Pipe Thread Clamp Calculations

[igidy]

I am working on a project for work that involves quickly plumbing up to pipe fixtures for pressure testing. The way we are doing this is by taking a female pipe fitting a sawing it in two. We have then fixed the non-threaded side of each half of the coupler to an air cylinder via a threaded rod (view attachment). The idea is to use the threaded pieces as a clamp to grab onto pipe threads quickly and easily. I have tested this set up and it works well. It finds the thread each time and provides good clamping.

The question I have is about thread calculations on this set up. Being that the pieces aren't actually threaded into each other and that these are pipe threads (which I have no experience with doing thread calculations for) and not straight threads. We are trying to increase our test pressures but before we do I need to figure out what we can consider our limiting pressure. Any input would be greatly appreciated!!! Thanks!

 

[hydtools]

If you know how to do stress calculations for straight threads, then you know how to do stress calculations for the pipe threads.

Ted

 

[MintJulep]

Or just use these:

http://www.flo-products.com/floprod/quikcntp4.html#FST

 

[redpicker]

I don't see how you would get that to seal for a pressure test. Maybe I'm missing something.

rp

 

[racookpe1978]

Check your welds on the cut-off ends of the female "pipe" connector to the system you are testing - apparently with high pressure air?

Losing that weld will be very dangerous - and bad welds (too little, porosity, incomplete, bad fillets, bad penetration, etc) are much more likely with this kind of cobbled up "temporay system for testing" than losing the pipe threaded end with thread failure. Are you checking with NDE (dye-penetrant or mag particle) the weld?

The pipe end threads are at least built to standard spec's to carry pressure..

 

[igidy]

What welds are you referring to? The rods will be threaded into the female pipe pieces.

Redpicker, I was also concerned that the "missing portion" of the pipe threads would create a leak path.

And yes, air pressure at 3700 PSI max. And we were thinking more in terms of a permanent system for testing.

 

[igidy]

regarding the sealing of the threads... there will actually be a piece that comes up into the male pipe piece to create a seal on the internal portion.

 

[Cockroach]

You could probably follow the paper from W.O. Clinedinst entitled Strength of Threaded Joints for Steel Pipes as presented to the Shell Conference, 26 October 1964. This gives a fairly decent discussion on the mathematics behind pipeline threads.

Hard to get an answer to you not knowing the dimensions of your piece. But reverse engineering this given the assumption that the thread is a pipeline female and that your picture is in proportion in the face plane looking down, I estimate you have something like a 3.0 NPT Female Box cut into a 5 1/4 OD steel plate, 1 5/8 inches thick (i.e. run-out of thread in engagement), flats are cut 4 1/4 inches, from which two 5/8 OD X 3 inch handles are welded, then saw cut 1/8 thru.

Noting your thread shear under these given circumstances, your thread would behave roughly similar to a 2 3/8 EUE oilfield tubing tool joint, 2055 ft lbf of torque input, about 97,817 lbf tensile yield for P110 material. This would be a pure guess since you also neglect to mention material specifications. I would therefore expect this to be 1/3 values if you are using mild steel, say 25 kips maximum sustaining load.

You inquire on "pressure" capable of sealing. With a split half dog nut and not understanding the nature of make-up with the interface of adjacent surfaces, impossible to determine.

Again, you hand waived a picture without proper documentation and hope to get an exact answer. I've provided you with an estimate based on that hand waive, expect large errors. But at least you have the thread source for initial study, the Clinedinst paper. That I would be absolutely sure of.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada