Gas Turbine Exhaust Pressure

We have finally got our old Frame V running and have excessive noise (94 dB) around the unit. We have some sound baffles in the exhaust and have good intake filtering, but we just can't get the dBs down to around 80 or less. Any help will be appreciated.

P.S. For those of you that were helping on the high EGT spread and ESD, we found the problem to be mostly caused by missing sections on the difusser section and the engine shroud. Thanks to all.

Good news (sort of) on the start-up spreads.

http://www.muellerenvironmental.com (A Texas company)

http://www.braden.com (Almost Texas; an Oklahoma company; hey, the Dallas Cowboys are owned by an Oklahoman)

Both found using Google (A TAFT company; "This Ain't Friggin' Texas")

Braden has a lot of experience with GE-design heavy-duty gas turbine exhaust systems, since you don't seem to want to contact GE.

As I am not aware of your level of experience with these machines,I am unaware to what level of checks I must suggest.

High sound levels can be associated with the following;

Poor alignment, Dynamic imbalance of Rotor, defective bearings, excessive fuel/speed caused by wrong calibration,worn-out parts, seal leaks,leaking bleed valve on the axial compressor, unsecure joints/plates, excessive or improper direction of thrust, passing or leaking relief valves.

Take a set of vibration readings all along the unit and make sure these are well within the permitted manufacturer levels. The vibration levels may give you a good indication if any problem exists.

One portable vibration monitor that I came across, has an attachment for sound level monitoring, this unit can compare the sound levels at different spots on the unit from which you can localise the fault.

Thanks for the information. I am aware of what is causing the high dB's and we have done all you suggested in the past months. Now that we have determined that the amount of noise being produced can not be corrected except by exhaust silencers, baffles or other media, we need a corrective action. The question was orginally intended to determine if we would create a problem by continuing to add baffles and media to correct the high dB. Thanks again.

Exhaust duct back-pressure is not an easy thing to accurately measure. Placement and type of probe are both very important.

Do you know what the current exhaust duct back pressure is?

Can you determine how many, if any, baffles were not re-installed during the relocation? One would have to believe if the unit were originally installed in Florida, and based on the age of the unit, the original exhaust duct and any silencers suffered some kind of damage (rust) over the years and probably couldn't stand removal, shipment, and relocation. Or, that even if they did, they might not be in the best condition. The design of the exhaust duct (straight up; horizontal, then up; etc.) would all have a determining factor on the amount of back-pressure.

Your best options would be to contact:

1) GE;

2) someone with experience with GE-design heavy-duty gas turbine exhaust systems; Braden has lots of experience and can show you exhaust systems they have provided to replace and retrofit similar units over many years.

Thanks for the reply. You are correct to assume that most of the exhaust duct work was not reuseable. We haven't attempted to measure the existing back pressure due to the fact that one of the engineers working on the project said there is no spec. on GT back pressure.

We haven't done a very good job with our new inserts or sound reduction media due to the still high existing dB's. I have the Braden web site and hope to make contact with them if our latest attempt doesn't fix the problem. One problem we have is limited funds on this project. We are using a half-pipe design that might work. Thanks again for the reply.

Don't always believe everything you're told. There's a limit on exhaust duct back-pressure for every gas turbine regardless of manufacturer. Since you don't seem to be dealing with the OEM you probably don't have access to the number(s). And since that "engineer" doesn't know the number, he's just saying there isn't one. (It's a safe bet this project is over-schedule as well as over-budget, and everyone is trying to clear out and get home as fast as they can, especially this time of year. Funny how these relocation projects often (always) go... Not. Not funny, that is.)

Restrict the air flow through the unit (which is basically a mass flow machine) and you will limit the power output.

Remember that many gas turbines exhaust into waste heat recovery boilers, sometimes called heat recovery steam generators, or HRSGs. (In Texas they like to call them "herzigs".) They usually have a higher back-pressure than a unit which exhausts directly to atmosphere, but only by a couple of inches of water or so.

Most of the exhaust duct silencer baffles (and inlet duct silencers) that I have seen in recent years are made of perforated stainless steel (very small holes, on the order of 1/16th-inch diameter or so, very closely spaced) with a filling of about 8-10 inches of some kind of insulation. These panels are the width of the exhaust (or inlet) duct and usually about the same dimension in heighth (in other words, they are basically square), and 8-10 inches or so thick. They are usually separated by about 12-14 inches or so. The exhaust gases between the panels as they exit the stack.

I have heard of baffling similar to what you described, but haven't seen it. It would seem it would take a lot of baffles, which is why I'm told they are used much any more.

Frame 5s are workhorses; they can "take a licking and keep on ticking" like few other gas turbines in the world.

Best wishes for a Happy and Prosperous New Year!

Sorry for interruption.

During my searches about the effect of HRSG on GT, from MW point of view, I have been guided here. But how does the back pressure change because of installing HRSG components?

And how does this increase in back pressure effects on the MW of GT?

Any HRSG will have pipes and in some cases some type of baffling which must be placed in the exhaust gas flow in order to transfer the heat of the exhaust gases to the fluid in the pipes. Also, the length of the exhaust duct will effectively be increased, and the stack will usually be a little different (taller) also. So, all these things will combine to increase the back-pressure on the gas turbine.

Imagine you place a filtration mask over your nose and mouth, a simple one that just removes dust, for example. Now, imagine trying to run a marathon or do a lot of physically demanding work with that mask over your mouth and nose. It will make breathing more difficult, which will have an effect on your ability to run that long distance or perform that work at the level you might be able were you not wearing that mask.

It's not just the extra effort of inhaling, but, most top athletes will tell you that exhaling is just as important as inhaling. It's just that we humans normally inhale and exhale through our mouths and noses; gas turbine "inhale" through one opening and "exhale" through another. But the concepts are the same in any case; restrict air flow, and power output is also restricted.)

Gas turbines are basically 'mass flow' machines. The more air one can get to flow through the machine, the more fuel one can burn and the more power one can produce. Restrict the air flow either at the inlet or the exhaust, and you reduce the power output, also.

There have been a couple of posts here on control.com about how new and more restrictive exhausts have reduced the power outputs of the gas turbines. If you're new here, you probably haven't tried the search function of control.com; it's second to none on the Internet for forums like this! It accepts multiple word searches, which most search engines don't handle very well. So, give it a try. We have a large body of information about GE-design heavy duty gas turbines and control systems; a lot of it is applicable to gas turbines of any manufacturer.