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Alternative norms to ASME Sec. VIII 1

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cloche

Mechanical
May 12, 2005
112
Hi,
I deal with a customer who has somewhat confused ideas about stress limit criteria for pressure vessels (in this case, it's a series of hydro-plant conduits).
We always use and refer to ASME Section VIII for this kind of calculations, but the customer asks if other less restrictive norms can be used.
I don't exactly know where the customer wants to go with "less restrictive", perhaps accept lower thicknesses in order to save money...
Are the ASME really so constraining as the customer is pointing out? Per our experience, no component we dimensioned following strictly the ASME failed because of design...
Any ideas? Any comments?
Thanks!
 
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I believe what your "customer" is asking is "are there cheaper alternatives to designing these hydro-plant conduits? ASME B&PV code uses design margins to assure reasonably safe and long service lives of pressure retaining components. So in a way, design margins could be construed as adding cost.

I am not that familiar with hydro-plant conduits. If they are not covered under the scope of ASME B&PV code or some other Code or Standard, your customer might be on to something. I will leave you with this thought – safety is most important and must be considered in every component design.
 
"Hydroplant conduits" sounds like "water piping"? In this case, check out AWWA's manuals and standards for steel piping. I believe the stress normally used for hoop stress is half the yield, or somewhat higher than ASME uses.
 
Thank you for your inputs!
I also do believe that safety must come before any other consideration. In the case of Hydro-electric power plants, the failure of a penstock part can have dramatic consequences. That's why I am so reticent to depart from ASME.
I'll look at the AWWA also. In the meantime I discovered that probably any country has its own technical directives as regards pressure vessels, and these can range from the severest you could imagine (one customer from Chile wrote in the tech requirements that hoop stress had to be less than 0.25 times the strength limit, where ASME wants 0.33!), up to the most "fantasist". In fact, in Italy a very old directive (dated 1966...) prescribes limits apparently greatly higher than ASME, but without any distinction btw pure membrane stress, local stress, bending, peak...!!! Sincerely, it's the first time a customer in Italy does not want at least the ASME limits in order to be safe...
 
I think what your Customer is asking is to refer to other design codes which are less expensive to implement when the manufacturer does not reside in USA.
To exemplify, you will find out that in Europe the pressure vessels are usually manufactured in accordance with PED - Pressure Equipment Directive - which is the european equivalent to ASME.
There will be no safety compromise - but the cost of Certification (for example) is vastly cheaper.
 
cloche,
ASME VIII is known to be more conservative than many other rules for pressure vessels, especially for room temperature components, as in your case: recent editions of ASME have addressed this point by lowering some safety margins with respect to what was applicable a few years ago.
So I understand your client's worryings and think that he is addressing the above situation.
BTW italian rules have been updated in 1995 (but the way pressure interacts with a containing wall is the same from the time of the big bang!). Today in Europe you can use one of many different codes, and among them of course the european code EN 13445.
Basically with this one (and also the italian code) the allowable stress is the minimum of yield/1.5 and tensile/2.4, but of course you'll have to decide how to handle the efficiency of longitudinal welds too.
Can't see a reason why you shouldn't use the european code. Of course if you increase the safety margin, generally speaking you'll be safer, but the codes are there exactly to specify an acceptable margin...

prex

Online tools for structural design
 
1) Pumpkind, we have no certification problems, our company is a multi-national corporation already certified at corporation level. Generally speaking we internally follow the ASME standards for all components assimilated to pressure vessels (spiral cases, penstocks, distributors, etc...)

2) Prex, our company has chosen the ASME because of two main advantages:
- they are the most widely accepted norms around the world
- we think that the "degree of conservativeness" on which they are based is the most suitable for critical components like the ones in hydraulic power plants (human lives can easily be lost in case of disaster). Based upon more than 100 years of internal experience, we could ourselves establish internal norms, but they wouldn't be usable for contracting of course...

3) We already are updated as regards ASME (we have the latest edition of Section VIII) and we use part 2 which is refered to the complete analysis of the stress field in the component, which we achieve with FEM); thanks to the suggestions of you all, the interesting thing is now that:
- we can perform our calculations and tell the customer what would be the most adapted criteria based upon our internal experience (almost coincident with ASME)
- we can make comparisons to the EuroNorms and other international norms, and estimate the possible loss of security margin between them. The customer will be then able to choose the right balance of cost/risk. In other terms, surely the best thing in this particular case is for me to "parametrize" the criteria in the strength analysis report.
 
It only comes down to corrossion;
you can very much save a bunddle on material thickness if you cut to te minimum,
but the real engineering will be in the corrossion add-on
it is what you can live with? and save in accidents later.
ER
 
Well, in this case corrosion would be a little bit difficult to precisely estimate, as the water carried by the penstock has an "unknown" (I mean, estimated but not exactly known...) content of sand and other aggressive particles. It's more a matter of erosion than of corrosion. Surely we can make additional exams on the water, but perhaps it would be more expensive than to keep a "bigger" safety margin...
 
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