Boiler Tube Bends 1

[Sharik]

Does anyone know what the maximum thinning of the outer radius of a boiler tube bend is? I seem to remember the thickness after bending had to be within 10% of the original nominal thickness. But I can't find this anywhere in the ASME Codes or the NBIC.

Thanks

 

[metengr]

Two criteria for boiler tube bends are ovality and wall thickness. Using ASME Section II, SA 450 - "Specification for General Requirements for Carbon, Ferritic Alloy and Austenitic Alloy Steel Tubes", section 7 "Permissible Variations in Wall Thickness", and Section 8 "Permissible Variations in Outside Diameter" give the necessary guidance for acceptable tube tolerances.

I have heard people use 5% or 10% variations, but I have never been able to locate any specific requirements in the ASME B&PV Code. Therefore, I revert back to SA 450 and treat tube bends using the permissible ovality and wall thickness variations listed in Table 2.

 

[rmw]

I am with metengr on the 5% and 10% ranges, and have even seen 5% stipulated in specifications as the maximum, and tube bending vendors eliminated because they could not guarantee that. But... I cannot quote a source or code.

rmw

 

[davefitz]

ASME B31.1 has a table of recommended assumed thinning as a function of bend radius- unless you can prove the thinning is less than this table, then you must add extra wall thickness to account for the thinning . This only applies to nonboiler external piping due to its safety implications , and is not applied to boiler tubes within the boiler setting since that is a lower safety hazard and years of practice has shown it is nto a problem.

European codes specifically allow the thinnning, because the toroidal equations demonstrate that the use of 2 radii of curvature enhances the strength and allows the use of a thinner wall to restrain membrane stresses, without taking credit for work hardening. See EN 12952-2:2001 par A.4.2

 

[athomas236]

davefitz

The star is for knowing about EN 12952 of which there are 15 parts.

The formula that you refer to comes from TRD the German boiler code who are the secretariat for EN 12952. This probably explains why they look so much like DIN/TRD's. This is especially true for part 15.


regards

athomas236