Teflon tape is not good pipe thread sealant

[Psi]

I've never had any problems getting a pipe to seal with Teflon tape.

I can imagine problems if someone is always using a fixed number of turns of Teflon and expecting it to work.
It needs to be adjusted to take up the gap which can be a bit variable between fittings/brands. Especially a join with two different branded ends. Sometimes you need like 20+ turns. It's pretty obvious when you screw it together if it's not tight enough and you need to remove and add more tape. And it's a bit of a learned skill to screw it on with no tape and know by the feel of how loose it is how much tape you need to add.

Hemp has the advantage that it swells up so it's more forgiving of random gap sizes than with Teflon.

[Monkeh]

TL;DR you bought the wrong fitting and blame the tape. 3/4" BSPT male x 1/4" BSPP female are quite available. You could also just get an unequal tee..

[MisterHeadache]

My two cents from a few decades of DIY plumbing repairs:

Pipe thread connections: always use Teflon tape (wound in the correct direction) or pipe dope. A connection without either is risking a future leak, especially the evil 'drop an hour' leak.  And especially on old work to new work connections.

Compression fittings (copper ferrule onto copper pipe): do not use Teflon tape or pipe dope on new connections.  On old connections where you are not removing the existing ferrule, I use a small smear of pipe dope around the O.D. of the ferrule.  I know this is controversial, there are folks who are adamant that ferrules must be replaced every time, but IMHO that is pearl clutching, in particular for replacing shut-off valves under sinks etc. If the nut fits the new valve and the ferrule isn't chewed up, I leave it on.  Sometimes I have had to cut 3-4mm off the end of the copper pipe because the new valve was slightly shallower than the old valve. There's nothing wrong with replacing the ferrule, but to do it right you need to cut the pipe behind the ferrule so you have virgin copper pipe for the I.D. of the ferrule to deform into. That usually means cutting the pipe 20-30mm shorter.  Obviously this moves the new valve closer to the wall, and in some instances you may not have enough copper pipe stub remaining to do this.   

[Monkeh]

Compression fittings (copper ferrule onto copper pipe): do not use Teflon tape or pipe dope on new connections.

A lubricant is highly beneficial in making proper compression connections. PTFE and 'dope' are both lubricants. Doing it dry risks needing to overtighten and necking the pipe.

to do it right you need to cut the pipe behind the ferrule so you have virgin copper pipe for the I.D. of the ferrule to deform into.

If the joint is made off properly instead of being overtightened, the olive is easily removed and replaced without needing to alter the pipe.

[tszaboo]

I've never had any problems getting a pipe to seal with Teflon tape.

I can imagine problems if someone is always using a fixed number of turns of Teflon and expecting it to work.
It needs to be adjusted to take up the gap which can be a bit variable between fittings/brands. Especially a join with two different branded ends. Sometimes you need like 20+ turns. It's pretty obvious when you screw it together if it's not tight enough and you need to remove and add more tape. And it's a bit of a learned skill to screw it on with no tape and know by the feel of how loose it is how much tape you need to add.

Hemp has the advantage that it swells up so it's more forgiving of random gap sizes than with Teflon.

We might be using different threads, but IMHO when you need more than 3 turns, it means that the thread is borked and should be replaced.
And the tape has to be placed the right direction, opposing the connection.
Otherwise it seems to work with pressure or water, so IDK... I don't like working on water in my house as I don't want to flood it in case I mess up something, so I usually call the professional. But then they've lately acted like the world's biggest idiots, so maybe I have to stop doing that.

[Siwastaja]

Good old hemp is just excellent, with track record of something like, I don't know, 200 years or what? The process margin for water pipes is just so wide.

I redid much of our old pipework (because of changes in system design I wanted to apply) using good old iron pipe + hemp. As a total beginner who never did any of that, I read different conflicting instructions online and did every joint in some different way, applying different amount of hemp, different amount of jointing compound (Unipak) before or after the hemp, winding the hemp in different directions, tightening "until the end" or "just a few turns"... Did the whole system, 50 or more joints, and finally filled with water and pressure tested - exactly 0 dripping joints. Not a single drop of water on any of them.

Never had similar beginner success rate on any other pipe joining system, really. I have had good results with Teflon tape too, but the amount is critical really (I guess usual mistake is to use too little, but you can also use too much of it). It doesn't stay in place as easily but starts to rotate when tightening. If loosening the joint, you then need to loosen it all the way, replace the teflon tape and try again. While with hemp, loosening and retightening even to finetune the pipe length is all OK.

So in closed HVAC circuits with no worries about slow oxidation, I still prefer black iron pipe, iron fittings, hemp and Unipak jointing compound because I always succeed with that. Lowest cost parts (pipe and fittings) is an added benefit. It is of course more work to make threads to the pipe ends, which is the primary reason this fell out of fashion.

Teflon tape isn't bad, just more difficult to get right.

[MisterHeadache]

Compression fittings (copper ferrule onto copper pipe): do not use Teflon tape or pipe dope on new connections.

A lubricant is highly beneficial in making proper compression connections. PTFE and 'dope' are both lubricants. Doing it dry risks needing to overtighten and necking the pipe.

to do it right you need to cut the pipe behind the ferrule so you have virgin copper pipe for the I.D. of the ferrule to deform into.

If the joint is made off properly instead of being overtightened, the olive is easily removed and replaced without needing to alter the pipe.

For clarity, I am talking only about copper pipes, copper ferrules/olives, and brass valves/nuts.  A bit of dope or lubricant won't hurt, yes, but it isn't necessary to make the seal.  Definitely do not put teflon tape wrapped around the OD of the ferrule.  I see on that on YouTube and it is WRONG. The connection requires metal on metal contact to seal.  I embellished an image below - the yellow lines show where the sealing happens, that must be metal on metal.  Compression fittings were invented decades before there was even such a thing as teflon tape.

Also, if you can easily slide the ferrule/olive off of the pipe when removing a valve, that means the nut wasn't tighened enough.  The ferrule needs to squish into the pipe slightly, but not too much.  If you undertighten a compression fitting, there is a chance that the valve will blow off the pipe when you turn the water pressure back on. But it's also likely to be leaking immediately before the blow-off, too.

Haven't seen it mentioned on this thread, but some folks believe the sealing happens at the pipe to valve interface, the purple circles.  That's not true.
 

[soldar]

Good old hemp is just excellent, with track record of something like, I don't know, 200 years or what? The process margin for water pipes is just so wide.

I redid much of our old pipework (because of changes in system design I wanted to apply) using good old iron pipe + hemp. As a total beginner who never did any of that, I read different conflicting instructions online and did every joint in some different way, applying different amount of hemp, different amount of jointing compound (Unipak) before or after the hemp, winding the hemp in different directions, tightening "until the end" or "just a few turns"... Did the whole system, 50 or more joints, and finally filled with water and pressure tested - exactly 0 dripping joints. Not a single drop of water on any of them.

Never had similar beginner success rate on any other pipe joining system, really. I have had good results with Teflon tape too, but the amount is critical really (I guess usual mistake is to use too little, but you can also use too much of it). It doesn't stay in place as easily but starts to rotate when tightening. If loosening the joint, you then need to loosen it all the way, replace the teflon tape and try again. While with hemp, loosening and retightening even to finetune the pipe length is all OK.

So in closed HVAC circuits with no worries about slow oxidation, I still prefer black iron pipe, iron fittings, hemp and Unipak jointing compound because I always succeed with that. Lowest cost parts (pipe and fittings) is an added benefit. It is of course more work to make threads to the pipe ends, which is the primary reason this fell out of fashion.

Teflon tape isn't bad, just more difficult to get right.

I think what we call "iron" pipe is actually steel.

Regarding the Teflon or hemp rotating the trick I learned from pros was to score (dent, cut) the male threads so the sealing material would not slide. Now they sell all fittings already scored so no need to do it.

What I find amazing is to what level the material will compress. Put a thick layer, tighten the thread, take the thread out and it's like the material has almost disappeared.

[nctnico]

For really soft pipes, you might not get a good seal using the ferrules because the ferrule compresses too much before it makes a good seal. In such cases it helps to put a winding of teflon tape over the ferrule before putting the pipe into the fittings. I got this tip from a plumber.

[Electrodynamic]

Soldar

I have been doing plumbing repairs for many decades now. Long before Teflon tape existed I, we, used hemp and I would put a few drops of paint on it and those threads sealed reliably every single time.

Now we have this newfangled Teflon tape which is supposed to be better but it just does not work well for me. It does not seal as well and even if I use a lot of tape I very often get a micro drip. Not much, a drop every hour or less but still unacceptable.

Cool, I was doing commercial/industrial HVAC and oil field pipe fitting on the side for about 35 years.

My goto solution is teflon and copper pipe dope for HVAC iron pipe. Some contractors were using hemp and some kind of grey dope which was a real nightmare. The pipe dope got hard as a rock and glued the connection together. It was a real gong show and we were snapping nipples off, egging connections and having to tap and thread equipment.

In one case, we had to remove a 6" bushing from a 6 MBtu boiler which required a 48" pipe wrench with a 10' snipe with two people standing on it. This was after we heated it up with a torch. We were not happy about the dope the contractors were using.

So in closed HVAC circuits with no worries about slow oxidation, I still prefer black iron pipe, iron fittings, hemp and Unipak jointing compound because I always succeed with that. Lowest cost parts (pipe and fittings) is an added benefit. It is of course more work to make threads to the pipe ends, which is the primary reason this fell out of fashion.

To be honest with HVAC stuff I had no problems with leaking using teflon/copper dope on iron pipe. One time I wanted to test my theory so I hand tightened a 1" nipple into a boiler running 15 psi steam and another 50 psi hot water which never leaked. What does tend to leak is when a pipe is threaded on site with the wrong thread taper. Where the threaded pipe end is too small to contact any threads and only a 1/4" of the threads are in contact with the fitting. Easy to spot, it goes in hard and only half way.

[helius]

I'm not as experienced as some correspondents here, but I have had good luck with using the yellow gas-rated teflon tape on water connections: it works just the same but it is made thicker; also a pipe dope called Gasoila Soft-Set, which does not harden. To reassemble old connections I also cleaned and lapped the mating surfaces, and wasn't afraid to use antiseize on the threads, like on unions.
I mention these products simply because they worked well for me, you can do with this info what you will.

[Analog Kid]

I tried teflon tape before, but had the same microdrips you are describing. Switched to Loctite 55, had no problems ever since.

(Attachment Link)

That's just Teflon tape in a different size.

[Monkeh]

Compression fittings (copper ferrule onto copper pipe): do not use Teflon tape or pipe dope on new connections.

A lubricant is highly beneficial in making proper compression connections. PTFE and 'dope' are both lubricants. Doing it dry risks needing to overtighten and necking the pipe.

to do it right you need to cut the pipe behind the ferrule so you have virgin copper pipe for the I.D. of the ferrule to deform into.

If the joint is made off properly instead of being overtightened, the olive is easily removed and replaced without needing to alter the pipe.

For clarity, I am talking only about copper pipes, copper ferrules/olives, and brass valves/nuts.  A bit of dope or lubricant won't hurt, yes, but it isn't necessary to make the seal.  Definitely do not put teflon tape wrapped around the OD of the ferrule.  I see on that on YouTube and it is WRONG. The connection requires metal on metal contact to seal.  I embellished an image below - the yellow lines show where the sealing happens, that must be metal on metal.  Compression fittings were invented decades before there was even such a thing as teflon tape.

Also, if you can easily slide the ferrule/olive off of the pipe when removing a valve, that means the nut wasn't tighened enough.  The ferrule needs to squish into the pipe slightly, but not too much.  If you undertighten a compression fitting, there is a chance that the valve will blow off the pipe when you turn the water pressure back on. But it's also likely to be leaking immediately before the blow-off, too.

Yes, I'm quite aware of how they seal. Teflon against the faces of the olive will lubricate and prevent it from galling against the nut, allowing it to be properly compressed. Any other form of lubricant does the same job - 'pipe dope' (sealant compound), silicone grease, etc. It will be entirely excluded as the joint is tightened, not cause a leak. It's beneficial on the threads, too, as it reduces friction and thus applied torque.

Easily removable does not mean it just falls off - a tool is required. This fails when, due to lack of lubrication when tightening, the joint is overtightened until it actually changes the OD of the pipe and requires cutting it back to rejoint successfully.

[Psi]

Sometimes you need like 20+ turns.

We might be using different threads, but IMHO when you need more than 3 turns, it means that the thread is borked and should be replaced.

For new installs yeah, but there's always a mix of job types where thread tape is needed.
Sometimes you're working on a tap or pipe that has been outside for 30 years and it's still working fine just, so no need to replace.
But you need to remove it, do some work, then put it back on with new tape. Sometimes the thread gaps are just larger than they should be.
I do agree 20 turns is excessive, but sometimes people get into the habit of always doing ~5 turns and then wonder why it doesnt always work and blame the Teflon.

[jheissjr]

Do countries using parallel thread use teflon on the threads? I never heard that before.

[Perrold]

The issues with tapered threads usually come down to PTFE tape quality, user error, and thread quality, in that order. With good ~0.003" thick PTFE tape the first issue is resolved entirely. The really thin stuff that sometimes floats around in a toolbox is worthless. For some reason tapered threads are a constant source of user error, even with people who should know better. Some lessons never sink in. The endless myths and outright bad advice that gets passed down as gospel does not help. Thread quality matters and the quality of some hardware store fittings is just not suitable. With good tape, proper skill, and quality fittings a connection with tapered threads will hold high pressure gas and be entirely leak free every time. On a big project with countless NPT connections holding in ~5000psi gas I recall only two leaks and the issue came down to thread quality. Thread lubrication is important and should not be done with PTFE tape or pipe dope. Of course the best way to guarantee an NPT connection will not leak is to not use NPT in the first place. It's a connection choice of last resort.

[Analog Kid]

I think one reason I had such good luck with Teflon tape on pipe threads is that when I was working (in a past life) as a handyman, I generally had my pipe cut and threaded to order for a particular job. So I was working with very cleanly cut threads (lubricated even), not whatever was on the pre-cut pipe lengths in the bin at the home-improvement store.

[tautech]

The issues with tapered threads usually come down to PTFE tape quality, user error, and thread quality, in that order. With good ~0.003" thick PTFE tape the first issue is resolved entirely. The really thin stuff that sometimes floats around in a toolbox is worthless. For some reason tapered threads are a constant source of user error, even with people who should know better. Some lessons never sink in. The endless myths and outright bad advice that gets passed down as gospel does not help. Thread quality matters and the quality of some hardware store fittings is just not suitable. With good tape, proper skill, and quality fittings a connection with tapered threads will hold high pressure gas and be entirely leak free every time. On a big project with countless NPT connections holding in ~5000psi gas I recall only two leaks and the issue came down to thread quality. Thread lubrication is important and should not be done with PTFE tape or pipe dope. Of course the best way to guarantee an NPT connection will not leak is to not use NPT in the first place. It's a connection choice of last resort.

Tape, hemp and the like must be used with tapered fittings.
Testing the taper thread fit quickly is very important as the thread taper should tighten before fittings are fully engaged.
There are so many types of fittings, gas, water and hydraulic to name a few and they all have some common types of threads, some of which are not tapered and rely on crush washers (normally copper) or Dowdy type washers most commonly found in high pressure machinery hydraulics.

Decades ago a old plumber friend taught me how to wrangle hemp and yes, there are right and wrong ways to apply it. After spinning and wrapping it some would apply a little spit to keep it in place but he prefered graphite grease which after decades will never lock up or corrode.
Decades later for any BSPT I have taken to using hemp and anti seize grease as it's always handy.
Whenever encountering rough threads hemp is the preferred solution and I still prefer it over any seal or O ring for croxed copper fittings.
Another little trick the old plumber mate had and shared it after watching me battle with hemp slipping on a smooth fitting when trying to wind it was to gently wipe the hacksaw sideways on the thread just enough to give the hemp something to grip on.

There's Teflon tapes and Teflon tapes, and all the thin ones are garbage but you can get away with the junk on modern ag good fit fittings such as Hansen here in NZ.
https://www.hansenproducts.com/products/pipe-fittings

The thicker Ceelon Teflon tape has always been my #1 choice.
YMMV


 

[Analog Kid]

If your roll of Teflon tape rolling around in your toolbox is that thin stuff, then just use more of it, right?
I mean, it almost becomes molten anyway when squished between the threads.

[tautech]

If your roll of Teflon tape rolling around in your toolbox is that thin stuff, then just use more of it, right?
I mean, it almost becomes molten anyway when squished between the threads.

Molten, no.
It's a gap filler like hemp is but I prefer to use Teflon on hydraulic fittings to 2500 psi and a crush washer parallel thread type fitting for anything higher pressure.
A tapered joint applies a lot of pressure to a fitting whereas a parallel thread and sealing washer much less which is why they are used on high pressure applications as the burst/fracture pressure on the fitting is less by not using a tapered fitting and its wedging of the fitting apart action.

[soldar]

This thread has taken off in all different directions on other types of sealing pipes which have nothing to do with my point.

Parallel threads are not designed or meant to provide any sealing per se. I am not talking about parallel threads.

Plumbing fittings and pipes use tapered threads for sealing and that is what I am talking about. Those tapered threads need some kind of dope or sealant for the final sealing. That is what I am talking about and nothing else. Tapered threads.

Tapered threads need to engage 4 or 5 threads at a bare minimum, 6 or 7 is better and 8 I would say is great. This allows the two sides to be pulled together and the conical shapes to engage well. A well formed thread engaged with enough threads is extremely easy to seal and pretty much anything will work.

In my real world though I run into fittings that don't have so many threads, (short length of thread so they can be made more compact and cheaper) and very loose tolerances. These two things mean much sealing compound is needed.

In this situation, which is the only one I am referring to, my experience is that thicker materials like hemp seal better than Teflon tape. Hemp has more "body" to it and fills more space and holds together better. Plus it swells when it gets wet.

It also may be that with such low numbers of threads engaged it may be easier for Teflon to be "spit out" the ends whereas hemp holds together better.

In my real world experience of short, loose threads, hemp provides better sealing than Teflon tape.

I have not tried special sealing compounds made specifically for sealing pipes but I would be confident they work well.

[tautech]

Plumbing and LP hydraulic fittings have one thing in common, BSPT taper threads where fittings from the same supplier with the same taper tolerances will easily be sealed with Teflon tape.

Mix suppliers up and you will have issues that might be corrected with a healthy application of Teflon tape and personally I have never had an issue with it to 2500 psi providing the thread engagement is sufficient and not excessive.
Properly applied even reuse without reapplication is without leakage.

You cannot rubbish quality Teflon sealing tapes when properly applied.

[Ground_Loop]

I gave up the tape decades ago. Only use pipe dope on NPT fittings.

[TimFox]

Plumbing and LP hydraulic fittings have one thing in common, BSPT taper threads where fittings from the same supplier with the same taper tolerances will easily be sealed with Teflon tape.

Mix suppliers up and you will have issues that might be corrected with a healthy application of Teflon tape and personally I have never had an issue with it to 2500 psi providing the thread engagement is sufficient and not excessive.
Properly applied even reuse without reapplication is without leakage.

You cannot rubbish quality Teflon sealing tapes when properly applied.

I did run into a problem once (before retirement) with cooling-water plumbing from Japan (with NPT threads) where the Japanese and US fittings to be mated seemed to be at opposite ends of the tolerance range, and Teflon tape sealing was iffy.
Otherwise, at university we used US brass NPT fittings for "roughing vacuum", down to maybe 10^-2 Torr without difficulty, sealing carefully with Teflon tape.