oil and static electricity build up.

[Skogsgurra]

Hello, electric guy on the loose in the oil fields...

I have had quite a few recent experiences with ball and roller bearing problems - all seem to be related to the swithing from mineral oil to synthetic oil.

There are also other "funny" phenomena that seem to be coupled to synthetic oil. Powerful electric charge build-up and subsequent discharging through filters, plastic tank wall, across ceramic sealings destroying the sealing surfaces etcetera etcetera.

I have very little knowledge about oils. Does anyone with a better understanding have a good explanation to this? I know about the extremely low conductivity of synthetic oils, but is that alone the explanation? What can be done to reduce the problem?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Compositepro]

There is no simple answer to your question except that you have to apply the basic principles of electro-statics to each situation.

Charge is created when two molecules with differing affinity for electrons are first in contact and then separated. Once the charge is separated then there are forces that will try to put them back together. So flow will create charge but it must be fast enough to keep the charges from recombining. Any conductivity in the liquid will help recombination. You are dealing with charged particles and like-charges will repel each other and opposite-charges attract.

In a tank the charge will be higher at the outside surfaces because the charge in the center is repeling other charges. If the tank is conductive the charge will flow to the wall and to ground. In a non-conductive tank the charge will concentrate at the wall. If a grounded object is outside the wall the charge will concentrate near this object because the charge in the tank attracts the opposite charge in the grounded object. The non-conductive tank is basically a capacitor and if you move a grounded object closer to it you increase its capacitance and the voltage in the tank will go down (if charge is constant). If the ground is moved away the voltage in the tank will rise.

Charged liquids can and will move so the charge is not really "static".

Remember that an electron-volt is a unit of work that corresponds to an electron falling through a potential difference on one volt.

Most liquids are conductive enough that one does not observe the effect of charge separation. In conductive pipes and tanks charge usually does not stay separated for long.

Is there a specific problem you need help with?

 

[Skogsgurra]

Thanks a lot for your fast and thorough answer.

The oils I have come in touch with are 220 Cst synthetic oils and also some lighter grades. The ball/roller bearing problems are 220 Cst central lubrication applications in gears. We know that we have some electric activity coming from the motor (VFD fed and capacitive coupling from stator to rotor/shaft) but that did not cause any problems when there was mineral oil in the lubrication system. We now seem to get a problem and we are not sure if that is because the synthetic oil produces a thinner oil film (with a lower break-down voltage, and hence EDM in the bearing) or if the problems are "helped" by extra charge separation in the bearing oil.

Most observations speak for the "thinner film" theory. But the oil guys keep telling me that a synthetic oil shall have a thicker film than a mineral oil with the same viscosity. Especially when running at elevated (+70 centigrades) temperature.

So, I do need some views on all this. And also things that I may not have considered.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Compositepro]

Interesting problem. It is hard to imagine not having conductivity between the inner and outer race of a bearing through the balls or rollers. On the other hand the purpose of a lubricant is to form a hydro-dynamic film that prevents metal to metal contact. That would make the rollers or balls isolated from ground and certainly susceptable to accumulating charge. A thicker film will mean a more intense spark when it does occurs. A higher dielectric strength oil would also cause hotter sparks. Lower oil conductivity would make the problem worse. Different oils chemistries will have different tendencies for charge separation. Another possibility is that the sparking is degrading the oil into corrosive compounds.

 

[Skogsgurra]

It may be hard to imagine not having conductivity between inner and outer race in a ball bearing. But that is a fact. A typical (shaft dia 25 - 200 mm) ball or roller bearing running at normal (1000 - 2000 rpm) usually has a break-down voltage between 5 and 15 volts. More if shaft temperature is low and mineral oil is used (thicker oil film).

So, that part is rather well understood. It is the paradox that we have with seemingly more damages in systems with synthetic oils that we do not understand.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[tomwalz]

We ran into problems filtering oils in plastic housings. What follows is about all we could find on it.

http://www.fiberglasstankandpipe.com/static.htm

Tom

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[Skogsgurra]

Thanks a lot for that link! The references included are also valuable.

I think that the opening clause should be changed from "...fuels that may accumulate electrostatic charges because of their relatively high electrical conductivity" to read "...fuels that may accumulate electrostatic charges because of their relatively LOW electrical conductivity"

It is the low conductivity that causes the problem.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...