The common cause for this type of failure with poly fittings would be due to someone trying to stop a leak by nipping up the fitting a bit tighter.
Sometimes the weld holding the fitting in the tank will break but more often the thread will strip and very rarely the fitting will break and only then because it was totally inferior (junk) to start with.
A decent fitting will quite happily hold over 200psi in a pressure application for many years.
Threads in Polyethylene (plastic) have to be treated with kid gloves although when properly install will be incredibly efficient.
The torque required should be finger tight with a nip at the end.
Thread tape is paramount.
If a fitting leaks
after doing up repeat the process but use more thread tape. Use two spanners when doing up so that the torque used isn't being transferred into the weld of the fitting in the tank.
Female threaded fittings are the worst and should be avoided as they expand when anything is threaded into them. There are fittings with stainless steel
rings around the outside of the threaded part of the fitting which stop the thing expanding. They work well.
Welding polyethylene is relatively easy but as with most things in life preparation is everything.
All polyethylene oxidizes over time and this oxidized layer needs to be removed back to fresh raw material before welding.
The easiest way to explain this is with black poly as it's the easiest to
see say using a pipe. The outside surfaces develop a greyish look to them but if you lightly scrape it you'll find a jet black glossy material underneath.
That's fresh raw poly.
Whatever you're trying to weld including new fittings should all be washed (water is fine but leave to dry before continuing) and then scraped/ground before welding and these surfaces you've prepared should be kept pristine including not putting your fingers on them.
There are solvents that can be used for cleaning
but are not readily available from the local hardware
store. Acetone, Metho etc. are not appropriate solvents as they all leave a residue when they dry. If something gets dirty again lightly re scrape it.
Unless you are certain of the quality of an old fitting I would toss it and get a new one and if the old one is broken I'd replace it every time.
Check out the fitting in the store as it will have printed on it some where what it's made from ie. PE - Polyethylene, PP - Polypropylene. If it doesn't it's probably made from 10 year old ground up milk crates (junk). Many will also have a standards mark or number.
They are not expensive and the best fittings are most easily found in the
, irrigation or pump shop.
Try to get something with a high pressure rating like SDR 11 or PN16 which will be the easiest to find anyway as they are easier to weld and will distort less (thicker wall) when the required amount of heat is put into them in the welding process
You can't weld dissimilar materials ie PE can't be welded to PP.
Such a fitting could be socket, butt, wired or extrusion welded onto a tank.
For a Rolls Royce job I'd socket weld it and then beef it up with an extrusion weld. This allows for a full thickness (tank wall) together with a mass I suppose of gusseting externally onto the tank and fitting.
Butt welding would require some skill in probably heating
the tank with a hot air gun and the fitting on a hot plate before bringing together. This wouldn't be very strong by itself as it'd be just a surface weld (on the tank)
Wire welding wouldn't be very strong either although the hole could be bevelled to get better penetration into the tank wall.
Extrusion and wire welding are basically the same thing except that a wire weld produces a slug of approx. 4mm while an extrusion slug could easily be 5 times the size increasing the size of the welded area by the same ratio.
Would this be a diy job.
The tank will have been roto moulded and additives will have been included into the raw material to make the molten poly flow better in the mould and these additives won't be present to the same extent in the fitting. Applying equal heat into both with for this case an equal wall thickness would see the material in the tank weld zone turn to mush before the fitting was heated sufficiently.
Dissimilar polyethylene's ie. PE100 or PE80 can be welded together as can MDPE (medium density) or HDPE (high density). Your local water
authority would have a heart attack to hear that but for the purpose of a low or no pressure application on a boat any half decent weld will easily out live the owner and boat combined.
The diy'ers will be fuming but many plumbers these days would have the necessary gear
to fix a new fitting into a tank and it won't be a long process. Most of their time will be spent waiting for various things to heat up. In a workshop situation such a weld once everything was hot enough would probably take about 15 minutes so you won't be paying all the college fees
for the plumbers kids
I tried to get some pics into this but I'm afraid it's beyond an old bloke.
As a heads up when buying
something like a tank try to get a black one as they will have the greatest UV resistance. The black colour is caused by the addition of carbon black to the raw material which is the only reason poly has any UV resistance at all. Natural poly will break down very quickly when exposed to UV. The pretty coloured ones also have this added but as has been discovered their protection isn't as good. Probably still last in excess of 50 years yet black poly's working life span still hasn't been determined but it's in excess of 100 years.
Also supporting whatever is hanging off a fitting is very important especially if theirs any vibration involved as poly will fatigue break just like most things if burdened by un unequal weight or being allowed to move back and forth over time.