when we run the GT FRAME 9E with natural gas at low load we noticed heavy white/blue smoke emitted from the outlet of the oil mist eliminator. But, if it carry high load, the oil mist disappeared.

why is that

norazman182 [at] hotmail.com

 

How long has this problem been noted? Did it begin after a maintenance outage? Did it begin after someone made an adjustment to the L.O. demister setup?

It's interesting that you report the smoke when you are running the turbine on natural gas at low load. Do you see the same smoke when you run the unit on liquid fuel at the same load?

Let's talk about why the demisters are there to begin with. There is always vapour present in the L.O. Tank when the unit is running, or after it has been shut down. This is normal because of the temperature of the L.O. The L.O. in the tank is NOT cooled until it is pumped through the cooler on it's way to the bearing header. The L.O. carries heat away from the bearings back to the tank, and that heat will cause oil vapours to be present above the L.O. level inside the tank. Without some kind of vent, pressure will build up inside the tank as the vapours and heat increase, so there needs to be a vent to atmosphere.

In the early days of GE-design heavy duty gas turbines, that vent was a pipe to the top of the exhaust stack. Oil vapours were "vapourized" by the exhaust heat when the unit was running. During cooldown operation after shutdown, the vapours would be visibly exiting the pipe until the L.O. tank temperature decreased.

In many parts of the world visible oil vapour discharge into the air (atmosphere) is unacceptable. Besides contributing to air pollution, it can leave a dirty film on nearby structures, and that film is also slippery which can be dangerous. In many parts of the world visible oil vapour discharges can lead to citations and fines. Now, those fines can even be criminal, meaning that individual operators and their supervisors can be jailed, convicted of the crime of air pollution for allowing uncontrolled visible oil vapour discharges.

So, over the years there have been many various types of lube oil "demisters" (sometimes called L.O. vapour extractors, among other obnoxious and lengthy names) provided with GE-design heavy duty gas turbines over the years. Some early demisters were high voltage devices (14KV!), but most were some kind of separator/filter and used a centrifugal fan to either draw oil vapours through the separator/filter canister or to push them through the canister.

There is usually a positive pressure (in mm of H2O/inches of H2O) on the L.O. tank from the cooling and sealing air to the bearing enclosures in addition to the pressure caused by the heat of vapourization in the L.O. tank. There is usually a manually-operated butterfly valve either in the suction or the discharge of the motor-operated fan on the separator/filter enclosure. The operative (hyphenated) word is manually-operated. It's usually a device that is set at one load (usually Base Load) to maintain a certain negative pressure (usually 2.5 mm of H2O/1 inch of H2O) on the L.O. Tank.

[It's useful to note that most GE-design heavy duty gas turbines don't have any means for monitoring the pressure on the L.O. Tank. So, it will usually be necessary to install some type of gauge or water-tube manometer to monitor the L.O. Tank pressure.]

The problem comes from the fact that as load is increased the cooling and sealing air flow/pressure increases, so the pressure on the L.O. tank increases. If the butterfly valve is set at one load, Base Load usually, then the suction on the L.O. Tank will be much higher at low loads, and that high suction can cause the blue-white smoke that is seen at the outlet of the separator/filter canister/fan arrangement.

So, to properly control the vapour discharge at all loads, the butterfly valve should really be adjusted as necessary to maintain a constant slight negative pressure on the L.O. Tank. But, that valve is usually very high above the ground, and there is usually no ladder or stairs to access the valve, so, it's just set once and forgotten. Or, it's adjusted at some low load to reduce vapour discharge and then at a higher load the vapours appear again.

One thing that system designers don't do well with GE-design heavy duty gas turbines is to consider all the various operating scenarios for the units. Many are started and loaded to Base Load and left there for days, weeks, months. Most of the auxiliaries used with GE-design heavy duty gas turbines are designed with this type of operation in mind.

But, particularly in recent years, they are used as load-following units and are started, and operated at various loads less then Base Load, and in many cases they are even started and stopped once, sometimes twice, per day, and operated at various loads for short periods of time when running. Auxiliaries designed for continuous Base Load operation don't work well at part load conditions.

I saw one site that purchased some chain operators to replace the handle on the butterfly valve to allow easier adjustment of the valve. But, it still requires manual operation of the chains. I'm sure that some kind of pneumatic operator, set to maintain a constant slight negative pressure on the L.O. tank whenever the L.O. demister is operating by modulating a butterfly-type valve, but that would be relatively expensive. Having said that, depending on the environmental regulations it could be very cost-effective to avoid citations and fines for visible vapour discharges.

Lastly, all types of L.O. demisters require periodic maintenance. And, there should be some kind of L.O. return to the L.O. tank, for condensed L.O. vapour "condensate" to return to the tank. For most of the separator/filter canister types of demisters there is a loop-seal return line which must be properly filled with oil prior to start-up or after maintenance for proper operation. There is supposed to be a sight-glass in the loop-seal piping to allow an operator to verify the presence of oil in the piping. If there is no oil in the loop-seal, then there will usually be a continuous vapour discharge from the demister outlet until the seal fills.

So, don't forget to read the demister manufacturer's instructions and include maintenance procedures in your maintenance planning. Don't forget to monitor the loop seal between the canister and the L.O. tank. Install some means of monitoring the L.O. Tank pressure if one doesn't exist, and don't forget to make it accessible and visible to the person adjusting the butterfly valve. And, then either erect a permanent ladder or stairs to the butterfly valve (or source and install some automatic means of controlling L.O. Tank pressure at various load conditions) to control the L.O. Tank pressure at various loads.

 

Well now, that wasn't a very good response was it? Sorry about that.

If the vacuum (negative pressure) on the L.O. Tank is too high at part load, there will usually be visible oil vapours from the outlet (discharge) of the demister.

Most of the time, the de-mister's "control valve" (the butterfly valve) is set when the unit is at Base Load, and that usually means the vacuum is too high at part load operation.

Unfortunately, it's very difficult to get to the "control valve" while also monitoring the L.O. Tank pressure/vacuum, especially when there usually isn't a gauge or means for monitoring L.O. Tank pressure.

 

Our frame 6B's oil mist eliminator initially had issues. The adjusting spring was in question. We replaced the plate the handle was passing through with a piece of plexiglass. We could view operation and found a variety of issues over the years.

 

Can you tell us some of the issues and how you resolved them?

 

We have 2 units, one was not operating correctly and it was found the flapper would stick half way. Easy to view with the Plexiglas. The filter elements deteriorated and we could visually see them also. We have found dead birds (have since screened the output piping). The visual inspection while the unit is running really eliminates any questions. It is a cheap and reliable aid.

 

This is good information. Thanks!

 

>This is good information. Thanks!

i have a question on a GE POWER PLANT 120 M/WATT pg9171 that is set up for dual fuel! what is the cost to install lines and new nozzles to operate on heavy fuel oil? also the tank that separates the heavy metals and other solid and contaminants from that result from h.f.o
(iraq hfo)

 

GREAT question for GE, or Wood Group, or Thomassen, or any other similar vendor.

They can provide "washing" skids, and treatment skids; all the necessary auxiliaries.

This would also entail a review of the hot gas path components (liners, transition pieces, nozzles, etc.) to be sure they can stand fuel and any deposits it might leave behind. Also the firing curve would need to be reviewed and possibly adjusted.

But, this is not something you get an estimate of from a World Wide Web forum and then plan to spend that amount of money, and only that amount of money. Because, I'd bet any estimate you might receive would either be excessively high, or ridiculously low. If you budgeted for the latter and had to pay a lot more, there would be consequences. And, getting approval for the former would probably be difficult, if not impossible.

And don't expect an answer in two weeks, or a month, even. Way too many intangibles and a lot of information required. Unless, you buy from the OEM packager who might have all the necessary information readily available.

Best of luck!