Does the order of the tape & pipe sealant matter on 2" pipes

[rock]

Does the order of the tape & pipe sealant matter?

Starting with this:

Just wondering, if, for 2" PVC schedule 40 pool equipment, when using *both* pipe sealant & teflon tape:

Does it matter if we put the teflon tape first:

And then the pipe sealant?

Before screwing the fitting into the housing:

Just curious:
Does the order of the tape & pipe sealant matter?

 

[duraleigh]

Re: Does the order of the tape & pipe sealant matter on 2" p

Just like wearing a belt and suspenders, you only need one so the sequence is irrelevant.

 

[rock]

Re: Does the order of the tape & pipe sealant matter on 2" p

Just like wearing a belt and suspenders, you only need one so the sequence is irrelevant.

Makes sense.
Asking around, I found this 1-pae reference:
http://www.lascofittings.com/supportcen ... lastic.asp
Which appears to conclude:
a. Don't use teflon tape; use non-lubricating sealant instead (note the requirement for non-lubricating)
b. Don't use any schedule 80 fittings with schedule 40 PVC pipe
c. Only turn schedule 40 PVC fittings a maximum of 2 turns after finger tight

Here's an interesting quote:

The resistance of PVC is 400,000 pounds per square inch (psi). The strain per turn past finger tight for one-inch PVC pipe is .00447, so the stress per turn is 1,788 psi. Thus, a one-inch threaded PVC joint that is tightened four turns past finger tight will develop a tensile stress of 7,152 psi. The joint is bound to fail since the stress exceeds the 7,000 psi tensile strength of PVC, without even adding the tensile stress caused by the pressure inside the system

Other details:
a. Adding even a single threaded fitting (versus glued), reduces the strength of the system by 50%
b. Smaller pipe exhibits more strain per turn (hence splits more) and is easier to turn
c. Schedule 40 PVC NPT thread taper is 1 3?4° per thread

Interesting quote:

most plastics, including PVC, are "notch sensitive." When the smooth wall of a plastic part is notched, the part loses a significant portion of its original strength, just as a thick sheet of glass will break along a scribed line on its surface. This is why the presence of even one threaded fitting in a system requires a 50% reduction.

Given the numbers above, for my 2" PVC plumbing,
1. Resistance of PVC = 400,000 pounds psi
2. Strain per turn past finger tight for two-inch PVC pipe is 0.00239
3. 400,000 psi x 0.00239 = 956 psi stress per turn
4. 956 psi x 2 = 1,912 psi tensile stress for two turns (which is less than the 7,000 psi tensile strength of PVC - but you need to add the stress of the fluid under pressure)

The number I don't know is what the pool pump hydrostatic pressure is:
For example, two turns past finger tight plus the stress of the system pressure has to be within the tensile strength of two-inch PVC:
956 psi x 2 turns + ?,000 psi must be < 7,000 psi tensile strength of PVC

 

[duraleigh]

Re: Does the order of the tape & pipe sealant matter on 2" p

Yikes! I simply used pipe dope throughout my entire system and tightened them all to what I felt was secure.

I didn't read all that stuff.....good thing, because I haven't had a pvc threaded leak in 9 years and I probably would have if I had realized I was doing it wrong.

 

[rock]

Re: Does the order of the tape & pipe sealant matter on 2" p

I simply used pipe dope throughout my entire system and tightened them all to what I felt was secure

Me too!

But, I like to know the right way to do things - even if I don't do them right the first time.

I thought that article very interesting, especially since I (apparently) have glued gray schedule 80 fittings at the Jandy valve junctions:

And, I have schedule 80 threaded fittings (apparently) at the pump junctions:

What that article intimates is that whoever used those Schedule 80 fittings was treating the symptom, not the disease. Apparently, the disease of plastic female threaded fittings splitting when tightened can be cured by using a proper thread sealant instead of using lubricating pipe dope or Teflon tape on plastic threads. Specifically, that article implies that the use of either Teflon tape or pipe dope on schedule 40 plastic threaded fittings will increase the wedging action of the male pipe threads, thereby increasing the liklihood that the female fitting might split.

It seems they're postulating that if people used a non-lubricating thread sealant instead of pipe dope or Teflon tape, there wouldn't be the increased splitting force on the female fitting, and there wouldn't
be any splitting problem. So, switching to Schedule 80 female plastic fittings to avoid splitting is not really solving the problem. The real solution, they say, is found in using non-lubricating thread sealant instead of pipe dope and/or Teflon tape to avoid the splitting problem altogether, rather than adding the stronger schedule 80 fittings that will stand up to the greater splitting force.

In summary, they implied the use of Teflon tape or lubricating pipe dope can lead to splitting of the female threaded fitting so that's why they banned tape and suggested non-lubricating pipe sealant.
And, they implied that the thicker schedule 80 PVC threaded fittings will be just as stressed (to the first order approximation) as the thinner schedule 40 fittings because both are wedged open to the same extent when tightened the same number of turns .

The article emphasized that a thicker fitting is not the answer - proper non-lubricating sealant (not Teflon tape or lubricating sealant) and tightening only two turns after finger tight was the answer, they said.

Below is another article that deprecates Teflon tape for PVC male fittings, even as it explains how to properly put on the tape if it must be used in emergencies:
http://www.spearsmfg.com/how_to/FG-3B-0 ... nglish.pdf

 

[rock]

Re: Does the order of the tape & pipe sealant matter on 2" p

By way of update, I removed all the home-made schedule 80 fittings, and cleaned out the combination Teflon tape and sealant.

On this second pass, I put only the sealant + lubricant paste on the male threads:

I then tightened the fitting two turns after hand tight:

I also replaced the schedule 40 fitting by putting the thread sealer on it:

I spread it around with my finger (but I wasn't sure how thick to make it):

Then I screwed the fitting on hand tight:

And needed a pipe wrench for the last two turns after hand tight:

With this being the end result:

BTW, does a union go together dry?

And, is there any special reason why it says it's for schedule 80 pipe?