TRUE min wall thickness of seamless tubing 1

[gilbertgrape87]

Hi everyone, this is my first post, so sorry if I'm not posting in the correct forum.

Situation: I have two pipes, similar material, rubbing on eachother. At this point, I am not trying to calculate the RATE of degradation of the piping, but rather just trying to find how much margin I have in the thickness of my hydraulic line.

Line 1:
1.25" OD x 0.12" wall thickness, seamless tubing, A269 Grade 304SS. 5400psi hydraulic fluid.

Line 2:
0.5", Schedule 80 pipe nipple, 304SS. 120psi air.

Due to the nature of line 1, I am not sure if I should be treating this as a pipe or pressure vessel. In either case, the calculations that I have attempted are stating that I only have about .005" margin in my thickness of the hydraulic line -- which can't be right. I believe the calculations I am using are giving me the min wall thickness ALLOWED by code and not the TRUE min wall the tubing will bear.

Any help would be greatly appreciated.

Thanks

 

[MikeHalloran]

Move the damn air line, NOW.

Your margin calculation sounds about right.



Mike Halloran
Pembroke Pines, FL, USA

 

[gilbertgrape87]

Thanks for your input Mike.

In order to complete this thread and make it useful for others, I solved this using Von Mises Stress calculations and compared them to the yield strength of the material.

Von Mises combines all of the radial, hoop, and axial stresses as well as the torsion that is being acted upon the material.

For this hydraulic tubing, it is considered a thick walled cylinder because the ratio of the radius to the wall thickness is greater than 10

The radial stress will be max at the internal radius of the tube and is equal to the internal pressure of the system (in this case, -5400psi -- negative because radial stress is a compressive stress)

The hoop stress is calculated as the equation dictates for thick walled cylinders. In this case, the selected radial point will simply be the outer radius since that is what is changing in this scenario.

There was essentially no axial stress other than what is being acted upon it by gravity (even though it is pressurized because it is open ended).

There is also no torsion, so that is negated.

Combining all stresses via pythagoreans principle (Euclideans distance) will give the total Von Mises Stress which can be compared to the Yield Stress. in my case, this was found to occur at approximately .103 wall thickness -- resulting in .017" margin.

Still not a happy scenario to be in, but at least I know what kind of margin I'm working with.

 

[MikeHalloran]

>>>
There was essentially no axial stress other than what is being acted upon it by gravity (even though it is pressurized because it is open ended).
<<<
You may have misinterpreted what your textbook says.
You cannot contain pressurized fluid in an open ended pipe.



Mike Halloran
Pembroke Pines, FL, USA

 

[gilbertgrape87]

True, an open-ended pipe can't hold pressure -- however the axial forces can be absorbed by other equipment. The tubing connects a pressurized accumulator to a manifold with a downstream valve. The outward forces are placed on the accumulator and the valve/manifold assembly -- leaving the tubing relatively unaffected.