A Guide To Water Well Casing and Screen Selection   -   26

8.8  Casing Guides

Casing Guides Casing guides are used to center the screen within the borehole. They should be of sufficient strength and surface area to provide support, yet not impede the installation of gravel. These are conflicting requirements, and some compromise is required. A simple guide, which has proven effective, is manufactured from 5/16 x 2 inch steel, 30 inches long, bent to provide the proper centering distance. This distance or bend is usually the theoretical borehole radius minus the screen radius minus one inch. The guides are attached to the screen by welding. Three or four guides are placed equidistantly around the screen at 40 foot intervals. Normally, they are not installed on the upper or pump housing casing.


8.9  Float Plates

Float Plates Float plates may be installed in casing strings, where the weight of the casing and screen exceed the safe lifting capacity of the installing rig. The plate, which must be manufactured from a frangible or breakable material, is installed between two joints of pump housing casing at a predetermined depth, where the collapsing strength of the casing is not exceed and the rig is not overloaded. The weight of the casing is reduced by the weight of the fluid displaced. Cast iron plates of the design shown, machined to provide a watertight fit between the casing joints, have been found satisfactory.

The use of welding collars simplifies the installation of float plates considerably. To insure that hydrostatic forces on the empty casing above the float plate do not exceed its collapsing strength, the casing may be partially filled with water during installation as the buoyancy increases. Once the casing and screen is installed the upper pump housing casing is completely filled with water and the plate removed by striking with a bailer, drill pipe or tubing. Float plates must be used with great caution, and under no circumstances are as safe a procedure as direct installation with equipment of adequate weight bearing capacity.


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