dear everyone:
in the technical document, there just has widely spaced mitter and closely spaced mitter, I think single mitter doesn't belong to any one .
so how to build it? just two pipes connect perpendicularly and do nothing?

thanks

Hello "new comer",

First be sure that it is a mitered bend. B31.3 paragraph 304.2.3 tells us that if the angular offset is 3 degrees or less it “does not require design consideration as a miter bend".

Single girth weld miters are not good practice. I have NEVER seen a case where a single girth weld miter could not have been avoided in a design. There is good reason for this, they will be highly stressed. Caesar II will let you make a change in direction at a node point but it will warn you that you have not described a "bend type" if you do this. However, you can place stress intensification factors and flexibility factors at a node. Now comes the real question - what FF and SIF will be appropriate? We have no data developed by testing that will provide appropriate FF and SIF for this construct.

The next issue is what rules do you use to address the internal pressure design. I believe that B31.3 paragraph 304.3.2 (b) does address single girth weld miter "bends" but there are limits prescribed for geometry. The real issue here though is what must you do to avoid the single girth weld miter.

The cross section of the single girth weld "miter" will be an ellipse (the cross section will have a long axis and a short axis) and when it is pressurized the short axis and the long axis will "want" to equalize - the cross section will "want" to become a circle. So, much stress will result from the material "trying to" maintain structural stability. Consider a wind tunnel bend (sometime a VERY large single cut miter). Wind tunnels are routinely operated at 4 to 6 atmospheres pressure. Such corners MUST be well designed with significant reinforcement to maintain structural stability (the "turning vanes" that vector the air in the corners also act as "stays" as they become tension members to reinforce the elliptical shape). Yes, this is an exaggerated case but it gives you some idea of the forces that act on single girth weld pipe miters. So, a single girth weld miter may need to be reinforced (depending of course upon its diameter, wall thickness and internal pressure).

In the past, to evaluate existing mitered bends in piping and large duct sytems for their fitness for continued service I have used finite element analysis. The highest stresses are on the intrados and it will be necessary to make a "sub-model" for the area of the intrados that includes the girth weld bead (the geometry of the weld greatly influences the calculation of the forces (stresses)). But what do we really learn from this? I REALLY like 5 X radius bends!

Is it worth it?

Regards, John.

thanks John!
Your post enhance my understanding about the mitter . thanks very much.

as for my calculation I wiil use the FF and SIF formula in APPENDIX D of asme 31.3 to dealed with the single miter.

thanks again.