Hook-strip screens have been mounted with both underslung and overslung supports. Some previous generations of oilfield shale shaker designs used screens that were underslung, or pulled up from the bottom of a group of support, or ‘‘bucker,’’ bars (Figure 7.15). These support bars would divide the flow of material down the screen. Some problem is experienced occasionally when solids are trapped under the rubber bar supports.
Some linear motion shale shakers utilize overslung screens (Figure 7.16). With this approach, screens are attached to the bed of the shaker by being pulled down onto the bed from the top. This results in a screening area completely free of obstacles. Modern shale shaker bed design has also increased the number of support ribs located beneath the screen to aid in fine-screen support and to reduce the amount of ‘‘crown,’’ or ‘‘bow,’’ necessary to properly tension screen panels. Some problem is experienced occasionally when the fluid leaves the high center of the screen and flows down the sides of the screen.
Most circular motion shale shakers were built with a double deck, meaning that fluid flowed over and through the top screen onto a finer screen immediately below. This arrangement led to some problems in operation, because the bottom screen was not easily visible. (Generally a flashlight was needed to inspect it.) A torn screen could remain in operation for a long time before it was noticed and changed. This created problems with solids removal because the bottom screen would not provide
the intended finer screening. Some manufacturers installed backflow pans under the top screen to direct the flow through the entire screen area of the bottom screen, but these just made it even more difficult to see the bottom screen.
Most manufacturers of linear motion shale shakers have adopted a single-deck design. The units have clear visibility for ease of care and maintenance. This unobstructed approach also makes screen changing much easier. The fluid pool tends to obscure any torn screens until drill pipe connections are made. Therefore, a torn screen on a single deck shaker reduces solids-removal efficiency until a new screen is installed.
Crews need to be alert to torn screens no matter what shaker is used. This is especially true during slow drilling, when drill pipe connections are infrequently made. When riser-assist pumps are used, flow should be periodically directed to different shakers during connections. This allows screens to be properly inspected and replaced, if needed.