What is the Future of ASME B31.3, Appendix P?

Posted by Chuck Becht on Tuesday, 22 October 2013 in Piping

Appendix P was written to introduce piping flexibility analysis rules that are more suitable for computer piping

flexibility analysis, in contrast to the rules in the base Code which were written when piping analysis was done by

hand calculations. Most of the elements of Appendix P have been, or are proposed to be, added to the base

Code. So what does the future hold?

The primary reason Appendix P was written was to introduce the concept of taking differences in operating stress

states. This is because there is an interaction between sustained and displacement loads with nonlinear

systems. Supports, for example, can be engaged in one operating condition, and not another, which effects the

stress range of the system. The base Code was written based on looking at stressed due to sustained loads and

displacement loads separately. Although commercial pipe stress software may take differences in operating

conditions to determine the stress range, this is not actually how the Code directs it be done.

A change to para 319.2.3 will be balloted, which will require looking at differences in operating conditions to

determine the stress range when the piping system has nonlinear elements.

Other elements that were included in Appendix P that have been added to the base Code, or are will soon be

balloted to change in the base Code, include the following:

Including stress due to axial loads in the calculated displacement stress range, SE

Including axial load stress intensification factors. As a result, there has been work by Paulin Research,

partially funded by ASME, to develop these based on testing and finite element analysis. This was done

as part of a larger project to update stress intensification and flexibility factors for the B31 codes

Rewriting the method for calculating the effect of cold spring on reaction loads on equipment

Explicitly stating that equipment loads should be calculated using the elastic modulus at temperature

As part of integrating Appendix P into the base Code, Becht Engineering conducted a study to determine if the

equation for allowable stress range, SA = f [1.25(Sc + Sh ) SL] could be changed to SA = f [1.25(Sc + Sh ) ] . This would provide a consistent margin of two on stress against fatigue failure (based on average fatigue performance of piping components), but the question was whether there would be protection against ratchet, which is progressive plastic deformation with cycles. The change that was considered was without including the

operating stress limit that was included in Appendix P.

To study this, Dr Nadarajah of Becht Engineering ran cyclic, elastic-plastic analyses of some simple piping systems. It was determined that ratchet did not occur with straight pipe, but ratchet did occur in elbows. The

reason is because elbows are somewhat unusual in how they are treated in the B31 codes.

The actual stress in an elbow is about twice the calculated value. This is not unusual as it is a

characteristic of the B31 flexibility analysis rules, since the fatigue limit is based on the nominal bending

stress in butt-welded pipe.

The calculated stress, including the stress intensification factor, is through wall bending in the elbow

sidewalls due to ovalization of the elbow when subjected to moment loading. This stress is likely to behave as a secondary stress, versus the calculated stress in components such as fabricated tees, which

are likely to include significant peak affects.

In calculation of sustained stresses in the Code, the longitudinal stress due to pressure is added to the

stress calculated for the elbow due to bending moment resulting from weight loads. However, the stress

due to moment loading is actually through wall bending in the hoop direction, and would add to the hoop

stress due to internal pressure.

Considering the complications associated with elbows, it was decided to leave the allowable limit in the base

Code for SA as is rather than try to modify it.

Given that the base Code is being updated to include the features of Appendix P, other than the stress limit

therein, Appendix P will most likely be dropped from a future edition, probably the 2016 edition, since the

changes to the 2014 edition are already finalized.