Wall Thickness Calculation as per ASME B313

Hello Fellow Stressers!!!

What is correct way to calculate the wall thickness of pipe based on design pressure considering C.A & mill tolerance. I have seen two approaches:

1) Approach 1:

t = 0.195 in. (calculated thickness as per 304.1.2 t=PD/[(2*(SEW+PY)]
c = 0.125 in.
tmin = (t+c)/0.875 = 0.366 in (after mill tolerance)

2) Approach 2:

t = 0.195 in. (calculated thickness as per 304.1.2 t=PD/[(2*(SEW+PY)]
c = 0.125 in.
tmin = (1.125*t+c) = 0.344 in (after mill tolerance)

I think approach 1 is correct considering 12.5% mill tolerance, but when I divide 1/0.875 equals 1.143 not 1.125. Please share the correct interpretation for this formula.

Warm Regards,

If you ask the manufacturer for 1" thickness with 12.5% MT, they're providing as thin as 0.875".

And when you put it into service, your corrosion/erosion is going to eat away from that. So, let's say CA is 0.0625".

The only pressure capacity you can take credit for is that of 0.8125".

It only makes sense that if you are sizing a pipe that you go in reverse.

e.g. if t=PD/((2*(SEW+PY)) then -> 0.8025" -> + CA = 0.865 -> / (1-MT) -> 0.989" -> round up to 1".

Thanks Michael for that very clear explanation. It makes sense.

If,

t(manufacturer)= thickness offered by manufacturer considering minus 12.5% tolerance = t(nominal) * (1-0.125) = t+c

Therefore:

t(nominal) = (t+c)/(0.875) ----> select the next higher schedule.

Cheers!!!