Shale shaker screens have changed as demands on the shale shaker have increased. Shaker screens have three primary requirements:
. High liquid and solids handling capacity
. Acceptable life
. Ability to be easily identified and compared
Early shale shaker screens had to last a long time. This demand was consistent with the shaker designs and solids-removal philosophies of the period. Shakers could remove only the large, coarse solids from the drilling fluid, sand trap, and reserve pit, while downstream hydrocyclones (if utilized) removed the bulk of the drilled solids.
Drilling-fluid changes, environmental constraints, and a better understanding of solids/liquid separation have modified the role of the shale shaker. Generally, the effectiveness of the downstream equipment is greater when more solids are removed at the flowline. Reserve pits can be smaller or in most cases eliminated. Cleanup costs are lower than not removing the solids at the flowline and overall drilling efficiency is increased.
As important as the mechanical aspects of new-design shale shakers may be, improvements in screen panels and screen cloths have also significantly increased shaker performance—shakers of older design have benefited from these improvements as well. Two design changes have been made to extend the economic limit of fine-screen operation:
(1) a coarse backing screen, which protects the fine screen from being damaged, extends life, and provides additional support for heavy solids loading.
(2) tensioned cloth bonded to a screen panel.
The most important advance in screen panel technology has been the development of pretensioned screen panels. Similar panels have been used on mud cleaners since their introduction. Earlier shakers did not possess the engineering design to allow their use. With the advent of linear motion machines, the pretensioned panels extended screen life and permitted more routine use of API 200 screens.
Pretensioned panels consist of a fine-screen layer (or layers) and a coarse backing cloth bonded to a support grid. The screen cloths are pulled tight, or tensioned, in both directions during the fabrication process. This ensures the beginning of proper tension of every screen. Correct installation procedures and post-run retightening of screen panels can add significantly to shaker performance and screen life.
Manufacturers employ different geometric apertures in screen panel design. Some of the more common panel shapes are square, rectangular, hexagonal, and oval. The apertures in the panels can vary from 1-inch to 3-inch squares to 733-inch rectangles to 1.94-inch hexagons to 26-inch ovals.
The panels can be flexible (of thin-gauge metal or plastic) to be stretched over crowned shakers, or they can be flat (of heavy-gauge mechanical tubing) for installation, as on flat-decked (noncrowned) shakers.
Regardless of configuration, the function of the pretensioned panel is to provide mechanical support for the fine-screen cloth bonded to it, and at the same time occlude as little potential flow area as possible with the supporting grid structure.
Some screened panels are made with no support grid at all, but simply by bonding of the finer-mesh cloth directly to a coarser backing wire using a heat-sensitive adhesive. Essentially this becomes a hook-strip design, with certain support refinements.
Pretensioned screen panels address two of the three original design goals: capacity and screen life. The remaining goal of easy identification is a function of better labeling techniques to display important screen characteristics.