API 610 1n/2n Torsional Excitation

[VibFrank]

Hy,
I have in a torsional analysis an intersection of 2n with a natural frequency (Campbell diagram). It is a (asysnchronous) motor-gear-pump-system. According to API 610 in such a case a harmonic analysis with 2n has to be done to determine the existing stresses. But is there an excitation with 2n in such a system ? From the pump there are only much higher frequencies, from the gear also. Is there a 2n excitation from an asynchronous motor or a hydrodynamic coupling (as I know, the frequencies for such a coupling are also much higher..)

If I think about API 610 and the paragraph, that excitation with 1n and 2n should be examined: Which excitations in a torsional system come with 1n an 2n ?

Thanks a lot

 

[Tmoose]

Can't speak for API, but 2 bladed impellers are radially axially and torsionally alive at 2X.

 

[VibFrank]

Is this independent of the number of blades in the diffuser ?

 

[rob768]

no, you have an interference at 2x Z-diffuser also. Excitation forces are low however