Accounting for Pipe Bend thinning

How does Caesar account for pipe bend thinning on in the extrados of the pipe on induction bends. B31.3 304.2.1 covers the minimum thickness requirements for the finished bend and Caesar provides a warning when you drop below this value however this does not really help in Caesar for determining expansion stresses.

If you reduce the wall thickness at the bend the flexibility of the bend goes up and your stresses remain roughly the same. This does not reflect reality as what really happens is the intrdos gets thicker and the extrados gets thinner so the average flexibility probably remains similar but the stress level in the thinner material would be higher as there is less material to distriute the loading.

So my current thought is to check the stress in the bends and increase the value by 10% (our maximum thinning in our standard) then compare those values against the allowable.

Does anyone have any thoughts on how to approch this. Am I missing a setting in Caesar that can take this into account?

It doesn't.

That is what I thought, Now for the more difficult question.

Do I need to account for bend thinning on pipe bends for secondary stresses or since the bends will meet the requirements of B31.3 304.2.1 will modeling the bend at the same wall thickness as the pipe (pre-bend) produce valid results?

I've been developing and supporting c2 for 26 years. This is the first time anyone has asked this particular question. So, if you do need to account for this, a great many people have missed this for a great many years.

This would have to be a local analysis using FEA techniques. c2 utilizes the 3D Beam Element - essentially an infinitely thin stick. This assumes the cross section is both uniform and constant. If these assumptions are not valid, neither is the analysis.

Thanks Richard,

I am probably over thinking it, I just have always had used fabricated fittings and haven't dealt with pipe bends before I will discuss this with a few more people around my office. The more I think about fabricated fittings have the same issues as B16.9 doesn't specify wall thickness so the fitting could be stiffer or less stiff than anticipated and capable of handling more or less loading. This really just falls into the long list of assumptions we make about the pipe to use a 3D beam element.

When I have some spare time I might run an shell type fea analysis on it to see if there are any meaningful differences.

Hello,

As it happens, I just read about this issue. Briefly, the influences you mention tend to get balanced out by other influences, and they are accounted for in the conservative assumptions, the SIF, and the safety factor of the code design.

Peng's _Pipe Stress Engineering_ 2009 pp46&69 addresses this issue.

One of the conservative assumptions the code uses is that the hoop stress is calculated based on the outside radius of the pipe, even though the pressure is applied on the inner radius, then the stress is nonuniform through the thickness. This assumption eats up many smaller effects.

Also, Code SIFs (used in the EXP case) are conservative and based on real tests using real pipe, so the thinner wall property is already accounted for. For the sustained case, I understand that using code SIFs or other SIFs tend to take this and other factors into account (but some people don't use SIFs in the SUS case...)

Lastly, and most importantly, there is an offsetting principle called Lorenz factors. If you look at the section of a pipe bend, you'll see that on the intrados, the internal face experiencing pressure is larger than the external face (think trapezoid), while on the extrados, the effect is reversed. That means that when compared to a straight pipe, for each region exposed to pressure, the intrados has less metal area to resist the load, while the extrados has more.

So for a bend of even thickness, the stresses on the intrados are slightly higher than the extrados... which means that in a real-life forged or rolled bend, the thinning on the extrados tends to balance with the Lorenz factors.

Aaaand as I'm not surprised to discover, the issue is a little more complex. It seems the discussion above applies most appropriately to 31.3 piping below creep range, focusing mostly on pressure (hoop stress) design with forged elbows.

The general take is that under typical service, the thinning issue is absorbed by the mill tolerance. But bent-tubing applications in the creep range might require further scrutiny.

B31.1, which can be more conservative, also deals with systems more likely to be in the creep range, more likely to be made of tubing that has been bent (resulting in thinner extrados than typical forged elbows). So 31.1 includes a "bend thinning allowance" in Table 102.4.5. For a 3D bend, it's 1.25*tm. (tm is the design thickness before applying corrosion allowance).

Some other related posts by folks more knowledgeable than I am:
http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=13586

http://65.57.255.42/ubbthreads/ubbthread...=true#Post23511

http://65.57.255.42/ubbthreads/ubbthread...=true#Post21623

http://65.57.255.42/ubbthreads/ubbthread...=true#Post11717

http://65.57.255.42/ubbthreads/ubbthread...h=true#Post9418