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Gas Turbine Mist Eliminator
Purpose of Mist eliminator in Gas Turbine Lube oil system


Experts in this forum. I am working in frame 9E GT, if anybody can help me.
I would like to know what is the purpose of using OIL mist eliminator in Gas Turbine LO system

thanks in advance

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The L.O. Tank must be "vented" somehow, and if that's just a simple pipe open to atmosphere then there would be a constant plume of oil vapors emanating from the open end of the pipe. And, those vapors usually condense on nearby equipment, causing a very unsightly mess.

In some parts of the world, any visible plume of hydrocarbon vapors emitting from a piece of equipment is cause for citation and fines, in some cases even criminal prosecution.

So, the L.O. Mist Eliminator was conceived, and its purpose is to cause the visible vapors from the L.O. Tank vent to be removed from the air stream and returned to the tank through a loop-seal drain line.

Most L.O. Mist Eliminators are incorrectly operated, and most do not receive any maintenance (even though they do require periodic maintenance; see the manufacturer's instructions).

There is usually a note on the L.O. System Piping Schematic (P&ID) drawing about how to set the butterfly valve on the top of the Mist Eliminator.

Hope this helps!


Thank you for your above explanation.

I have also been told that lube oil mist eliminators assist with the following:

1) Provides a sub-atmospheric condition inside the tank, to help the oil flowing back to tank from bearings and generator seals, with a slight draw

2) Prevent a possible build up of hydrogen inside the tank, if the lube oil is also used for sealing oil for a hydrogen cooled generator (I don't know the mechanism how hydrogen can end up in the tank - can it be absorbed by the oil and then separated in the tank somehow? I don't know).

Are these statements true or have I been misinformed?

My experience is with a 9FA STAG, not a 9E as per Superman.


If you define -1-2 in H2O as sub-atmospheric....

Oil flows back to the tank because of gravity, presuming the interconnecting piping was installed correctly.

As for "removing" hydrogen which can be entrained in the lube oil because it is exposed to the hydrogen because it's used as the sealing fluid around the generator shaft, the enlargement tanks on the generator are supposed to do most of that. There used to be a seal drain enlargement tank for each end of the generator, and a large bearing drain enlargement tank. Those are connected back to the L.O. Tank through a loop-seal ("P-trap") to prevent the sub-atmospheric draw of the L.O. Mist Eliminator from pulling large quantities of hydrogen-enriched air back into the L.O. Tank.

And, there has been at least one TIL about reconfiguring the L.O. Tanks of F-class machines to prevent accumulation of hydrogen in high points which are necessarily under sub-atmospheric "pressure" and -flow inducements....

Thank-you CSA for your further explanation.

You're most welcome.

By the way, there have been many types of L.O. mist eliminators over three decades or so. One of the first was an electrostatic precipitator, but they were not very efficient. Most of the more recent types have been "cyclone" separator-types of devices (causing the air/vapor being drawn through the device by an electric motor-operated fan) to make many quick changes of direction forcing the heavier L.O. vapors to "fall out" of the air stream), to filter-type mechanism.

Prior to the use of precipitators/mist eliminators, the vent pipe of the L.O. Tank was routed to the top of the exhaust stack in the hopes that the vapors would be further "vaporized" by the heat of the gas turbine exhaust. But the problem with this method was that when the unit was not running there was still a visible plume for hours after the unit was shut down.

in addition to that, we have Frame 5 machine and we have improved oil tank vacuum by making more contact area between the hot air on turbine exhaust on the stack and mist oil by cutting the pipe end by 45 degree instead of 90 degree.

good luck

Lube oil mist eliminator

i have a point to add to what CSA

"Oil flows back to the tank because of gravity, presuming the interconnecting piping was installed correctly."

The oil in circulation, in liquid form flows back due to gravity. But a percentage of the oil ported into the turbine, load gear and generator bearings turn into lube oil fumes, this increases when the load goes higher. The mist eliminator maintains a "vaccum" in the LO tank (usually the vaccum is of the order of -100 - -150 mmwcs). this is specifically for drawing out these LO fumes. This is the reason why the LO return lines are connected to the tank above the "LO level High". You can take a look at the PID of the lube oil circuit to confirm this. You can also witness this phenomenon when you turn off a mist eliminator on high machine loads, there will be LO fume generation in the Bearings, especially in the load gear compartment.

Good Day,

I have a concern with regards to the sub atmospheric condition in the L. O Tank.

Some context:
At my facility we have frame 5D gas turbine all equip with mist eliminators. Presently we are having the issues of the L.O tanks with positive pressure values; example +30mmH20. Note the alarm and trip setting is 40 and 50mmH20 respectively.

During our recent plan outage, we changed all the filters, butterfly valve, clean and inspect the fan,replace the motor, and clean all the drain piping. However upon startup the L.O Tank pressure dropped but still in the Positive value. and adjusting the butterfly valve is not helping.

Can anyone share some light on this issue?

Check your LO tank pressure which shall be sub atmospheric due to vapor extractor fan operation.The purpose of mist eliminator is to remove the oil mixed to air to be discharged to atmosphere.So you will be saving oil :) if that vacuum pres in the tank is disturbed, you might be encountered with burned oil fumes and smell coming out of the bearings

Thanks for your feed back and good answers


@ Process value: : Can the vacuum in Main oil tank (MOT) be of the range -100 to -150mmWC?? In our machine even when the vacuum goes below -70mmWC, oil rather than mist is being sucked up into piping. Is this because of wrongly tapped mist eliminator suction??


I just want to clear my view regarding oil mist vapor in LO tank. I believe that any vapor is formed due to heat. And in GT LO system, LO to be cooled down. Mist eliminator removes the heat from the oil mist to atmosphere and saves the oil and send back to tank.

Is this correct? Please clear my doubt.


Dear bhavik

The mist eliminator job is to remove the oil fine droplets from air.

As the lube oil cools and lubricates the bearings, oil will get hot and oil vapor will be present in bearing housing. This oil vapor along with the air will mix and form "oily air". This oily air is drown to back to the lube oil tank thanks to the vacuum created by the mist eliminator blowers. The oily air then passes through the mist eliminator media where the droplets will be separated from air and routed back to the lube oil tank.

The air discharging to atmosphere after passing through the mist eliminator media is a clean air.

Left uncooled, L.O. will get so hot that it quickly turn to vapours in the bearing housings and it can be easily ignited especially in the exhaust area (research 'flash point'). This could also occur in the L.O. tank, also, simply by a hot enough surface; again research 'flash point.'

The L.O. in the tank is only cooled before it is fed to the turbine-generator bearings, not the entire tank, at least on GE-design heavy duty gas turbines.

The original "mist eliminator" was a pipe to the top of the exhaust stack where vapours would be further vapourized by the heat of the gas turbine exhaust--when the gas turbine was running, and for a short period of time after it was shut down while the air flow through the turbine and exhaust were hot enough.

BUT, when the exhaust temperature cooled the oil vapours would condense on nearby structures and surfaces, attract dust and generally make a bloody mess.

Subsequent to that, it became illegal in some parts of the world to allow hydrocarbon vapours to vent freely to the atmosphere, along with fines and criminal penalties. So, it became imperative to devise a way of "capturing" the naturally-occurring vapours from the L.O. tank and preventing them--to the extent possible--from escaping to the atmosphere (visibly). Not to mention, owners and operators didn't like cleaning up dirty, oily messes. (Okay, some owners and operators didn't like cleaning up dirty, oily messes; most didn't care.)

In its current incarnation the Mist Eliminator helps to condense the vapours into liquid oil and the "natural" thing to do with the liquid oil is to return it to the tank in a kind of closed-loop process.

ALL of this is explained in the previous posts made to this thread. Maybe not in this detail or chronology, but it's all there. Basic physics and a minimal cursory understanding of how hydrocarbons combust (burn)--and how oil vapours can make an ugly mess of surrounding structures and surfaces if left unchecked (common sense, in other words)--should be all that's required to logically reason one's way through this subject.

There should be absolutely be no doubt about this. Doubt is an awful word; see your Oxford's English Dictionary.