McDermott1711
Mechanical
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?
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Compressor Piping Natural Frequencies 5
- Thread starter McDermott1711
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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.
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.
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- #3
McDermott1711
Mechanical
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?
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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.
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.
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.
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.
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- #7
McDermott1711
Mechanical
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?
LittleInch
Petroleum
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
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
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LittleInch
Petroleum
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
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
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- #11
McDermott1711
Mechanical
So, it seems you (BigInch) believe that no excitation frequency is transmitted to the suction or discharge piping in this regard. Is it true?BigInch (Petroleum) said:
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!!!!
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!!!!
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- #14
McDermott1711
Mechanical
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?
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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,
and,
I hate Windowz 8!!!!
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,
and,
I hate Windowz 8!!!!
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- #16
McDermott1711
Mechanical
Thank you BigInch for your mentoring.
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:
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.
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:
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.
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
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
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- #19
McDermott1711
Mechanical
hacksaw said:
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.
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