RE: High temperature Piping in Refineries

From: <Christopher>
Date: Sun Sep 30 2001 - 14:38:00 EDT

>Too much flexibility in a piping system is not desirable since it will have
>difficulty in meeting the stringent allowable load imposed by the equipment
>(he's of course refering to the nozzle allowable loads per API 610 for
>centrifugal pump, API 617 for compressor and NEMA SM 23 for steam tubrine).
Flexible pipe will decrease the nozzle loading from thermal stress. Nozzle loading from external loads such as wind loads is a little more complicated. If the pipe is soft (flexible) with respect to the nozzle (think about a piece of pipe mounted to a very stiff support which will be substantially clamped at the ends), the actual nozzle loads will be a maximum but the stiffer vessel may be able to carry them more easily. If the vessel is soft the end loads carried into the vessel may be small (think about a piece of pipe supported by soft springs--almost no end moment) be the vessel may not be strong enough to support the loading.

>The use of expansion loop makes the pipe prone to vibration since it will
>get easily excited by a small disturbing fluid force (normally we term this
>as focring function in vibration). In addtion it enhances the internal fluid
>diturbenace by creating cavities due to excessive delta P.

The expansion loop makes the pipe more flexible so that thermal deformations can be absorbed with less force (A soft spring requires less force to deform a given distance). It's correct that a softer pipe run is more susceptible to vibration but that's because the pipe has lower natural frequencies so that deformations are higher when resonance occurs. A softer system deforms more under a given force than a stiffer system and the bigger the deformation, the higher the stress.

Sorry for the engineer-speak. I'm not really trying to split hairs, Ralph, but sometimes it's easier using rules of thumb if the physics is a little more solid. I find it's a lot easier thinking in terms of stiff springs and soft springs rather than writing down a bunch of equations or resorting to FEA.

Christopher Wright P.E. |"They couldn't hit an elephant from <a href="/group/PipingDesign/post?postID=i53ZJmQCwDw0gknqWpj_2v0qkxS2j0B8XtyM7pOGlZWIisNIbRLWETfPEcck229WP3_tSnfyo8de">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 Sun Sep 30 14:38:00 2001