Water is not compressible but it can be pressurized or non-pressurized. Below the check valve, water in a submersible pump is not pressurized when the pump is off. Above the check valve, water is pressurized even when the pump is off.
At the instant of pump start, high velocity is induced so the impellers stack, and even the rotor in the motor go into up-thrust condition. When the pressure in the pump is high enough to push the check valve open, the impeller stack and rotor are pushed back down.
This all happens in a fraction of a second. But each time the pump is started the impellers, shaft, and rotor fly up, and then get slammed back down. This is another reason why multiple starts are not good for a pump and/or motor.
I am working on a case now where the installer is regularly shattering thrust bearings in a 50 HP submersible motor. It is set at 1200’ and has 5 check valves spaced evenly in the drop pipe. The manufacturers are telling him he must have a check valve every 200’. I am telling him the multiple check valves are the cause of the fractured thrust bearings.
A pump that is designed to work from 1200’ will be in an up-thrust condition until it gets 1200’ of head applied to it. Each check valve up the drop pipe isolates the pump from the 1200’ of head. Until the bottom check valve is pushed open, the pump only sees 200’ of head. Until the second check valve from the bottom is pushed open, the pump only sees 400’ of head, and so on. Not until the fifth check valve at the top is pushed open will the pump see the 1200’ of head it needs to keep from being in up-thrust.
For each check valve in the drop pipe the shaft and rotor go from complete up-thrust to being hammered back down as soon as the water hits the next check valve. Water hammer can easily fracture a thrust bearing. And the 5 check valves are causing 5 water hammer events before the water ever starts flowing out the top of the well. The thrust bearings look like they have been busted with a sledgehammer.
Having only one check valve attached directly to the pump will solve this problem. Sure the pump will go into up-thrust until this check valve is pushed open. But the short distance between the pump and the check valve will cause much less shock to the thrust bearing than check valves higher in the drop pipe. When the pump is started, each additional check valve causes the pump to go from an up-thrust condition to hammering the rotor down against the thrust bearing.
