RE: pressure test thoughts

Hello Chris

Youre a bit of a worry. 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. The one I occasionally get involved in is the derating of economic ( firetube + furnace tube) boilers when changing fuel (e.g coal to gas). The derating applies to the firing (heat release) rate which is necessary with a low luminosity (gas) firing system because the furnace tube absorbs less heat which means that the back end tube plate gets hotter at a given output. By rerating to preserve the original back end temperature tubeplate stresses are inferred to remain the same, although it is transparent that the tubesheet thermal gradient will decrease axially and radially. So with this class of boilers a pro-rata derate will be a conservative change while an uprate will be aggressive and requires extra care. Now the curious thing is that when I am dealing with a " boiler", an uprate is almost impossible, but if I am dealing with a "process heater" I can do nearly anything I want to. In my opinion the difference is due to the the inability of the regulators and their servants to understand the process and regulate accordingly.

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

Cheers

Steve

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From: <a href="/group/PipingDesign/post?postID=u4afE-K6sVd_xU1k68amZEHW-P5eO6FqlUK32L0GiltvQ5QhBpqvWSoWe_O45MnBeUvI0B-wvMTfq_Li1T7F9phU">PipingDesign@yahoogroups.com</a> [mailto:<a href="/group/PipingDesign/post?postID=u4afE-K6sVd_xU1k68amZEHW-P5eO6FqlUK32L0GiltvQ5QhBpqvWSoWe_O45MnBeUvI0B-wvMTfq_Li1T7F9phU">PipingDesign@yahoogroups.com</a>] On Behalf Of Christopher Wright
Sent: Monday, September 12, 2005 10:06 AM To: <a href="/group/PipingDesign/post?postID=u4afE-K6sVd_xU1k68amZEHW-P5eO6FqlUK32L0GiltvQ5QhBpqvWSoWe_O45MnBeUvI0B-wvMTfq_Li1T7F9phU">PipingDesign@yahoogroups.com</a> Subject: Re: [PipingDesign]pressure test thoughts

On Sep 11, 2005, at 8:41 AM, John Luf wrote:

> In general they cannot be re-rated (FEA has been used
> by some for this but frankly I am a strong dissenter
> in such practices...

There's and interesting distinction I hadn't thought about. The ANSI flange ratings are pretty conservative because the gasket loading assumed in design is high and the rating, ie. CL 600 applies to a pretty high temperature. I wouldn't have any qualms about using a CL600 flange at a higher pressure than 600 psi if the ASME Code App 2 calculations worked out. I don't think that's re-rating, though.

BTW, I heard from someone in an authorized inspection agency who knows for sure about such things, and the 're-rating' process he describes is what I've always thought it was--repairs or alterations done in accordance with the NB Code. That Code requires that the vessel carry an NB 'R' plate alongside the original Code plate, with the new pressure and temperature rating. Anyway that's how his agency does it. Received on Mon Sep 12 06:18:00 2005