Tim Van
New Member
See the attached table, which I had handy. Note the shutoff PSI, which represents the deadhead pressure at a given depth to water.
There are charts available for other pump sizes. In 10 GPM pumps, I think 1 HP would be plenty. Even 3/4 HP might be a good fit. If you add the CSV, allow maybe 10 ft or 5 PSI to compensate for a bit of resistance .
For irrigation, it is best if the load consumes most of the available GPM. For mixed use, that is not as easy to balance. I think there is a need for a valve to shut off irrigation loads if the water pressure drops below some setpoint, such as 45 PSI. Then it should turn back on when the pressure rises. That way household use would have priority. I don't know if such a valve exists at consumer prices. The CSV is nice for irrigation because it makes up for the varying load.
http://www.pressure-drop.com/Online-Calculator/ lets you predict a pressure drop. I would use 1 mm for the pipe roughness unless you have more info. You can select the units with the drop-downs. Take defaults, and don't forget to select US units at the bottom. I got 11.3 PSI drop for 20 GPM and 50 ft. I used 50 ft instead of 20 ft to compensate for fittings or whatever, and to make a more conservative estimate.
Good stuff! Thanks Reach4!