Turning A FV Into A Boiler

[Befuddler]

Perhaps the guy in question mounted the kettle element too low and overheated that part of the plastic.

That can't happen unless it's actually touching. The element is surrounded by liquid which will never go above it's boiling point, so effectively the plastic is insulated from the element whenever it's submerged. Technically, you're overheating all of the plastic beyond "safe" operating temperatures in an hdpe boiler.

It's actually a bad idea to fit the element high up the side of the boiler anyway, you'll create a pretty bad thermal gradient and the liquid below won't be heated properly. Once the liquid at the top starts to boil, it'll all churn around and lessen the problem, but I still don't think you'll get a very good hot break like that - the liquid below the element will probably be several degrees below boiling.

[Kev888]

I agree. There's no great mystery here, and people don't need to have done anything different/wrong with their build; its simply that some HDPE barrels are clearly very close to the edge whilst some (many) can cope somewhat better.

Its entirely what you'd expect from manufacturing variation if you choose to push them so far beyond their safe design rating. They aren't precision engineered, and so vary; the weaker examples will show it first. This is why people saying 'mine works' is pretty meaningless as a guide; until you have built and tested a specific barrel (over time) you won't know if its a good one or a weak one.

Cheers
Kev