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Vibe-Rater

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Hi All,
There is a wet gas compressor where I am working now and it runs 8547 rpm.  Every 18 months or there abouts the centrifugal impellor fowls with stuff and cuases it to show increasing trend in micron.  They stop and clean - back to normal.

I have been asked. If we replace thhe gearbox with o/p speed 900 rpm slower can you predict the new vibration levels.  Old levels say 125 micron.

From memorry there's a formula involving a rpm squared.  knowing the new rpm may dividing somehow..

I haven't googled yet - thought Id ask here first. rgds
John from PA

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The RPM squared function primarily applies to forces related to unbalance.  In other words, double the speed and the effect (in terms of force) of any unbalance would go up by a factor of four.  In your case (7647/8547)^2 is about 0.72.  That does not mean the vibration will be 0.72 * 125 or 89 micron, basically because  it fails to take into account any proximity to critical speeds.  As you can well imagine, if for instance the machine had a critical near 7600 RPM, reducing the speed to that region would increase the vibration, and perhaps by a considerable amount.
Noknroll

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just a thought from a level 2 newby, if the limit were to change with speed, wouldn't lower speeds allow higher levels of displacement? and another thought, if that were the case would it be acceptable to use the same 125 micron displacement regardless of speed?........I'll go now

Vibe-Rater

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The enquiry came from the viewpoint that fouling would still occur but would take longer to reach unacceptable levels hence extending T/A (turn around) intervals which would save them millions.  rgds
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Quote:
Originally Posted by John from PA
The RPM squared function primarily applies to forces related to unbalance.  In other words, double the speed and the effect (in terms of force) of any unbalance would go up by a factor of four.  In your case (7647/8547)^2 is about 0.72.  That does not mean the vibration will be 0.72 * 125 or 89 micron, basically because  it fails to take into account any proximity to critical speeds.  As you can well imagine, if for instance the machine had a critical near 7600 RPM, reducing the speed to that region would increase the vibration, and perhaps by a considerable amount.


Hi John, I already warned them about potentially running into a resonance(s) at lower speed. But didn't understand re other stuff so thanks for that.  Makes sense with the squared relating to unbalance.  Thanks for quick reply. I guess it must be close to midnoght where you are. rgds


MarkL

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From a different point of view, is the cause of the 'runs ok for 18 months, stops due to fouling, then repeat' cycle understood? Can it be mitigated? Or is it just a Wear and tear thing?...or is it a process issue, say build up of crap from the normal operation of the machine?.

From another humble level 2 :-)
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Quote:
Originally Posted by MarkL
From a different point of view, is the cause of the 'runs ok for 18 months, stops due to fouling, then repeat' cycle understood? Can it be mitigated? Or is it just a Wear and tear thing?...or is it a process issue, say build up of crap from the normal operation of the machine?.

From another humble level 2 :-)


Hi MarkL, the latter, stuff in the wet gas it compresses apparently.  Ill ask more detail tomorrow. rfgds
John from PA

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Quote:
Originally Posted by Noknroll

just a thought from a level 2 newby, if the limit were to change with speed, wouldn't lower speeds allow higher levels of displacement? and another thought, if that were the case would it be acceptable to use the same 125 micron displacement regardless of speed?........I'll go now



API has limits for a new machine on the test stand (see below).  In the field limits are often higher for various reasons although test stand values are often the desire.  As you can see this machine, at 125 microns, already well exceeds the limits.  The level of 125 microns should probably be compared against the bearing available clearance.  Some guidelines state that once amplitude exceeds 1/3 the bearing clearance, then the life of the bearing begins to suffer.

Your question, at the original speed of 8547 RPM, the API formula calculates to 30 microns (1.18 mil).  At the reduced speed of 7647 RPM, API would allow 32 microns (1.25 mil).  But a portion of the clause "shall not exceed the following value or 25 (1 mil) whichever is less" applies so at either speed the 25 microns or 1 mil applies.

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