Hi,

I have modelled a DN200 (8"), Sch-120 (18.26mm) line, material to A106C, having 12.5% mill tolerance and 0.75mm CA for 187 barg @361ºC. Piping code is B31.1

Why does Caesar give a warning that the computed wall thickness is greater than the entered thickness ?

If I manually calculate the min wall, using same allowable as used by Caesar (129.947 MPa), I get a min wall thickness of 17.8935mm. This is also confirmed by the fact that if I run the analysis (ignoring the warning), Caesar reports a max hoop stress of ~93% only.

Must be missing something elemenatry - can anybody point out please?

regards,

Dear Sanjay,

Below is also the related link. Also try using search function on above right using any related keyword combination.

http://www.coade.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=20049&page=4#Post20049

Dear Samsul,

The link you mentioned, does not give much clear suggestion on the above topic.

Does your element end with a bend? If so, use the bent pipe wall thickness formula instead of the straight pipe equation for thickness.

The Miscellaneous Data report in the output processor displays the calculated thickenss for each piping element.

Thx Sir Dave Diehl...

Dave - good point, my element does have a bend at the end. I am going to check if the thinning allowance I as per Cl 102.4.5 (B) of B31.1 is causing the problem, as you suspected.

I am, however, not fully convinced though. As per B31.1 Cl 102.4.5 (A), is it not sufficient to meet the bend wall thickness requirement as per Cl 104.1.2 (A), ie, the calculated wall thickness of straight pipe only? When the schedule of bend (or any fitting) is decided, it is specified as the finished thickness (not including thinning allowance) and is gfenerally not higher than the pipe schedule.

Secondly, if the warning is due to the above, why does Caesar i) not show code compliance failure for hoop stresses in the final report and ii) not give warnings for all other bends in the model which have the same schedule and press/ temp condition too (the warning is only at the starting element)?

Samsul - the topic you refrred to is on a different subject and has nothing to do with minimum wall thickness due to hoop stress only.

regards,

Hi Dave - you were right. I have checked the calculated thicknesses for bends in the misc data report and realized CII determines the bend thicknesses as per the eqn in para 102.4.5. (B) of Code.

However, is that section of Code not applicable to bends formed of pipes only? I thought butt weld bends are excluded (governed by butt weld fitting codes, which requires the fittings not to burst than the pipes they are attached to and are therefore thicker thasn pipes anyway).

What is the exact requirment of the Code? If the minimum wall thickness at bends must be determined as per the above eqn, why does Caesar not report a Code compliance failure (only for hoop stress criterion) ?

regards,

Still there is a relation ;), except I just realize that CII does min. WT checking for the bend not only pipe element according to above post.

Please note that Hoop stress is NOT a code stress. CII only does checking for the code stress. in B31.3 for example, If you observe the equation to calculate tm = t + CA; "t" is based on hoop stress with a little bit modified equation. If your WT is enough you don't need to worry about hoop stress. The warrning helps us to make sure if we input correct value of WT or the document for calculation of min WT is correct. I've also ever saw that hoop stress value for OPE and SUS case are different in value. Which one should you take if you want to compare with Sh? I will not compare it (my opinion) against Sh.CII also I think doesn't know the represent of external load here which may affect to min WT (opinion)...CMIW.

...This is my opinion...
Code rule number One: Ensure piping components do not burst under pressure load. Most piping components are manufactured fittings - elbows, tees, flanges, etc. These components are burst tested by the manufacturer in accordance with their relevant standard (B16.5, B16.9, etc.). Given a design temperature and a design pressure, a piping designer can assemble a safe system simply by picking these "listed" components out of the standards. No pressure analysis of these complex shapes are required since they are "certified" acceptable for the design conditions.
But what about "home made" pressure components? The Code offers rules to set minimum thickness. The Codes do not calculate hoop stresses and compare them to allowable limits; they specify minimum thickness. You will see these thickness equations for straight pipe, bent pipe, miters and fabricated tees. This is where the CAESAR II check comes in. We check straight pipe and bent pipe. If all your elbows come from B16.9, simply ignore the CAESAR II warning as it does not apply.
For other "unlisted" components, the Code points to testing or more rigourous (ASME VIII Div. 2) analysis.

Agreed Dave

There should be option in Caesar to turn off separate WT calc for bends - can somebody advise please?

Else, as you said, I can simply ignore the warnings as they are not relevant for BW fittings to B16.9

Thanks again.