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SELECTION OFF SCREEN DIAMETERS
The designed yield of a tubewell is 150m²/h (1.5 Cusecs) and a screen having 20% open area is proposed to be used in a length of 20m to tap an acquifer of 25m thick. Diameter, of the screen is to be worked out.
  As Q = A x V

Where   Q = discharge in m²/s,    A = effective open area of the screen   
   V = entrance velocity in m/s (to be ltd. to 0.03 m/s)

Therfore, A = 150/3600 x 1/0.03 = 1.39m²
Taking effective area as 50% of that provided, in order to taking into account the reduction of the effective area of the screen opening with time due to incrustation rearrangement of the acquifer particles around the screen and coverage by gravel, etc.

	Grains size curves of a typical aquifer materials covering sand are shown in figure :
	Very Fine Sand Fine sand Medium Sand Coarse Sand very Coarse Sand Gravel All Dimensions are in Millimeters

The required open area = 2 x 1.39 = 2.78m²
Open area of pipes = x d x l x open area
Where d = dia of pipe,   l = length of pipe
Therfore, d = 2.78/ x 20 x 20 = 0.22m or 220mm

The required sceen dia works out 220mm.

SELECTION OF SLOT SIZE
Acquifer Material Classification - The acqifer material may be classified into various categories according to the following ranges of the particle size:

Fine Gravel Coarse sand Medium sand Fine Sand Silt Clay

- above 2.0 mm 0.5 - 2.0 mm 0.25 - 0.5 mm 0.05 - 0.25 mm 0.002 - 0.06 mm below 0.000 mm

Typical Slot Pattern
ALL DIMENSIONS ARE IN MILLIMETERS Developed view of a typical well screen plan slotted type	ALL DIMENSIONS ARE IN MILLIMETERS Slotting Arrangements

All dimensions are in millimetres

All dimensions are in millimetres

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