Mitered bends in Pipeline

[nanil]

"Mitered bends are prohibited". This is a statement seen in some of the client specifications related to High Pressure Gas Pipelines. What would be the reason's for this.
Please advice.

 

[Latexman]

High pressure drop.

Good Luck,
Latexman

 

[csk62]

Prone to leakage and failure compared to elbows or hydraulically bent pipe. The cross-section of miters may not be circular and is difficult to prevent leakage if there are non-negligle pressure and forces on the pipe. Also if you think from a flexibility/stress analysis viewpoint, a miter bend is worse than an elbow or bent pipe as well.

Let's say you had a single 45deg miter joint, this creates a sharp intrado and extrado corner which tends to act to open up the weld seam and leak with any type of bending load.



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Not making a decision is a decision in itself

 

[goutam_freelance]

Because of nonuniformity of geometry there will be large stress concentrations at corner welds and so these tend to leak. Also Miter joints are mostly fabricated at site where control of weld and root gap is difficult increasing the risk of leakage

 

[LittleInch]

"What would be the reason's for this"

Well because these tended to be field made with a certain lack of precision, inability to get full wall thickness welding, the fact that the stress concentration factor goes through the roof, Gas lines in particular tend to suffer from fatigue due to diurnal changes in gas pressures, it makes pigging a lot more difficult, especially intelligent pigging and basically these went out with the arc.

I only ever saw one gas line in the former Soviet Union when the engineers told me that the design called for a formed 5D bend, but no bends were sent. So rather than be sent to somewhere even more inhospitable because they couldn't finish the pipeline they did what they needed to....

I was still fascinated by them.

I think they are still "legal" in ASME codes but only for pipelines with stress levels below 20 or 30% of SMYS

Oh and below 3 degrees it isn't a miter or mitre bend.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[nanil]

Thank you all for the responses.

 

[Dhurjati Sen]

Too many weld joints at very close proximity. We need to avoid / minimize weld joints everywhere, not only in high pressure systems.

DHURJATI SEN

 

[KevinNZ]

Mitres area allowed in piping codes, but you have to design for them. SIFs and increased wall thickness.