Sealing Pipe Threads

[Willie Makeit]

I also don't give a rip as to your opinion either. My "inside knowledge" is based design/engineering and practical applications in the nuclear, petro, aerospace and chemical-processing fields.

NPT thread forms are designed as a mechanical seal when installed to the proper torque/rotations and Teflon is not a sealant, it is a lubricant to allow for better thread engagement using less torque.

I stand by my original statement.

 

[MushCreek]

If teflon tape isn't a sealant, why do they make extra-thick tape? We used to use it at work for 'problem' threads.

 

[cannuck]

I also don't give a rip as to your opinion either. My "inside knowledge" is based design/engineering and practical applications in the nuclear, petro, aerospace and chemical-processing fields.

NPT thread forms are designed as a mechanical seal when installed to the proper torque/rotations and NPT is not a sealant, it is a lubricant to allow for better thread engagement using less torque.

I stand by my original statement.

You are entitled to your opinion, but it in no way affects the facts.

What you believe about being designed for a mechanical seal IS true of NPTF thread forms, but is NOT true of NPT. NPT thread crests do NOT reach the bottom of the female thread and fill the spiral groove. Again: you don't need to take my word for it, READ THE SPECIFICATIONS for the thread form.

BTW: I had to go back and do that because I had remembered the tapers M to F being slightly different, but they are not. Amazing how memory can fade after a half century of doing something.

 

[Willie Makeit]

ok ladies, argue amongst yourselves. I 'm fully aware of the differences in engineering "facts" versus a bunch of jack-leg plumbers on some irrelevant hand-tool forum.

Carry on ... lol

p.s. one should study the term "pitch diameter" and then study where NPT threads "make their seal" ... hint: it isn't in the crest/root contact.

 

[rlitman]

...p.s. one should study the term "pitch diameter" and then study where NPT threads "make their seal" ... hint: it isn't in the crest/root contact.

That's because NPT threads do not seal. And unless the contact is all the way from crest to root (or put another way, the male crest must crust against the female root), a helical (often incorrectly called spiral) leak path exists. Wink all you want.

 

[cannuck]

I would sure as heck not want to refer to this as any kind of "irrelevant" forum. There is a massive amount of skill and experience here that many members seem prepared to put up and stand behind. I was delighted to find it and try to be informative and respectful when presenting my personal opinions and knowledge - that often differs greatly from the status quo. BUT: as to the "jack-leg plumber" comment: if the individual really wants to put it on the table and measure up - I am the guy who designs what I build and sell and the one who strokes the cheque when I am wrong. I have to answer to several hundred engineers and engineering technologists as to why I write and teach the standards and procedures that they use on equipment I have designed or devices we must build or repair (BTW where a fluid leak can and does cost in the millions to repair). I will be the very first to admit what I do not know, but be damned if I will dig my heels in without knowing what I DO.

 

[SARG]

Rule #2 applies : Your opinion only matters to you. No one else cares what you think.

 

[speed bump]

I love how these threads always go exactly how this one has.

Someone asks
Someone says tape (probably specifies blue monster)
Someone says dope
Someone declares both is the only way to go
Someone says do this many wraps, brush strokes, hand stands, and 48" wrench turns I've never had a leak in some statistically improbable amount of time
Several idiots want to argue semantics about pipe thread forms.
There are several opinions about why it is called dope.
Original poster becomes confused and installs PVC air lines instead because it doesn't involve threaded couplings that he is now convinced are Satan's work.

 

[American Locomotive]

NPT threads as defined by ANSI B1.20.1 are not designed to seal solely based on their thread form. If you want a guaranteed leak-free seal, you HAVE to use some kind of sealant - be it teflon, dope, or otherwise. The specs and tolerances are too relaxed with NPT. You can get fittings you can tighten with a 6 foot pipe wrench, all the way until there's no thread left and it will still leak. You can see this more easily on smaller NPT fittings. I've screwed countless NPT air fittings into tools without any tape. I get out my big pliers and bottom that sucker out in the tool, and it will still leak. I pull the fitting out, put a few wraps of tape, and then it will seal tight just a few turns into the tool. These threads MAY or MAY NOT seal without sealant.

NPTF or "Dryseal" threads, are NPT threads that ARE designed to seal mechanically as defined by ANSI B1.20.3. These threads have more carefully designed roots/crest height, and those specifications are much more tightly controlled. These threads WILL seal without any sealant.

https://global.ihs.com/doc_detail.cfm?document_name=ANSI%20B2%2E2&item_s_key=00007281

If NPT were designed to seal mechanically with the threads alone, NPTF wouldn't need to exist.

I personally just use tape for junk at home, but when I worked in industry we used to use Loctite 567 exclusively without tape. Always seemed to work really well.

I don't give a rip what a webpage from a company thats trying to sell me a product says. I have worked in the NPT fitting business longer than that webpage has been around. Teflon tape is NOT a sealant. If you disagree, please show me one application of TEFLON outside of pipe tape, where its sold for anything other than lubricity applications.

Teflon is widely used in the aerospace and petrochemical industries as sealing rings. It's durable, withstands a wide temperature range and its chemically resistant to basically everything.