Q re Brass Compression Fittings

I have a question for the experts regarding the all brass (metal to metal) compression fittings as used on the water supply lines to the behind the tile, shower & tub valve sets.
Briefly, is there a right or wrong way to install these things?
In our remodel, the contractor's plumber installed the rough-in valve sets, then was called back when we discovered a leak from the fittings on one set. He added more teflon tape to the compression fitting threads and reinstalled. Still leaking. He cranked down harder and managed to pull the nipple right through the nut, shearing the brass shoulder inside the nut. He then declared the valves defective. These are NOT cheap valves (EXPO Dolphin brand).
There were 2 identical sets installed, tub & shower. Later we found the other set also leaking. At that point, I had the remaining valve set removed & the pipes temporarily capped.
I'm now having problems with EXPO on warranty return because the plumber apparently threw out the bottom parts of the fittings.
I'd appreciate comments. My suspicion is that the plumber has more muscle than brains or experience, but I'm not an expert. Probably not likely we can prove one way or the other, if it even comes to that. Since then, I've inspected similar fittings on known brands; some were more robust looking but others were essentially identical to the failed valves.
Is it common practice to use teflon tape on these fittings? Seems to me that if that's needed to stop a leak, the game's already lost & the teflon may only be a short term fix since the seal is not supposed to be at the threads. I have no desire to criticise someone's work, but I don't want time bombs installed in my walls either.
Thanks for any help, Jack
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The threaded nut on compression fittings is only intended to compress the ferrule around the tubing and to hold in place against the fitting--the threads are not part of the sealing system.
Threads designed for sealing are tapered. When I've had compression fitting problems they are usually caused by misalgnment or undersized tubing which allows the ferrule to cock as it is being compressed. I have never succeeded in getting a leaker to seal without just cutting off the spent ferrule and replacting it with a new one.
Regards,
John
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jack wrote:

plumber, that is his job... if the plumber ruins the fittings, the general contractor has to make good with another plumber to get the job done.....
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