Re: Reinforcement Pad Weep hole

From: <Christopher>
Date: Thu Oct 07 2004 - 11:52:00 EDT

On Oct 7, 2004, at 8:26 AM, harihara.v ramanan wrote:

> this is intended to facilitate both the escape of gases during
> welding

Perish the thought that I should argue with B31.x but when a pad is placed, the area between the pad and the pipe wall isn't closed until the instant the weld is completed. Seems very unlikely that pressure can build up in the space. It's also hard to imagine that any gas involved with welding would be anything but benign.

> Since the reinforcement pad is not designed as a pressure part
This isn't the case with ASME Code Section VIII, which also forbids welding on a vessel after the shop hydrotest. Pad materials are indeed a pressure part because they provide necessary reinforcement to counter the weakness resulting from the nozzle opening. Proper design and weld procedures are necessary and the material must be supplied according to an applicable ASME Spec.

> Since the pad thickness and material is same as the shell the pad can
> take
> the shell design pressure. But the integrity of the shell to pad
> welding is not sure

Seems to be a distinction with no difference. The consequences of the failure are the same, namely that the reinforcement is ineffective, whether the pad or the attachment weld fails.

> Can the exchanger/vessel be operated by plugging the hole and dP all
> the welding joints to avoid an immediate shut down of the
> plant? In this case the repad need to be designed and tested as a
> pressure part which is not done presently.
My experience with a lot of plant guys is that they'll operate anything that isn't actively discharging flames or poisonous vapor. They'll say with a straight face that it always does that and anyway it'll straighten out once the system gets up to temperature. If your guys are different, I'm sorry if I seem to be slandering them. When someone asks me about keeping a leaking vessel in service, I always tell them a leak is a sign that something is wrong. A leak in a welded joint is especially bad because it's likely to be a crack, and it is never acceptable to operate a vessel with a crack in it. If you have a leak through a nozzle wall under the pad, there's no way in hell you tell the extent of the problem without looking at the nozzle weld and fixing it. Dye penetrant doesn't fix leaks. All you'd be doing by sealing the weep hole is making it look like it was fixed, when the crack in the nozzle wall just keeps going. You need to fix the problem, not cover it over. Sure as God made little green apples, once it appears to stop leaking, it'll be forgotten until the crack grown to point where the nozzle breaks loose.

And once more (maybe you should make a note of this)--the reinforcing pad is part of the pressure boundary, so it is a pressure part, and should have been designed that way before the vessel got its Code plate. When you repair the leak and replace the pad, it has to be done to the same standards as the original construction.

I have almost no doubt that there's someone reading this and saying, 'Huh…just another pain-in-the-ass engineer out to cost us production and act important.' But I've seen too many little things glossed over until they become big things. I've also seen more stuff ruined with hasty weld repairs than was ever fixed. Just remember--when that nozzle lets go late one night you're going to have a helluva lot bigger mess on your hands than a shut-down you can plan for and control.

Christopher Wright P.E. |"They couldn't hit an elephant at <a href="/group/PipingDesign/post?postID=kFZih1AlZZUVauDy3mjsGzgEZDs0Rql8hYAjZ05JfsNGJG3M9ZHY6bGKRvQFjFluVlXxM-6KP_HE">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 Thu Oct 07 11:52:00 2004