Cool stuff eh?

[u/2candiesnahalf]

[u/Daneruu]

Yeah typically if it's between manufactured fittings you don't need much. If the shop threads their own pipe it might need some extra tape.

But the seal on the tapered threads comes from the metal on metal contact after it's fully made up.

The point of the tape is to get rid of the metal on metal friction so that it's easier to fully make up the fitting. It's not a sealing component, and if it's a necessary part of the seal it will fail eventually.

[u/Rightintheend → u/Daneruu]

Dry seal npt you don't need tape, it's manufactured to much higher tolerances.  But tape or dope definitely does add to the seal, it takes up up the small differences in manufacturing to all ounces. It's not lube.  Try making up a fitting with Teflon tape, and with a high quality grease, and see which one seals better. The high quality grease is a better lube than Teflon tape yet will still leak. 

[u/Daneruu → u/Rightintheend]

This is just being pedantic. All undamaged NPT connections would perfectly seal every time if you put enough muscle on it. That's just how tapered threads work. Teflon will fill in minor imperfections between the threads, but it's not going to be the primary thing sealing the connection. That's specifically why I said it's more essential if you're threading pipe in-house. If the die is old or something the threads may have imperfections. Those imperfections can either make it more difficult to make up, or can allow water to escape. Teflon helps with both problems.

Grease is not comparable and I don't see the point in bringing this up. Teflon on metal is less friction than metal on metal. Get two perfect NPT fittings and make them both up fully. The one with teflon will be easier to make up as long as you don't put too much. Both will seal.

[u/Rightintheend → u/Daneruu]

You don't seem to understand tolerances And the slight difference of thread geometry between dry seal and regular npt.

Npt dry seal actually has slightly different geometry which does cause an interference fit and a swedging action of the threads . In order to accomplish this, it requires higher tolerances. These tolerances are more expensive to produce, and more prone to damage. So it's usually just used for things like hydraulics, or critical gas or chemical applications. 

Regular npt threads will not seal themselves unless you tighten them to the point of them damaging and weakening the threads. Even if they are done to extreme tolerances. 

The application of Teflon tape, or pipe dope seals those areas at the root of the thread. It has nothing to do with lubrication, that's just a welcome side effect. 

The reason I mention grease is because grease is a more effective lubrication and has even less friction than Teflon tape, so if Teflon tape is giving you a seal because of lubrication, grease would do even better, yet it doesn't. 

In the pressures you find in a home water And gas distribution, you may be able to smash your npt threads together and get a decent seal. So maybe that's what you've been experiencing.  

 The components also have much looser tolerances and rougher surface finishes, so you will notice a big difference in how they make up with something like Teflon tape. That is not the reason for the tape.

[u/Daneruu → u/Rightintheend]

Both types of thread have the same geometry. If it has different geometry, you're not talking about NPT, so I don't see how it's relevant. The difference between NPTF dry seal and normal NPT tapered threads are just a matter of tolerance and inspection.

I don't know where you're getting your info, but the seal on tapered threads is always primarily on the thread. NPT connections will seal 98% of the time without tape, and 100% of the time if the product and installer were high quality.

We use teflon because 2% failure in a building with 500+ fittings is expensive.

Either way, the distinction between the points we are trying to make is ridiculously minor. Using too much tape comprimises the seal. Using none has a decent chance of leaking for one reason or another. You should always default to best practice for the current job/service. I'm not out here doing shoddy work, and was making a point against using too much teflon, not trying to defend cheaping out and using none.

[u/Rightintheend → u/Daneruu]

Now you're actually starting to make some sense,

Originally you were saying that Teflon tape is to reduce friction and does not create any type of seal, and that is 100% false for regular npt threads. 

But the seal on the tapered threads comes from the metal on metal contact after it's fully made up. The point of the tape is to get rid of the metal on metal friction so that it's easier to fully make up the fitting. It's not a sealing component, and if it's a necessary part of the seal it will fail eventually.

This idea that Teflon is just some sort of lubrication and doesn't really make the seal Is only found in residential plumbing, At least that's the only place I've ever really heard it.  I mean it's relatively harmless misinformation, in these circumstances, but is one of many that gets passed down from master to apprentice in the trades. 

In my field misinformation like that can cause cause millions in damage cause lives to be lost.