Hook-strip shaker screens (named for the method of hooked edging that
provides the tension along the screen) are also available. Because of the
superior life characteristics of panel mount units, Hook-strip shaker screen has been relegated to a minor role on linear motion machines. They are used
extensively on circular and unbalanced elliptical motion machines.
Proper tensioning (and frequent retensioning) of all types of screens is
good screen management and adds significantly to screen life. Individual
manufacturer’s operation manuals should be consulted to obtain the
proper installation methods and torque requirements, where applicable,
for specific screens/panels.
Several types of bonded screens are available. The repairable perforated
plate screen has one or more layers of fine mesh cloth bonded to a sheet
of metal or plastic with punched, patterned holes. Perforated plate
designs are available in various opening sizes and patterns. Additional
designs include a special application in which backing and fine-screen
materials are bonded together, eliminating the need for perforated
plates. Flat-surfaced pretensioned screen panels are becoming more even
tensioned, easy to install, and capable of even distribution of liquids and
solids across the screen deck.
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.
The materials used to weave the cloth screens are quite varied. Screens are made from metal wires, plastic wires, and molded plastic cloths.
Alloys that are most weavable and resistant to corrosion are nickel/chrome steels; 304, 304L, 316, and 316L. These alloy wires are available in sizes down to 20 microns. The finest wire available is 304L, which is available to 16 microns. Other materials, including phosphor bronze, brass, copper, monel, nickel, aluminum alloys, plain steel, and plated steel, are also available. Within the drilling industry, 304 stainless cloth is the most common.
Two types of synthetic screens are available: woven synthetic polymer
and molded one-piece cloth, called a platform.
Conventional looms can be used to weave synthetic polymer screens. Polymers, such as polyesters, polypropylene, and nylon, are drawn into strings having diameters comparable to those of wire gauges and woven into screen cloth. Synthetic shaker screens exhibit substantial stretch when mounted and used on shale shakers. Because of this, plastic screen openings are not as precise, although this variability is not nearly as great as in layered metal steel screens.
One-piece injection molded synthetic cloths are typically made from urethane compounds. These synthetic cloths have limited chemical and heat resistance but display excellent abrasion resistance. The designs range from simply supported molded parts having very few open areas to complex structures with up to 55% open area. Molded cloths are
very popular in the mining industry, where abrasion resistance is important.
These screens make a coarser separation than screens used in the oilfield. Development of molded cloth screens capable of making a fine separation that have heat and chemical resistance necessary for oilfield application is under way.
Cloth selection for shale shaker screens involves compromises among separation, throughput, and screen life.
Screen blinding occurs when grains of solids being screened find a hole in the screen just large enough to get stuck in. This often occurs during the drilling of fine sands in the Gulf of Mexico. The following sequence is
often observed during screen blinding:
. When a new screen is installed, the circulating drilling fluid falls through the screen a short distance. . After anywhere from a few minutes to even several hours, the fluid endpoint slowly or even quickly travels to the end of the shaker. . Once this occurs, the screens must be changed to eliminate the rapid discharge of drilling mud off the end of the shaker. . After the screens have been washed, fine grains of sand are observed stuck in the screen.
. The surface of the screen will feel like fine sandpaper because of the
sand particles stuck in the openings.
Most every screen used in the oilfield is blinded to some extent by the time it has worn out. This is the reason that when the same screen size is reinstalled, the fluid falls through the screen closer to the feed.
A common solution to screen blinding is to change to a finer or coarser screen than the one being blinded. This tactic is successful if the sand that is being drilled has a narrow size distribution. Another solution is to
change to a rectangular screen, although rectangular screens can also blind, with multiple grains of sand. Unfortunately, the process of finding a screen that will not blind is expensive.
In the late seventies the layered screen was introduced to avoid screen blinding. This hook-strip type of screen was mounted on a downhill sloping unbalanced elliptical motion shale shaker vibrating at 3600 rpm. The two fine layers of screening cloth, supported at 4-inch intervals, tended to dislodge fine grains of sand and would blind only about 25% of the screen in severe laboratory tests, leaving 75% of the screen nonblinded! The nonblinding feature is assumed to be the result of the deceleration of the two screens. The wire diameter is in the range of
0.002 inch and the opening sizes are in the range of 0.004 inch. In the upward thrust of a layered screen, the screens must come to a stop at the upward end of the motion. They would tend to each have an inertia that
would prevent them from stopping at exactly the same time. This would create an opening size slightly larger than the original opening size of the layered screen during the upward part of the thrust. Solids would be expelled from the screen. On the downward thrust of the motion, the two layers remain together until the screen starts deceleration. At the bottom of the stroke, again the inertial forces could cause the screens to slightly
separate, allowing larger solids to pass through the screen. This probably also explains why the separation cut point curve shows poorer separation characteristics for a layered screen than for a single square mesh
screen. Many particles larger and smaller than the median opening size are found in the discard from a layered screen.
Unfortunately, the downhill sloping basket and high frequency limitsthe amount of liquid that can pass through the screen. Furthermore,lost circulation material has a high propensity to get stuck in the screen due to the high-frequency, short-stroke vibration. These problems have been ameliorated by reducing the vibration to 1800 rpm and flattening the basket slope. In the early 1980s, linear motion was introduced so that solids could march up an incline out of a pool of liquid. This fluid pool provided additional pressure to force fluid through the screen.
Unfortunately, linear motion, combined with marginal support, tore layered screens apart. The only way to obtain satisfactory screen life ona linear motion machine was to support the layered screen in 1-inch squares.