Maximum Bending Stress in a Pipe? 1

[ekuest]

i was just told at work that i need to find out if some pipes can withstand a force or not, but dont have much to go off of yet. they just want some formulas. im wondering what info i'll need. one end of each pipe goes into an engine, and the other ends either go straight out to a hanger where theyre held in place, or else turn 90 degree and go to a hanger where theyre fixed. the lengths and diameters of the pipes are all different. the pipes are welded and brazed, so no threads to turn to give more movement. they want to jack the engine up an inch temporarity, and want to know if the pipes will be able to flex that inch or if they'll break.

not much to go off of i know, which is why im so lost here. can anyone tell me what info i will need, or have any idea how to start tackling this problem?

 

[flash3780]

Sounds like standard structural mechanics type of stuff. Start by drawing a free body diagram of your problem so you have a handle on the loading. You'll have to dust off a strength of mechanics book. Here are a couple of formulas that may get you started:
Pulling/pushing: sigma = P/A (

http://en.wikiversity.org/wiki/Introduction_to_Elasticity/Torsion_of_circular_cylinders)

Bending: sigma = My/I (

http://en.wikipedia.org/wiki/Euler–Bernoulli_beam_equation)

Twisting: tau = Tc/J (

http://en.wikiversity.org/wiki/Introduction_to_Elasticity/Torsion_of_circular_cylinders)

Pressure in a thick walled cylinder:

http://www.engineeringtoolbox.com/stress-thick-walled-tube-d_949.html

Buckling:

http://en.wikipedia.org/wiki/Buckling

What's the application here? Often plant/process sort of stuff needs to be signed off by a professional engineer. Do you have a PE looking at your work? Sounds like someone might get hurt if there's a failure - it'd be good if someone with some experience in structural mechanics checked over your calculations. What sort of background do you have?

 

[racookpe1978]

"Flex an inch or two before they break?"

You (you and your supervisor) need to think about what will bend first (engine casting, engine flange, valve, support, fitting, pipe, etc) BEFORE trying this lift.

The pipe is relatively "soft" compared to a casting or an "engine" and so the flex of the pipe will help cause yielding and failure elsewhere that you don't intend. Disconnect the piece, support the pipes and fittings that will "droop" or sag from their weight with jigs, chainfalls or temporary brackets. This is more than a theoretical exercise.

 

[BigInch]

Failure can be defined as unacceptable deflection, which is what you will have if you go with that criteria.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[DLite30]

Sounds to me one of those jobs, where operations is too lazy or doesn't want to take the time to unbolt things, drain coolant, etc from a component (engine) whose connections weren't designed to be in a stressed condition in the first place, and are looking to the newb engineer to tell them it's okay, and the newb engineer being too newb to tell operations that they shouldn't do it in the first place, and sweats himself to death to come up with a way to calculate bending moments, stresses, ad nauseum, to come to a conclusion that it's way beyond a reasonalbe time frame and far from accurate to mean anything.

Unless you have flexible connections, that seem to have enough give for this much movement (such as braided hose, bellows, or the like) at all the points; I'd tell them to suck it up and disconnect everthing, or they'll break something, and let them live with the consequences if they do.

 

[osoloco]

I agree with DLite!
If you are concerned that something will go wrong, most likely it will! Murphy's Law is based on experience.

 

[ekuest]

haha, i didnt know murphys law was an accepted engineering formula.

so i guess my boss realized that the whole idea of calculating this is not really feasible because of the fact that there are so many pipes and so many variables, so theyve decided to disconnect all of them and not risk it. thanks for the help though everyone!