Allowable Stress of ASME A312-TP321 Seamless Pipe

[onrush]

Dear members,

According to ASME B31.3 Table A-1 (Basic Allowable Stresses in Tension for Metals), the allowable stress of ASTM A312-TP321 seamless pipe is different depending on its thickness ( greater than 3/8 in. or 3/8 in & below).

My questions are followings;

1. Normally, we use this allowable stress values to find out the applicable thickness under pressure/temperature condition by calculation. It means that we do not know the thickness in advance before calculation. In this case, how can I use this allowable stress values for ASTM A312-TP321 seamless pipe when calculating to find out the applicable thickness?

2. What is the technical background for different allowable stress per its thickness for ASTM A312-TP321 seamless pipe?
(Why did ASME committee assign different allowable stress per its thickness for ASTM A312-TP321 seamless pipe?)

Thanks always....and Merry christmas...

 

[BigInch]

START:
Assume your thickness will be 3/8" or below.
Calculate thickness using allowable stress for < 3/8" thk.
IF calculated thickness <= 3/8",
use that thickness
GOTO EXIT:
ELSE
(calculated thickness is > 3/8")
use allowable stress for thickness > 3/8",
recalculate new thickness.
IF calculated new thickness is > 3/8",
use that thickness END IF
Else
GOTO START:
ENDIF
ENDIF
EXIT:

I hope you don't get caught in an infinite loop.

A different thickness may have different allowable stresses due to cold working procedures and/or cool down rates, post work heat treatments, potential for microscopic surface cractks, inclusions, laminations, etc. as a result of the process used in forming that material.

Because its extruded seamless as opposed to being rolled up from plate with its edges welded together.

**********************
"Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[metengr]

2. What is the technical background for different allowable stress per its thickness for ASTM A312-TP321 seamless pipe?
(Why did ASME committee assign different allowable stress per its thickness for ASTM A312-TP321 seamless pipe?)

This applies in general for most materials listed in ASME B&PV Code as well. The reason is that as materials increase in thickness, wrought product forms like pipe versus tube may not achieve uniform through thickness mechanical properties. Cold forming for example may not reach the depth that would allow benefit from increased strength through thickness. So, based on actual mechanical testing that is performed to qualify material for code approval, the committee selects the lowest value among several variables used to establish allowable stress lines.

 

[onrush]

I very appreciate.

Your replies helped me understand very much!

 

[onrush]

If possibel,
could you explain me why only some materials are applicable for this requirement, not all?

For example, why for 321SS seamless, not for 304SS seamless?