B31.3 Allowable Stress for High Pressure Piping

[TXLSUCHE]

I am calculating the W.T. for 7" OD 10,000# prssure piping using 4130 mechanical tubing. Unless I am misunderstanding B31.3, Section K302.3.2, it appears that it is acceptable to use 2/3 the SMYS for the allowable stress. However, everyone I have conferred with uses 2/3 the SMYS or 1/3 the tensile, whichever is less. Am I missing something in B31.3 that says I have to use the lower of the two? Even most of the allowable stress values given for the materials in Table K-1 use 2/3 SMYS. The piping system I am designing will be operating at about 75 F. Please help.

 

[ijzer]

Normal piping per B31.3 use lower of 2/3Yield and 1/3 of the tensile , as per par. 302.3.2.
High pressure piping is per par. K302.3.2 , the lower of 2/3 Sy and 2/3Syt.

 

[TXLSUCHE]

Thanks for the response ijzer. But what worries me about that is for higher pressures, the safety factor is actually decreasing because 2/3 yield is greater than 1/3 tensile.

The pipe I am considering (I have the MTR's) is 7" OD x .9375 W.T., 4130, 104,600 TS, 80,900 YS. As you can see, which of the two allowable stresses you choose can make a significant difference in determining if the tubing will be "rated" for 10,000#.

 

[DSB123]

TXLSUCHE,
Suggest you post this in the "Piping and Fluid Mechanics engineering" Forum or the "ASME(Mechanical) Code Issues" forum.