New Pump on Suspended Slab 1

[someengineer]

Hello!

I have been tasked with investigating the feasibility of installing a new pump (and concrete pump base) on a suspended slab operating floor. The pump is 600HP and the suspended slab is only 6" thick supported by concrete beams.

I was hoping someone may be able to shed some light on any special considerations or rules of thumb for this type of design. I would imagine that in addition to the vertical gravity loads there may be some additional vibration loads. How can these loads be quantified? Are there any standard methods to dampen the possible vibrations? Would you just use a large concrete mass typically?

Any insight would be most appreciated.

Thanks!

 

[SlideRuleEra]

The pump is 600HP and the suspended slab is only 6" thick supported by concrete beams.

...no way. For starters, expect a 600 HP electric motor to weigh over 3000 lb. Add in the pump/piping weight, a suitable mounting for both, then the dynamic forces experienced during operation.

Both the beams and the floor almost certainly will have to be replaced with more robust structure.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[LittleInch]

You would hope that a motor of that size was installed and aligned to prevent any vibration otherwise it would destroy itself and the coupling fairly fast.

Of more importance is the fact that the bedplate of the pump and motor are not really structural things and need to sit on concrete which doesn't flex or move.

You can also get reaction loads from the pump vendor to take into account as start/ stop and occasional loads.

This doesn't sound like a good thing to be doing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[XR250]

Maybe put some spreaders underneath to transfer the load to the beams

 

[andriver]

Sliderule - how can you blanketly say no way without knowing more details (ie concrete compressive strength, span of slab, steel yield strength)? This is an honest question, I don't have much experience with concrete more of a steel guy. 3 kips doesn't sound like much to me depending on the existing loads.

If the loads are too great for the slab configuration, XR250 makes a good point. Steel beams beneath the pump running to perpendicular steel beams over the concrete beams would be a possible solution.

 

[Brad805]

You need to look at SRE's website. He has some useful links regarding this subject. Also, you need ACI351 Foundations for Dynamic Equipment. You may be able to use vibration isolation devices, but be forewarned that if you want to prove the system is adequate this is not a simple task. I have added a few useful references below and there are many other very good references you will find with a search. The general rules of thumb tend to apply to equipment at or near grade. I do not recall many simple rules for foundations for elevated equipment. Half of the battle is getting good equipment data and determining realistic forces once the motor is out of balance. I too doubt this will be a simple task. That is a big motor.

K G Bhatia, "Foundations for Industrial Machines."
Suresh Arya et al, "Design of Structures and Foundations for Vibrating Machines"

 

[someengineer]

I wanted to provide a few more details to see if this sheds a bit more light -

We are looking at a couple different locations for the pump, one where the pump would be centered on a large 14" x 34" concrete girder (running parallel to the girder span) and the other where the pump runs over top of a 10" x 20" concrete beam (running perpendicular to the beam span).

The 2nd proposed pump location is actually where there was one in the past that was demoed (similar HP). There are also several similarly sized pumps on the operating floor adjacent to the proposed location.

I would say that having the pump on the girder is the best location. Spreading the loading out an option but if we are taking the loads into the girder directly, is it really necessary (provided the girder can take the loads)?

Thoughts?

 

[Brad805]

You should post a schematic of the basic framing. Gravity loads are not usually the problem. That is the easy part.

 

[delagina]

google "table top compressor foundation" this type of structure is common in o&g industry.

http://engineering.benabraham.com/html/compressor_foundation_dynamics.html

 

[someengineer]

Please see the attached proposed pump location(s) as requested.

 

[SlideRuleEra]

andriver - You bring up a good point. I was too quick to say "...no way". A better answer would have been: "A 6" slab will not be adequate "as-is". Details of the beams are needed to determine if the slab can be modified into more-or-less suitable support." Thank you for asking about that.

As LittleInch and Brad805 pointed out the concern is vibration, structural support can be addressed. No doubt the pump/motor can be installed, and it will operate properly... the consequence would be much more frequent major overhauls than would be the case for a machine with a proper foundation.

Steel beams under the pump/motor solve the structural problem by distributing the load, but make the vibration problem worse. The pump/motor would now be somewhat isolated from the mass of the building. Mass (inertia block) is the easiest way to "tame" vibration. Connecting the beams to the building would not help much, the inertia block needs be almost "bonded" to machine so that they all vibrate together - at the same frequency(s).

For large elevated rotating machinery, the link that delagina provided is to the type foundation also used at electric generating stations for the turbine/generator. This massive concrete "turbine/generator pedestal" is actually designed to vibrate at the precise frequency needed to cancel out vibration of the turbine/generator operating as it's synchronous speed. That is an example of how important minimizing vibration can be for a machine expected to operate for decades.

For reference, here is the page of my website about machinery foundations that Brad805 mentioned: Link

There is one "trick" that someengineer could investigate, if the pump/motor have not been purchased:

Is the pump/motor a "Horizontal Pump" (motor shaft is horizontal) or is it a "Vertical Pump" (motor shaft vertical)?

A Vertical Pump is more tolerant of vibration issues than a Horizontal Pump. Has to do with the fact that in a vertical machine gravity is not causing the shaft to bow - even that small force can be important with machine vibration.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[someengineer]

I will investigate the use of a vertical pump. The pump has not been purchased yet, we are still in the feasibility stage. Thank you for all your help!

 

[delagina]

we just convert this to static load, 3x or 5x the weight.
mainly because nobody really knows how to add dynamic analysis of the pump inside an equipment structure.