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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