Plumbing mystery with PVC pipe
Had a buried 2.5″ PVC water feed lateral break on XMAS eve flooding area under apartment driveway lifting blacktop about 8″ up above the leak site. Crew worked until 2:30 AM to repair. The sch 40 PVC had a coupling on it that had the bottom edge ground smooth on about the lower 1/3 of the coupling. This lateral only runs about 30 feet T’ed off the main line. The line was buried about 6 feet down in 3/4 minus gravel. Is this grinding down of the PVC coupling caused by extreme expansion/contraction of the PVC pipe or what is going on? The backhoe operator damaged the pipe at the point of the leak so we couldn’t see why the pipe failed. Anyone have any experience with this type of failure? Should the PVC have an expansion loop or similar device to keep it from breaking again?
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Not sure this is what you want but, FYI.... On a golf course my father installed for a boss of his, (9 hole only one player, 0ne par 5) they installed the irrigation system about 7 years ago. 2&4" PVC @ 160 psi operating pressure. Found out the hard way that if you don't wipe off the excess primer before the glue, the primer continues to eat it's way through the pipe. 7 years later major fountains where none were designed to be. Just make sure you wipe the joints with a clean cloth after the primer on the fix.
Good luck
I've seen the integral couplings on PVC pipe can abraded in handling/shipping. I haven't seen that on discrete, regular couplings. Water hammer comes to mind if the pipe was up against a large stone or concrete slab. Getting bumped back and forth could do it, but not in 3/4 minus gravel. Expansion loops/bends? I've never done them at 6' temperatures are pretty stable at that depth (compared to 180F temperature ranges between radiant cooling on the coldest nights and sunlight heated on the hottest days.)
Working 6' underground is tricky. OSHA and common sense say to use shoring (or you could end up 6' underground, so to speak) but many contractors don't. Often the pipe is glued above grade and the whole run (or 50-100 feet at a time) is rolled into the hole. If the glue wasn't set (a couple hours) and often installers don't wait that long, then pipe can get bumped mostly out of a fitting. Also, when glueing, you need to hold some pressure on the fitting for 30-60 seconds after you push it in or it will sort of ooze out of the fitting a fair bit. Either of these installation errors could have left a weak spot just waiting to fail.
Another PVC failure mode I've seen is when the pipe was stored outside for some months. PVC gets UV-embrittled. Very noticeable to a thoughtful installer if they are pressure-cutting the pipe. Much less noticeable if they are sawing their cuts. I often test-break a few ends to check the batch for brittleness. I have occassional sent a batch back. Taught me to only buy from suppliers who store PVC indoors.
Back to water hammer: the way to avoid water hammer damage (which puts the largest stresses on long pipe runs) it to put big backstops in a preselected intervals. Like a poured concrete wall that takes the force and prevents longitutidal movement of each section. Also spec'ing gate valves versus ball valves avoids operator error. Not everyone knows to close ball valves slowly when dealing with long, straight piperuns. But only 30' seems far too short for water hammer to be an issue. I'd guess one of the installation errors mentioned above.