Need help on some fluid flow theory...

[SteveMitty79]

Ok, so I've read in a few threads about how having a closed line on submersible startup actually reduces pump stress due to certain dynamics which I cannot seem to wrap my head around. This is opposed to leaving it open against a pressure system, if I am correct. I can buy that.

My question is, since I'm installing an open system, which the pump would not be pushing again any pressure or even a closed valve, but just head, would it still be an advantage to start up against a closed valve or was that just vs a pressure system.

Thanks. I may really be confused after this. Steve.

 

[Valveman]

Even though it is advantageous to start a pump against a closed or almost closed valve, you don't HAVE to do it. But you do need enough head on the pump to keep it from going into an upthrust condition. If you size the pump correctly the lift in the well and friction losses will be enough head to keep the pump within it's curve. Just don't oversize the pump.

Say for instance you are using a 1HP, 10 GPM pump. You want at least 180' of head on that pump at all times. If the lift from the static water level to the surface is less than 180', that pump will try to produce more than 16 GPM, which will cause an upthrust condition in the pump. In a case like that I would throttle the discharge of the pump with a ball valve or Dole valve so that it cannot pump more than 15 GPM. It would still be easier on the pump to start it against a valve closed to 1 GPM, but it would complicate the control setup on a system that needs to pump wide open into a storage tank or something.

Another thing you can do is use the longest length of the smallest wire possible for that size pump. If you use 250' of #14 wire on a 1HP pump, it works like a reduced voltage soft starter, which reduces the torque by 36% and the starting current by 20%. This would make starting easier on a pump that has an open pipe to discharge. But you still need to have enough head to keep it out of upthrust.

 

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[Reach4]

For geothermal heating and cooling, you want to pay attention to pump efficiency. Don't oversize your pump. Consider using a 3-wire pump with a control box that has a run capacitor as well as a start capacitor.

For geothermal, which you did not explicitly point out, I would not want higher resistance wire for power efficiency reasons.