Hello Stressers!!!

Lately I've been reading a lot about sound waves. Anyway for those who are already familiar with the physics of sound waves I will go directly to my question...In API 618 there's a requirement for a peak to peak allowable pressure pulsation corresponding to a certain frequency.. We know that source of frequency is from compressor speed (rpm/60 (Hz)) which is the first fundamental frequency (n=1) due to compression/expansion stroke of a cylinder. I have two questions which a bit confuse me.

1. When we say harmonic frequencies, (n=1,2,3,4,5.....10.), does this correspond to the standing waves produced or it is the compressor speed, my guess is the former for a fixed speed compressor.

2. Where is the harmonic (n=10) comes from, if my guess to my question 1 is correct, does this mean that a 10 harmonic frequencies is sufficient to cover a whole piping system connected to a reciprocating compressor.

Anyway, I will appreciate those who are experts in this field,if they can give their expert opinion on this matter. I can feel that I am missing something in my understanding.

Basically, I am comparing my understanding to the experiment I've seen in you tube where a tuning fork say (1000Hz) is the same as the reciprocating compressor (rpm/60 Hz). Then the pressure pulsation wave will travel to the piping system where standing waves (which can be considered as peak-peak pulsations)will exist in the anti-node portion of the piping system. Knowing that the speed of sound is constant in a certain medium, then from given the frequency, the amplitude can be computed. So I was thinking the n=10 harmonics is some sort of safety factor (like a sweep to take into account uncertainties) say for a fixed speed compressor where the frequency is only one (meaning 1 speed rpm only.)

Cheers and Thanks in advance!!!!

Hello Stressers!!!

I think I have answered my own question, but further explanation is appreciated.. I have watched you tube videos about "harmonics" and a bit understand the concept behind it. Basically, the main source of vibration for a reciprocating compressor is it's fundamental frequency which is rpm/60 (Hz). But the sound waves (pressure pulsation waves) can be excited at higher frequencies from the multiples of the fundamental frequency (n=1,2,3,4...10). If one of this multiples of fundamental frequency coincides or excites the mechanical natural frequency of the piping system then resonance will occur at certain points in the piping system and causes vibration. With regards, to how much multiples n to be considered depends on the source of vibration, for a high amplitude low frequency vibration such as waterhammer beam bending modes of the piping system will be excited, but for a low amplitude high frequency vibration such as AIV from pressure reducing valves shell modes of vibration of piping system will be excited (FEA software will be used not the beam model softwares).
For a reciprocating compressor I've seen API 618 Pulsation Reports as typical value from the softwares I've read so far...


Cheers to all!!!! Enjoy stressing!!!Let's keep our stress level low!!
That's our job, others think our job is stressful (anyway that's from a political point of view...:))

n=10 as typical value I've read so far reciprocating compressor (my sentence is incomplete above).

To further elaborate multiples of frequency "n" depends on the boundary condition such as for an open closed system n=1,3,5,7.... and for an open open system n=1,2,3,4,5,6...

Sorry my bad...I've done further reading and found out that the API 618 pulsation requirement is done thru Fourier analysis of a sawtooth flowrate function, to determine the strength of the individual flow harmonics. The discrete Fourier analysis of the flow saw tooth indicates which harmonics of the compressor running speed produce the largest pressure amplitude. When the pulsation exceeds the API 618 allowable pulsation, that's the time a mechanical response study is performed like the one I've described above. Am I right in saying that a mechanical response study is only required if the API 618 pulsation levels are exceeded or it is a mandatory requirement regardless pulsation levels are exceeded or not?

Thanks a lot in advance...