flange: pressure vs. temp rating why?

[ruby67]

Since asme viii does not reduce allowable stress until 650f for carbon steel, why is there pressure vs, temperature ratings for standard 150#, 300#..... it seems as if there should not be a rating reduction until 650f. Of course we all know that there is a rating reduction with temperature for standard ANSI flanges?? Why?

 

[ijzer]

Because the piping Code ASME B31.3 does not have equal stresses up to 650 F. So the rating is more linked to this Code.

 

[jte]

ruby67-

You haven't checked Section IID in a few years. Typical vessel carbon steel (SA516 etc) allowables are not constant to 650°F.

Flanges have a different failure mode than vessels and pipes. Ever seen a flange split open down the long seam due to excessive internal pressure? They fail not due to being overstressed, but due to excessive deflection. The flanges rotate too much (a rule of thumb would be 0.2°) and your fluid escapes between the flange faces and gasket. This is kind of like structural design where deflection often governs over pure stress. People don't like to walk on a floor which bounces under them, even if from a stress perspective the structure is perfectly safe.

jt

 

[Raydel]

The temperature versus pressure rating of flanges has its origin lost in the mists of time. The values found in B16.5 pre-date any analytical method that later came onto the scene.

Designing flanges is a 'black art'. Some of the dimensions (especially the slope of the hub) prove very sensitive to the stresses that are generated in the flange. Doing the job by hand is not the best way. Trial and error helps to pin down an economical design, but reliable software is really required to do the work rapidly.

PVElite / CodeCalc I have found is fast and help nail the final design in a flash.