safety concern of foundation of vibrating equipment

[Zishan.A]

Hi All,
I work in industry as a civil engineer and recently came across a problem. Actually mechanical engineer asked me regarding safety of footing for generator with 1500 cpm frequency and vibration amplitude which is increased to 4.1mm/sec. If I look into plot by Richard el (1962) it is way off limit.
What are your opinions?

 

[WARose]

I'm kind of lost as to what the scenario is. (And what you are looking for.) Is this a existing foundation that this velocity was measured off of? Or is this a design someone has asked you to approve the resulting velocity?

 

[JLNJ]

Foundations for vibrating machinery is a broad and deep subject. The best way is to hire an expert structural engineer.

 

[SlideRuleEra]

Actually mechanical engineer asked me regarding safety of footing for generator with... vibration amplitude which is increased to 4.1mm/sec... it is way off limit.

There are two likely outcomes:

1) Whether amplitude continues to increase or stays the same, premature generator failure. Foundation ok.

2) If amplitude continues to increase, foundation failure immediately causing generator failure.
We had this happen in 1995 to a 270 megawatt steam turbine / generator. Operator error motorized/damaged the stationary turbine / generator causing rapid rotational acceleration and excessive vibration. Vibration overloaded the 4" diameter generator anchor bolts which cracked the reinforced concrete turbine / generator pedestal (foundation)... causing more vibration. At this point, the (still accelerating) generator self-destructed. All of this took a few seconds. Recovery and restoration of the unit took 1 1/2 years.

 

[Tmoose]

4.1mm/sec. ~ .16 in/sec.
probably RMS if European.

As others asked, what is the situation?
You mentioned the vibration amplitude as "increased to 4.1mm/sec. "
So recently increased vibration that attracted attention?

Is the 4.1 mm/sec vibration frequency also 1500 cpm?

Where is the vibration measured ?
I'd be interested in measurements on all the bearings , the mounting feet, horizontally up and down the bearing pedestals.
And many good clear pictures of the actual arrangement.

Also vibration Spectra with frequency range to at least 500 Hz f max.

Are there prox probes available tracking the shaft orbits etc?

Has any of the history been made available?

 

[Zishan.A]

Hi,
Thanks for your all responses, to make things clear to understand I let you know that it is existing generator (very heavy machine) on first floor of a building which is supported by concrete slab and columns. Vibration is measured in all direction of bearings. Last year vibration amplitude was 2.3 mm/s rms and now it is increased to 4.1 mm/s rms and currently not changing. I physically inspect the structure and currently there are no visual structure damage but you can definitely feel the columns and concrete slab vibrating (not unusual) as it is very heavy generator.
I could not find any reference in concrete design standards, only thing i found online is graph from Richart et al and to be honest i am not quite sure that it is relevant to that, any suggestions?

 

[JoshPlumSE]

I always point people towards the ACI report for vibrating equipment foundations. Mostly because there are some good suggestions for "rule of thumb" sizing of foundations without having to go into a lot of technical detail.

I summarized some of this back when I worked for RISA and the company was thinking about adding time history analysis. Attached is a write up for a training class / example I put together at the time.

https://files.engineering.com/getfi...84c-6246d51490d3&file=Vibrating_Equipment.pdf

 

[WARose]

Hi,
Thanks for your all responses, to make things clear to understand I let you know that it is existing generator (very heavy machine) on first floor of a building which is supported by concrete slab and columns. Vibration is measured in all direction of bearings. Last year vibration amplitude was 2.3 mm/s rms and now it is increased to 4.1 mm/s rms and currently not changing. I physically inspect the structure and currently there are no visual structure damage but you can definitely feel the columns and concrete slab vibrating (not unusual) as it is very heavy generator.
I could not find any reference in concrete design standards, only thing i found online is graph from Richart et al and to be honest i am not quite sure that it is relevant to that, any suggestions?

Thanks for clarifying the situation. When vibration all the sudden shows up with a existing piece of machinery...it typically means one of several things:

#1: Internal issues with the machine (i.e. wear or some kind of damage) is causing excessive vibration.

#2: There has been some modification of the existing structure to change the dynamics of the situation. This includes the addition/subtraction of mass from the floor...or framing change.

#3: The structure has some fatigue damage that can change the dynamics of the situation.

If you can eliminate #2 & #3 by your inspection (and talking to whomever runs the plant).....then you are left with #1. Sometimes #1 can be addressed with some balancing work (and there are people who specialize in that)....and sometimes it may go further. The possibilities in this are nearly endless.

Sometimes I've modeled the situation with FEA software and seen if excessive wear really explains the level of vibration I am seeing. But I don't know if you have the budget/software for that.

In any case, at least according to one vibration severity chart I have, that's rough running for most machines. Given that-and the fact it appeared suddenly-it definitely needs to be addressed soon.

 

[Zishan.A]

Hi WARose, thank you very much for response. The vibrations are caused by some faults in generator and they are planned for repair in few months. We just want to make sure that with these vibrations structure integrity is not compromised. Would you please mind sharing graph you have? If not whats the safe value for vibrations? I read on one website structure can sustain horizontal vibrations upto 10mm/sec in long run and thats making me confuse even more!

 

[WARose]

See the attached pdf. I found this for free on Research Gate. See p.20-22 for the various charts and permissible values for vibration for machines and people.

At the end of the day though, it is always best to contact the manufacturer to see what they think is ok as far as vibrations go.