UCS-66(b)(1)(b)_Understand the reduce the MDMT due to lower membrane stress

[staticmh]

Gents,

I'm little confused about the how compress is giving exemption of impact test at -55 deg. F by simply refereeing the clause UCS-(b)(1)(b).

My governing thickness is = 3.810"
Nozzle type = Heavy barrel
Nozzle material = SA-105N
Nozzle size = NPS-24"
Shell thickness = 4.250"
Required thickness tr = 1.0491"

As per figure UCS-66.1
Govrn. thk, tg = 3.81, tr = 1.0491, c = 0.125in, E✱ = 1
Thickness Ratio = tr * E✱/(tg - c) = 0.284.

Can someone expert guide me what is the actual picture behind this all clause and how compress is taking the exemption on the basis of UCS-(b)(1)(b) ?

 

[david339933]

Your ratio is small, therefore you get a lower exemption temperature. Normally below .35 is exempt to -155F.

 

[Cobra17]

Because you are normalized you move curves, assuming you meet the requirements, Compress is assuming your flanges are fine grain practice and normalized so they meet UCS-66(c)(1)(-a)&(-b)
You can see the difference between B and D on the attached (Curve B on page 1, page 2 has Curve D).