I just want to ask for your assistance/help regarding the minimum frequency (for the modal analysis) required for the piping connected/near compressor. I am more after for your experience point of view/ rule of thumb. Or maybe a reference I can read and use for my report. Thank you in advance.

from my experience,
we usually used 1.3*RPM/60 = cut off frequency for compressor

Thank you for your information rileykyrra.

Dear Marchie,
You should avoid coincidence of mechanical natural frequencies with 10 harmonics of compressor. A simplified analysis of the piping system shall be performed to determine critical piping lengths that may be in resonance with acoustical harmonics.
Make sure that there is adequate separation between the fundamental running speed of the compressor and its harmonics and the mechanical natural frequencies of the system. 15% separation is reasonable.
Compressor harmonics are RPM/60 times 1,2,3,...,10.

Dear Farhad,

Thank you for your information and advice.
Is it possible / okay for me to assume that the required frequency for reciprocating compressor is more critical or higher than centrifugal compressor's frequency? Based on my experience in the site, reciprocating compressor has higher vibration than centrifugal compressor. Please correct me if I am wrong about my assumption. Thank you.

I was just doing the assumption since the line I am analyzing now is not actualy main line of the compressor (Suc/Discharge)and it is actualy a spray nozzle on the casing of compressor(0.5" size). Thank you in advance.

Marchie,

You are right. Reciprocating compressors have more higher vibration due to pulsation. For small branches such as instrument or any other connections, you may use brackets of field support to eliminate vibration.

Calculate the m/c frequency (refer m/c datasheet). Run modal analysis & maintain frequencies +/- 20% of the m/c frequency for first 4 harmonics.
Stiffen your piping by adding supports or closing gaps to maintain this frequency. Its preferable to keep frequency on +20% of the m/c frequency.