Branch reinforcement

[rjstephens]

Hi all,

I have a 28" to 28" 8mm thk fabricated branch, the design temp is 175dec c & press =1000kPa LPsteam. CaesarII tells me that the sus. & exp. stress is acceptable, the B31.1 code tells me that the area reinforcement required is ok. I have since done and FEPipe analysis on the component & it fails, can anyone explain this & possibly verify the FEpipe does actually fail.

Many thanks to all who reply

Rob

 

[prex]

If you fulfill B31.1 criteria for pressure and sustained loads, you should be OK.
In the finite element calculation, depending on the type of model and elements used, you'll see stresses that are not necessarily relevant to the strength of your component. An example are the in thickness bending stresses, that you find if you use shell elements: they are not relevant to the pressure and sustained loads verifications of B31.1.
To assess the detailed results of a FE calculation you need to use ASME VIII Div.2 App.4. prex

http://www.xcalcs.com

Online tools for structural design

 

[LSThill]

rjstephens (Mechanical)

Ask Tony Paulin the Author of FE-Pipe

http://www.paulin.com/

tony@paulin.com., and or the FE-PIPE Discussion Group
Paulin Research

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Leonard

 

[paulint]

Regarding FEA (FE/Pipe in this case) showing overstress when CAESAR and Area Replacement does not:

The CAESAR calculation for an intersection only includes the effect of external loads through the application of moments and sif's. Pressure stresses are calculated by Pd/4t. Area replacement is a well-tuned approximation method, but even Section VIII Div 1 and Div 2 rules do not agree on the equivalent lengths to be used. Usually when we see this happen, the area replacement rules are close to the limit, leaving little extra room for external loads. Depending on the number of cycles you might also have a fatigue problem due to the varying pressure stress, which is not included in either the area replacement evaluation or in the B31 CAESAR calc.

Hopefully your FE/Pipe run does not show to be too overstressed. Stresses we ignore in more simplified analyses are hopefully "considered" by the size of the "safety factor." If you want to send the FE/Pipe model to us we can take a look at it here in Houston to be sure that there is not something flakey going on with the model.

 

[LSThill]

paulint (Mechanical)Tony Paulin, PAULIN RESEARCH GROUP

Thank you for your reply

Leonard

 

[rjstephens]

Thanks to all who have replied,

Sorry for being slow to respond, we have over come the problem by discussion with the AIA & they will accept the area replacement method as defined in B31.1.

To Tony, a friend of mine A Stewart says to say hi.