A Guide To Water Well Casing and Screen Selection   -   21

7.5  Shutter Screen

One type of screen designed many years ago for use in gravel envelope wells has enjoyed considerable success. Shutter screen was originally manufactured by punch-formed downward facing louver apertures into short lengths of pope, then welding them together forming sections up to 20 feet long. A more modern method has been developed which permits manufacture from tubes up to 50 feet in length. This process incorporates the use of an internal mandrel which perforates the shutter against external die blocks.

Shutter screen is manufactured in a variety of patterns with various surface percentage area of openings. This flexibility permits designing to meet real conditions and eliminates unnecessary cost.

One of the major advantages of shutter screen is that it has higher collapsing strength (up to 60%) than blank casing of the same wall thickness. This is due to the corrugating effect of the louver-shaped openings. Surprisingly, this strength increases as the percentage areas of opening found in commercially available shutter screen increases. Resistance to collapse does not vary according to aperture size, as in the case of wire-wrap screen.

Another important advantage of shutter screen is that tight swabbing can be employed to develop and redevelop wells without risk. This is due to high mechanical strength and a full circular cylinder interior. For the same reason, wells can be repaired and deepened more easily.

Shutter screens are available which afford much lower entrance velocities than required, minimizing head losses. The downward facing louver aperture is particularly suited for gravel envelope wells, and offers special advantages over other opening configurations. Use of down-the-hole cameras, model studies and years of experience confirm that the louver-shaped opening resists clogging as well as, or better than, any other type of screen. Its shape is the same for any wall thickness material. Better gravel control during installation, development and operation is characteristic of shutter screen because of the hood shaped, downward facing orifice. Tolerance of filter pack range is enhanced. Many wells have been successfully completed with greater than 80% of the pack capable of passing through the openings. While the recommended relationship is 35% to 20% passing, this latitude provides protection against variations in the gravel envelope due to segregation or other reasons.

Well screen is an important component of well design. It must be realized that it is not an answer to all well problems. In a gravel envelope well, it is the gravel that filters the formation material and not the screen. The screen simply retains the gravel pack. Screen selection must be based not only on theoretical considerations, but on a practical relationship to the design and construction of an efficient, durable installation. The best screen design cannot automatically correct incompleted gravel installation or well development. Screen can only facilitate the construction of an efficient well and help assure its satisfactory operation.

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