In 30 plus years I have never had a submersible pump I've installed come unscrewed. However I have pulled unscrewed pumps out that someone else had installed.
Did have one 5 hp pump I pulled on sch 120 pvc that was loose, you could unscrew it by hand. It had a 2 inch pump discharge, with a pvc 2"x 1 1/4" threaded bushing in the discharge. The 1 1/4" pvc drop pipe was loose in the pvc threaded bushing. I think the wires had kept from unscrewing. It was in a VFD, not that hard of a starting kick either.
The only split couplings I've seen on the PVC pipe has been on the surealign style with the integral female coupler. None of the distributors in my area even carry it any more. I think the problem was installer error though. I think they were over tightening the connections. I've used it on a few wells, but now I only use sch120 with stainless steel couplings.
Yes it could be possible to deadhead a pump or run it out of water and melt the pvc. Haven't had the drop pipe melt. However I've pulled more than one pump that the 4" pvc liner has softened and warped around the pump. What a mess that is to pull.
I've also seen plenty of above ground centrifugal pumps that were plumbed with pvc right off the discharge. If they are left running with no flow,They will heat the water up and soften the pvc enough that it will balloon and finally fail.
I've got a 1 1/2 hp centrifugal pump that I irrigate my yard with. It pumps out of the Irrigation ditch behind the house. It's been accidentally left running with all the lines shut off before. It's got a steel pipe discharge until it goes underground. The pump casing heats up enough you will burn your hand if you touch it. Don't ask me how I know that!
