Turbine Noise Control 1

[turboengineer]

I have been tacked by my employer to do some research into a noise abatement plan for a turbine power plant that we are designing (3.5Mwe). I’m not beginning my research until Friday the 9th of July, so before I begin I’d like to see if any of you, my esteemed colleagues, can help the process. There are numerous papers and other resources relating to turbine noise abetment, but do any of you know where I should begin?

Thanks for the Help

Tim, the turboengineer

 

[Barry1961]

I have heard that jet engines have done very well on noise abatement due mainly to govt. regs. on noise pollution. GE is supposed to be one of the best at making quite jet engines. You might start an additional thread in "aeronautic engineering" just for a different perspective.

Sounds like fun.

Barry1961

 

[MintJulep]

Look into active noise cancellation.

 

[turboengineer]

We have jokingly tossed the idea of active noise reduction about, but the cost would be prohibitive when compared to passive screening. I know of a lot of vendors of materials that will do the job, but what I am looking for is some form of authority to back up my recommendations.
Thanks for the help so far!

Tim, turboengineer

 

[yates]

Having worked extensively in noise abatement of Gas Turbine power stations up to 300MW, I can assure you the problem is not simple ! You do not mention your environmental noise level requirement. Stations built in industrial areas need not be heavily soundproofed, since the perceived emerging noise level is not great. In a residential area however, things are different.
Essential factors to your noise treatment solution are :
- Is the turbine a peaking station or does it run continually (in other words, at night) ?
- Is the gas turbine simple or mixed cycle (does it have a heat recovery boiler) ?
- Is the alternator driven directly or through a reduction gear ?
- What type of air filter is used ?
- How much pressure drop can you afford in inlet and exhaust ?
This last question will determine your lowest noise level possible, since cladding the turbine will not get you down to low noise levels without silencers (hence pressure drop) in inlet and exhaust.
To give a rough idea, my worst assignment was a 25 MW station with a requirement for 43 dB(A) acoustic pressure at 100 meters. Who lived 100 m away ? - a complaining senator ! Noise treatment equipment here represented a 10% increase of power station cost as compared to a 65 dB(A) at 100 m requirement.
Biggest problem is low frequency from exhaust - (emission depends on chimney height and slope of surrouding terrain) since low frequencies are much less attenuated by air. Ground transmission of vibrations can be another troublesome source of noise - particularly on an elastic clay subsoil. Direct transmission through the ground can be dealt with by incorporation of damping into the civil work. Putting up a soundproofed building around the turbine is no good if vibration is being transmitted through the concrete to a large surface of building cladding which then radiates out noise.

 

[turboengineer]

The turbine is designed to run more or less continuously. It is not in a residential area to my knowledge. It is an organic Rankine cycle waste heat recovery engine that uses isopentane as its working fluid. The engine will be located at a brick manufacturing facility. My spec requires a maximum noise level of 85 dBA at 3 feet (standard). The generator (induction) is driven through a gearbox that has noise issues of its own, but these are being addressed by the manufacturer of the box. I am not certain what the un-insulated noise level of the turbine is, but we’ve estimated it at about 110 dBA. Heat is not much of an issue as the maximum turbine temperature is only 400F. Since it is a closed cycle, there is no exhaust stack to contend with. I think it’s ridiculous to demand such a noise spec being that the plant is less then 100 feet from a brick factory, but there are regulatory requirements to fulfill. Let me know if this helps and if you need more info.

 

[yates]

Sorry, I went off on the wrong track, for me 'Gas Turbine' means a combustion turbine. Your problem is considerably simplified by the fact that there is no air inlet and exhaust to contend with. The 85 dB(A) at three feet is the normal standard for operating personnel who are not wearing protective gear. The reduction gear, in my experience, will give you the greatest problem. It's all very well saying it's out of your scope, but when it comes to measuring sound pressure levels at 3 feet from the machine base, how can you tell the proportion of noise coming from the turbine or the reduction gear ? I'm surprised that the given untreated acoustic power is given as high as 110 dB(A), where is the noise coming from ? shaft and bearing noise should not be of such a high level. It would seem to me that most noise produced would be of mechanical origin (not combustion noise) so the transmission of vibrations through the concrete base of the turbine will be the point to watch.
Your job is made easier since mechanical noise is of a discrete frequency (with harmonics) linked to rotational speed, and not the wide-band freqencies you get with large volumes of moving air (exhaust). Consequently, knowing the major frequency allows you to better design soundproofing equipment specifically for this frequency.
One of the most effective cladding materials I have used for close enclosures (as near the source as possible) to reduce noise at 3 feet is a rockwool panel about 6 inches thick with a lead sheet (foil) membrane in the middle which is free to vibrate and therefore absorb energy. The inner face is of course expanded metal grille to expose the greatest surface area of rockwool (better than round-hole perforated sheet).