My wife is very much into eco solutions - we are wanting to pump gray water from our washing machine out to our garden. (Aside: We do use a plant friendly detergent.)

We are wanting to pump a length of 20 meters. The diameter of the outlet / flow pipe is 22mm, but when we tried pumping this distance (using a 20 meter length of pipe @ 22mm) - it really sounded like the pump was struggling. We then tried 34mm (which was much much better) - before finally ended up using 38mm diameter diverter.

Could someone please let me know the basic Physics involved here ? Ideally we would like to know the "optimal" diameter to use - how can you calculate it ?

P.S. If this is the wrong stack exchange to be asking in please let me know. Maybe Engineering is a better fit.

With Thanks in Advance


You can use an Online Pressure Drop Calculator. I used the following assumptions:

Flow volume: 6 m^3/h (20 l/minute) Roughness (plastic pipe): 0.0014 mm

When I used 22 mm as the diameter, I get a pressure drop of 1610 mbar (1.6 atm) over 20 length. With 38 mm diameter, the drop is only 117 mbar, less than 10%.

The formula used for the calculation is the Darcy–Weisbach equation

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  • $\begingroup$ Wonderful - thank you very much for your help! $\endgroup$ – Patrick Rynhart Feb 25 at 3:54

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