Manual design of SAG/ball/rod mill foundations 3

[dbuzz]

Any handbooks/papers for the manual design of SAG/ball/rod mill foundations?

We are doing a preliminary design only and don’t want to use finite element software.

 

[youngstructural]

I have a bit of a long answer here, so I'll give you a synopsis: I don't think an design can be produced manually, and I think a preliminary design could be detrimental (ie: increase client expectations).

Personally I would base any preliminary design on previous experience. In the case of something like this, and lacking previous experience, I would not be willing to do a preliminary design. I would give conservative dimensions for what I was thinking would work, but I would not show any reinforcing or detailing whatsoever. I would leave the entire foundation greyed out, and explain the complexity of designing such a foundation to the client. In fact, I would go to great lengths to explain these complications to the client in any case.

Personal thoughts: These machines are vibration sensitive in addition to highly senstive to settlement (even more so when it is differential) and deflections which are perfectly normal and acceptable in a typical foundation designed for loads (including simple magnitude based load factors to compensate for dynamic loads) are usually not acceptable. You need to work closely with the Mechanical design engineers involved regarding forcing frequencies and the operating frequency ranges for the mill, and will almost certainly need to carry out a full dynamic analysis of the foundation in something like Calculix or ABAQUS. Remember that you need to avoid being close to the forcing frequencies and natural operating frequencies of the mill, as well as the first few harmonics of these frequencies. Your spring constants and/or any modelling you input to reflect the founding soil characteristics will also affect the results of the analysis; Again, working closely with the Geotechnical Eng. is called for.

Have personally never been invovled with the design of a SAG Mill foundation, but have done a number of rotler, balancer, very large CNC Mills... A number of these issues are common to both, but as I understand it the typical SAG Mill is very large and I would think most of these issues will be magnified. Pay close attention to how you're going to ensure that vibrations do not travel between the mill's foundation and surrounding pads.

Good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[FOETS]

Youngstructural a star for you.
I have direct experience of a SAG mill with conflicting frequencies causing resonance and vibration problems. I have also heard of many other other operations with similar issues. Once built these are incredibly difficult and expensive to rectify.

You mention the need to consult with the mechanical design engineers to analyse forcing frequencies, it is important they also pay special consideration to the torsional frequency aspects.
The other critical component is the electrical variable speed drive system which in itself can be a major source of frequencies and harmonics. These drives are infamous in the past for creating major issues with resonance. They have developed and improved vastly over time but should still be a major area of concern.

DBuzz these systems are incredibly susceptible to problems. The variables are usually very complex to model, and even the most experienced companies at designing this equipment have got it wrong. The load is forever changing with feed variations, and liner and lifter wear thrown in to complicate matters.

A thorough analysis needs to be done of the entire system to ensure that Mechanicals, electricals, Civils and structural are all in sync; as each of these elements are typically designed by different individuals or consultants.
Leave no stone unturned in your analysis (pertinent for a mill eh?)

Foets

 

[SlideRuleEra]

See the documents on this page of my website

http://www.slideruleera.net/foundations.html

http://www.SlideRuleEra.net

 

[youngstructural]

Thanks for that FOETS... If you'll forgive the pun, I guess I've always been a bit sensitive to vibration problems... *smiles*

SlideRuleEra: Great articles! Very glad to see you've posted them, since finding resources like these is always a challenge.

I have to say that I really look up to you on this site. Being a recent graduate, and only now nearing the end of my years as an intern, I can only hope to be such a fount of knowledge with time!

Cheers,

YS

P.S. DBuzz: Let us know what you decide to do!

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[SlideRuleEra]

Thanks, youngstructural - always glad to hear that old information can be useful.

http://www.SlideRuleEra.net

 

[dbuzz]

In the context of a study for a minerals processing plant worth in excess US$500m, a preliminary design (i.e. not FEM) approach to such a foundation is perfectly appropriate. We're not trying to design and build the thing, but make adequate provision in a quantity take-off and cost estimate.

BTW youngstructural, you say "I don't think a design can be produced manually". How do you think designs for these types of foundations were produced 30 or more years ago?! Our role as engineers is to use our experience and the body of knowledge to do the best job with what we've got, not to take the 'high technical ground' and pull out all the bells and whistles on a relatively small cost item in a much larger project. With no disrespect intended to respondents to this thread, unfortunately it's all too common to find engineers fixated on the "tools of the craft" rather than the "craft" itself.

As an engineer who has experience using FEM for the design and assessment of SAG and ball mill foundations, I know that this approach can be time-consuming, costly and -- in some cases -- not even a hope of being accurate if you don't have appropriate geotechnical and manufacturer information.

 

[youngstructural]

Sorry for the late reply all; I was on vacation.

dbuzz: Please do not misunderstand this reply: I appologize in advance if it seems that I am attacking your opinion, however I feel rather strongly on this matter.

I thought that I said quite clearly that if you've done it before, great; Go ahead and do a preliminary design. PERSONALLY, I would not produce a preliminary design for something like this without previously having completed a design for similar.

As for how things were designed 30 years ago, I would say that 30 years ago this type of foundation would have been strongly monolithic, but very well engineered for the knowledge of the time. I know for a fact (and it looks like FOETS has similar personal experience) that there were (and sometimes continue to be) alot of problems with older machine foundations. I know of one foundation that was built and then beefed up three times. The final product just became a massive block of concrete. We took one look at the foundation and permitted a modern SAG Mill to be installed on the old foundation. With respect for what it was, it's not going anywhere, and will not be experiencing any vibration issues anytime soon. My back-of-the-envelope check gave me a natural frequency somewhere in the 0.000X range. With respect for the environment, the foundation should have only needed a third of that concrete, at half of the expence, and certainly should not have needed to be re-worked three times.

I have read your replies to other postings with respect dbuzz, and honestly hope to see your replies to my future questions. I think you bring alot of experience and expertice to this forum. Even so, I stronly disagree with your view on this matter. I would not give a perliminary design for such a large machine foundation without prior experience in similar works. It is my belief that it only leads to unrealistic expectations and client disappointment. Perhaps we'll just have to agree to disagree here.

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[dbuzz]

I think we have a case of 'the forest' and 'the trees' here youngstructural. We'll have to agree to disagree on this one, but thanks for your input nonetheless.