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Compressor Piping Natural Frequencies
5

Compressor Piping Natural Frequencies

Compressor Piping Natural Frequencies

(OP)
A (natural gas) compressor vendor asked us to design suction and discharge piping system so that its natural frequencies go above 6 Hz. Can anybody tell me from where this 6 Hz is coming?

RE: Compressor Piping Natural Frequencies

Was 6Hz the only criteria they gave? usually the compressor designers would required the system components to be a considerable distance away from their range of operating speeds so as to avoid operating at Resonant conditions.

There are usually a couple natural frequencies that the compressors would have to go through as they increase their speeds, but the general normal speed is far away from a critical speed.

6Hz is 6*60 = 360 RPM which is no way near the likely operating speed of a compressor.

RE: Compressor Piping Natural Frequencies

(OP)
Thanks Ripz for your reply. It seems you (like me) expecting natural frequencies being higher than RPM of compressor (divided by 60), isn't it?

RE: Compressor Piping Natural Frequencies

you need to be specific, pipe size, horsepower,compressor design, etc, to engage a proper conversation

RE: Compressor Piping Natural Frequencies

Ripz,

I'm curious why you think 360 RPM isn't likely to be near the operating speed of the OP's compressor? 300 RPM is pretty typical for a lot of recip compressors I've seen in recent years. 6Hz would get your 20% above that, which is also pretty common.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

RE: Compressor Piping Natural Frequencies

StressGuy,

In your case, this sounds likely. I was thinking more along the lines of larger centrifugal compressors. My assumption being that Plant Designers don't spend the money to do Pipe Analysis on the smaller equipment. Unless you're referring to a large, critical Reciprocating Compressor.

Maybe McDermott can enlighten us.

RE: Compressor Piping Natural Frequencies

(OP)
Sorry, I was away of this thread for a while. Actually, in my case the compressor is centrifugal with 8400 rpm as rating speed. Suction diameter is 24" and discharge one is 20". According to DNV-RP-D101, "Structural Analysis of Piping Systems", a typical system supported in accordance with a good pipe support standard should result in a lowest natural frequency not less than 4 to 5 Hz. May be this frequency (as a basis) came from vortex-induced vibration from flow of gas in the piping. Am i right?

RE: Compressor Piping Natural Frequencies

A bit of a stab in the dark here, but this might just be a desire to avoid vortex induced vibration from external wind. Any time I've seen pipe vibrating in the wind it's between about 4 htz....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Compressor Piping Natural Frequencies

Looking this up further, i think it's external vortex shedding. 4 htz as a vortex frequency for a 20" pipe is around 10 m/sec, a fast wind, but not unreasonable.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

RE: Compressor Piping Natural Frequencies

I do believe LittleInch is correct.
Plus the 8400 rpm centrifugal compressor is not likely to cause piping excitation troubles.

I hate Windowz 8!!!!

RE: Compressor Piping Natural Frequencies

(OP)

Quote (BigInch (Petroleum))

Plus the 8400 rpm centrifugal compressor is not likely to cause piping excitation troubles
So, it seems you (BigInch) believe that no excitation frequency is transmitted to the suction or discharge piping in this regard. Is it true?

RE: Compressor Piping Natural Frequencies

your 6 Hz range is not meaningful for the application you've defined, suggest that need to get an engineer involved, if you are contuing to have or are having a problem





RE: Compressor Piping Natural Frequencies

There will be little to none pressure pulsation energy transmitted from such a compressor to piping.

There may be some vibratory energy transmitted from machine-foundation vibration, however that will also most likely be very small, if the foundation has been designed properly. Machine foundation system frequencies are often in the range of 500 to 900 rpm, 8 Hz to 15 Hz, if it is a larger machine on a block and mat foundation. As long as the pipe has natural frequencies outside the range of 0.7 * 8 and 1.3 * 15 Hz, there should be little chance for resonance.

I hate Windowz 8!!!!

RE: Compressor Piping Natural Frequencies

(OP)
BigInch, Is it possible to tell me more about those numbers? How do you relate natural frequencies of the connecting piping to ones of machine foundation?

RE: Compressor Piping Natural Frequencies

3
You would calculate the frequency and amplitudes of vibrations expected from the machine-foundation-soil system. Impose those as forced displacements on the connecting piping. You could calculate the shear and bending stresses in the pipe due to those displacements.

If the frequencies of the forced displacements were in resonance with the pipe, natural frequencies of both being nearly equal, displacements can become quite large and fatigue damage can occur quickly. In order to have a safety factor against resonance, it is usually suggested that pipe natural frequencies be made to be outside the range of 0.7 to 1.3 x the frequencies of forced displacements. That is typically done by arranging supports at odd spacings, no two spacings the same, such resulting spans giving the pipe between them natural frequencies outside the 0.7 to 1.3 range of the machine-foundation-soil's natural frequency.

You can find out more about that here,
http://www.slideshare.net/ved_ram/machine-foundati...
and,
http://www.google.ae/url?sa=t&rct=j&q=&...

I hate Windowz 8!!!!

RE: Compressor Piping Natural Frequencies

(OP)
Thank you BigInch for your mentoring.

RE: Compressor Piping Natural Frequencies

As usual, I'm in agreement with BigInch.

It does strike me as strange that a centrifugal compressor vendor is asking for a specific minimum piping system frequency. I usually only see such in relation to recips, hence the basis of my original reply.

6Hz specifically is hard to me to understand as a recommendation. As LittleInch mentioned, 4Hz is the frequency to avoid excitation due to typical wind, and there's some info on that back in this old thread:

http://www.eng-tips.com/viewthread.cfm?qid=245824

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

RE: Compressor Piping Natural Frequencies

rather suspect that your are mis-reading your compressor vendors requirements.

no doubt have already discovered from your contacts and meeting with them, 6Hz may be a a mis-print with 6KHz is more nearly the range of frequencies encountered with such equipment

RE: Compressor Piping Natural Frequencies

(OP)

Quote (hacksaw)

6Hz may be a a mis-print with 6KHz

But, as I mentioned earlier, DNV-RP-D101 also took 4 Hz (which it seems that's related to reciprocal ones rather than centrifugal compressors.

RE: Compressor Piping Natural Frequencies

Quite. Centrifugals arn't the ones causing the problems.
They would normally stop well in advance of causing vibrations that hurt the piping.

I hate Windowz 8!!!!

RE: Compressor Piping Natural Frequencies


don't you wish the centrifugals didn't have piping/duct vibration issues...

RE: Compressor Piping Natural Frequencies

BigInch,
My company is facing high vibration problem and i'm not piping engineer even, our compressor piping is designed for 8 Hz frequnecy but eqully supported at 20', is eqully support is creating problem for vibration in our case.
Thanks
10815L

RE: Compressor Piping Natural Frequencies

I'd be likely to believe, if it is a reciprocating compressor. I'd have to know much more about the system to form some opinion as to what might be causing the problem. Please start a new thread, post the relevant design information and a diagram and we'll have a look. But please start a new thread.

I hate Windowz 8!!!!

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