Hello Caesar users,

I am doing a manual calculation for Bend SIF from B31.3 Appendix S Example. I don't usually do this since this is already programmed in any piping stress analysis software. But when I compare my calculated results to any piping stress analysis software they are not matching. I was tasked to do some training to junior stress engineers, so for the sake of example I want to do it manually. I don't know if I misinterpreted something in B31.3 Appendix D formula for SIF calculation such as nomenclature interpretation & other factors. Anyway, these are the parameters:

1. This is my manual calculation

DN400 (NPS 16)
D=406.4mm----------------------->Pipe OD
T=9.53mm------------------------>Nominal wall thickness
R1=1.5xNPS=1.5x16x25.4=609.6mm--->Bend Radius of Elbow
r2=(D-T/2)=198.435mm------------->mean radius of header pipe
h=TxR1/(r2)^2= 0.147537122------->Flexibility Characteristic
SIFin=0.9/(h)^(2/3)=3.223356229-->In plane SIF
SIFout=0.75/(h)^(2/3)=2.686130191-> Out plane SIF

2. This is taken from piping stress analysis softwares:

SIFin ~ 2.572
SIFout ~ 2.143

Please note that the SIF is taken from Stress Report for Expansion stress not the sustained SIF.

I'm curious what did I miss in the formula or interpretation of appendix D for long radius elbows.

Many Thanks in advance for the help.


Cheers,

Did you apply the pressure correction, as specified in the "Notes"?

The Bend SIF utility will help here.

Thanks Dave & Richard.... Richard is right...I've calculated the pressure correction factor for SIF to be 1.23078...I now get the right SIF upon dividing my calculated value above to this pressure correction factor...That is:

SIFin=3.225/1.23078 ~ 2.6
SIFout=2.687/1.23078 ~ 2.18

I was thinking that note is for D/T > 100 (thin-walled elbows), but my interpretation is incorrect because D/T value is not specified in that note. The D/T in this case is (16/.375=43) which is a little bit higher than 40. From the previous webinar of Tony Paulin, he cited this rule of thumb for D/T greater than 40 as something to watch for in performing stress analysis.

Anyway my mind is now clear from the SIF calculation as per Appendix D of B31.3 for bends & elbows..

Cheers!!!