Elbow allowable stresses

[Chungstar]

Sorry I'm a novice with pipelines and have a very simple and possibly stupid question that I can't work out. Depending on whether you're in the US or Canada codes the design allowable is 72% or 80% SMYS, the question is if you've design the straight pipe wall thickness based on hoop stress compared to this allowable (assuming we're just accounting for internal pressure) how can an attached elbow have the same nominal thickness if its calculated (equivalent) stresses will be much higher for the same load?

Any help would be much appreciated and reference to a specific section of the code that might educate me on this subject.

Thanks.

 

[BigInch]

Hoop stress is not the same as equivalent stress. You design for hoop stress, then you design for equivalent stress and make sure that both cases work.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[Chungstar]

Thanks Big inch, would you happen to know a reference to an allowable equivalent for an elbow located at a pumping station. I've had a look at b31. 4 and 8 and can only find one relating to offshore piping.

 

[BigInch]

What section was that?

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[LittleInch]

chungstar,

perhaps you need to start again and explain better what your issue is, some numbers to go with it and how you've calculated the hoop stress and the equivalent stress. Ad diagram or sketch is very useful also as I'm not sure what you mean by an "attached elbow". Elbow implies piping which is commonly to B31.3 on and offshore and subject to completely different rules and formula than pipelines (pipeline have bends not elbows....).

In answer to you first question, if the equivalent stress check comes up with a stress higher than the maximum allowable for what is almost certainly an unrestrained section, then you need to increase the wall thickness. However until you know wheter the elbow is subject to additional forces or stresses, then start with the hoop stress criteria and work from there.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Chungstar]

Biginch, littleinch are you guys brothers/sisters!

Sorry, I think I confused myself initially, I assumed that pipe wall thickness for the whole pipeline was the same (including bends/elbows in the system) to prevent changes in product velocities (due to bore size) and was dictated by the 80%SMYS rule on hoop stress for a straight pipe. I've since determined that they don't necessarily have to have the same thickness. In which case bends/elbows still need to conform to this allowable but need to include stress intensification factors of the bending loads. I also initially thought that evaluation of an equivalent stress in elbows was more appropriate than hoop stress only since in this geometry you're also likely to see other directional stresses, however, I've since discovered this appears to be considered in Z662-07 Clause 4.8.

On a side issue, I presume the stress intensification factors account for the stress increase due to the geometry (at the intrados in particular), however, why is it only factored to the bending stress (Z662-07 Clause 4.8.3)? What if there were no bending loads to account for?

Biginch, the equivalent stresses I was referring to are mentioned CSA Z662-07 Section 11.8.4.5 which has a design factor of 0.77 SMYS for offshore pipelines against equivalent stresses. I now don't think this appropriate for my application and CSA Z662 Clause 4.8.5 is.

Littleinch I think you need to educate me where you say that pipelines have bends not elbows. If that's the case why are elbows mentioned frequently in codes such as Z662 and B31.4 for pipelines? Hope I'm not being rude I'm just a little confused with the terminology since I'm relatively new to pipelines/piping/tubes that transport stuff!

Really appreciate your input into this so far and please let me know if my understanding so far is at fault!

Chungstar.

 

[LittleInch]

I've never worked with the Canadian code so don't have a copy, but 31.4 is applicable for pipelines and piping in stations so covers both. The term elbows is predominantly used in piping (ASME B 16.9 applies). To differentiate between elbows which normally have a radius of 1.5D with bends which vary from 3D to 40D radius, many (but not all) pipeline engineers call bends which are part of a piggable pipeline system and elbows which are part of a piping system. "Bends" tend to be made the same wall thickness as the line pipe if possible as the stress factor is lower for 5D bends than it is for elbows which tend to be forged as single items.

Whenever anyone talks about elbows as part of a pipeline I tend to suspect that they are piping engineers masquerading as pipeline engineers. The same applies when people refer to wall thickness of a pipeline as schedule 40 or similar piping terminology, not wall thickness. Piping and Pipeline may look similar, but work and design quite differently and are not always compatible.

The BigInch / LittleInch thing has been said before - we're definitely not sisters!, just brothers in arms....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way