B31.8 Elastic Bending radius 3

[darkknightrises]

does any one know what basically is meant by elastic bending radius....?
what its significance...??

after all the calculation of elastic bending i got a value of 261.6 m for a 16 inch pipe .what does this show....?

and finally then what is the difference between the cold(24D) ,hot bending(5D) done on pipes and the elastic bending....??

 

[BigInch]

Elastic bending means that the pipe is curved without exceeding its yield strength at any point. If you pick up a 12 meter length of pipe by hoisting it at the center, the ends droop down. If you lower it back to ground, once on the ground again it will resume its straight shape, without having to jump on it. It was curved within its elastic limits.

If you bend the pipe into a curve and it won't return to straightness after you've released it, you have yielded the pipe and have made a cold bend. Cold bends are usually not very sharp bends, most pipe will need minimum radaii of 40 x diameter.

http://www.eng-tips.com/viewthread.cfm?qid=258983

Hot bends are made by heating the pipe to lower yield stresses, thus can be bent to tighter radaii.

Disadvantage of elastic bending is that if the pipe is bent elastically and held in that position forever (after construction has been completed), the elastic stresses, although less than yield stress) remain in the pipe forever. When you pressure the pipe, the pressure stresses will add to the elastic stresses, which could cause allowable stress to be exceeded when under pressure. That could require that your maximum allowable operating pressure be reduced, so the combined stresses remained below limits.

Cold and Hot bends have no remaining stress once the bending forces are released. Therefore hot and cold bends can be pressured without adding to any previous stress resulting from holding the elastic bend in place forever.

 

[darkknightrises]

thanks bro for the helpful reply...

 

[BigInch]

Guess every Dark Knight needs an assistant.
May the stress be with you.

 

[LittleInch]

Big Inch has it exactly right.

How did you get to the rather exact radios of 261.6 m?

The only think to add really is that elastic bends work fine in vertical profile, but are very hard to do horizontally so I wouldn't try.

24D cold bends at 16" diameter may be ok in terms of maximum strain, but I've never seen any. 40D is pretty much standard.

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

 

[darkknightrises]

LittleInch it a bit complicated , and highly confidential but anyways here it is

having all the pipe data (i presume u have)

first calculate the # hoop stress
then # maximum allowable stress
then # thermal stresses

now from code b31.8 we have
max bending stress= maximum allowable stress-0.3*hoop stress-thermal stresses

now to calculate the bending radius we have flexure formula

bending radius = Modulus of Elasticity*pipe Dia /2*max bending stress

i hope u got it

 

[LittleInch]

Don't forget some of the stress can be negative as well as positive.

You need to work out which is the smallest amount "left" from the max for the bending force. Sometimes its the inside of the bend, sometimes it's the outside.

You've ended up with a radius of about 640D, which is about the lowest you want to go. 1000D is more normal and will generally pass the stress checks.

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