Re: Modifications to B31.3 Piping w/ Overstress

On Feb 27, 2006, at 3:17 PM, Ken Nisly-Nagele wrote:

> And I understand that the primary stresses will not have a significant
> effect on fatigue strength for
> carbon steel piping experiencing less than 50,000 cycles, and this
> system has probably seen less than 10 FT cycles
Primary stresses never govern fatigue life in the absence of peak stress--fatigue is a problem only for concentrated stress where local plastic strain may accumulate. Primary stresses are invariably below the yield, so plastic strain can't accumulate.

> I have performed site investigations to verify the boundary
> conditions. I think the restraint modeling is realistic and have
> looked at a range of friction assumptions.
Sounds like you've done it exactly right. There's no substitute for a site inspection.

> What I understand you to suggest is that a remaining life can be
> determined for a low cycling system (under 7,000 cycles) with high
> overstress, provided I can obtain reliable thermal cycling history
> data.

There are papers discussing the evaluation of remaining safe life for pressure vessels and piping. I can't cite any specific examples--check the ASME web site or maybe www.scirus.com/. There may be some papers amongst the WRC bulletins.
Christopher Wright P.E. |"They couldn't hit an elephant at chrisw@skypoint.com | this distance" (last words of Gen.

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