B31E allowable stresses

[Matthew L]

thread378-516645
Hi All,
B31E allows a higher longitudinal stress than the B31.x codes by adopting vessel design codes. But it is limited to 60ksi, i.e. min(2.4S, 1.5Sy, 60ksi) for most piping materials.
Can anyone shed light on 60ksi limit?

Also there is no criteria for other stress components.

Cheers

 

[GD2]

OP,
This is my understanding:
ASME B31.x and B31E pipings are different. ASME B31.x pipes have low ductility factor (complement Response Factor) and hence the lower allowable. there is a big difference between the a[sub]p[/sub]/R[sub]p[/sub]factor.
Look at the span Table 2 of B31E. Span lengths are much longer than the B31.x span tables.
ASME B31E refers to ASCE-7.
Load combinations in B31.x and B31E are the same. However, seismic loads are @700% higher and allowable by @2x in B31E.


GDD
Canada

 

[XL83NL]

I dont think it directly answers the OP's question, but there's been more discussion recently here at ET on B31E vs B31x;

https://www.eng-tips.com/viewthread.cfm?qid=521690.

I think if you understand the differences in both Codes, you might find an answer more easily.

Huub
- You never get what you expect, you only get what you inspect.

 

[KevinNZ]

My guess, 60Ksi is to limit stress in high strength metals and avoid brille failure.

as above, important to note ap/Rp factor is different for B31.x and B31E.

 

[Matthew L]

Thank you GD2, XL83NL and Kevin for your passion and relies.

Lets put it into the context of code acceptance criteria, and I am trying to get the rationale of 60ksi maximum for B31E's longitudinal stress limit. Mitigation against Brittle failure seems reasonable.

However, for large collection of material API 5L X60 and higher, this 60ksi upper limit for longitudinal is more stringent than other piping codes, and the allowable limit is actually lower. For X60 and higher grades, the longitudinal limit a a function of Sy, rather than a fixed value.

On the other hand, for lower material grades, a min(2.4S, 1.5Sy, 60ksi) is extremely generous, similar to BPVC code limits for combined loading conditions.

So in a nutshell, it is extremely generous on one hand and and extremely stringent on the other than B31.X.

Personally for material grades lower than X 75 (Y75), no brittle failure should occur unless subjecting to CP or caustic chemicals H2S, for instance.

I would appreciate your thoughts.

Thnaks

 

[1503-44]

The allowable displacement stress range was limited to 60,000 psi in B31.3 2012 edition. Reason being that while high strength steels have higher yield and tensile strength, they have fatigue strengths similar to the more common steels, which is why the limit was added.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Matthew L]

Good thought (1503-44).
Fatigue due to displacement stress ranges is typically associated with system startup and shut down thermal and pressure cycles.
For seismic events, it is one off and ultimate limit state failure and damage is governing. It would appear to me it is a mixed up of two things.

 

[1503-44]

Your question was about why the 60 ksi limit is in place. That's why. Ae you trying to reason a way out of ignoring it? I don't understand the objective of your new comment.

Seismic loads occur on top of fatigue damage. Fatigue damages are a running cumulative lifetime total. It is always present. It does not suddenly just go away during an earthquake. It is not only from startup and shutdown. Fatigue may result from pressure pulse, wind or underwater current vibrations, any load variation, ocean platform movement, or traffic loads at a road crossing.

It is also possible that high strength materials experience a higher damage accumulation at low cycle numbers. Generally the more brittle materials are more sensitive to this.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Matthew L]

Hi,
I just want to argue the code is not rationale. For a seismic event, agree it is on top of other load constituents (e.g. pressure, thermal). The maximum design pressure is relevant for this as well (on top of operation conditions) but the acceptance criteria is limited by the UTS factor, not Sy.

I believe the code is open to justification, though.

 

[1503-44]

I have not used B31E.
You say longitudinal stress is limited to 60.
Then you say acceptance is the UTS factor.
Appears to be contradictory.
Perhaps stresses other than longitudinal are limited to UTS factor and S[sub]L[/sub] <=60 ?
See if that logic applies.

Could be its an artifact left over from 2012. Have you found any clarifications?
You can submit a request yourself as well..

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Matthew L]

I have not found any clarification on this. Yes will raise a request.
Thanks for your comments /suggestion.