As per API 618 following is the formulae for calculating frequency:

f = ( rpm * N ) / 60

where N = harmonic integer ( N = 1, 2 , 3 ..... )

in one project we reveived pulsation report from vendor where vendor used N = 2.

My question is how to select value of N while calculating frequency?

Thanks in advance

Regards,
AAD

As per my understanding the N represents the harmonics . 1f ,2f , 3f & 4f means first ,second ,third & fourth harmonic of the compressor respectively .Generally for reciprocating compressor we need to keep the natural frequency away from the +-20 % of the first to fourth harmonics of the compressor to avoid the mechanical vibration in the piping system . This is done by compressor vendor with the use of other software like ANSYS .
As a detailed engineering phase ,However in Caesar with the help of Modal analysis, we can eliminate the frequency with in the range of +-20% of first and second harmonic .Generally beyond second harmonic , the compliance in static analysis may not allow to eliminate the frequency .

Note :The simulation(modelling) of piping system in the Caesar II should be correct so that it show the exact Natural frequency & mode shapes .Read the pulsation study report carefully , i think they have provided some recommendation like minimum stiffness required , additional support requirement also .

Corrct me if i am wrong .

thank you for your reply.

but i want to know is there any basis for selection of harmonic integer.

means in our case vendor taken N= 2 how to select this harmonic integer.

or it is generally selected as 2.

please advise.

regards,
AAD

My guess is that you will not get significant harmonic response from the higher multiples. Remember, we are focused on beam bending and, with higher frequencies, there is less distortion for the same amount of energy provided.

Some vendors use 2*2rest to 0.5 also to get the higher frequencies...