Centralizers( Centralizers for pipes ) 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 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 . www.bjfilter.com/