Pumping data
How much water can you get through a well screen?
Well screens have different open areas, depending upon the construction and design of the screen. There has been much research and field experimentation in the last 80 years to try to work out the optimum flow rate through the slots in a screen, for different grain size and permeability aquifers, to minimise friction head losses, corrosion, erosion by fine sand or silt, or encrustation.
Various authorities have presented safe, optimum or recommended entrance velocities. These range from a generally accepted optimum of 3cm/second to an upper limit of 45cm/second. It is not possible to be more precise because every borehole and aquifer is different.
All authorities stress the value of local knowledge amongst drillers, hydrogeologists and engineers. Experience of what works best locally is usually more successful than theoretical calculations.
The well screen slots in contact with the natural or artificial gravel pack are always partially blocked by the granular material in the gravel pack. Therefore the entrance velocity, or the amount of water that can get through the screen slot is not determined solely by the aperture of the slot.
Instead, it is controlled by the apertures between the grains of sand or gravel jammed up against, or partially, into the slot. Biofilms or encrustation can build up over time and further reduce the size of open holes either in the gravel pack or the screen. These factors cannot be controlled or predicted accurately. Therefore the table below is a simple conservative guide to the amount of water that can obtained for a length of one metre length of 1 mm slot screen and a two metre drawdown. These flow rates can be exceeded, if a clean gravel pack can be developed with large well rounded grains around a screen with wider slots.
Pumping Data For PVC Screen
Listed below is approximate data relating to the volume of water that can be pumped through a 1 metre length of screen with 1 mm slots (with a 2 metre drop in the water level), all based on practical experience and results.
| diameter mm | Ø inches | m3/h | l/min. | igpm | usgpm |
| 60 90 113 125 140 165 200 225 250 280 315 330 355 400 450 500 |
2 3 4 4.5 5 6 7 8 9 10 11 12 13 14 16 18 |
3 4.5 6 7.5 8.5 10 12 13.5 19.5 22 27 29 31 35 40 45 |
50 75 100 125 142 167 200 225 325 367 450 483 517 583 667 750 |
11 17 22 28 31 37 44 50 72 81 99 106 114 128 147 165 |
13 20 26 33 37 43 52 59 85 95 117 126 134 152 173 195 |