Why you shouldn't over-tighten threaded connections

[Sleepydoc]

I have a valve teed off my return plumbing that I use for water changes - to drain the water I close the valve on the return plumbing, hook up a hose to the side pipe and then turn on my return pump to pump the water to the drain. The other day I started the process and noticed water spraying out the back of the stand.

For those who don't know, NPT threaded connections are tapered, or funnel-shaped, so the tighter you screw it in, the more outward pressure the male fitting puts on the female fitting.

This is a standard, schedule 40 PVC valve that I got at Home Depot and has been in place for 2 or 3 years. I remember installing it and tightening it to try and get it square (and pretty) but I clearly had over-tightened it. The fortunate part is that when the valve on the return pipe was open there's relatively little pressure on the system so the leak was nothing more than a drip. When I closed the return valve and the pump had to increase the pressure to pump through the hose it caused a significant leak to spray out. Fortunately I had another valve left over from something else so it was a quick fix to swap it out.

 

[fachatga]

Thank you. That’s useful. I always read that but didn’t know they were tapered.

 

[BrianReefer]

I once did that on the bottom of a 55g RODI mixing station. You can imagine how that ended…

 

[ying yang]

Qualified plumber and heating engineer for over 20 years so know all to well shouldnt over tighten plastic fittings.
But on my first saltwater tank with first sump 8 months ago i probably over tightened the tank connector on return pipework ( i tell myself when fitting made it probably got stuck in mold or worker dropped it and stood on it causing hairline crack as was certain didnt over tighten to much) but probably did.anyway system full with 365 litres of saltwater and aquascape amd bottle bacteria so didnt want drain system down so rang manufacturer to try order just the one fitting i needed as online had to buy all pipework set costing £ 80 but didnt meed it all and thought could buy it all amd fit myself for £ 20 max so manufacturer said will do me favour as tank sat unused for 5 years so never used but out of warranty but i needed a lfs stockist to order it as they wouldnt let me so lfs rang manufacter but after a week im worrying so much as scared of massive leak as used a bike puncture repair kit to glue few patches onto fitting but lfs rang to order but apparently forgot to follow up and order it online so nearly 2 weeks i got system running all holding on with a puncture repair kit and could see pressure building up and bubble forming under the patch so lots anxious sleepless nights but luckily no leaks so all good that ends well.here few photos of my make shift repair ha ha