ASME B31.8 - Elastic Bend in Pipeline

[Engg.james]

For a pipeline project, the cold bends for smaller size angles have been avoided and pipe elastic bends have been considered. Our client has asked to calculate and check the minimum possible bending radius for the natural bending of pipe.
Is there a method to calculate the minimum radius?

 

[LittleInch]

Bending moment.

From that calculate longitudinal stress.

Add to pipe longitudi stree from internal pressure and thermal expansion or contraction and check combined strest.

Usually around 500D

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[LittleInch]

Usually the issue is how do you get the right radius and how do you lay it.

If it's too tight you risk buckling the pipe.

What's wrong with cold bends like everyone else does? ?

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

 

[Engg.james]

The drawing from client mentions that the “minimum elastic bend radius shall be 620.57m”.
What does this mean?

 

[LittleInch]

That's a ridiculously exact number. I thought it was going to be a conversion from 2000 feet, but it's not quite.
What size pipe?

What it means is what is says. The peak radius of the elastic bend is 620m.

Now that's a pretty big number so unless your pipe is over a metre diameter, works out to be >600D so pretty gentle elastic bend.

These are still a lot easier to calculate than build accurately....

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

 

[1503-44]

Can you even see a 600D bend? It is a limitation on deflection when installing straight pipe. It is not going to be practical for making an actual bend to change a pipelines direction. Find the bending machine.

 

[Engg.james]

Pipe size is 8” ( 219.1mm)
The minimum elastic bend radius as shown in the pipeline drawing is 620.57 m.
Can somebody help me convert this value to a bend angle as our client wants to find the allowable angle for natural elastic bends.

 

[1503-44]

360 / (620.6 x 2 x pi) is like a 10th of a degree per meter length of pipe.
You can only bend about 1 degree per 12 m pipe joint. Lotsa luck with those "bends". Thats a sag limit, not a bend limitation.

 

[LittleInch]

You can't convert a radius into an angle until you know the arc length.

I agree with ax1e. one degree is about 12m of arc length. So say a 5 degree change of direction would take 60m of pipe.

An 8" pipe once in the air held up by sidebooms, excavators etc will whip around like a piece of spaghetti. Getting any sort of elastic bend within a set horizontal radius is actually very difficult unless your surveyor has managed to mark out a trail 60m long. Usually you will end up with a bend radius much less than your magic 620m one you survey it. Then what? Have an overstressed pipe?

This practice is sometimes referred to as "roping" in a bend, but it really isn't good practice.

your client or whoever came up with this idea may need to consider the practical aspect of this choice.

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