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# Re: Vibration Problem

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Vibration Problem
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Fri, 25 Apr 2008 15:08:04 -0500

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On Apr 25, 2008, at 3:18 AM, Swami Raviraj ((Mumbai - Civil)) wrote:

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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.
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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.
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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.
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Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com   | this distance" (last words of Gen.
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.......................................| John Sedgwick, Spotsylvania 1864)
```http://www.skypoint.com/~chrisw/

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