Do you know about NPT threads?

e9999

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Dealing with an issue with a new water heater I've just gotten.
Question: any idea how many turns on a 3/4" NPT thread would be required to resist pushout under 150 psi of water pressure? Or more specifically, do you think that 3 turns would be enough?
TIA
 

e9999

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thanks. Interesting read. But I'm not concerned about leaking, I'm wondering about structural strength and what should be considered a minimum thread engagement to prevent pushout. I'm zeroing a bit on this, including with the Machinery Handbook but it's a bit unclear still. My impression in the past has been that 4 to 6 turns to wrench tight is "normal", so I'm wondering if 3 is too little for the max pressure to be expected in the water system.

I'm hoping I don't end up having to read the actual NPT standard cuz I'm sure that's gotta be painful... :) I may just go do pullups hanging from a pipe fitting and see what happens instead.
 

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What do you mean by "3 turns"?

If by 3 turns you mean just 3 threads engagement then there's something wrong, your NPT's are made wrong if they are tight at that point.

If you mean 3 turns past finger tight then that's a lot more than 3 threads engagement and that sounds good.
 

e9999

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I just get only 3 1/2 turns total with a wrench before I feel like I can't go further reasonably. Probably only 2 turns of that is finger tight. That's with Teflon too. I'm questioning whether either the male or female thread was not cut enough and whether that is an issue for my situation.
 

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A name brand npt tap is a good thread gauge. It should go in about 2/3 depth if the nptf threads are good.
 

e9999

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Yes, may do that. This is the TP valve engagement I'm questioning. I wonder if the threads may have been coated when they did the glass lining. Or just cut incorrectly. If so, I'd rather work on the valve thread first. I don't think I can use a regular pipe threading die as there may not be space, but maybe a thread chaser may help a bit.

added: You had me curious. So I went and checked a few 3/4 fittings I have, both brass and steel, with an older US-made tap (no brand logo visible). The tap did go in anywhere between 1/2 to 2/3 of its length just hand-tight in all cases. I will check with the tap -very lightly- next time I have the TP valve out, that will indeed be a good first check, and a good argument to argue with the manufacturer with if it comes to that, thanks for that suggestion! Interestingly, some of the brass ells did only have 5 or 6 threads total.
 
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e9999

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well, dang. I did some trials. Took some brass nipples and fittings and went to town on the threads with a thread cutter (long story). Something odd happened. After a fair bit of cutting new threads, the male thread barely changed in number of threads engaged. A bit more but less than a turn more. However, when I used the tap to dig a bit in a female thread, probably fewer turns than with the die, the effect was much more pronounced, the male nipple got 4 turns more engagement after that. I can't quite see why that would be. I must be visualizing the geometry wrong. Unfortunately, that is the opposite of what I was hoping for, cuz I'd rather die the valve than tap the tank. Odd.
 
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The tap is making the hole 'bigger' so your male part can go deeper.
Try to stay on topic here.
By using the die you are just adding more full threads at the top of the taper thats are above the engagement.
whereas the tap is carving away metal at the opening so that more of the taper will go in before the threads take hold.
Bob
 

e9999

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the issue may be how long the die and tap are, compared to the thread. Unfortunately, IIRC, the die was quite a bit shorter, and I did keep going for a bit with it, so I may have lost some of my taper doing that.

added: I measured, and yup, that was it, the die cutters were too short so I could not gain much taper at all. And I ended with a cylindrical section towards the end of the pipe and that was likely interfering. HF... sigh... And to think that I was ecstatic because the cheap HF cutters were not indexed so I could use them backwards, which solved the clearance problem. Unfortunately, that's the only threader set I could get my hands on on short notice. TBH, though, I'm not at all sure that Ridgid cutters are much longer. And those are indexed so I could not use them backwards for the valve anyway.
So, likely back to having to tap into my water heater tank. Not sure I want to do that myself, if I break the tap in there, there goes the warranty... Bummer!
 
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