Submersible Water Pump Torque Problem?

Installation: 3/4hp 1phase 230volt 5gpm Submersible Pump (7years old) has an 1-1/4 inch outlet (non-metallic hub standard righthand threading). There is an 1-1/4 to 1 inch pvc reducing bushing attached to the outlet with 100 feet of 1 inch schedule 80 pvc pipe vertically in a 4inch casing and 200 feet horizontally in the ground to a 1500gal storage tank. The storage tank has a dedicated float switch for on/off control and a motor controller for starting, time delay, and protection of the submersible pump. There is one flow valve at the pump and one 20feet above it. The original torque arrestor is 2 feet above submersible pump and secure. There is a support cable for the submersible pump attached at the top of pump and secured at the top of well.
Checked: Voltage & Amperage to Motor Okay. Cleaned inlet to pump and temporarily installed pump in surface tank to verify operation. The submersible pump works but has a lower output than originally experienced. The torque looking at it from the top is clockwise to the rotation of the pump
Problem: The Submersible Pump unscrews itself from the 1-1/4 to 1 inch reducing bushing. This happened twice. The first time unknown and the second time within a week.
Solution: Pump Failing? Torque Arrestor Failing? Replace Pump Due To Reduced Output Which May Be Causing Additional Torque Problems? UNKNOWN SOLUTION???

That is why I do not use plastic head pumps. You either can't get it tight enough to keep from unscrewing and when you do you end up cracking the head.

I would go get a new "wet" end if the motor is still good....either in brass or stainless.

Sch 80 threaded is good. However, it is best when screwed into metallic fittings. I would use a brass or stainless 1¼ X 1 bushing between the pump and sch 80. I would also use brass or stainless couplings between the joints of pipe. I have had plastic couplings come unscrewed the same way.

Torque arrestor is not necessary. Just something you don’t need that could cause the pump to hang up in the well. You shouldn’t have many cycles (torque events) anyway since it is just filling a tank with a float switch.

My guess is either your float switch is bouncing or the pump is getting hot, or you wouldn’t be having this problem.

Filling a storage tank from the top can cause waves that bounce the float switch, bouncing the pump on and off, which is what makes it come unscrewed. Either fill from below the water level in the tank or use a stilling well for the float switch. A stilling well is just a large diameter piece of pipe with slots cut in it. Float switch inside the stilling well won’t see waves.

If the well is pumping dry it will heat up the water and make it unscrew easier.

Why would there be the additional check valve 20' above the one at/in the pump? It might have something to do with the problem too.

Clean all the threads well and use EPOXY for pipe dope. Dont use so much that it runs into the check valve. Stand the pump upside down if you want to be sure while it sets.

Check out the cycle time and if its running dry as noted above.

I've worked with many plastic discharge submersible pumps over the years. I've seen them spit out the 1.25"x1 pvc reducer several times. valveman is right, sch 80 wants to thread into something metallic at the pump, and the plastic pump seems to want metallic going into it. i found that if I used a galvanized 1.25x1 reducer in the pump, then screw the 1" pvc pipe into that.. it never spits it out again.

I think the day of tapered fittings is coming to a close. A steel bushing into a nylon or PVC head too tight is a crack soon to be. And lots of faulty fittings on the market from Asia now.

A set screw would be nice on the pump head so that one could just snug it up and then lock it.