% Allowable Stress
% Allowable Stress
(OP)
Sometimes back I encountered a pipe stress model that generated one of the highest at 95% of the allowable on operating load case. I was then a bit worry of the closeness to the maximum allowable limit untill somebody told me that it is okey. Technically it is stil within the allowable and is supposed to be consider safe. My mentor has told me 2 years ago the lower the better and of course it makes sense to me. My question from the experts around is: Is the % allowable be limited to figures below 100% for any longterm effect (say not > 90%? Any effect on fatigue stress, creep etc. that may cause failure on the future? Thanks again for a professional help.
Eduard
Eduard





RE: % Allowable Stress
RE: % Allowable Stress
Eduard
RE: % Allowable Stress
What sometimes happens is that people doing calculations and limit checks with rules of thumb prefer to take a margin with respect to code limits to somewhat cover their errors!
Of course these behaviours are not covered by code: only an as far as necessary detailed analysis done by a professional can be used to satisfy code limits.
prex
motori@xcalcsREMOVE.com
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RE: % Allowable Stress
RE: % Allowable Stress
Well, look at S-N curves for various materials. The lower the stress, generally the more stress CYCLES can be tolerated in the piping system's design life.
If you look at the B31.3 Code, paragraph 300, Severe Cyclic Service, you will see that you should limit your calculated stress range (Se)to 0.8 Allowable syress range (Sa) if you will have more than 7000 cycles in the design life.
Again, I advize seminars on B31 Piping Design Concepts and the book by Glynn Woods and Roy Baguley (CASTI Publishing) and the book by Dr. Chuck Becht (ASME Publications) to get a good understanding of the background and basic principles.
Best regards, John.
RE: % Allowable Stress
One other thing after reading the original question again.
The ASME/ANSI B31 Pressure Piping Codes provide an allowable stress range for the calculated expansion/contraction (displacement) load cases and they also provide an allowable stress for the calculated sustained longitudinal stresses due to weight and internal longitudinal pressure (i.e., "additive stresses"). The B31 Codes DO NOT provide an allowable stress for the "Operating case" which would include thermal, weight and pressure loadings all together. The Mechanical Design Committee is currently looking at providing an allowable stress (range) for operating case loadings but it will be a while before it gets into the Code "books".
regards, John.
RE: % Allowable Stress
In structural designs we consider the calculated stress between 80 to 90% as safe and satisfactory. In some cases where the loads are defined more accurately with less probability to change during the service, values upto 95% can be acceptable.
Of course, it is better to have the stress levels lower for specific systems subject to load cycles considering the fatigue strength.
However, the value is left to the discretion of the designer.
RE: % Allowable Stress
Eduard
RE: % Allowable Stress
The actual installation may be different from the original design (due to field adjust or some modification at site). The desiner should re-check the pipe stress analysis model after installation in the case of close to allowable stress in the first stage.
This is not quite often to modify again after recheck the model. However, in my experience, the contractor run pipe stress close to allowable stress. After the installation and recheck the model was changed and exceeded the allowable stress.
RE: % Allowable Stress
how does eduard47 thread jive with para 302.2.4(1) that state "... its permissible to exceed the pressure rating or the allowable stress for pressure design at the design temp of ...". thats over 100% .
RE: % Allowable Stress
Allowable stress is just that - allowable as defined by the code. That doesn't mean that exceeding that stress automatically results in catastrophic failure.
Q for John Breen:
While the code doesn't explicitly define an operating allowable, isn't it essentially given (refering to B31.3) by equation 1b in 302.3.5d
Sa = f(1.25(Sc+Sh)) - Sl
This equation incorporates both the displacement and sustained stresses and can through some simple rearrangment be setup to say that:
Sl + Se < f(1.25(Sc+Sh))
Obviously, the displacement and sustained stresses need to be checked against their individual equations since this equation doesn't tell you whether or not you've satisfied the sustained and displacement stress criteria.
However, it does seem like f(1.25(Sc+Sh)) is going to be the maximum combined stress allowable. Is the committee considering a lower value?
Edward L. Klein
Pipe Stress Engineer
Houston, Texas
All opinions expressed here are my own and not my company's.
RE: % Allowable Stress
Just a word about the second para of your post.
It is true that allowable stress is not the ultimate and exceeding the value will not cause failure. The effect is only a reduction in the Factor of Safety. However, the Factors of Safety have been provided to take care of other uncertainities such as material imperfection and unnoticed defects in fabrication and erection. These factors have been prescribed in codes as a result of considerable studies and statistical data analysis.
Hence, we, design engineers generally do not encroach into that. The margin we keep below the allowable stress is to take care of the uncertainities in loading only.
RE: % Allowable Stress
again, revisiting 302.2.4 (f).1
Subject to owners approval, its is permissible to exceed allowable by 33% for 10 hours incursion not once but 10x over. Thats 100 hrs events/year. It may not be your operating case but thats still count a lot and exceeding the allowable has been permitted.
RE: % Allowable Stress
As a good engineer / designer we have to consider some unseen aspects like fabrication changes, any changes that happen on site, material inconsistency. So I consider, it better not to cross 90% for the expansion stresses and may be 60 to 70% in sustained stresses. Especially when you have a high temperature, high pressure system, why to take risk?
Regards,
Kaustubh
RE: % Allowable Stress
Introduction of B31.3-1996 Edition
Second paragraph opens .. "Rules for each section reflect ..."
Then follows by the statement "The designer is cautioned that the Code is not (repeat not) a design handbook; .."
Let me cite the thought of glyn woods (type in bold by him) page 13 of his PIping lecture (asme education and training).
The Code does not provide any instruction as to how to design anything.
RE: % Allowable Stress
RE: % Allowable Stress
I think your post needs a correction. Basically, the allowable stress we discuss here is for the design, whereas finite element method is a method of analysis which gives the actual stress in a loaded system.The discussion is about how much close these stress (as calculated through FEM or otherwise) can be to the value allowed by the codes.
RE: % Allowable Stress