> when we start this compressor the structure also starts vibrating
> (& its really much more like earthquake at 25m level)
This isn't a trivil problem and you need someone who knows a good
deal about structural dynamics. Normally what's done is to find
what's actually vibrating and locate the excitation. It sounds like
you haven't done either.
Without a lot better description that you've provided there's no way
to determine even a starting point, but a modal analysis of the
structure will give you something to begin with. It may feel like the
whole thing is shaking, but what often happens is that some
relatively soft element (say, an unsupported length of pipe or a
motor on a soft foundation) is resonating and the reactions where it
connects are moving the rest of it. You may be able to find the soft
points by inspection and estimate the corresponding frequencies
manually if you know what you're about.
The source of the excitation is a little trickier. Unbalanced
rotating machinery is frequently at fault, and the fix is to
eliminate the imbalance. Snoop around for rotational frequencies that
correspond to some structural frequency. Flow-induced vibrations can
also be a problem. Sometimes you can change the speed of rotation to
shift the excitation frequency away from the structural resonance or
change the support conditions for whatever vibrates to shift the
structural frequency. Best bet, though, is to eliminate the source of
the excitation.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/~chrisw/
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