Re: (unknown)

From: <Christopher>
Date: Tue Aug 23 2005 - 12:13:00 EDT

On Aug 23, 2005, at 12:23 AM, Pankaj Mandal wrote:

> Erik is right. B31.1, Para 102.3.2 (D) permits the use of liberal
> stress for piping calculations. In simple terms liberal stress allows
> use of unutilized stress margin in sustained case as an additional
> stress allowable for expansion case.

Be a little careful about this. Codes recognize higher allowables for certain combinations of primary + secondary stresses only. Be careful not to go using your 'liberal' allowables with primary calculated stress. Same thing holds for fatigue analysis.

Primary stresses result from statically determinate loads, like pipe walls under internal or external pressure. When a primary stress exceeds the yield stress, the pipe will deform without limit. Secondary stresses are self-limiting and include discontinuity stress and certain thermal stresses, where the load is statically indeterminate. An example is the discontinuity stress that develops between the head and the shell of a pressure vessel. In that case yielding tends to redistribute the stress without causing plastic collapse.

Bolted flange calculations have similar provisions for hub stresses, which involve secondary (discontinuity) stress. The remainder of the calculated flange stresses are primary stresses and the primary stress allowables apply. You don't want to mix them up.

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=lFuvKEnjI4lzmGgXu5LCKODI_DlOZoaoRx3BqRTgPpGZhpfOwN2UF2iDZJDBHDdzrS0ecdSu-y3qlSfQdmU">chrisw@skypoint.com</a> | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania

1864)
<a href="http://www.skypoint.com/~chrisw/">http://www.skypoint.com/~chrisw/</a> Received on Tue Aug 23 12:13:00 2005