Re: Modifications to B31.3 Piping w/ Overstress

On Feb 24, 2006, at 10:32 AM, Ken Nisly-Nagele wrote:

> Stress modeling of existing pressure reducing station that is within
> the
> scope of the piping receiving the planned modifications indicates
> expansion
> stresses are 7 times and occasional stresses are 9 times above the
> allowable
> stresses in portions of the station.

If these are primary or secondary stresses, you don't need any further comment. At the very least you're headed for a significant problem. If I were to guess, your system is over-restrained or your restraint assumptions in the FE model aren't realistic. The latter situation isn't all that uncommon, but don't just assume that's the situation.

> My understand is that just because the piping has held up for 10 years
> does
> not mean the piping is okay,

At best it means that the first failure is 10 years closer to happening than it was when the system was new. At worst, it means that there have been failures and either no one remembers or they're not talking. Or maybe that you're asking the wrong people.

> 1. Concerned about the fatigue stress that is resident in the existing
> piping, it seems prudent to not simply make restraint or piping
> modifications that reduce the stress, but to demo the piping that is
> shown
> with expansion overstress. Comments?

You should estimate the remaining life and show that additional stress cycles after the mod is installed will satisfy system life expectations. You can estimate the remaining life of the system with more or less standard techniques. Your problem will be getting reliable estimates of loading cycle from plant people. I've had some unfortunate experiences with selective memory on the part of plant people or people who and simply made up things in an attempt to be helpful.

> 2. It also seem prudent to remove the piping that is indicated as
> overstressed by occasional loads too unless it can be established that
> occasional loads have not occurred. Comments?
Again, you should make a good-faith attempt to mitigate the overstress, and show how it affects system life.

I'm sure there are plant engineering people reading this who now think I'm an impractical techno-snob, who just wants to turn a simple upgrade into a PhD thesis. They wouldn't be the first. But I figure I'm straddling a line between making impossible demands on a client and aggravating an unsafe situation. Either way it's problems for me and the client, and it's very much a judgment call. I tend to put a lot of weight on my impressions of the client before I start using his input to make engineering assessments.

Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

.......................................| John Sedgwick, Spotsylvania