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Old 11-12-2010, 11:10 AM   #1
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PVC plumbing and Teflon Tape vs Cement

I was reading up on Durso Standpipes and there was a part in one DIY page that suggested the use of teflon tape wrapped 3 times around a pipe to go into another fitting, saying it would ensure a tight fit.

Exploded Parts List - Page 4

This leads me to my question. For my tank to sump piping, I have always used PVC cement at all connection points. The problem I keep running into is that for certain requirements and dimension matching for routing the pipe, it is very difficult to get everything together exactly right, if you're off by the slightest bit, you either have to deal with it or make it all over again. For instance, 2 90's and a T on top of one facing a certain direction. If your 90's are not perfectly aligned, the pipes going in and out will not be parallel. If you T is not twisted right, the pipe out of the side will not be in the direction you intended.

Previously, the solution I saw was to just leave them friction fit. It works, but SW will work into the joint and drip out, and eventually it self-seals, essentially. And this works fine under a low pressure overflow.

But with a system I am working with (the ATS) it requires a little more pressure to build up, no a whole lot, but enough that I didn't want to risk a friction fit.

Would I be able to use this teflon tape wrap idea that is used for a Durso Standpipe to make the fittings tight enough where they wouldn't loosen?

This is a pic of how the PVC goes together



and how it actually turned out



What happened with this first attempt is that the bottom T didn't get on there precisely right, so the whole acrylic box is cocked to one side on the tank. It still works, but I don't want to re-make the thing again and have it not turn out just right.

It would sure make it a heck of a lot easier to get the pipes exactly where I need them to be using a teflon/friction fit, and I would think this would eliminate the slow leak issue.

Has anyone tried this? Also, should I even worry about a friction fit connection letting loose at all? Most times, I need a pipe wrench to loosen really tight friction fit PVC connections, after tapping in with a rubber mallet. So maybe it's not a big deal.

I estimate the pressure to be only about 1 foot of head (max) above the bottom of the tank, so maybe 2-3 feet of head above the 90 below the union right under the tank connection.

Any thoughts?
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Old 11-12-2010, 12:28 PM   #2
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The one advantage to glued joints is that you can have fittings place very closely together. Often times you will need a union to complete an assembly, that you wouldn't need with a glued connection. It is a little more difficult with threaded joints, as well as a little more expensive. Perhaps you just need to refine your assembly techniques to ensure you get proper right angles. This could be accomplished with a few right angle squares, made from dimensional lumber. This is the type of technique we used when manufacturing with steel (steel squares and jigs), for the same reasons.
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Old 11-12-2010, 12:30 PM   #3
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I wouldnt as there is more pressure in the piping and on the joints then at the stand pipes.
Always glue, use some unions to be able to take it apart if needed.
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Old 11-12-2010, 12:41 PM   #4
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I'm not talking about a threaded connection. I'm talking about taking a solvent fitting and wrapping it in teflon tape, then inserting and tapping with a rubber mallet to get it in there tight.

I have figured out that most all solvent fittings allow for 3/4" to 1" of the pipe to be inserted during the gluing process. I seriously doubt that under 3' of max head pressure (which is very, very low) that any fitting would work itself loose if it was tapped in to that depth.

Like I said, I have to use a wrench to pull out a friction test-fitting, and that's at 1/2" depth. So I don't think it will actually be a problem, I'm just trying to find out if anyone else has tried it and had failure or success.
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Old 11-12-2010, 12:53 PM   #5
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You might be able to get away with the interference fit the tape creates, but I know I'd never be comfortable with it.

You may have some friction-based coupler options though. I know for iron pipe there are fitting called no-hub clamps. You butt two pipes together inside this clamp and tighten it down. It uses rubber as the friction material. I don't know if there's something similar for plastic. There are also rubber fittings called mission couplings. Basically they're a black rubber tube with hose clamps.
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Old 11-12-2010, 01:22 PM   #6
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What about this:

Could you friction fit all the pieces together, then run a bead of superglue around the joint?
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Old 11-12-2010, 01:48 PM   #7
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I don't like the teflon tape idea ... the fittings are so tight that the tape simply bunches up when I inset it, the joint is less secure than plain friction fit.

I use plain friction fit on non-critical areas, & either glue or threaded joints elsewhere. To get around the alignment problem, I use a bit of creativity in the design. I use a length of flex hose connection to allow for adjustments of the entire assembly. You can also use a slip-union for the same purpose.

I would not use super glue like you suggested. The joint is too tight for the glue to penetrate. What you need to do is to dry fit everything so it is perfect. Then mark all the joints with depth & orientation marks. You then take the joint apart & glue it with cement, making sure that all your marks line up. You should get a perfect result that way. <This is how I redid the plumbing in my laundry room, both the PVC's & the copper pipes. Everything is perfectly lined up after assembly.>
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Old 11-12-2010, 02:13 PM   #8
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That's what I tried to do, and have found it to be difficult. I'm really good working with the stuff, but getting it perfectly right seems to be the trouble. I've done what you suggested, the problem is that when you dry fit something, the pipe doesn't go as far into the fitting as it does after you apply the glue. Dry fitting a pipe 3/4" into an elbow or tee holds it so tight in some cases that I need 2 wrenches to get the dry fit pipe out. So what I was doing was dry fitting with shorter pieces, then cutting another pipe that was longer and using that for the weld and going in as far as 7/8".

Maybe I should not worry about inserting the pipe any more than 1/2"? Also was running into problems marking the orientation and depth. Was using a marker and it you're off by a degree or two that gets amplified down the line. Maybe I will just deal with the 2-wrench issue and mark with a razor blade. I was hoping that friction fit would be good enough.

I considered using a pipe joiner (one that lets you insert pipe in both ends, then has a screw-on compression fitting) but I don't have the room for it (6 inches long)

I have a hard time believing that if you hammer a fitting together well enough that it will ever loosen up on it's own.

Just looking to a good way to seal a friction fitted joint so that it won't loosen up - just in case. What about running a bead of cement around the joint after the friction fit? Would that fall into the same category as the superglue, it won't leech into the joint itself?
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Old 11-13-2010, 07:11 PM   #9
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Once you mark a depth on dry fitting, you don't go deeper than the mark on gluing, or the length won't be right.

I don't think you need to bottom out the joint when gluing. Inserting it 1/2 to 3/4" should be plenty for gluing. Also try lubing the pipe with water when dry fitting, it will slide in & out easier. <Using mineral oil would be even better, but then you would have to get all the oil off with wiping & the cleaner .... Never tried the oil so can't vouch for how this will work ... but plain water works well enough for me.>

On my setup, there is only one or 2 places where the joining had to be precise. I have designed things so there is a bit of a give, so I can be off by 1/4" or so.

A really precise way of doing this would be making a full size drawing & a wood assembly jig. You draw out the assembly on a board & nail pieces of wood on it so that the pipes are held down precisely while gluing. This is a lot of work for a one off piece, but saves time when you have to make many copies. I sometimes do this in my wood working when I have to make many identical components (a bunch of drawers, eg) ... but for plumbing, I find I can get it pretty good just by the fitting & marking method.
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