Separate Low RPM gensets

[AusLee]

Hello,

Please i have a threefold question:

1. I have a generator room near an auditorium, someone proposed the use of "low RPM" gensets as a first measure towards noise reduction. Is it true that low RPM gensets are less noisy than normal 1500/1600RPM generators? I have a financial study that shows their fincancial efficacy overt the service hours, but nothing electrical?

2. The maintenance company wants to change the oil every 250 hours. My question is: is an hour of service counted as 60 minutes of runtime, or are there other conditions such as: 10 minutes at Full Load count more than half an hour at 30% of the load? The genset is 1,000kVA.

3. The installation was served by a 1,000kVA generator. The client took that out and put 2x500kVA generators (in case of low load, only one will go in service). Now he changed his idea: he wants to separate the main distribution board in 2, and each 500kVA genset supplies one half. So i will take out the 1,600A breaker in the MDB and replace it with two 800A breakers, one for each side, plus a third one as a "tie-bus" breaker to link the two parts in case of supply from the 1x1,000kVA transformer. My question is: is there any weird neutral arrangemets i should watch out againt? the system is supposedly a TN-S.

Thanks!

 

[itsmoked]

I hope you are a contractor that is getting a piece of this crazy customers weekly whims and flip flops.

Lower speed generators usually have a longer life time and are quieter. That said, many companies make very, VERY, quiet generators based on "critical 1" mufflers and sound attenuation enclosures that are first rate. I first saw this on a trailer generator running next to my old high school that was supporting a movie production. I didn't realize it was even running until I read the oil pressure, ammeter and voltmeter. (about 90kW 208)

I am sorry to see your customer complicating the installation. When you start with the splitting games and multiple units you jump way up in the required skill level of on-the-site personnel. Often to the point of someone making a mistake and losing all power until someone else shows up to puzzle it all out.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

There is noise and then there's noise. The low speed units should have lower noise levels in the weighted dB range that is supposed to relate to human response to sound. However, they can have a more noticeable low frequency thump that is more felt than heard.

Agree with itsmoked, you can specify "residential" grade mufflers and sound-attenuating housing that can make a generator surprisingly quiet.

If I could only fit one on my neighbor's diesel pickup truck that starts up about 5:00 am. A real "box of rocks".

 

[MikeHalloran]

You can also get "hospital grade" mufflers, even quieter than residential. The only serious downside is the volume of the muffler; figure on about equal to the engine's volume. Not cylinder displacement; gross volume of the entire engine.



Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

I agree 100% with everyything itsmoked and dpc have advised.
A couple of additional observations.
With the generators running seperately you will probably get a beat frequency. A very low frequency increase and decrease in the sound level. To get rid of it, synchronize and close the tie breaker.
If there's any way to close the tie breaker or a generator breaker when the breakers are out of sync. someone will do it one day.
I see a couple of big Cats from time to time. They have an onboard computer that calculates the oil change interval depending on loading, but I don't know the specific algorithm.
The botton line on oil changes is either the manufacturers specs or regular oil analysis or both.
Neutrals. Don't get fancy. Run the neutrals to the neutral bus in the main panel or the neutral connection in the transfer switch(s) and from there to the main panel neutral bus. The main service ground on the main service will be the only ground tie point. The generator neutrals should not be tied to ground at the generators.
Noise, in addition to the critical muffler, sound absorbing material on the walls will help a lot. The quiet trailer mounted gensets have the inside of the enclosure lined with a sound absorbing foam. Some sets generate a lot of noise with the fan. An air duct with a couple of 90 deg. turns lined with sound absorbing material will help a lot.
I had a customer whose neigbour phoned my customer's mother every time his set started to complain about the noise. A big part of the noise abatement was an air discharge duct with two 90 deg. bends. I know, bends add restriction and increase back pressure. Fortunately, the increased restriction and back pressure help the sound absorbing material work better.
respectfully

 

[itsmoked]

Industrial Grade (15-22 dBA Attenuation)
Residential Grade (18-23 dBA Attenuation)
Critical Grade (18-32 dBA Attenuation)
Hospital Grade (30-42 dBA Attenuation)
Super Critical Grade (30-42 dBA Attenuation)
Extreme Grade (45-52 dBA Attenuation)


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[UWH]

AusLee,

One more thing to consider: if you go to a lower speed engine you may have to upgrade the engine size (i.e. add a few cylinders) which will increase your overall project cost.

At the same load (1000 kVA) a larger engine may produce a slightly lower exhaust flow rate, possibly letting you get away with a smaller silencer. And as MikeHalloran stated the hospital grade silencers are BIG.

However, I don't believe mechanical noise created by the engine would be much different for say a 1200 rpm generator vs. an 1800 rpm (as is common here in North America). Usually this is in the order of 120 dB(A) at the engine. You need to ensure there is good sound attenuation in your walls, and vibration isolation between the engine and foundation.

One additional problem is with the large louvres required for engine room ventilation. Sound-dampening louvres (in my experience) are expensive, create a lot of flow restriction and still let a lot of noise through. Sound-deadening barriers around the outside of the room work well.

cheers.