Re: pressure test thoughts

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

On Sep 12, 2005, at 5:18 AM, Steve McKenzie wrote:

> If you can derate then logic says uprating must
> be possible and both terms are a subset of the rerate family.
> And you will be aware derating is a fact of life.
What logic says that? You derate a vessel to account for degradation over time like corrosion or erosion. By that logic you'd upgrade a vessel because it got thicker over time. My experience is that vessels don't get thicker over time--unlike worm gearing, vessels wear out, not in. ;->

I don't have any heartburn over de-rating. Vessels have a finite life and service degradation is a fact. I do have some problems with upgrading a vessel from the MAWP on the Code plate. And I know how repairs and alterations should work, at least in the US and Canada, and it's not a matter for fårting around. There's a sound process by which changes are documented, so no one gets any unpleasant surprises, and the original design intent is maintained. Paul's anonymous letter really got my attention--I'll be the first to admit that I don't know every dumbass notion ever concocted to keep from buying equipment, but I've run into a few, and I do know of several where it's cost someone a bundle. I'm beginning to suspect that I've run into another.

I got hold of a few paragraphs from the piping Code (I think--usually Code pages are all identified with the Code designation and date. This one only has 302.2.4-302.3.2 at the top) reading in part-- (1) Subject to the owner's approval it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than (a) 33% for no more than 10 hr at any one time and no more than 100 hr/yr; or
(b) 20% for no more than 50 hr at any one time and no more than 500 hr/yr.

There's obviously more to this (Code sub-paragraphs don't start at (1), for instance) and I think that it's not any sort of 're-rating' but an allowance for temperature excursions, probably intended to be used during design. For high temperatures, allowables are based on a 100,000 hr time to failure by creep rupture. If the time at temperature is less than 100,000 hr the permissible stress would be higher, but the Code allowables don't reflect shorter temperature exposures. (a) and (b) in the the foregoing paragraph are creep-rupture or creep rate criteria for short term exposure.

Further down you see "(h) Temperature variations below the minimum temperature shown in Appendix A are not permitted unless the requirements of para 323.2.2 are met for the lowest temperature during the variation." That particular provision appears to limit (a) and (b) above to temperature excursions above the point where creep-rupture starts to govern the allowable stress.

I'm sticking my neck way out, since I haven't done the homework I'm always nagging about, but I suspect that using the foregoing for 're-rating' is dead wrong, and amounts to cherry-picking Code exerpts without understanding the application. The meaning of 'increased condition' is probably set out at the beginning of that particular sub-part from which my quote has been extracted, but even without seeing it, I'll bet that it refers to a short temperature--pressure excursion, not a new operating condition. I daresay the rest of the exerpt dealing with how the designer needs to determine that the remainder of the system using methods acceptable to the owner is one of those motherhood statements that amounts to a license to steal. Not one owner in a thousand knows or cares jack about creep-rupture, and even if they did, squeezing more product out of a system is a lot more interesting than a bunch of geek-speak.

So I'm sticking to my guns for the time being about 're-rating.' Luf is right about the kind of work that might be involved--not for amateurs. I'm wondering aloud if anyone who re-rates a system using (a) or (b) above actually goes through a real thermal analysis to determine the effects of creep-rupture or whether it's something a little cruder. Hands please... ;->

> That New Orleans thing sounds dreadful. Hope you guys are in a
> position to contribute.

It seems to be going better than the initial horror stories might lead you to believe, but it's no walk in the park. The city is getting pumped out faster that first estimates and they aren't finding bodies as fast as they'd thought. I've already contributed money. As for contributions to refineries to allow them to ignore possible damage so they can flog gasoline at inflated prices, this post probably puts paid to that. ;->

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=PbGUcz0C8ZMvB1YziYP1sMmD0mH9wq2PHWUJF7EgS8OknRJ9br5ScnZolKkLp6ZpyrOBwTfYob1tex8">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 Sep 13 12:00:00 2005