B31.3 Stress ratio per 323.2.2(b)(1)

[Cobra17]

I'm trying to calculate the stress ratio per 323.2.2(b)(1) to determine the MDMT reduction in figure 323.2.2B.

per B31.3
(1) circumferential pressure stress for the condition
under consideration (based on minimum pipe wall thickness
less allowances) divided by the basic allowable stress
at the condition under consideration.

I have:
(P*(D-(2*Y)*t))/(2*t)/S

P= pressure
D= outside diameter
Y= coefficient from table 304.1.1
t = thickness with UT and corrosion removed
S = material allowable at ambient

I think it is correct, but I don't have any way of checking, AKA commercial software to compare MDMT's to and no one I work with knows. Any help would be appreciated.

 

[GBTorpenhow]

My interpretation would be to use the 'actual' hoop stress for the condition under consideration (PD/2t) and that the Y factor, used for calculation of the code required minimum wall thickness, shouldn't factor in. Also make sure to use the allowable at the condition under consideration, if your piping operates above ambient.

 

[GD2]

The condition under consideration is almost always the design condition. At this condition, the design pressure, design temperature, design wall thickness, are clearly stated, allowable stress for the pipe material at design temperature is determined from the stress table and therefore, becomes the direct route and almost accurate for the stress ratio calculation.
Without these parameters, if the calculation is based on operating condition, the calculation will be an estimate and approximate.

GDD
Canada

 

[Cobra17]

GBTorpenhow: That seems to be a slightly more conservative approach, so that's what I'll use, thanks.

 

[GD2]

Cobra17,
How about the stress ratio per (a)(3) of Fig 323.2.2B. Did you find that out? It's more complex.

GDD
Canada

 

[Cobra17]

GD2: I typically only do fittings or small section of piping where I don't need to take the combined loads into consideration. If pipe stress analysis is required, someone else does it. It might be something I could incorporate into my spreadsheets. Got a sample calc?

If anyone is interested, I attached the way I was calculating the SR vs GBTorpenhow's way. His is the 'Alternate Method' that gets 0.5901, and the other way gets 0.5781, 2.4°F difference.

 

[Alex Matveeff]

It is described here:

https://blog.passuite.com/minimum-design-metal-temperature-mdmt-and-impact-test/

And actually it is usually done automatically. No need in manual calculations

I'm the PASS/START-PROF Pipe Stress Analysis Software Developer