B31.3 2004 Questions 1

[StressGuy]

Alright, first question is - will they actually get this thing out in 2004? There's basically two weeks to the end of the year and still no official word on it.

But, here's the real question - I obtained notes from a presentation on changes in the 2004 edition, and a question has been raised. Specifically regarding the new Appendix P, providing alternate rules for flexibility analysis. The new equation intends to consider the effect of axial stresses on the pipe, which have typically not been considered.

Including the axial effects isn't so much a problem, we have to consider it's effects anyway for cases where axial stress is significant, like jacketed piping systems. The question come from the definition of Fa as the axial force, including that due to pressure. Pressure has been considered a primary loading, so there's a question now why it is being considered as part of a secondary stress evaluation. For a fully pressurized line, the axial component due to pressure can eat up a significant percentange of the stress.

Maybe when the actual code comes out, it will be clearer. Right now I'm just able to operate from notes to a free seminar what I wasn't able to attend in person.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[jte]

Mr. Klein-

No, there's no chance it will be published in 2004. In essence, it was in line behind the Boiler and Pressure Vessel Code which was delayed for some reason.

Charles Becht published a paper at the PVP conference this summer titled "Technical Changes in ASME B31.3 2004 Edition." Available for $15 at

http://store.asme.org/product.asp?c...apers&category_name=&product_id=PVP2004-2707.

Though you probably need more info, the formula for Appendix P stresses is presented as S=sqrt[(|Sa|+Sb)^2 + 4*St^2] where S = SE (stress range) or Som (max operating stress).

jt

 

[ApC2Kp]

The paper presented at the PVP meeting might have the same information as the item on

http://www.becht.com

website, that has some insights on appendix P, and other B31.32004 changes.
It will take some changes in thinking and design to have the operating stress range less than the SA allowable stress.

 

[StressGuy]

Alright, this makes things even more confusing. While B31.3 has never had an explicity defined "operating" stress allowable, it has generally been understood, thanks the to "liberal" definition of Sa (B31.3 2002 equation 1b on page 17)

Sa = f[1.25(Sc + Sh) - Sl]

You can rearrange this to produce an "operating" equation that looks like this:

Se + Sl < f{1.25(Sc + Sh))

As such, the stress due to pressure has always already been accounted for as part of the Sl term. Even if you use the 1a version (i.e. the conservative allowable) you've deducted a full 1xSh assuming the sustained stresses are at the full Sh value. Even then, the pressure stress is accounted for.

Now, we're saying that we're going to be adding the effect of the pressure stress to the Se term as well. That means, unless the definition of Sl or Sa is being changed, that pressure is going to count twice against the total allowable. In fact, even more so, since there's an SIF applied to the pressure stress in the Se equation.

The guys in my office are having a hard time with this. Maybe something else is getting redefined so the pressure stress isn't counted twice, But I haven't seen anything in Becht's writeup to suggest that.

Can someone clear this up?

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[bvi]

For Appendix P purposes, SA=1.25f(Sh+Sc). Sl is not subtracted, so pressure and other sustained stresses are not double counted.