ACME / Stub ACME pressurized limitations?

[blckwtr]

If you consider a pipe with internal pressure connected together with a stub acme thread, and the o-ring is not on the pressure side. Then you will have a burst effect on the female thread and maybe a small collapse effect on the male. So what I want to know is the reduction in capacity (thread shear capacity). Please see attached picture.

 

[israelkk]

You neglected the effect of the linear force and the thread slope angle that add to the radial forces and increase the tendency to enlarge the inner diameter or the female thread and reduce the outside diameter of the male thread. Basically, a design with the O-ring NOT in the pressure side is risky and should be avoided.

 

[blckwtr]

No, we allways take into account these effects, and calculate a "resultant" radial displacement. From there we can say how much the threads are engaged and estimate the strength from these "new" parameters. This is not science, only engineering assumptions. The reason for asking is that I haven't so far found a standardized Acme or Stub Acme calculation. The ones I have are from my own head, and combined with API RP 7G. But I don't have anything to check this against, or to verify that my assumptions are correct.

PS.: The reason for having the o-ring on the low-pressure side is to verify the o-ring entrance.

 

[hydtools]

My 20th edition of Machinery's Handbook has a calculation for the shear area per inch of thread engagement of 29--degree acme threads. Is that what you are looking for? The equation could be modified to account for the pressure dilation of the female thread.

Ted

 

[blckwtr]

thank's hydtools. I guess this could be an option.

We did actually "blow" a Stub Acme thread once during a pressuree test long before we could have dreamt it, and we found out during the investigation that since the acme threads are not self centralizing only one side had begun the separation process, and finally BOOM!!! So theory and practice doesn't always go together. We could clearly see that on the "upper half" of the female thread the threads were destroyed on the inner edge, while on the "lower half" of the female thread the threads were destroyed on the outer edge. So the box had tilted as result of the un-symmetric load scenario.