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RE: Equipment Vibrations

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In doing a little of research since I wrote the message below, I have found
the following information relating to some of my own questions:

For concrete structures, ACI 350R-89 states "The natural frequency of a
structure should be calculated for the vertical direction and for the two
principal horizontal directions."; "If the machinery is installed on an
upper floor, the use of vibration isolators is recommended, but this should
not be considered a substitute for dynamic structural design, since all
isolators transmit some vibration."  Their recommendations relate to
"centrifugal pumps, fans, centrifuges, blowers, generator engines, and
compressors".  

>From vibration guidelines for a private engineering firm for heavy
industrial structures: "In general, it can be stated that a motor is a well
balanced item of equipment and will not operate satisfactorily with an
internal unbalanced condition.  For this reason we do not usually tune for
the rpm of the motor."; for trusses and braced structures: "It may be
pointed out that individual members such as columns, verticals and diagonals
should be investigated in both directions as well as the structure as a
whole."; "The dampening ratio ... can be neglected for structural steel and
concrete structures...". 

And from a book on "Pumping Station Design" (by Sanks): "Avoid resonance
frequencies by supporting pipes at frequent intervals and using deep beams
and thick floor slabs.  Isolate diesel engines by setting them on spring
supports."; "In centrifugal equipment, vibration is caused by unbalanced
rotating parts, misaligned shafts, and fluctuations in the load."; "There
are no established legal limits for vibration levels of equipment and piping
associated with pumping stations, but there are guidelines for normally
acceptable vibration levels of major pieces of equipment."; "...a
peak-to-peak level of 1.0 in/sec is generally regarded as an upper limit for
safe, continuous vibration levels of structural floors."; "When vibrating
equipment is rigidly mounted to a massive structure (such as a building
floor slab) the majority of the vibrational energy generated by the machine
is transmitted into the structure...If the rotational speed of the equipment
happens to match one of the resonance frequencies of the supporting
structure, the structure may vibrate at excessive levels."; "Vibration
isolators reduce the amount of vibrational energy to the structure
supporting the equipment and, hence, can reduce the vibrational levels of
the structure."; "Most pumps do not require vibration isolation in
well-designed pumping stations.  Equipment that should be vibration-isolated
in most pumping stations includes generator sets, fans, and air compressors.
This equipment usually contains sufficient low- and high-frequency energy to
cause excessive structure-borne noise in other areas of the building.";
Pad-type vibration isolators are effective only for equipment with driving
frequencies greater than 1500 rpm, springs can be effective for all
equipment with driving frequencies greater than 300 rpm; vibration isolators
primarily serve to reduce structure-borne noise for equipment with higher
frequencies.

Other than the "rule of thumb" that motors generally aren't critical for
structure vibration, most of the above just confirms that a frequency
analysis is required on all primary axes for various equipment.  But I've
run into problems where it is questionable whether a full vibrational
analysis is warranted for a relatively small piece of equipment.  It is for
such cases I am looking for "rules of thumb" where a simplified analysis is
warranted, such as simply applying an impact factor. 


> -----Original Message-----
> From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
> Sent: Tuesday, November 09, 1999 6:25 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: RE: International Conference on Vibrations
> 
> I can't make it to the conference on vibration, but that 
> brings up some
> questions I have struggled with: 
> 
> 1. Are there any rules of thumb on when a structural 
> vibration analysis
> needs to be done or can be waived, say for a steel support 
> frame, based on
> readily available equipment data?  (equipment weight, RPM, 
> centrifugal vs
> reciprocating, etc).  Typically I don't know the "disturbing force"
> magnitude, amplitude, etc.  And it is usually quite difficult 
> to achieve
> required stiffness to provide a high natural frequency of a 
> support for
> rotating equipment.  When can design be based on an increased 
> load (such as
> impact percentages per AISC for supports for machinery) in lieu of a
> frequency analysis? 
> 
> 2. Does vibration excite all vibration modes/directions or 
> only directions
> associated with the direction of the disturbing force?  For 
> centrifugal
> equipment (pumps, fans, etc), must vibration be considered on 
> all principal
> axes, i.e. vertical and two lateral directions for a support frame?  
> 
> I am currently working on the vibration analysis for a steel 
> support frame
> for some vertical turbine pumps and have pondered the above 
> questions.  A
> vibration analysis appears reasonable for this case as the 
> pumps are fairly
> heavy, but I haven't come across any good published rules of 
> thumb for the
> above questions.