fan stack vibration

[spike]

Background: A cooling tower fan stack that is 18" tall. The fan is a 14' propeller fan with five blades.
There is of course a noticeable amplitude of vibration that occurs at blade pass frequency.A bump test indicates a natural frequency of about three times blade pass frequency.
Problem: The amplitude at three times the blade pass frequency is much higher than that occuring at blade pass frequency. Lesser amplitudes occur at 2,4,5,6 & 7 times blade pass frequency. I do not understand this as there is no exciting force at three times blade pass frequency.
Is the stack really vibrating at this amplitude and frequency or is it just an indication of the looseness of the stack.

 

[MachineryWatch]

Spike,

I am not sure if I really understand the question, but I will give it a shot. Yes, there really is vibration amplitude at the harmonics of the fundamental forcing frequency (in this case 3X BPF). If you have a natural frequency near or at one of the harmonics of the fundamental it will be excited. The harmonic is considered the forcing frequency and a resonance is created by the response of the natural frequency. Looseness may be causing the harmonics to have more energy than they should have and this will ultimately impact the measured amplitude of the resonant-amplified vibration.

Skip Hartman

http://www.machinerywatch.com

 

[GregLocock]

Looseness is one possibility. I think it is more likely that the pressure pulse generated by a fan blade will not be sinusoidal, therefore the blade passing pulse will generate all harmonics of BPF. Third order is higher because it is exciting the resonance. Cheers

Greg Locock