It all depends on how you define factor of safety for flanges. If you are referring to the point at which the flanged joint would leak there is a small factor of safety. If the reference is to factor of safety on ultimate tensile stress then there is a large factor of safety. Flange design is usually governed by deflection and rigidity and not allowable stress even though the design formulas use allowable stresses. It really depends upon how you define a failure of the flanged joint.
Here is the introductory paragraph for Annex B of ASME B16.5 (FYI it is no longer called ANSI B16.5)This annex describes the methods utilized for establishing the pressure temperature ratings.
ANNEX B METHOD USED FOR ESTABLISHING PRESSURE–TEMPERATURE RATINGS
Pressure–temperature ratings for this Standard have
been determined by the procedures described in this
Annex. These procedures are counterpart to those identified
as Standard Class in ASME B16.34. The primary
considerations in establishing ratings are component
dimensions and material properties to sustain pressure
and other loads. Other considerations affecting or limiting
ratings include
(a) stresses in flanges resulting from bolt-up necessary
to maintain a gasket seal
(b) distortion of flanges and flanged fittings due to
loads transmitted through attached piping
(c) limitations applying primarily to flanged components
(e.g., valves), but also imposed on flanges in order
to preserve compatible ratings
I highly recommend obtaining this ASME Standard if you seek the answers to your questions.
There are three kinds of people in this world; those who can Google and those who can't.
