ASME B31.3 Chapter IX High Pressure Piping

[chincheetan]

Hi all,

I am wondering if someone could kindly help me with this problem I am facing here. It's with regards ASME B31.3 Chapter IX High Pressure Piping. The formula for the minimum pressure design thickness is given as t = (D - 2c_o)/2 * [1 - exp (-p/s)]. From my understanding, the reason behind this formula is the fact that we are no longer treating the pipe using thin wall theory, rather we are looking at it as a thick walled pipe. So, I am trying to derive the formula by integrating the hoop stress from T = 0 -> T = t. Such that: s dA = P[D - 2(T + c_o)]. However, from my derivation it appears that the final formula will be t = (D - 2c_o)/2 * [1 - exp (-2p/s)] (With the hope that my knowledge on integration is still correct. ). When I refer to the Charles Becht book (Complete guide to ASME B31.3), the comment is the formula is based on limit load pressure rather than the hoop stress - or am I misunderstanding it? Could anyone please kindly help me with it?


Secondly, the footnote 5 says that the intent of the equation is to provide a factor of not less than 1.732 (SQRT 3). Is that based on the fact that according to Von-Mises failure criterion, the failure will be 1.732 times lower in a pure shear case than a pure tensile case. In a thick walled cylinder, the inclusion of radial stress in the consideration will give rise to shear stress and hence making the case of pure shear a valid consideration. So is that the reason behind the 1.732? And if so, how is that built-in to the equation?


Many thanks!