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NPT thread tightening

NPT thread tightening

NPT thread tightening


I have a question for experienced thread fitters. I have rarely used NPT threads (I am in a part of Europe where parallel fittings are used), but now I had to install some 1/2 NPT threaded male adapter to 1/2 NPT female valves.

However, after wrench tightening the adapter, I see some sort of thin metal scrap coming out of the thread. See attached picture, the thin metal scrap hanging.

Have you encountered this before? Could this be any problem or it should work fine?

The parts are made of 316 stainless steel, the adapter is made by Swagelok and the thread sealant used is Swagelok SWAK. The thread fitting was tightened about 2 turns past finger tight. It still wasn't totally locked, it can be tightened even more, but decided to stop and inquire first about that scrap.

Can you give me some advice?

Thank you for your help.

RE: NPT thread tightening

If all threads are within specification, clean, and undamaged, 1-1/2 to 2 turns past finger tight are sufficient.

The metal string may be from the female thread lead-in being sharp instead of chamfered. Should not be a problem.


RE: NPT thread tightening

I would agree with hydtools. If the debris is metal, it is likely from the female threads. Possibly machining swarf that was not cleaned out after the threads were cut. Or the un-broken tip edge of the lead-in thread that sheared off when the fitting was installed. 316 cres can be a bit gummy and thin, poorly supported edges (like thread tips) can tend to roll over from the cutter pressure rather than shear cleanly. Also it is always good practice to relieve/chamfer the first thread pitch of tapped holes.

Two turns after finger tight seems reasonable for a 1/2" NPT thread. However, since there is clearly a presence of metallic debris it would be good practice to disassemble the joint, clean the threads of both parts thoroughly, inspect the threads for any burrs or sharp edges, remove any burrs or sharp edges that are found, and then re-assemble the parts.

RE: NPT thread tightening

My first instinct is to think that the female thread is cylindrical and that there is an error in delivery. There is a chip like that when you screw the BSPT male in the female BSPP or NPT male while in the female BSPP. You should know that only the dimensions ½ and ¾ have the same step that BSP NPT. Easy to check: unscrew your connection. If only 2-3 male threads to large diameter are marked, is that the female part is cylindrical and not conical.

What you need to know:
1) pluparts of suppliers deliver the same tapered male fitting for ½ NPT or BSPT ½ and ¾ for.
2) When you have the choice to order taps always prefer NPTF rather than the NPT. Male and female parts by NPT are identical profiles (to within manufacturing tolerances) to as TPH. The advantage of NPTF is that the female profile is different from the male: when we might see what are first the tips of nets that will make a permanent deformation of the opposing party while rest of the game on the flanks. Continuing to tighten the torque increases sharply when the sides meet. Thus there is no risk of leaking into helical by the tips of the net.

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