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nfcoetzee 02-12-2012 01:08 PM

Deep well pump power
Does anyone know the basis for the recommended 2 HP limit on 1.25" sch 120 pvc deep well drop pipe? I assume it is due to stress on the pipe caused by torque during motor startup. If so, what is the peak power ratio assumed during startup? I'd like to use a 3HP pump on 1.25" pipe and use a VFD/soft starter to minimize startup loads. I have asked 2 or 3 of the sch 120 pipe manufacturers this question and they have so far not been able to answer it......

joecaption 02-12-2012 01:20 PM

My question is why do you think you need the added HP?

nfcoetzee 02-12-2012 01:31 PM

To achieve the flow rate that I need.

Akpsdvan 02-12-2012 02:11 PM

And the flow rate needed?

On the constant pressure pumps of today, it is more about the stages along with the HP..

Larger HP does not always mean more gpm.

joecaption 02-12-2012 02:21 PM

Has you needs changed that require more flow or is the problum really just low pressure in the house?

nfcoetzee 02-12-2012 02:50 PM

Thanks, but we're kind of getting away from the original question. Does anybody know the technical basis for the 2 HP limit on 1.25" sch 120 pvc pipe?

Yoyizit 02-12-2012 02:59 PM


Originally Posted by nfcoetzee (Post 851600)
Thanks, but we're kind of getting away from the original question. Does anybody know the technical basis for the 2 HP limit on 1.25" sch 120 pvc pipe?

Probably repeated flexing of the pipe through some arc limits pipe life, so it would be a pipe failure due to fatigue.
If you've ever energized a starter motor that was not restrained, conservation of momentum gives you a pretty violent twist.
See if you can get through to a pump applications engineer. It's in their interest to help you.
Soft start is the only way I know of solving this and the slower the motor speed ramps up, the less flexing.

nfcoetzee 02-12-2012 03:16 PM

Thanks - you're probably right that it is related to repeated flexure. When I calculate the angle of twist it is about .1 degree per foot of pipe length for the 3 HP motor.

Akpsdvan 02-12-2012 04:20 PM

Has no one put any Adjustable torque stops on the pipe between the pump and the pitless adapter?

Yoyizit 02-12-2012 04:28 PM

Or maybe someone makes a motor with a second, counter-rotating, armature.

Akpsdvan 02-12-2012 04:31 PM

There might be on the large commercial pumps, but none of the well drillers that I know have said any thing about that for the smaller pumps, they use a few well placed torque stops on deep wells if needed.

nfcoetzee 02-12-2012 05:41 PM

Thanks. From what I can gather, the torque arrestor's main function is to keep the pump centered in the casing - is that not correct? And as far as I can tell, the 2HP limit is placed by the manufacturer without relating it to the torque arrestors. It would be interesting to know if the power limit is based on angle of twist as suggested earlier, shear stress or tensile stress. I assume that all of these peak during startup - probably reaching 3 or 4 times the steady state levels, which I calculate to be a little over 50 psi for the 2 HP. That seems pretty low compared to pvc yield of 8000 to 10000 psi.

Javiles 02-12-2012 06:44 PM

They make dampers for that problem torque at start up and run vibration.
I always use schedule 80 PVC not 40 on pump installs

nfcoetzee 02-12-2012 08:21 PM

Thanks. I'm using Sch 120, not 40 or 80.

Javiles 02-12-2012 08:34 PM

The last time we used 120 was on an industrial lift station and a 3 phase 30 hp dual pump configuration. Is this for you residential domestic water pump? Now just curious.

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