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
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
RE: Compressor Piping Natural Frequencies
RE: Compressor Piping Natural Frequencies
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
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
RE: Compressor Piping Natural Frequencies
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
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
Plus the 8400 rpm centrifugal compressor is not likely to cause piping excitation troubles.
I hate Windowz 8!!!!
RE: Compressor Piping Natural Frequencies
RE: Compressor Piping Natural Frequencies
RE: Compressor Piping Natural Frequencies
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
RE: Compressor Piping Natural Frequencies
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
RE: Compressor Piping Natural Frequencies
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
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
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
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
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 hate Windowz 8!!!!