Re-Rating summary

From: <Christopher>
Date: Wed Sep 14 2005 - 17:13:00 EDT

Looking over the code, finally, it's obvious that 302.2.4 doesn't address re-rating. In fact B31.3 only applies to new construction, as noted in the foreword. 302.2.4 permits occasional non-sustained variations above the design conditions *only* if sub-paragraphs (a)-(i) are met. You can only have a total of 1000 such variations over the life of the system; the pressure must not exceed the hydro-test pressure and longitudinal stress can't exceed 1.33S as provided in 302.6 and in no case can the stress exceed the yield strength at temperature. There's no way that anyone can justify re-rating from 302.2.4.

302.2.4(1)(a) and 302.2.4(1)(b) don't explicitly address creep-rupture as I thought, but I think the intent is to exclude long-term over-loading and (a) and (b) would certainly apply to a situation where allowable stresses are set according to creep criteria. The reference to use of Appendix V also addresses high temperature service. Note also that (i) further limits pressures to maximum differential ratings established by valve manufacturers.

I'll stand by what I said--
> Hi Experts
>
> What if an existing piping is rerated to a higher pressure and the
> available
> pipe thickness just satisfies the new operating pressure, Is the
> 1.5xtimes test
> pressure is mandatory in this case? Can the piping withstand the new
> test
> pressure? Operating temprerature is around 350 deg F. Code allows
> only upto 33%
> overpressure for less than 10 hrs at any one time.
You can't use 302.2.4 to re-rate anything; it addresses occasional pressure-temperature excursions only and only within carefully specified limits. The 33% overload for 10 hr criteria is a condition for occasional overstress, not a hydro-test criterion. Hydrotesting is set by the design pressure and temperature of the system, not occasional over-stress.

Probably most of us old fårts have had to cope with people cherry-picking sections from the Codes in order to justify something silly. I remember a post to the ASME PV forum by a guy who only had welders qualified to make 3/8 in groove welds and wanted to use two different welders to make a 1/2 in weld, each one making half the weld. That's either criminally stupid or abysmally stupid--I can't decide which. When someone comes up to you with a request like this, read the damn Code carefully, including the reference paragraphs. The piping and pressure vessel codes are slavishly self-consistent and aren't likely to say one thing that contradicts another provision, without some carefully laid-out conditions. Picking and choosing just the sentences that you prefer is a good way to get yourself fired or get someone killed.

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=FukCGLbcbqf24TGllWBSiPmk6w7u1LKZyD6q38I1Abg-iDLxWeXv5Hi1zQt_RkPZV_ynkGWQoITKCxb3">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 Wed Sep 14 17:13:00 2005