Choosing screens

Choosing screens for water wells

Choise water screens for filter pack in water wells Paparelli

The choosing of screen slot dimensions for water wells depends essentially on the gravel pack grain-size distribution and this latter dependes on the grainsize distribuzion of the water-beraing soil.

In case of wells where it is not possible to have an artificial gravel pack (rotary drilled wells) it is neessary to choose the screen so that 30-40%of the water-bearing soil can be extracted in order to make an area of high permeability around the screens; therefore, the size of the slots must be nearly equal to the valve d30 - d40 on the soil grain-size distribution.

In case of artificial gravel pack, there are various methods, but the easier one is the following:
multiply the value d30 by 4 if the soil is fine and homogeneous (U < 2,5), by 6 if the soil is big and heterogeneus (U> 2,5) and by 9 if the soil is fine and heterogeneus; dans the value obtained on the same diagram in corrispondence with d30 and the trace a grain-size line passing throught this point and having a value of U< 2,5 for soil U>2,5 or U=2,5 for soil U>2,5.
The screen slots for water wells must be such as to retain not less than 90% gravel, that means it is necessary to have b<= D10