A Guide To Water Well Casing and Screen Selection   -   18

7.3  Wire Wrap

One well-known well screen is manufactured by wrapping a wire around longitudinal rods. The wire is welded to the rods by resistance welding producing a cage-shaped cylindrical configuration. This type of screen, commonly known as wire-wrap or continuous slot, is usually manufactured from type 304 stainless steel, galvanized steel and carbon steel.

The continuous slot design originated in the early part of this century to overcome the problems of ground-water development from distinctive aquifers associated with the North Central United States. These aquifers were generated from rock picked up, broken, and pulverized by advancing glaciers during the Ice Ages. While glacial till is not well sorted, occasionally thin layers of fine-grained, uniform sands were washed from the original deposits. Such materials are high yielding water producing formations. Prior to the advent of rotary well construction and the gravel envelope well design, it was difficult to produce the full capacity of sand free water from these aquifers. A well design incorporating wire-wrap screen was developed to meet these conditions, and proved successful.

The characteristics of wire-wrap screen are well suited for its original purpose. This design offers the highest surface area of opening of any screen. Consequently, with very small aperture sized (.005 in. to .035 in.) necessary to control fine sands from thin aquifers without a gravel envelope, sufficient area of opening is still available to minimize frictional head losses through the screen. However, under such circumstances, stainless steel must be used since enlargement of openings result in sand pumping. The manufacturing process lends itself to close tolerances required for very fine aperture sized and the V-shaped slot configuration reduces clogging.

Careful consideration must be made of the use of wire-wrap screen in situations or under conditions for which it was not originally designed. It is generally more costly than other types without necessarily providing higher production, better efficiency or greater durability. Screen of this design, including most higher cost, heavy duty versions with bigger rods, wire or both, has lower collapsing strength than other screen types and usually lower than the well casing it is installed with. Special care should be taken in installation.

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