structural verification 1

[mikycampo]

I'm doing a structural verification of a piping system.
Is it correct to check for allowable stresses in separeted cases: deadweight+prssure nad thermal loads?
I mean: i've checked if maximum stress intensity is under the allowable stress for deadweight+pressure and than in a separate analysis I've checked for maximum stress intensity due "only" to thermal loads

thanks a lot Michele

 

[BigInch]

No, you must check for all possible combinations as described in your design code.

ie.,

deadweight + Liveloads (not only live weight, could be waves, winds etc.) + operating pressure, operating thermal + seismic

would be a perfectly valid combination under most codes.

http://virtualpipeline.spaces.msn.com

 

[mikycampo]

Thanks BigInch,

I've red several times my code and now your answer, but I cannot find the combination "op.pressure + op.temperature" in stress verification.
On the contrary, the combination "op.pressure + op.temperature" must be used in supports' loads and foundations' loads calculation.

is it correct?
why?

thanks again michele

 

[BigInch]

For the record, exactly what code are you using?

If I'm reading your question correctly, I think you have to ask the question, what could be included under "loads and forces" from "other causes" in conjunction with par 301.6

For example B31.4,

301.6 Weight Effects
The following weight effects, combined with loads
and forces from other causes , shall be taken into account
in the design of piping.

of the medium transported or the medium used for
test. Snow and ice loads due to both environmental
and operating conditions shall be considered.
301.6.2 Dead Loads. These loads consist of the
weight of piping components, insulation, and other
superimposed permanent loads supported by the piping.

Continue reading through paragraph 301.11

302.3.5 (c) Longitudinal Stresses SL. The sum of longitudinal
stresses in any component in a piping system, due to
pressure, weight, and other sustained loadings SL shall
not exceed Sh in (d) below.

B31.4
You will find similar wording in section 402

In the end, it makes no sense to design pipe only for pressure and thermal effects, when it could easily buckle from an axial dead load, a lateral wind, seismic or wave load, nor would it make sense to design a support only for pressures and thermals when they obviously must support the pipe dead and live loads and simultaneously resist any forces generated by operating pressures and temperatures.

The intent of the code is much more important than the actual text.

http://virtualpipeline.spaces.msn.com

 

[mikycampo]

Dear BigInch,

My explanation was quite bad (my english is quite bad). You understand correctly my answer: I'd like to know "loads and forces from other causes" as stated in 301.6.

In my analysis, I've considered TOGETHER dead loads, live loads(quake and wind in my application), internal pressure. I've calculate dthe stress intensity (Tresca) and I've compared this value with 1.5 the allowable stress at design temperature(Sh). I've used the stress intensity because I'm using a fem software nad this results it's simpler to compute.

Then, as stated in "Part 5 flexibility and support", I've considered only the constraints and the thermal loads (par 319.2.1). In this case again, I've compared the stress intensity with 1.5 the allowable stress at design temperature. Do you think this is correct?

The analysis, with all the loads of the two cases, seems not to be requested by B31.3. I've made this analysis and, using the limit 1.5 Sh for stress intensity, I've found a lot of pipes overcoming the stress limit.
So, I've decided to use 3 Sh as limit in this global case, and I've found that now no pipes overcome the stress limit.
Is it this last assumption correct in your advice?

Thanks a lot,
Michele

 

[BigInch]

You're losing me with this talk of 1.5 and 3.

The code states defines Sh which can't be adjusted except as allowed for displacement stress as permitted in part d for calculating Displacement Stress Range SA, but Sh is always the basic allowable stress.

I think the best thing to do here might be to refer you to this commentary by CAEPIPE authors.

"Most piping codes incorporated into piping stress analysis programs stipulate that piping systems (new or existing) meet certain minimum engineering requirements thought necessary for safe design. Users are well-advised to follow their own more rigorous analysis (in lieu of such code requirements) if the validity of such an approach can be shown. Either way, the code does not limit conservatism, nor on the other hand, does it do away with the need for competent engineering judgment.

With regard to stresses, there are three main types of stresses that a designer must check for:"

the rest of which you will find here,

http://www.sstusa.com/06julsep.htm

http://virtualpipeline.spaces.msn.com

 

[mikycampo]

Finally, if I've understand, I must check for allowable stress in three separate cases:

1. dead weight + internal pressure (primary stress) and check my stress intenity levels with Sh

2. thermal, imposed displacement and constraints (secondary s.) and check my stress intensity level with Sh

3. Quake, Snow, Wind load (peak stress) and check my stress intensity level with Sh

Thanks again, Michele

 

[prex]

No, mikycampo, you are certainly not correct!
1)stress limits are defined, for various categories of loads, by the code of reference
2)piping codes that I know of do not use stress intensities as the basis for stress limits, you'll have to check circumferential stresses due to pressure, longitudinal stresses due to pressure + imposed loadings, and longitudinal stress (or also stress intensity) ranges due to thermal and other imposed displacements; if you want to use stress intensities, you'll need to justify that their use is on safe side.
3)please be sure to first understand clearly the concept of stress range: I may be wrong, but you do not seem to be aware of it
4)the allowable stress range, in ASME codes for pressure piping, is normally calculated based on the actual value of the longitudinal stress from imposed loadings: you definitely need to consult your code for that
5)when you go to occasional loadings (wind, earthquake, etc.) the code of reference could define a higher allowable when those loadings are combined to the normal ones: once again you need to consult the code.
Sorry, but there is no simple answer to your questions.

prex

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