ST Lube oil mist eliminator issue

[npower0073]

On site we have a steam turbine. The Lube oil skid is equiped with 2 mist eliminator fans. The OEM of ST states that there should be a 5mbar vacuum to ambient. in order to get the oil bak in the tank and not lose it. We usually worked with higher vacuum (20mbars). Also we experience the fact that the pipes of the discharge of the mist eliminators (common for both fans) have not only fumes but also oil coming out. This means that oil does not condensates and goes out on the atmosphere. Can ypou advise on that issue?

 

[byrdj]

blowers or fans that pull a slight vacuum on the tank, I refer to as "vapor extractors".

Most units I have seen have a "mist eliminator" tank between the vapor extractor and the atmosphere vent. this tank is several layers of fine mesh and also has a drain back to waste oil. there needs to be some distance to allow the vapors to cool.

the vent pipe is tee'd to a vertical run from basement to turbine roof with a bottom loop seal, with drain also to waste oil

I have seen some devices that were the fan and wire mesh together for smaller coupling drives. the wire mesh would even spin to help collect mist and let it drain back into tank.

by operating with 4X the vacuum, you will have an increased amount of air sucked by the oil deflectors and this increase the vapor discharge velocity and not allow it to cool

 

[stgrme]

Vacuum should be measured in each bearing housing (pedestal) at 5 to 10 mbar (2 to 4 inches of water) rather than in the oil tank. I would measure the vacuum in each pedestal and throttle the damper at the inlet to each exhauster to obtain 5 to 10 mbar at each pedestal.

A mist eliminator can be added to the discharge line from the exhausters as noted by byrdj. One manufacturer of mist eliminators is Koch's Otto-York division.

http://www.koch-ottoyork.com/

Best of luck!

 

[rmw]

From rereading your original post several times I take it that you have lots of carryover and operate at 20 mbars while the ST MFG is recommending only 5 mbar. The purpose of the slight vacuum pulled by the VME fan is not to get the oil back, gravity does that, but to pull the vapors created by the oil out of the tank and out of the bearing pedestals while creating a small flow of air across the bearing seals.

Too much air flow (caused by operating at too deep of a vacuum) will entrain oil mist droplets that might otherwise have dropped out by gravity on their way through the system to the tank if not actually pick up oil droplets due to the high vapor velocities in the LO piping from the pedestals back to the tank.

Some VME mfgr's have bypasses built into their fan system to prevent excessive vacuum in the LO tank.

It is hard to see in the picture in this link, but at the bottom of the unit where the inlet is, there is an elbow open to atmosphere that will bypass air into the fan suction via the filter to prevent over-pressuring (under-pressuring??) the tank.

http://www.hilliardcorp.com/mist-eliminator.html

I suspect that you are pulling so much air through the VME's when pulling that much vacuum that you are overrunning the VME's capability to entrain the droplets and they are carrying over to your outlet piping, but that's just me.

rmw

 

[byrdj]

Another negative consequense of operating with excessive vacuum will be pulling contaminates into the lube oil drains and oil deflectors.

I don't have any notes or drawings at this time, but I seem to recall only 1/2" H2O being sufficient

IF you have oil leakage by deflectors with the minumn vacuum, then there is a problem with the deflector or bearing that needs to be corrected. increasing vacuum as a long term fix will create these other problems