Gas turbine lube oil tank level cyclic fluctuation experienced in 2 units of GE Frame 7 gas turbine used as a driver for MCR and Propane compressors respectively.

Recently I noticed that the lube oil level is fluctuating and upon checking the historic trending, the fluctuations had started as early as 2005! As of 2013, we have had multiple shutdowns (MI, HGPI, CI) in between but unfortunately no investigation was done as nobody notices this problem until recently.

The lube oil tank level (measured by Rosemount transmitter using wet leg) of a GE FR7 is experiencing a cyclic level fluctuation of about 20%. Another similar unit of FR7 is also experiencing cyclic fluctuation of about 10%. It was confirmed that the sight glass level tallies with the transmitter measurement. The oil tank has a capacity of 14000 liters so 20% of level fluctuation is a lot of oil!

Both machine's lube oil tank level flucutations are very consistent, reached the high of 60% normally around early morning (1am - 7am) and gradually reduced to 40% normally around late afternoon (2pm-6pm). The cyclic period (from high peak to next high peak) is around 24 hours.

Really appreciate any suggestions/comments to help solve the problem. I will try to provide more information so please feel free to ask if there is any specific questions for better understanding of the problem.

Thank you!

 

eszy23,

One has to remember that there is a LOT of piping and some large cylindrical components in the typical GE-design Frame 7E/EA heavy duty L.O. tank/reservoir. The L.O. Coolers, the L.O. Filters, and a lot of piping are inside the tank, so L.O. volume is not directly proportional to L.O. level.

You have said the sight glass indication tallies with the level transmitter, but again, volume is not directly proportional to level because of the various components (and their shapes) and amounts of piping in the L.O. Tank. So, while the visible level may be changing by approximately, the actual volume in the tank is probably not changing by 20%.

It would be very interesting to know if anyone has recorded how much the sight glass level and the level transmitter changed when adding oil to the L.O. tank--in other words, when the tank was full, and then the level dropped, by how much did the level increase for each barrel of oil that was added? I've seem some level indicators that went from 'Empty' to 'Full' when just 2.5 barrels of oil (55 US Gallons/barrel) were added to the L.O. Tank.

I would imagine multiple things are happening, which may or may not be directly related. The early morning "high" level may correspond to the coolest ambient temperatures of the day in your region of the world, and the early afternoon "lows" may correspond to the warmest ambient temperatures of the day.

Also, I might suggest that the load on the units might be at the highest in the morning, and the lowest in the mid-to-late afternoon.

I would also suggest that the pressure on the L.O. system varies throughout the day at about the same frequency, and/or the L.O. Demister Vent Fan suction on the L.O. system varies at the about the same frequency for some reason through the day.

So, if possible can you trend all of these parameters (ambient temperature; cooling water system temperature; load; L.O. tank pressure (most times there is not a pressure transducer installed by the packager to monitor this parameter, so if it's possible to temporarily connect one to an available tap on the L.O. tank (the "open" area above the L.O. level) that would be very helpful); bearing drain temperatures; L.O. header temperature; L.O. tank temperature (again; sometimes there is not a sensor installed by the packager to measure this parameter, so it may be necessary to temporarily connect one); and CPD (Compressor Discharge Pressure).

It would also be helpful to know if turbine L.O. is used as the lubricating and/or seal oil for the compressors, and if there's some change in compressor oil demand during the typical day that corresponds to these level fluctuations.

It doesn't seem possible that the oil can actually increase and decrease in volume by that much during a typical day even if the L.O. tank temperature is changing by a large amount (oil volume changes with temperature--which is why it's important to know what the L.O. tank temperature is doing through the day!).

It would also seem unlikely that the oil flow-rate could vary by that much throughout a typical day, with oil being used and/or not returning so quickly to the L.O. tank. Without being able so see the P&IDs for the entire L.O. system at your site, it's difficult to know what the configuration is and what might be the possible causes.

The "dry" (Low) leg of the L.O. Tank level differential transmitter must be connected to the pressure which the L.O. tank experiences (which is a function of the vapor temperature/pressure and the L.O. Demister fan suction pressure, and the Cooling and Sealing Air pressure). There is also a #2 Bearing vent that is connected to the "top" of the L.O. tank, which is a function of Cooling and Sealing Air pressure (which is not usually monitored on a GE-design heavy duty gas turbine).

When something like this happens, and it's ultimately proven that the level change is "normal"--for whatever reason--it's important to check the L.O. Tank level at the same time each day when the turbine is running, so as to get the best possible indication of a problem (oil leak; cooler vessel leak; etc.) as early as possible.

Please write back to let us know what you find and how the troubleshooting progresses.