This is my first post so please be gentle…

I have been asked by a client to analyse a piping system using the now defunct BS 806 Code - a Code that i am not familiar with.

I first analysed the system using the ASME B31.3 Code, which produced Code stresses within the prescribed allowables. However, when i switched to the BS 806 Code and re-ran the job, the sustained stresses were over by 300% in some places (with Code stresses

It would seem as though i am doing something fundamentally wrong!

Please can anyone advise me on how to correctly approach this analysis?

In particular the significance of the term “non-intensified dead-weight” for sustained combined stresses.


Best Regards,

Anthony

Hi Anthony,

The last time we spoke you were at PM Ltd. It is a very long time since I used the BS 806 code but I remember that it certainly has it's quirks. Firstly, I suggest you read the code specific notes for BS 806 in the Technical Reference Manual (Technical Discussions section page 6-105). Note particularly that CAESAR II will not address the sectionalised systems requirements of BS 806 (which is an extremely conservative approach).

The Non Intensified DeadWeight (NIDW) stress requirement is specific to junctions and it means just what it says (i.e. check the deadweight stress at branches but without any SIF's applied). In order to check this requirement I suggest you do a 'save as' on your input and run a separate analysis purely for the NIDW case. In this run you will need to remove all tee definitions (i.e. no SIF's) and use an allowable stress of 15% of the design stress (para 4.11.2 (c) of the code). Unless my very old copy of the code has been amended the design stress is found in Table 3.2. Check that the SUST stress levels at the tees are within your defined limit.

I hope this helps a little.

Steve,

I haven’t worked for PM Ltd so I possibly have a namesake who is also a pipe stress analyst?

…Thanks for the advice regarding BS 806, it is much appreciated.

I have done a ‘save-as’ for the NIDW case, removing all SIFs, as per your advice.

My pipe material is BS 3602 Gr.410 / design temp 449ºC.

I have set my NIDW allowable to 7.65 N/mm² (15% of 51 N/mm² (from table 3.2 of the Code) - as per 4.11.2 (c) of the code).

Section 4.11.2 (b) of the Code states the following for sustained allowables:-

“(c) Sustained combined stresses. Except at branch junctions, the sustained combined stress shall not exceed the lower of 0.8 times the proof stress value or the creep rupture design stress value given in table 3.2 under cold or any other operating condition”

Therefore:- 0.8 x proof stress value = 0.8 x 128 N/mm² = 102.4 N/mm²
(from table E.1 (1) 0.2% proof stress, in Appendix E of the Code)

Or:- Design stress valve from table 3.2 = 51 N/mm² x 0.8 = 40.8 N/mm²

Taking the smaller value = 40.8 N/mm²

So for my NIDW case do I only apply the 7.65 N/mm² local to branches and the higher allowable of 40.8 N/mm² to the rest of the pipework/bends – or do I apply the lower 7.65N/mm² to the whole system?

Final question, should pressure at the branches be included in the NIDW case?

Best Regards,

Anthony