piping stress developed in operating cases 2

[ibj2527]

Hello,

first time posting on this forum.

I am a junior piping engineer with 3 years of experience, so I know there is a lot out there for me to learn.
My understanding is that the piping code (ASME B31.3) addresses the following failure modes: excessive plastic deformation, plastic instability or incremental collapse, or high-strain-low-cycle fatigue.
Each of these modes of failure is caused by a different kinds of stresses and loading. So it is necessary to place theses stresses into different categories, such as primary or secondary, and set limits to them.

Primary stresses are not self-limiting. They are developed by imposed mechanical loadings such as weight, pressure, etc. A system under primary stress will continue unabated until force equilibrium is achieved, or until failure of the cross section results.
Secondary stresses are self-limiting. They are caused by constraints of displacements of a structure, where displacements are caused by thermal expansion. Distortion of piping system or local yielding relives stresses. That's why the yield stress of secondary stress is twice as high as that of primary stress: repetitively applied load which initially stresses the pipe into plastic yielding will "shake it down" to elastic action.

Sorry for the lengthy introduction, here goes my question.

So primary and secondary stresses are classified into two different categories by the code because their mechanisms are different. So that is why we segregate SUS and EXP and evaluate them separately - not to mention that they have different allowables.
My boss says we also have to look at the overall "energy level" in a piping system in OPE cases, and see how much the operating stresses are compared to material's allowable stress. He sets the stress type of his operating case in the load case to either SUS or OCC in order to compare the operating stresses to the allowables. I have no idea how CAESAR II calculates stresses in OPE cases where we have Weight, Temperature, and Pressure all together though. When the operating stresses are too high, he says extra flexibility needs to be considered even though EXP cases pass.

Anyway, I get his idea but I don't entirely agree with him because by doing so we are mixing two different stresses together, which I believe the code does not allow.

I know this is a sticky topic on this forum but I had no luck finding a thread that truly answers my question.

 

[1503-44]

If you are sure your design meets code, I wouldn't worry too much about your boss' extra criteria. The code allows him to make additional requirements and provisions above and beyond its specifics. In fact, the code requires such provisions be included whenever the EOR determines it is necessary to provide and ensure a safe and functional design. I myself always require that a blowdown vent riser be fabricated using XXS pipe, even if Sch 10 meets the provisions of the code. I also require that anchors not be used, unless it can be proven that they are necessary. My opinion is that any increase in flexibility will be advantageous, especially if the pipe is highly stressed.

It is more interesting for me to know if there might be any particular reasons that make additional provisions to the code necessary, prudent or otherwise highly desirable.

If you think that the resulting design is too conservative, try to prove it and then discuss the case with your boss. Maybe you both learn something. Other than that, "Happy boss = happy life".

 

[saplanti]

ibj2527,

In case you are running CAESARII, go to the directory on your computer where the application is. You will find all the documentation as pdf files there, you will see that all the answers are there.

When you develop experience over the years you will develop your own approach on the stress analysis, some of them will come from your clients, some from bosses. You may not get the answer immediately, just be patient and follow the rules.

 

[KevinNZ]

The B31 codes do include a case for SUS combined EXP. In B31.1 at least, what allowable stress is left over from the SUS case you can use in the EXP case. See equation 1B.

 

[StressGuy]

I don't know what your boss is referring regarding operating stresses, there's no "allowable" operating stress to compare against. As KevinNZ indicates, there's the "liberal" allowable for expansion stresses, where you can borrow the leftover allowable stress from your SUS evaluation to increase the EXP allowable stress. It's a de facto operating stress check, but it's still looking at expansion stress ranges and the allowables on that basis.

For myself, the operating results are of interest for things like equipment nozzle loads and flange leakage, not really for stresses in the pipe itself.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.