TDH Calculation review

[rogerstv]

I have to check the TDH calculations for fourteen extraction well pumps discharging to a treatment system building. It has been a long time since I performed any calculations of this nature. And, I have no one at my place of employment to review my work. Will someone please double check that I am doing the work correctly?

Here is one of the scenarios...

Submersible pump is 85 feet below ground surface.
Ground surface elevation = 847 feet
Discharge elevation = 810 feet
Discharge is to a vented equalization tank. So, discharge pressure = 0
Design Q = 10 gpm
Well pipe is 1.25" diameter
219 feet of 2" HDPE
1287 feet of 3" HDPE
125 feet of 2" equivalent length losses from fittings

TDH = 89.9 feet = 85 + 3.02 + 1.24 + 0.65 = static head + FL well pipe + FL 2" + FL 3"

I used a static head of 85 feet since the pump has to lift the water to the ground surface before it flows downhill to the treatment building. Am I thinking correctly?

 

[bimr]

You are pumping from elevation 762 ft (847-85) to elevation 810 ft, so the static head is 48 ft.
.
However, your pump must be supplied with enough power to pump against the 81 ft when the pump starts. Once the pump starts, the fluid is recovering the energy as it moves downhill (assuming you have a full discharge pipe).

The pump operating point will move leftward (less capacity) on the pump curve when the pump starts, and then as the pipe fills, the operating point will mover rightward (more capacity) on the curve.

 

[cvg]

don't forget about the height of the tank.

 

[rogerstv]

Thank you. Will do.

 

[77JQX]

I think I find myself in the rare circumstance of disagreeing with bimr. You can only recover pressure going downhill if you don't go vapor at high points. It would be helpful to plot a profile of your pipe elevation and head profile to see if the elevation is high enough to go vapor ("Going vapor" means the pipe elevation is higher than the head at that point in the pipe (accounting for friction, atmosphgeric pressure and vapor pressure). Since your 2" and 3" pipe friction only amounts to 1.89 feet, I think it's safe to say that you go vapor at the high point. I see two choices - move the tank to the high point, or use a pressure sustain at the outlet to push the head profile high enough to avoid going vapor.

 

[bimr]

Don't think we disagree and you have made many fine posts in the past. The flow may be going gravity once he passes the high point. That is why I said assuming a full pipe.

 

[cvg]

you will need a vacuum break at the top of the hill if you want to rely on gravity flow. or just throttle a valve at the tank inlet to sustain pressure