Many times I have heard people make a statement such as "In a spool with pipe and flanges, the flange will always be the weakest link with regard to MAWP".

I am wondering whether this statement is wrong or at least should be worded less general.

From ASME B31.3 I can calculate what pressure a pipe with a wall thickness t can withstand:

P = (2*S*E*t)/(D-2*y*t),

where t = t_nom(1-tol)-ca-threaded-ea

tol: manufacturing tolerance (usually 12.5/100)
ca: corosion allowance
ea: erosion allowance

As an example consider the following:
2" XS CS pipe with #600 CS flanges at 100F, 1/8"ca, no threading, no erosion allowance.
The above formula gives me an MAWP of 1132 Psig, while the flange can withstand 1480 (according to ASME B16.5 table 2, page 22)


So, I think, the above statement should be revised towards:

"In a non-corrosive, non-erosive, unthreaded pipe spool with flanges, the MAWP of the flange is always lower than the MAWP of the associated piping"

This statement is much more restrictive, as it should be.

The statement you made is a statement of design intent rather than one of universal truth.  It is possible and sometimes sensible to select flanges which have a higher rating than the piping welded or threaded to them.  A common example is the use of 300# flanges on low pressure hot oil piping to minimize leakage.

However, some build their pipe specs using a pipe schedule which exceeds the MAWP of the flange class selected throughout its pressure/temperature range, such that the design MAWP is "limited by flanges".  Some do the same with ASME vessel design.  The idea is that overpressure is more likely to lead to flange leakage than to pipe rupture.  But it is acceptable to design piping such that the pipe itself has the lowest MAWP of the assembly.

Actually, and seriously, this is only true when you need enough pipe to equal a minimum mill order.  Buying directly from a pipe mill allows you to roll your own wall thickness, so we are not limited to using any particular standard wall of a pipe suppliers list and can spec our very own, down to the last 1/1000 inch.

You give a perfect example of pipeline economic decision making, when you mentioned that any additional pressure costs money.  Designing pipe to ANSI#600 (1460 psig), instead of 1000 is a difference of 460 psi.  For pipeliners, that 460 psi is about 64 X over what qualifies as extra/ball valve, where 1 mil of wall thickness is probably going to equal around 7.2 psi of additional operating pressure.

There's a reason why its cheaper to transport just about anything via pipeline!

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