Question just about the order of pipe dope sealant and teflon tape

I'd use one or the other, not both.

Teflon is primarily a lubricant and is most effective on NPT fittings. NPT fittings change their pitch so that the connected pieces deform the pipe threads more and more making the seal.

Other types of threading do not change the thread pitch (if there is any involved in the seal), so any imperfections are better dealth with by a pipe sealant.

a plumber friend uses tape and then dope, on every joint to avoid leaks

Does he wear a belt and suspenders too?

I know people that use both, but I know people that use one or the other and don't have leaks. One advantage of tape is when you take it apart five or ten years later, it is probably going to be easier. Joint compound can get hard.

Does he wear a belt and suspenders too?

I know people that use both, but I know people that use one or the other and don't have leaks. One advantage of tape is when you take it apart five or ten years later, it is probably going to be easier. Joint compound can get hard.

I use one or the other too, never both. Usually I use tape. And I rarely have a problem.

According to this, tape is a no-no

That's a good find. What they are saying makes sense and they should know. I've used tape for years and rarely had a problem. But part of it is probably because I know how much to tighten them and don't over tighten a fitting to crack it. For the future, I'll use joint compound.

Also, the part about Sched 80 vs 40 was interesting. Particularly the part about how any threaded PVC joint reduced the overall pressure rating by 50% compared to all glued connections.

Pipe or dope - not both. Read the instructions on either one.

Interesting conclusions.

I noticed a LOT of my threaded fittings are gray (schedule 80?) and most of the glued "end" fittings (near the pumps & valves) are gray thicker stuff (schedule 200?).

Asking at a pool store, they said they use gray fittings near the pump "because they handle heat better".

Yet, that article says both teflon tape and schedule 80 fittings are a no no.

Interesting though the schedule 40 PVC tensile strength of 7K psi and the PVC thread "resistance" of 400K PSI.

For 2" pipe, apparently each turn past finger tight adds 0.00239 inches per inch, times that 400K psi, nets almost a thousand psi per turn (956 psi per turn).

The chart gives us only about 4,000 PSI as the maximum allowable (3912 psi); so it seems that 4 turns past finger tight is the maximum one can go with schedule 40 PVC.

In summary, that article recommends, for my 2" PVC plumbing:

1. Pipe dope (not teflon tape).
2. Two turns after finger tight (never more than 4).
3. Only schedule 40 plastic (no schedule 80).

It was also interesting that a single threaded fitting reduces the strength of the system by half the original strength!

The whole treatise was very informative, I'm sending the web address to several DIY'ers I know.

While basically true there are some fallacies there, particularly wrt to the Sch 80 vs Sch 40 discussion on stresses.

The stress on a Sch 80 fitting is _not_ the same as that for a Sch 40 fitting--the extra thickness _DOES_ make a difference because the remaining cross-sectional area remaining after the threads are cut is thicker and that increases the denominator (the per sq in portion of the stress which has units of psi).

I've not gone and looked up actual dimensions, but the percentage increase in remaining thickness will be quite significant.

I saw no place that says "no chedule 80". It says DO NOT MIX 40 and

Harry K

A lot incorrect information.....

Bingo!

Vic has posted an article that would have been my next comment....

The use of tape can lead to splitting of the female threaded fitting.

Loctite makes a product specifically aimed at plastic threads..

No-More-Leaks

As T4 points out, the article notes that threaded joints reduce pressure capacity by 50% (due to wall thickness reduction). A side note...in steel piping threading also greatly reduces corrosion life and that's why we see corrosion evidnence and leaks at the threaded joints. :(

Specifically, the article's comments about sch 40 vs 80 were related to the behavior of threaded fittings.

SInce the threaded behavior & subsequent resulting stress is driven by the wedging action.... a thicker fitting will be just as stressed (to the first order approximation) as the thinner sch 40 fitting. Both are "wedged" open to the same extent when tightened the same number of turns .

The articles's comment was to emphasize that a thicker fitting is not the answer.... proper sealant (not tape) is the answer.

cheers Bob

I'm actually surprised that I apparently put too much on, according to your comments.

I thought that's how much you're supposed to use.

I agree, it didn't say "no sch 80" ..... it said going to sch 80 & expecting that to avoid a splitting problem was wrong.

Sch 80 will give you higher system pressure & better resistance to misc mechanical loads but it won't solve the tape or over tightening problem. :(

cheers Bob

dpb-

I was wondering if they were only looking at stress induced from tightening & doing their tightening stress approximation as merely an E * radial strain based wedging action? Would that not be independent of wall thickness?

Total fitting load is tightening load + pressure load.

Of course the system pressure capacity would be higher with Sch 80 vs sch 40 but perhaps they were trying to emphasize the point that sch 80 was not more over-tightening resistant than sch 40?

Am I missing something here?

cheers Bob