Pull Strength of 2-inch NPT Threads

[zdas04]

I'm designing a gas-knockout that has a 2-inch NPT coupling right where I would put a lifting eye. The knockout weighs about 1,000 lbm and I was thinking about fabricating a lifting eye that would thread into the NPT threads. I've been searching for data on the pull strength of cold-rolled NPT threads all morning without much success.

Anyone have this information? I know that I've lifted more weight than that with piping that was screwed tight into NPT threads, but I don't want to have to screw the lifting eye in till it is pressure tight.

David

 

[JJPellin]

I would not recommend using an NPT connection for any lift. We used to allow this on pump heads that had a cooling water outlet connection at top dead-center. We had at least one incident where the threads failed and the part was dropped. If you can't place a lifting eye at top dead-center, I would suggest that you place two eyes spread equally on either side of the pipe connection and rig with a two leg bridle. If you have to spread the two eyes too far apart for a standard shoulder eye, then an appropriate swivel eye (side-pull or center-pull) could be purchased to allow for the side load without exceeding the rating or capability of the rigging.

If you do manage to find a value for pull-strength, it could be based on perfect conditions. But, since you cannot properly torque that fitting and since the condition of the threads may deteriorate over time, you could be creating a hazard for future lifts.

I was not aware that there was a machine to cold-roll a female NPT thread. All of the ones I have seen have been cut threads. If you are referring to the male threads on the lifting eye, I assume that this would have to be straight threads. Are you proposing using straight threads on the male into tapered threads on the female? If so, I am sure that there are other experts out there who could speak to that point.


Johnny Pellin

 

[zdas04]

No, I'm looking at the female threads being a fabricated pipe coupling and I thought those were cold-rolled, maybe I was wrong but they definately are better than we get with a field tap and die set. The male threads would be a purchased pipe nipple with a lifting eye fabricated into the other end. The intention of this gas knock-out is to go in the ground and stay there for decades, I was just trying to come up with an easy way to get it in the ground plumb. I can lift it like a piece of pipe, but getting it plumb for install is a bit of a hassle.

Thanks for your input.

David

 

[jte]

I guess one way to look at it is to take a look at what the MAWP of a pipe with a coupling would be. Using S=20 ksi and a remaining wall after threading of 0.10" you'd get roughly 2000 psi MAWP. This in turn trnaslates to about 6000 lbf end thrust. So... the threads should be good for a 6 kip load. Plenty of margin if the actual load is 1 kip.

jt

 

[unclesyd]

This is just a little anecdotal evidence but may give you another shot at the windmill.
During WWII the lifting plugs for bombs were made from Malleable Iron. From my recollection the early ones were NPT threads while the later ones were straight pipe. These plugs plugs were screwed into both CI and CS in up to 1000 pound bombs. The testing of same used a tool steel die as a hold down and as I recall the safety factor was better than 2::1 and the actual test were at least 3::1.
I tend to recall the AAR also has a pull out requirement on their Malleable iron fittings.

To the more modern stuff. Flat face pipe flanges are use for various construction items, like railings and rack tie downs.

 

[zdas04]

jte,
That is an interesting way of looking at it. I started off thinking that this was a trivial question that I just wasn't getting my head around. Sounds like there are multiple ways to go at it. Thanks.

David

 

[unclesyd]

Working off jte's post couldn't you just work off the hydraulic end force as seen by a pipe plug at the MAWP for the material and fitting size.

 

[zdas04]

That is what I'm thinking. Those MAWP calculations have a LOT of safety factor built in so that calc should work. Last year I assumed that the fabricator would look at the size of the flanges on a drawing and know that they were ANSI 300 (I actually thought I noted flange rating on the drawing, but it got overlooked), the thing was built with ANSI 150 flanges and tested to 900 psig (which I did note on the drawing). The flanges held and the studs didn't yeild. I'm betting that the thread calcs are at least as over designed.

David

 

[unclesyd]

Pipe plugs have no class. The same plug can be used within reason, up to and including Cass 6000 forged steel fittings. So with the proper installed coupling and a round head plug with a proper eye bolt you should be OK.

 

[jte]

Pipe plugs have no class.

Agreed. That's why I prefer end caps. They're much classier!

jt