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How to find the thickness of pipe after bending
6

How to find the thickness of pipe after bending

How to find the thickness of pipe after bending

(OP)
Dear sir,

I want to know how to find the thickness of pipe after bending

Thank & Regards  

RE: How to find the thickness of pipe after bending

Cut it up and measure it.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: How to find the thickness of pipe after bending

(OP)
Didn't have the equation to find it ?

RE: How to find the thickness of pipe after bending

I don't think there's a single equation that covers it.

Among other things, it has to depend on the material and temper, the type of bender, the geometry of the unbent pipe including nonuniformity, the geometry of the bend, the condition of the tooling, presence/type of lubricant, skill of the operator, and phase of the moon.

Also, there isn't 'a' thickness after bending; some areas get thinner, some get thicker, some don't change.


Can you pose a more specific question?

  

Mike Halloran
Pembroke Pines, FL, USA

RE: How to find the thickness of pipe after bending


GOBLINTECHNIC
I think its not possible to exactly determine the
degree of thinning. However, it can be approximated by multiplying the thickness before bending by the ratio:

R/R+r


where r the radius of the pipe
R the radius of the bend

sendel kumar

RE: How to find the thickness of pipe after bending

2
1+r  ?

Try Hooks Law and Poisson's Rratio.  The thinning is proportional to the axial stress applied.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

If you know someone close who has Ultrasonic NDT  they could measure thickness for you pretty easily.

RE: How to find the thickness of pipe after bending

Hooke's law doesn't apply...bending creates plastic deformation and Hooke's law applies in the elastic range.

JHanson717 has it...measure the resulting thickness ultrasonically.

RE: How to find the thickness of pipe after bending

You'd at least know the thickness before it went plastic. So you're saying that after it goes plasdtic there is no relationship we can use?  

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

Plastic solid mechanics is not well known and still under research.

peace

Fe

RE: How to find the thickness of pipe after bending

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending



 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

Zero E means movement without force increase. In reality it's somewhere between zero and elastic E. (nonlinearly)
I guess if you really want to you can approx. it.

peace

Fe

RE: How to find the thickness of pipe after bending

Yup.  If something's not well known, that's about the only alternative we have.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

I like your persistence. Star for BigInch.

peace

Fe

RE: How to find the thickness of pipe after bending

Thanks.  That would be my dominant bulldog gene.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

Although I don't think it would be as simple as those curves show.  We should probably integrate along some lines more typical for steel.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

You're welcome.
The last plot seems more like it. Should be able to give a good approximation.

 

Fe

RE: How to find the thickness of pipe after bending

Ah, true - but is the second set of graphs correct for the outside fibers of a real pipe?  

It'd be mostly correct for a round (solid) bar because the neutral axis of the bend is near the middle of the bend - between the stretched outer fibers and th einner fibers getting pushed sideways into the mandrel  and lengthwise by the two ends of the bar.

But the inside of the pipe is hollow, so a good bit - but not all! - of the pipe outside fiber is deformed inward towards the centerline of the bend as the pipe wall deforms.   The inner wall of the pipe tends to crush and deform into ridges - so it is not under simple compression either.    The net stretch of the pipe outer wall will expand in length a little bit, but will also get pulled back "inwardly" towards the pipe tube centerline.  

The overall thickness of the final pipe wall (inside as well as outside parts), cannot therefore be accurately calculated directly by a combination of 2% elastic fiber stretch, steady-state elastic movement as the pipe deforms, and the final wall thickness decrease as the length increases.   

Besides, the variation of pipe wall thickness due to fabrication tolerance and final corrosion/erosion restrictions/allowances for your actual services  means you wouldn't  be able to use this information even if you could calculate it accurately enough to make an assumption about the rest of your pipe bends to allow a "safe" calculation of allowable internal pressures anyway.

Make sense?  Calculating final pipe thicknesses due to bending is not likely going to give you (your company) enough savings to give you the pay raise you want/need/would like to have.    

RE: How to find the thickness of pipe after bending

I would imagine that you could ask one of the shop bend manufacturers or experienced field bend operators what they're seeing.  From what I understand they have been using computer control to do these for many years.  Maybe they'll share a secret or two.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

Whats wrong with using a thickness approach.  Straight pipe = 1, if radius is longer then the same material must still be there but thinner no?

RE: How to find the thickness of pipe after bending

An approximation I recently tried successfully for a boiler tube bend repair simply applied conservation of mass and density before and after bending relative to the outermost band of fibers.  I rationalized that the tube diameter(s) would be more or less equal before and after bending, so that the radial (circumferential) strain would be negligible.  The change in thickness was then approximated as a function of the change in average fiber length between the bend radius to the outside ID and the bend radius to the outside OD of the pipe.

The Regulator accepted the calculation on the premise that the assumption of constant density - while probably not entirely accurate - produced a more conservative result relative to the wall thickness reduction.

Regards,

SNORGY.

RE: How to find the thickness of pipe after bending

I'm sure it did pretty well.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

Yes, but sadly...

I didn't get a "star" for it.

sadeyes

Regards,

SNORGY.

RE: How to find the thickness of pipe after bending

Post the calcs. Maybe you will get a star. Or not, since I don't agree with the const. density assumption. wink
jk

peace

Fe

RE: How to find the thickness of pipe after bending

Not sure why.  Even my wife agrees with the "constant density" assumption.  At least, that's what I conclude when she's constantly telling me:

"You're always just so dense."

Regards,

SNORGY.

RE: How to find the thickness of pipe after bending

I was kidding. Although it wasn't me that gave the star. At first I thought that dislocation motion (caused by plastic deformation) would change the density enough to matter. However, I don't believe it does matter.

I get the dense thing too. But, its more of a " your soo god dam hard headed"

peace

Fe

RE: How to find the thickness of pipe after bending

(OP)
Thank for all comment
  

RE: How to find the thickness of pipe after bending

If you want to do a bit more math and get what some of my profs* believed to be a more accurate answer, try checking in a mechanics of materials textbook. A good one is Intermediate mechanics of Materials by Vable.  There are definitely ways to approach the problem that will get you closer than Hooke's law, though of course you'll never get a real world situation that looks just like the formulas.


*I am a recent grad with an engineering job, please don't kick me off the site for mentioning profs.

RE: How to find the thickness of pipe after bending

* OK, but you're getting close smile

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

why not perform FEA ?  
That should provide some good guess or rough figures

Wimple

RE: How to find the thickness of pipe after bending

Is your FEA's mechanics valid in the plastic range?

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: How to find the thickness of pipe after bending

Material models shall be entered as inelastic (plastic in this case) and Non Linear solver is to be activated in FEA software.

RE: How to find the thickness of pipe after bending

Constant density?  Sure.  The metal is the same.
Constant weight (total weight, that is.)?  Certainly!  
Constant outside radius?  Probably not - The outside is  deforming inward as the metal deforms plastically.
Constant inside radius?  Close to constant, but small waves and compression zones/heights are likely.
Constant thickness?  Well, we don't know.
Constant strain?  Nah.
Constant stress?  Nah.
Consistent position of the neutral axis at pipe CL?  Probably not.   

So, though my wife agrees with my constant density of mind, I'll say "Bend it, drill it, mic it."   Can't see how FEA could be assumed accurate enough for all of the approximations.   (Whether it is worth paying for the FEA modelling time and computer time is another question all again....)

RE: How to find the thickness of pipe after bending

"Bend it, drill it, mic it."
Forget about dislocations. Material science brothers...

My ass is so 'strain hardened' my gf can't break it. wink
  

peace
Fe

RE: How to find the thickness of pipe after bending

racookpe1978

I guess that your point, is to clearly avoid use of calculation/simulations to guess the thickness? you are considering that measurement (e.g. NDT) is almost the only proper way. Do you confirm?

Frankly speaking, I am not specialist of such problems. I have suggested FEM, as it is a tool that engineers have in hand, so why not consider it?

In my optinion, we know the initial geometry before bending (probably straight pipe) and the final one (loaded). So Starting from this, one should try to simulate with FEM the behavior on an iteratively manner. That can be a possibility.

I guess the problem stands in the non linear nature of equations and especially the material deformation occuring in the plastic range.

Plasticity being "non nonservative", the final state is dependant from the path of the load vs. time. Thats add a complexity. I understand however that FEM codes are now quite confortable for solving such non-linear and non-conservative problems.

My point is that in any case, we need to know what is level of accuracy required, what is the type of application involved (e.g. lethal service?) and what are the applicable Codes & Standard? Afterward one could evaluate wether FEM simulation could produce some orientation to Engineer or is clearly jumping in the dark...

Wimple


 

RE: How to find the thickness of pipe after bending

Well, there is seldom any problem with guessing (more accurately, making well-founded assumptions based on experience and based on reasonable extrapolations from previously tested models) ...  But.   

A FEA model is best used within its rannge of accuracy: nothing can come closer to showing the high stress areas when a good, close-fitting model is subjected to outside strains.   Then what?  

Well, the usual FEA model exaggerates the movement (distortion) from an original shape to a distorted shape based on outside forces.  The exaggerated movement (strain, thus stress) is usually shown in colors - but those forces are applied up to the elastic limit, within the 2% limit for permanent plastic deformation in all the results I've seen.   (Which is NOT a large sample of more than a few 100's, mind you!)  Pipe bending will be at least to a 90 degree permanent angle, permanently stretching the steel wall well past the small movement where the "rules" of the FEA equations from cell to cell apply.   For example, what happens to the FEA "cubes" when the material moves through the first FEA box all the way through the next 24 into a 25th "cube"?  Is the original equation of strain in one cell that affects adjacent cells still valid when there is no material left "in" the first, second, or third cell?   

Will FEA be accurate over that large a movement?   Before we assume it will, we should (I would, at least) actually test the validity of the model over the range in question.   

But, returning to the original focus of the question: Why is this thickness being questioned?  What are the budget/time/resources available to his project team and WHY are these resources beng spent?  How many times will this problem be faced, and in how many different configurations (what combinations of pipe wall, pipe material, pipe diameter, bend radius, and bend angle) will this problem be faced?)  If there is only one bend, test that configuration properly and be sure.   An FEA model will only tell you how accurate the program "thinks" its answer is after x number of iterations with a cube size of y and material properies of wxyz, but that answer has nothing to do with realty.

The tone of the original question doesn't appear to warrant assuming that questioner is facing a situation where determining the presence (or absence!) of a 1/100 of a millimeter (a few 1/1000 of an inch) in a pipe wall bent into a 90 (?? - how far is he concerned?) will affect the operable pressure in his system.   

RE: How to find the thickness of pipe after bending

Good one Ibrahim  

peace
Fe

RE: How to find the thickness of pipe after bending

And, bottom line from that paper, was a decrease in wall thickness at the outside of the 90 bend from 18 mm (baseline) to 16 mm.

Generic issue for all pipes in all circumstances under all fluids and pressures and flowrates, will be, over time, how much additional material will be removed from the inside of the pipe at that point from erosion and corrosion?   As the "original" 16 mm pipe wall becomes  14 mm, 12, 10 ... when will the fluid break through?   

RE: How to find the thickness of pipe after bending

I would have thought that the real question is "is the bend as strong as the straight". If this is the case then we as a company would not accept any FEA analysis but require the supplier to hydro test three samples of each type of bend to supplied. If the bend fails before the straight then it is not acceptable.

Regards,

athomas236

 

RE: How to find the thickness of pipe after bending

athomas236

Do you mean FEA Analysis to estimate deformations and Stress of the Pipe under loading conditions to check against allowable limit, or do you mean FEA to estimate the pipe thickness after bending ?

Rgds
Wimple

 

RE: How to find the thickness of pipe after bending

athomas236

Refering to your comment "is the bend as strong as the straight".  I would ask you as well what is typical L to D ratio of the pipe to consider it is the case?

Thanks
Rgds
Wimple

RE: How to find the thickness of pipe after bending

I wonder why no one bent a pipe and cut it open.  

Richard A. Cornelius, P.E.
WWW.amlinereast.com

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