We have at our installation a “G” type turbine driven by 450 psig steam, at 3500 rpm.The original lubrication to the turbine bearings was wet sump via oil rings. And the oil grade recommended was R&O150, this turbine was re-furbished some years ago and a pressurized lubrication system was installed, an auxiliary oil pump, and sump etc was installed. My question is ...should the oil grade remain the same? Or should a lower grade oil such as R&O 68 be used, if so why, if not why not? There were issues relating to the oil temperatures before. Hence the reason for re-refurbishment.
We have turbines being operated in other plants under the same conditions and lubrication system and R&O 68 is used. I am eagerly awaiting your responses, as we have to advise operations on what is the right oil to use.
Thanks all guys

Two questions:
1 - What size is the shaft?
2 - When you say pressurized oil system, do you mean A - pressurized to pump it to the bearing where it is picked up by oil ring? Or B - pressurized as in hydrostatic lubrication (pumped into grooves near bottom of bearing)?

If you're not sure about question 2, what is the typical pressure at the discharge of the oil pump? I think for A your discharge pressure will be certainly under 100 psig (more likely around 30), for B your discharge pressure will be several hundred psig or more.

Thanks for the response electricpete
The shaft size is 2.5 inches. The lube system is more of a circulating system than pressurized my apologies.It is more as described in A.

Neale's Tribology Handbook page C7.2 lists minimum viscosity for sliding sleeve and thrust bearings based on bearing loading (pounds per square inch) and surface speed. Your surface speed is about 11.5 meters/sec for your sleeve bearings and higher than that for any thrust bearing that acts on a collar at a larger diameter. Your loading I'm guessing has got to be less than 1,000 psi. (just a guess... most motor sleeve bearings are far far lower and I don't think many babbit bearing machines are designed to operate beyond 1000 psi... if you have rotor weight, we can calculate horizontal sleeve bearing loading from that although I suspect you may have other sources of loading). For >= 11.5 m/sec and 1,000 psi, the chart tells us that minimum viscosity at operating temperature is 4.5 cSt. For VG 68 oil, you can increase the temperature all the way to 130C and without getting the operating viscosity down to 4.5 cSt. I'm assuming your bearing runs well below 130C now (with the VG150). It should run even cooler if you reduce the viscosity grade, so I think you will meet that minimum operating viscosity requirement.

So, unless your bearing loading is dramatically more than 1000 psi or I have made a gross miscalculation, I think you should be able to use the VG68. It should reduce your bearing temperatures and slightly improve your machine efficiency. I assumed these are sleeve (vs rolling) bearings and there is nothing else lubricated by this oil other than sliding sleeve and thrust bearings.

You might ask why was the viscosity grade so high to begin with. A higher viscosity grade would be needed if it was assumed the bearing would run hot due to the very limited cooling afforded by a very small reservoir in the machine. When you improve the cooling by circulating the oil to a large sump with much larging surface areas available to transfer heat to the environment, you lower the oil supply temperature to the bearing. That should mean that you can use a lower viscosity grade at a lower oil temperature and still achieve comparable operating viscosity. In other words, the viscosity had to be higher before the oil system modificaiton because the bearing was running hotter.

Another small consideration is the law of unintended consequences... any time things are running good and you make a change, you never know what you're gonna get. One thing that comes to mind is oil whirl/whip... relatively rare but when it occurs it is sensitive to oil viscosity and temperature and could start if you change the oil viscosity. If you are able run the changes by the OEM that of course helps. If you are aware of other very similar machines operating trouble-free with the VG68, it helps address this type of concern also.

This message has been edited. Last edited by: electricpete,

Thank you electricpete for such an excellent reply to my question.
I now have a clearer understanding of how we should go about lubricant selection/ queries.
In my case where VG 150 oil is being used in the circulating system, what are the detrimental effects?

The only obvious disadvantage of using a higher viscosity grade is that the bearing runs hotter and the machine runs less efficient. Hotter bearing has less margin to high temperautre alarms in the event that conditions change such as increased loading.

Running hotter and colder may have other effects that are desirable or undesirable. For one thing, temperature affects the way moisture behaves in the system. It's good if the oil sump remains hotter than ambient temperature... helps prevent condensations and keeps the moisture levels low. So if switching to lower viscosity makes your bearing runs cooler and your sump gets cooler, you may have more moisture problems and more long-term corrosion problems.

All in all, if it was our machine and it seems to be running good with temperautres well below alarms with existing VG150 and I was confident that the temperature alarms are set correctly and the temperautre monitoring is effective, I think I might just leave well enough alone. A small improvement in efficiency doesn't attract much notice (high probability, low positive consequences), but introducing a change with unanticipated negative consequences does (low probability, higher negative consequences). But I don't have a strong preference one way or the other from the information we have.

Maybe others have comments/suggestions or opinions?

This message has been edited. Last edited by: electricpete,

electricpete you mentioned before "A higher viscosity grade would be needed if it was assumed the bearing would run hot due to the very limited cooling afforded by a very small reservoir in the machine". My question is; what property of the oil allows for this? is it higher concentrations of anti oxidizing agents? and what really determines the Viscosity grade of lubricating oils?

I meant, if you have poor oil cooling, you have to use a higher viscosity grade, because high temperature lower the operating viscosity. If you improve the cooling, this effect is not as significant and you can go to a lower viscosity grade while still maintaining the required minimum viscosity.

What determines the viscosity grade? - I vaguely remember that higher viscosity oils have more of the longer polymer chains and lower have more of the shorter. But could be wrong on that. Maybe some of the more chemically-inclined folks can jump in on that question?