Neophyte plumbing question here – hope this is the right place:
In a threaded plumbing connection, I have seen instructions to put teflon tape on the male threads, and pipe dope on the inside of the female threads, but I can’t tell whether these are either-or or if you use both together?
Kevin
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Replies
Kevin,
Pipe dope is usually only put onto the male threads. The rationale for this is that you don't want a plug of dope pushed into the fitting where it can break loose and plug up some valve downstream. Any dope applied to the female side needs to be done sparingly, since any excess is pushed into the pipe.
Tape or dope alone will work fine on good machined threads. Problem threads sometimes require both together in order to seal. Forged copper male adaptors for copper tubing are often problem threads, in my experience.
I'll bet you get a few replies to this one.
Bill
I have used both methods, tape alone and both. It is dependent on the quality of the threads. Many times you can see problem threads, they generally aren't as sharp, although this is not always the case. We never dope a female thread because of the potential problem down the line. Believe it or not there is a difference in tapes too. Some are just poor quality, some are differnent colors, (I have used yellow for gas piping). I say use tape and if you use dope with it use it sparingly.
Is that a special Teflon tape? I have always been told that you can't use Teflon tape on gas fittings.
wyatt
They have yellow tape that you can use on gas lines.
The yellow tape is made for gas piping and it's more expensive, too. I can't swear to it, but I don't think that white teflon tape is allowed in gas lines.
69265.4 in reply to 69265.3
Is that a special Teflon tape? I have always been told that you can't use Teflon tape on gas fittings."
Yellow tape is the same composition as white, it just indicates the thickness (it's not super thin like the stuff from china). Years ago they all where white. Just like romex was, but is now color coded by gauge.
WSJ
Edited 2/5/2006 10:53 am ET by WorkshopJon
experienced plumber here ----- no need to do both, as you will see from comments following. NEVER NEVER put anything on female threads for the reason pointed out --- it gets into the system and creates "trouble"!!
Remember --- tape or dope do not do anything to seal the joint!!!! if the joint won't seal --- it won't seal!!!! there are some specialty compounds that will HARDEN and fill voids..... but you don't want them if you can avoid it!!!!
now for the important part ------ tape or dope is a LUBRICANT -- NOT -- a sealant!!!! Its whole purpose is to make it easier to get the threads fully and deeply engaged so the mechanical joint in also a fluid tight joint. Remember this and many things will work better over time....
also --- the concern with gas service used to be worry that the gas would degrade the teflon ( early concern while learning to use something new!! ) and that little "ribbons" of teflon would easily float along to a combustion port whereas the low pressure of the gas couldn't move stiff dope that easily....
happy plumbing!!! cvjxn
LOL I should have scrolled all the way down before I posted.
Here's an area I can speak to with authority!
Your post is incorrect. An NPT threaded joint should always be thought of as a spiral path connecting what's inside the pipe to the outside world. That spiral joint must be packed with a thread sealant or it is extremely likely to leak.
An NPT joint is not designed to dry seal (i.e. without using a sealant or gasket). Dry seal threaded joint designs do exist but are seldom used because they're unreliable long-term. If the parts heat up and cool down again repeatedly, or if they see cycles of pressure, or corrode etc., the joint can open up and leak.
Thread sealants fall into two categories: tapes and pastes or "dopes". Both serve two purposes: to lubricate the two parts so that they don't gall (weld or seize together) while being tightened, and to fill the spiral gap between the two parts to prevent leakage.
Tape sealants are generally teflon and can range in density. The cheap crap you get at the hardware store has been stretched to the point where it may have a density as low as 0.25 g/cm3. It feels "thin" but can actually be just as many mils thick as the other tape. It's not the thickness but the density that determines how it performs. The "yellow" tape for natural gas service is unstretched and has a density of 1.3 to 1.5 g/cm3. And yes, you can get this tape in different colours, including white.
Pipe dopes vary in design and function, but they're all basically just viscous liquids with a solid particle filler. Their primary purpose is to fill large voids. The best of these sealants will flex with changes in the joint geometry as the joint heats, cools, flexes and corrodes.
For household water plumbing, either tape or paste is sufficient if used properly. The sealant should be applied only to the male part, and not to the first thread. One major caution though: if you use paste on plastic fittings, make SURE the paste is suitable for it. Many, indeed MOST good paste sealants are incompatible with PVC fittings and may cause them to crack later.
My company uses high density teflon tape with a layer of a Loctite paste thread sealant on top. We use this combination for all services that can tolerate both, up to the temperature limit of the paste and at pressures to 10,000 psig. Our pipefitters rarely see leaks with this system, even with cheap Chinese stainless steel 150# threaded fittings where the threads are poorly cut.
thanks for your additions to my primary point about the need for lube to get a taper thread (npt) properly seated. [do people realize its national pipe TAPER ??] I do definitely understand that a npt will not seat as designed without lube to prevent galling happening before the threads bottom out as the taper engages (and then there is the field cut threads of varying lengths :-) :-) )...... So yes, in actual practice the dope/tape does actually end up filling some voids.love the comments about big wrenches egging fittings. in fact I would venture a guess that most problems with copper thread adapters applied to iron pipe are exactly that as the copper eggs so much easier than an iron fitting!!! also look at the iron thread --- was it cut a little long??? the thread cup in the copper is not very deep usually and the end of the male thread can bottom befor the taper is sufficioentluy engaged.also thanks very much for the quantification about teflon tape density--- I have sensed the differences many times but now know how to look for a spec number....... Even us "pushing 70 er's" can still learn something every day....cvjxn
I repeat- it's not just lubrication that you need to seat a taper pipe thread so it seals. NPT is not a dryseal system- it cannot be reliably sealed with just grease or wax or any other lubricant. An NPT joint is a PACKED JOINT- it needs a permanent sealant to keep it reliably leak-free. The joint is tapered so that it can be tightened by interference, but that does NOT reliably generate a seal long-term!
You may get away with grease or wax on the threads, because if the threads are cut carefully enough even grease or wax can act as a sealant. But they're poor sealant choices- they tend to "creep", flowing or dissolving out of the joint over time. Good paste sealants are crosslinking ("curing") high viscosity liquids that resist creep and redissolution.
Soft, low-galling materials like leaded brasses permit the parts to deform to one another's contours to seal with only lubrication. That's how cylinder regulator joints are sometimes done. But reliable joints NEED a sealant, period.
Which is first? Dope or tape?
I always put the dope on first, just enough to cover the grooves of the threads. Then I wrap tape over the dope. It keeps the dope from spreading all over my hands, the countertop, my clothes, and eventually the inside of the washing machine.
I have found that if you have to disassemble the joint it leaves the female threads cleaner.
I feel that it helps get the dope farther into the connection.
SamT
Tape first, then dope on top. Tape should be well burnished into the male threads first. The high density teflon tape takes a fair bit of pull to get it properly seated into the bottoms of the threads, and putting dope on first will make that job harder.
I can see where putting dope on before the tape could result in dope being squeezed out as it was tightened. This could result in a slug of dope going down the line and plugging a n orifice. Some auto engine builders dont like tape because they found tiny strips plugging oil passages. Thanks for the excellent post Molten
Good observation. We don't use tape downstream of the last filter on any air line for exactly the same reason. Even if you're careful, little strands of tape tend to get stuck in solenoid valves, flowmeters, check valves, needle valves, valve positioners, paint gun jets etc. and gum up the works. Paste is actually less likely to move, especially if you apply it properly.
About the Lubriplate, or Thomas Lubriseal, or anti-seize compound, or vacuum grease etc.- they all do the same thing- a good job of lubrication, and a bad job of acting like a thread sealant. At a few inches of water column pressure like in a low pressure natural gas system maybe that's ok, but personally I wouldn't trust it on anything long term. Grease, regardless what kind, melts when it gets warm!
I watched the gas company guy move my meter. He used lubriplate for everything. (He only worked on the low pressure side.) Though theoretically it's just a lubricant, apparantly the gas co. relies on it to stick around forever and block the gas.
As for bad threads, I touch them up with taps and dies. A gentle touch is all it takes.
-- J.S.
Me & my co-workers always use both if we have both.
You don't HAVE too
You don't HAVE to use either-- makes for some tough screwing if ya don't though.
They used to use fels-naptha soap on the threads.
Tape & dope are lubricants not sealents the taper of the threads is what makes the seal.
Never put on the female threads causes alot of problems & against code.
We tape first then dope over tape, it's just more lubricant on the threads.
I'm not trying to be difficult, but it seems to me that tape and dope are more than "just lubricants."The solids in either fill in minor imperfections in the tapered thread pipe joint, improving the seal. Surely a can of 3-in-1 is a lubricant. I'll bet it does not work as well as pipe dope on a threaded pipe joint. (Dryseal joints are manufactured to tighter tolerances so as to seal without any compound.)Bill
I use Rector-seal T-plus(?) dope with Teflon added.
I'm not a plumber but I play one @ work...
Mr. T.
There's a steering-wheel in me pants and it's driving me nuts!!!
I use yellow tape and Rector Seal #5 on gas pipe threads. The Rector hardens up a bit after a while and I have no doubt that it has the effect of filling very minor gaps, if any, at least at the <1 lb. pressures in gas lines. An aggressive pressure test would probably blow it out. The can says either 'soft-set' or medium-set' on it.
You are correct, tape dope and regular dope are both sealants and lubricants, if someone would read the manufactors specs. they would understand this. "May the force be with you".
the problem is you can't trust the fitting to be perfect. When I was doing my apprentiship, we had to set up a block of dies, hand thread a pipe and then screw it into a test rig by hand with no dope. Good threads will hold 200 psi.
The other problem that can happen is using too big a wrench, it's easy to egg a pipe.
You're not being difficult.
Yes we know it will help on imperfect threads, but not always.
But it's not the purpose. I was working on a Boeing job about 15yrs ago had to screw some 2" galv together, had a 90 that would not hold at all.
Tape & dope didn't make a seal----- 36" wrench didn't help either.
I ended up candle wicking the threads & that finally made a seal sshhhh don't tell the inspector.
plumbill-
What, ya never heard of X-pando?
When all else fails, X-pando will stop the leak. But don't expect it to be easy to get the parts apart again! It's an expanding cementitious pipethread sealant, and the trick with using it is to not tighten the joint too much. You need to leave enough space for the sealant to fill the joint and expand, locking the joint in place. Works like a charm, but not for the faint of heart! It is possible to get joints apart again, but you need to shock the joint or it's going to stay put, permanently.
Oh yeah I have heard of it & have used it on more than one occasion.
But 15yrs ago on that Boeing job we didn't have any
I'm always looking for ways to prevent leaks so I looked up Gasoila and it seems that they make different types of pipe dope - soft set, hard set, and several others. If possible, could you elaborate on the pros and cons of these variations? Thanks.
Get some Gasoila Softset with PTFE.....You will never use anything else again. You can use it on all types of gas, all pipe (even plastic), boilers, air lines, you name it!
I was complaining to a friend of mine, who is a pipeline welder, about problems with leaks in boiler piping with the influx of junky fittings. He gave me a can of that stuff 7 years ago and I haven't had a leak in anything since.
The soft set is used if you may someday want to take the fittings apart. The other stuff pretty much welds them together.
Edited 2/5/2006 5:13 pm ET by BoJangles
which plane was that one? just so I know ehen planning my next flight.
LOL
On the old paint hangor.
Friggin vent wouldn't hold test
Thanks everyone for the information - I'm sure glad I asked the question!
Billbrennan said
"Forged copper male adaptors for copper tubing are often problem threads"
I have 4 of those to fit, so now I know to use tape AND dope on them. Belt AND suspenders :-)
plumbbill said
"We tape first then dope over tape, it's just more lubricant on the threads"
That was going to be my next question, which first if you use both. Thanks for the insta-answer :-)
Further question: the new tub going in is enamelled steel (American Standard Salem). One of the other (cheaper, also steel) tubs I considered comes with a foam slab to go under the tub, and I have read about setting a tub on top of mortar. The old tub was also enamelled steel, about 1950 vintage, and its only support was the ledger on the back wall and the tub's apron. Should I put anything under the new tub to support it?
Kevin
Edited 2/5/2006 12:44 am ET by KevinW
absolutely -- you must use a mortar bed for any tub or shower base. Even those new ones that say its not needed... But be very careful with those newer ones with exrensive patterns in the bottom --- you make the mortar too stiff or put too much and you won't get it to push around and the floor won't be down where it should be and then nothing else fits right forever!! the reason why you must use the bed of mortar is to prevent flexing.... think about all the weight of water (8lbs per gal!!) and 200 lbs of person concentrated on just a small foot print.... the bottom and or the sides flex on EVERY tub or shower material ( except maybe the really old true cast iron ones!!) that flexing will sooner or later start stres cracks in the base material and/or the surface enamel or plastic...cvjxn
The best material I have used under showers and tubs is Structo-Lite basecoat plaster, usually available at good lumberyards and masonry suppliers. It mixes up like cottage cheese. The bed you lay on the floor will not slump, but when you set the pan down and put some weight on it the plaster spreads out nicely, and then is hard within maybe 20 minutes. I have seen guys struggling to get masons mix and readymix concrete to do the same thing, and it looks a helluva lot harder.
presumably if you use the Structo-Lite or mortar, you have to put something waterproof under it.... I've got an old wooden floor here, w a diagonal-boarded subfloor and more boards for the floor surface under the tub (ratty boards with gaps between, they didn't care what it looked like under the tub I guess!). And a dam to stop it going down the hole for the waste pipe.
Kevin
Put a layer of 15lb or 30lb felt under the tub if the joints between the sheathing boards are large. You are not trying to glue the tub down, just prevent it from being deformed by weight during use.
Final answerRector-seal T-plus(?)(Yellow tube) dope with Teflon tape also. You don't need to crank as hard to get a good seal, the dope will allow the joint to come apart without corosion in the future.This belt and suspenders approach always works, and since I started using this technique, I have never had a leak.Kirk
Would vapor barrier poly work? I hate to go buy a whole roll of felt just for a few feet (though maybe I can buy just a few feet off the roll, have to check).
Kevin
thanks for the hint on the structo-lite --- I will look for that next time I set one ---- maybe it doesn't need as much mixing either?????regarding all the concerns about holes in the underfloor -- one, if there are two layers without obvious big gaps and you don't mix too sloppy --- I've never needed to line the floor --- and by the way -- I almost always mix the motar in place on the floor ---- no bucket to clean later!!!!!if big holes ==== yes::: a piece of roof felt, tyvec wrap , the plastic wrap from the fixtures being installed, scrap of luan, even sheet of cardboard carton, scrap of the old vinyl floor just ripped up..... All have been successful inthe past --- and maybe a couple more I have forgotten about!!!learn the principles and why things work, learn the codes and why they call for the steps they do, keep your brain engaged and your eyes open for creative solutions, and this business can really be a lot of fun!!!!cvjxn
PS to my comments on mortar under tubs ---- I just caught the comment about guys using concrete mix!!!! Never Never Never try to use concrete mix ---- the gravel pieces will lodge where you don't want them every time!!!! sand mix or mortar mix ONLY !!! and mortar is better because it is softer and slower setting so it moves easily early on when you want it to move and is easier to break loose from at the next change out... cvjxn
I use Gasoila pipe dopes. Without a doubt the best stuff you can put on a threaded connection. You can use it on any connection and I guarantee that it won't leak!!!
I should be a salesman for them...but actually it is terrific stuff. It takes a few days to get off of your hands tho.
Available at professional supply outlets.
Ok, I am JUST A HOME INSPECTOR....not a plumber. I am not sure if we are talking about DWV or not in this thread. BUT....I do sewer camera inspections and I have run into several PVC caps on PVC pipe that have had pipe dope put on them and I cannot get the cap off. If the pipe/cap is metal, I can pound on my wrench a bit and break it loose but NOT on PVC. PVC Pipe will break before the dope/paste does. Can't believe code allows pipe dope on PVC sewer caps.