Are you sure its not Sch 120 pipe? Sch 20 does not sound right to me. Do you have any info on the pump. (size, model number, horse power, anything) My best guess from what you are saying is that the pump is just barely big enough to pump from 441'. So after you run it a little and draw the water deeper in the well it may not be able to build up to the shut off point of the pressure switch. Pump still running, water in pump gets hot, pipe starts melting and blows off. The galv pipe won't melt so the pump probably got hot enough to melt the impellers and lock up this time. The plastic pipe blowing off probably saved the pump first two times. Doesn't matter though, if this is what is happening you need a bigger pump anyway.