Hello. I'm in Venezuela. Our Stack Baffles Started comming apart after one year. Suspect a manufacture defect or bad welding procedures. The material is 409 Stainless.We have already launched 1 plate from the stack.Unit down 1 week already while a decision is trying to be made on what to do.

From what I see we have 2 choices. Repair in place or take apart stack,take baffles out for now and run without baffles until a new set is made which takes 4 months. Any ideas? And is it OK to run without baffles due to less back pressure on turbine.

Thanks Tom Murphy

 

Tom,

I want to qualify what I'm about to say by saying that you are working on a Frame 7FA, and F-class units are much "finicky" about exhaust duct back-pressure and inlet differential pressure and many other things which other GE-design heavy duty gas turbines aren't so affected by relatively small changes in such factors.

F-class turbines, in my personal opinion, are operating at the very limits of materials and ambient factors in order to produce as much power as possible. (Some early F-class units were operating past the limits... and that required a LOT of cost to remedy, which GE did do, but in fairness many of the early F-class turbines were not operated per original design (meaning early designs (not the earliest!) were intended to be Base Load units but were operated in Load Following and cyclical applications and the thermal stresses involved were very high).)

I've been to some Frame 6B and Frame 7B/E-class sites (older turbines in humid environments) with almost no baffles left to speak of, having rusted away many years ago. Yes; they were difficult to start, and in my opinion this was because of a lack of back-pressure, but they also produced a little more power at Base Load (though I can't say that wasn't entirely a result of some over-zealous tweaking of operating parameters). I've also seen some Frame 5 applications where the operators weren't even aware the exhaust baffling was non-existent, and probably hadn't existed for more than 10 years, and the units were running just fine (apart from the usual issues of failed spark plugs or solenoid valves or failing torque converters or extremely dirty compressors/bellmouths).

If I recall correctly, the unit(s) at your site aren't operated at or near Base Load, and only run on liquid fuel at Part Load. I don't believe exhaust back-pressure is as important when the combustors at your site are running in diffusion flame and at Part Load.

I also believe that a higher-than-normal exhaust back-pressure is worse than a lower-than-normal exhaust back pressure, but that's an unsubstantiated belief.

If you've lost part of the exhaust baffles due to poor construction, it's probably a safe bet the rest of them aren't going to last much longer, either. In my personal opinion, and especially if the units are not being operated at higher loads/Base Load, the absence of "some" exhaust baffling might not be a problem, but that's a SWAG (Scientific Wild-Arsed Guess).

There is a safety issue with this approach; flying plates of perforated stainless steel can be hazardous to human health.

You said you were considering operating with all of the baffling removed. If the unit has working exhaust duct back-pressure sensing, you might try running the unit without the baffling (while you might be affecting some repairs to the removed baffling which could then be re-installed whilst waiting for any new baffles which might be necessary) and noting the difference in the back-pressure and operation, if any. If you have historical data capability, you could compare operating parameters before and after during a limited run for data-gathering purposes.

Let us know how you proceed!

 

Tom,

I believe the primary purpose of baffling in the exhaust is for sound attenuation (deadening), not so much to provide back-pressure for the turbine. I believe higher back-pressures (such as those experienced with HRSGs ("boilers") on the exhaust, or very tall exhaust stacks, or very long transition sections and tall exhaust stacks, or stacks with diverter dampers, all are taken into account when calculating exhaust temperature control values (Base Load, Peak Load, etc.). But, I'm not sure if they are taken too much into account at Part Load, and again, if I recall correctly the unit(s) where you are don't run at Base Load, or they didn't when you last wrote.

Again, let us know how you proceed!

 

Thanks so much for the info. Very helpful. Just an FYI we do run Base Load a lot now sometimes 17 hours a day. Take care.