Turboexpander Lubricant problems..

We at our Gas plant are having issues with the turboexpander lubricant (Turbo T 100) getting low viscosity and low flash point due to gas contamination in the lubricated bearings. This has caused several bearing failures in past years. I have come to know that this is a common problem with many other operators for the lubricated bearings of turboexpanders.
We are replacing the contaminated lubricant on frequent basis to avoid failures.

Although new turboexpanders are manufactured using non lubricated bearings, i.e. magnetic bearings, however we have to find a solution for the old units.

Can others share their experience and ways to resolve this issue ?

Regards
Mohammad

Regards,

Muhammad

Original Post
What is the MW of the seal gas? Is it possible to find a leaner source of seal gas?

The higher the operating pressure of the lube oil reservoir and the heavier MW of seal gas the worse the lube oil dilution will be.

Does the machine have any mechanical issues (ie high vibration or high bearing temperature)?
While true that magnetic bearing machines will not have lube oil dilution or process contamination issues, they can have problems as well.

The first applications of magnetic bearing machines were all in low MW, low pressure ethylene processing applications due to the high cost of lube oil contamination in the process. As they are applied to high pressure, high MW (and high density) applications they can have issues.

Magnetic bearings are not very stiff, with higher density process gases the forces on the rotor are much higher and can become an issue. Also, all turboexpander manufacturers are at the mercy of the magnetic bearing supplier and the tuning loops that they can provide. This can add another level of complexity when there is an issue and it needs to be resolved quickly.

Given Mohammad's location, I am guessing that his machine is fairly high pressure (hence the lube oil dilution issue) and also potentially sour/wet which is not a good combination for magnetic bearings.
William / turb1,

Thanks for your inputs.

Last week the T/expander OEM, Mafi Trench recommended us to use Synthetic lube Oil ISO VG 46 for our application. The key benefit of Synthetic lubricant is due to its thermal stability, since this machine is subjected to higher bearing temps.

Regards
Mohammad
Mohammad,

I thought you have issue on gas contamination into lube oil. What did mafi told you about the issue? Yes, we all aware that syntetic oil is superior than mineral in many ways. but you are still living with the issue right? the only changes you will see is reduction in oil change frequency. you can just hope the frequency reduction will be significant
Mohammad CMRP, MLA1 posted:
William / turb1,

Thanks for your inputs.

Last week the T/expander OEM, Mafi Trench recommended us to use Synthetic lube Oil ISO VG 46 for our application. The key benefit of Synthetic lubricant is due to its thermal stability, since this machine is subjected to higher bearing temps.

Regards
Mohammad

Mohammad!

You said that you change Lube oil viscosity 100 to 46 and temp bearing increase?

I think when you reduce viscosity machine will lower temp bearing because more flow thro bearing.

Regards

Assuming the bearing operational temperature = 92 Degree C

The oil viscosity will be:

Mineral VG 100 (VI 100) = 14.6 cSt at 92 degree C.

Synthetic PAO 46 (VI 170) = 10.28 cSt at 92 degree C.

Here we have to consider:

1. We lost 4 cSt, and hopefully it is still acceptable by tribology calculation of the bearing.

2. If We choose synthetic PAO (PolyAlfa Olefin), I don't think so it will helping much. Because PAO is still "Non-Polar" which having same characteristic as Mineral Oil.

3. I prefer to advise to use "Polar" oil which is Synthetic PAG (Poly Alkelene Glycol), then the leakage gas will not easy to dilute to the lubricating oil. But it is little bit expensive

4. Closed loop Vacuum Dehydration system may good to be utilized, but WITHOUT HEATER. Oil tank vacuum extractor can be utilized also.
5. As explained by VALVE, changing oil type and utilizing vacuum dehydration system is not solving the root of the leakage gas. The seal has to be improved.

Good luck, and regards.

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