Equipment Vibration
Equipment Vibration
(OP)
I am designing a 20' tall elevated concrete platform that supports two centrifuges designed to extract sludge from treated wastewater. The equipment operates at a speed of 2500-4200 RPM. The equipment criteria states "the stiffness of the supporting structure should be adjusted so that the natural frequency of the structure with machine is 30% above the frequency at operating speed".
I have calculated the frequency of a relatively stout concrete structure to be less than 2 Hz. For purposes of wind design, ASCE 7-98 defines a rigid structure as one with a natural frequency greater than or equal to 1 Hz. There is no way I can achieve a frequency 30% above 4200 RPM (70 Hz)!
What's the deal? Anybody out there ever run into this? I've tried looking it up on the Internet without much success, except to be able to purchase expensive books on the subject.
I have calculated the frequency of a relatively stout concrete structure to be less than 2 Hz. For purposes of wind design, ASCE 7-98 defines a rigid structure as one with a natural frequency greater than or equal to 1 Hz. There is no way I can achieve a frequency 30% above 4200 RPM (70 Hz)!
What's the deal? Anybody out there ever run into this? I've tried looking it up on the Internet without much success, except to be able to purchase expensive books on the subject.






RE: Equipment Vibration
"Basic Vibration & Vibration Isolation Theory"
"Principles of Foundation Design for Engines & Compressors"
"Don't Gamble on Machinery Foundations"
Then go to my Portland Cement Association page; this document may be of interest:
"Fastening & Bedding Columns & Machine Bases"
May not be exactly what you need, but it is a start.
www.SlideRuleEra.net
RE: Equipment Vibration
RE: Equipment Vibration
The vertical frequency of the stucture is only about twice the lateral frequencies, so even it is not close. I can't add columns directly under the equipment anyway.
Shear walls are also out of the question.
My platform is 18'x25' in plan, has four 18" square concrete columns with 18"x32" deep perimeter beams, and an 8" one-way slab. No reasonable amount of beefing up is going to work. There is no "$xxx" that would satify the criteria because there is virually no possible solution.
Could you explain the 50% rule in more detail?
RE: Equipment Vibration
1) Change the required frequency of the supporting structure.
2) Realise he needs a beefier structure, and he won't be able to have a 12' cieling under the support structure.
3) Explain to him what aggman mentioned. During operation, the structure will be sufficient for the stiffness requirements. During power-up and shutdown, there will be frequency problems. If he's ok with it, then you're ok with it. If not, see options 1 and 2.
RE: Equipment Vibration
RE: Equipment Vibration
If you have to provide a natural frequency of 30% over 4200 rpm you're going to have a real problem. When I get into the office after the weekend I'll take a look at my designs and see what the I ended up with.
RE: Equipment Vibration
RE: Equipment Vibration
The operating speed of your equipment is 42 Hz to 70 Hz which much higher I think than you can get as a fundamental mode of a normal structure. The objective when designing a support for equipment is to keep the natural frequencies of the support/foundation system out of the operating range of the machinery or you will end up with resonance problems. The technical reasons for this are as described by aggman and the 50% rule sounds about right to me.
One consequence of having a natural mode lower that the operating speed is that you pass through it on startup and shutdown. The owner needs to understand this. If ok make sure the structure is strong enough for a short duration period of heavy vibration, otherwise at operating speeds it should be ok. Vibration could be a problem however if the centrifuges are near office areas or other sensitive equipment.
Be careful with vibration isolators. These are basically springs, and when put on top of structural supports (which are springs) you can get undesired results. I think you should start with a rudimentary dynamic analysis to predict the first few natural frequencies (3 to 5) to assess roughly where you are. Remember that the foundation and soil stiffness are part of the model.
Regards,
-Mike
RE: Equipment Vibration
I think you and the spec writer need to have a short discussion. I suspect he meant the frequency of the floor and so on, not the whole building.
You and he might decide that a better approach is to define an allowable vibration limit in the rest of the building, then you can do the job properly with an isolating floor.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Equipment Vibration
The natural frequency increases as the foundation weight decreases and as the soil bearing contact area increases. So you could try minimising the foundation mass and maximising the plan area -- a long, broad, thin slab (which may not actually be that desirable). As a rule of thumb for preliminary sizing, footings for reciprocating machinery should weigh approximately five times as much as the machine.
The British code of practice CP2012 Code of Practice for Foundations for Machinery: Part 1. Foundations for Reciprocating Machines is a good guideline.
RE: Equipment Vibration
I concur with all the previous posts about keeping the natural frequencies away from the operating frequencies of the equipment.
One reason this will work for you is that there is internal damping present in the concrete structure. If the damping is present, this is what will prevent the resonance that was also mentioned. When you analyze your mathematical model, be sure to include the efffets of damping. That will demonstrate for you and your client that the actual vibrations may be small enough not to cause problems even if the natural frequencies are relatively close to the operating frequencies. The 50%/150% rules are guidelines, not hard and fast rules.
Regards,
chichuck
RE: Equipment Vibration
-Mike