G. Archibald,
Dave Diehl suggests just flexibility factor k. Not SIF.
SIF please look at these articles:
Stress indices for straight pipe with trunnion attachments
http://www.epriweb.com/public/TR-110162.pdfStress Indices for Elbows with Trunnion Attachments
http://www.epriweb.com/public/TR-107453.pdf