Hi everyone,

At our site we have a GE9FA gas turbine and V94.2 Siemens gas turbine. I observed that when the temperature of the Lube oil increases the level in the Lube Oil tank decreases. Which according to me just does not fit in.

I took the example of oil inside a 200ml container. At 15 degC it will be approx. 150ml but if you heat the same container it comes back to 200ml. This is what it should happen with the lube oil tank.
But in my case, For 9FA machine the lube oil tank level (with one seal oil pump running) at 30 degC the level is 410mm of water.

As the temperature raised to 44 degC (with the same pump running) the level dropped to 389mm of water.

I checked for leakages at all the places but there were no leakages.

Kindly, give me vivid reply

 

Yassar,

Are you concerned about a leak or leaks? Increased consumption when the oil is hot? Are you having to top off the tank more often than it would otherwise seem necessary?

It's true--most liquids expand when heated. But, then most everything expands when heated, right? Including metal?

I can't comment on the Siemens turbine's oil tank(s)--and you didn't, either--but for the GE system when pumps are running there is a LOT of churn in the oil tank, along with a lot of oil vapors and pressure. If only the Seal Oil pump is running, there will still be turbulence/churn, but it would be less, for certain.

However, again, as the oil heats up, so does the "tank" (the metal that forms the enclosure. The tank is not usually insulated, and part of the reason for that is to allow some radiation of heat so that when the Cooling Water system is not in operation the oil temperature won't increase too much.

If the oil were static, then, yes--I would agree that under normal circumstances it should expand with an increase in temperature. It probably is increasing, but not by a noticeable amount, again possibly due to the expansion of the metal used for the enclosure.

Oil tankers and barges do correct for temperature when considering oil level--but then, there's big money involved and much larger volumes, usually.

When the turbine is running, there is a lot of oil in the local bearing drains and return piping that, while it's moving back towards the tank is not in the tank. And, again, there is a lot more turbulence (churn) in the tank when the turbine is running and that affects the level detection--as does the oil mist eliminator and cooling and sealing air pressure/vacuum.

So, there are a lot of things to consider, and to really compare the oil level at one temperature to the oil level at another temperature you would need to take levels either with the same pumps/mist eliminator running or not running--preferably some time after the oil has had a chance to return to the tank and settle. This could be done with the Seal Oil Pump running, which, again, should be a lot less turbulence/churn.

 

As long as it comes back, why worry?

Regards
cww

 

just to clairify a few questions
are both machines using the same oil? Is that oil mineral or synthetic?
are both machines measuring oil level the same way?

Noticing the oil level was expressed as a pressure, I'll assume the level is measure with a pressure transmitter at the bottom of the tank. did you measure the actual level, say from the top of the tank?

Since you find this a concern, I would suggest

1)measuring the tank dimensions and calculate the change in volume for its thermal expansion (my gut says this would not be sufficient to account for your observations, but since I have not performed the math I can not make that assumption)

2)calculate or determine the density change in the lube oil for the temperature change

3)detail the tank shape to determine the change in height base upon the volumetric increase in oil. I would expected the tank shape to have a larger area at the top versus the bottom so the change in hieght will not be as great as the % change in volume.

4) using the expected reduce density to calculate the height and I expect the physical level DID increase, but the density of the oil decreased so your pressure measurement is less

sounds like a good project for a slow day!

 

CSA,

Thanks for your reply. No, I am not concerned about the leaks and there were no leaks in the system.

Increased consumption when the oil is hot is what was of my concern.
The Siemens Lube oil tank also has the same variation approx. in level .i.e when the temperature increased the level dropped. Sorry I didn't comment regarding the Siemens machine.
Your answer gave me a clear picture.

Kurt,

Thank you for your reply. I was just getting very curious and wanted to know the actual reason. Even after researching on the net it didn't convince me.

JFB,

Thank you for your reply.

The oil used for GE Machine is IOCL Servo Prime 32 and for Siemens machine the oil is IOCL Servo Prime 46.
Both are mineral based oils.

Yes you are right level is measured in terms of pressure. And also the bottom is kind of curved so the volume at the bottom of the tank is going to be less compared to the top.

And as far as the project is concerned I will go ahead finish it and get back.

 

I’ll simplify

From your observations, a 200ml of hot oil cools to 150ml. Its volume changes, but not its weight, therefore the density of hot oil is 75% of cold oil. So if you maintained the exact same column height the pressure gauge measurement would show 75% less when hot versus cold.

Now if the W X L X H of your tank was 1 X 1 X 1, then when oil goes from cold to hot, the actual H of the oil will increase to 1.33, however the density of the oil decreased to 0.75%, so the pressure gauge would still show a level of 1 (1.33 X 0.75).

But your tank is not a cube, but more of a rectangle. Assume a simple area to height of 2:1 so when the oil goes from cold to hot, the H of the oil level only increases to 1.17 with the same decrease of density, now the pressure gauge will show a level of 0.88 (1.17 X 0.75) when hot.

So for an accurate indication of level when monitoring with a pressure gauge, fluid density Should be included.

 

I did not look at the validity of the information you provided with your concern or considered the accuracy in your experiment to determine the thermal volumetric expansion of the oil.

The expected volumetric thermal expansion coefficient for oil would be 0.001 which would give an increase of 1.03 for your 30C rise. To match your observation a coefficient of 0.01 would be required.

I am not saying what you observed is not true because I also experienced severe expansion of turbine lube oil that was determined to be caused by contamination of the oil with improper additives.

I would recommend you contact your oil supplier.