Modeling a Groundwater Well 2

[esass13]

How do you model a pump in a groundwater well?

I'm thinking you set a resevoir elevation at the normal water drawdown level and you set the pump elevation at it's actual elevation (ground elevation - pump setting depth).

Our consultant is setting the resevoir below the pump and that creates a negative intake pressure for the pump. That just doesn't seem right even for a model.

Any ideas? Thanks for the help!

 

[cvg]

you are correct, your consultant is confused. Maybe he is confusing the motor with the actual pump which is located below ground.

 

[thewellguy]

I'm not sure I understand your question.

There is the static level, the non-pumping water level.
The pumping level, the level that the well drops to when pump is running.


There are three types of pumps that are used in wells
1)Jet pumps, motor above ground with a suction pipe.
2)Submersibles, pump and motor underwater.
3)Line shaft turbines, motor above pump below.

All pumps have to have the intake below the pumping level.

 

[LHA]

I, like thewellguy, am not sure I understand your question.

It sounds like both you and your consultant are thinking correctly. There would be a negative beginning pressure (NPSH available) and the pumps will have to overcome that, for the reasons you've stated. That value is the amount, as you've stated, "...the normal water drawdown level and...the pump elevation at it's actual elevation...". This sounds like what the consultant's model has done.

The intake has to be below the water, but the pump does not.

For hydraulic analysis, the elevation at which the energy is added to the water is the pump elevation. The elevation at which the potential energy exists is reservoir elevation is the drawn-down water surface. That is the beginning head.

Engineering is the practice of the art of science - Steve

 

[kchayfie]

You are correct. If the pump is physically located below the water level (i.e. option 2 or 3 from thewellguy's list) then your consultant is mistaken. If you are using a pump in the option 1 style then you are both correct.

 

[esass13]

Maybe I can clear my question up a bit. As an example:
ground elevation = 500'
actual pump elevation = 300' (200' deep in well)
actual water pumping level elev. = 350'

This is a submersible pump/motor situation

I would think in the WaterCAD model you set the pump elev. at 300' and the resevoir elev. at 350'.

My consultant is setting the pump at 300' and the resevoir at some lower elev. like 240'

This is just an example, not actual numbers.

thanks for the help

 

[LHA]

Ask where he got the lower elev. like 240'. Maybe it is justified for conservative design, perhaps it is a verifiable historical low water level, during a drought. You'll need to flush the toilets, even if it doesn't rain

If that is the case, make sure the motor choosen some type of internal cooling provision. Submersibles often depend on external cooling, and ground water might be a lot colder than the ambient air, or even the air in the well shaft.


Engineering is the practice of the art of science - Steve

 

[DMcGrath]

In your example you are correct at how the model should be set up. I connect the "reservoir" to my submersible pump with a big (12"), short (1') pipe to essentially eliminate any suction head. What I have yet to figure out is how to model well drawdown without just manually adjusting the reservoir level each time.

 

[esass13]

Thanks DMcGrath.

Well drawdowns are so variable (in my experience anyway) that I don't know how you can model them accurately.

 

[cvg]

In my experience, trying to simulate well drawdown in a water system network analysis was unnecessary. For the systems I have modeled, generally the wells pumped into transmission mains to fill gravity reservoirs. The reservoirs provided the supply at a relatively constant pressure to the service zones. So the modeling of a well with drawdown might only be useful for sizing the pump and motor to be installed.

 

[thewellguy]

What is your goal with the model? Do just want of "picture" of how it works or are you trying to determine something else? Also what type of well is it? e.g. screen, limestone, sandstone