Vibrating screens (shale shakers) are the first mechanical devices in the solids control system to process the fluid. Maximizing the removal of solids at this point will increase the efficiency of the remainder of the solids equipment utilized.
In unweighted fluid systems, maximum removal with shakers will reduce the solids loading on the desander, thus lowering the chance of an overload condition, which in turn, will lower the chance of overloading the downstream desilter. The cascading effect of a solids control system points out the importance of having each piece of equipment operate to its maximum potential, thus allowing the downstream units to do the same. Since the operation of the shaker affects the remainder of the equipment, it must be sized and utilized correctly for the system to remove the maximum amount of solids.
Fine screen shakers should be used on any operation where optimum solids control is desired, but they are particularly suited for operations employing weighted fluids, high-cost fluid-phase fluids, and oil-base fluids. When weight material is carried in the fluid system, the options for removing solids without removing the weight material are limited to screening and centrifuging. If weighted oil fluids are used, then even centrifuging is not feasible because of economic and environmental reasons. Dual centrifuging an oil or synthetic is done to strip and clean mud plant fluids In this case, it is very important to have optimum solids removal at the shale shaker. It would be contrary to sound engineering practices to spend thousands of dollars for an oil-base fluid system or any inhibited system to retain “cuttings integrity” without utilizing the most efficient solids control equipment available. An inhibited system cannot achieve its full potential and be cost-effective if the cuttings are re-circulated and ground down into very fine particle sizes.
Two basic designations are used for shakers presently in the field – rig (scalper shaker) and high-speed shakers. Normal operation of scalper shakers would typically employ the use of screens in the 10- to 40-mesh range. The removal of large drill cuttings at this stage will increase the efficiency of the downstream high-speed shakers. The term high-speed refers to a unit that is capable of operating with fine screens. These units also are commonly referred to as linear motion, premium, and high-efficiency shakers. The term high-speed had its origin with some of the early improved shakers that operated with speeds in the range of 3600+ rpm. Although very few shakers have this speed on the vibrator shaft at present, the term stuck. Linear motion refers to the action that the vibrating assembly transmits to the screen deck, in this case a reciprocating motion. Figure 10-2 demonstrates three typical screen motions.
Regardless of the particulars of the machine, the most important point to remember is that the goal is to remove the maximum amount of solids, and this is done by running a screen with the smallest openings. If we assumed the average particle size of formation solids (in fluid returns) was 80% finer than 838 microns and 70% larger than 178 microns, theoretically only 20% could be removed with the 20-mesh screen, whereas 70% could be rejected with the 80-mesh screen.
Fine shaker screens come in a multitude of designations, and no single measurement adequately describes a screen’s solids removal capabilities and throughput. Mesh is the linear measurement of the number of openings per inch. The aperture is the actual opening dimension which controls the maximum size particle that can pass through a screen. Two screens with the same mesh, if woven with different diameter wires, will have different apertures.
The current API screen designation system states that no fewer than the following minimum elements be stated:
- Equivalent Aperture in microns
- Conductance (Kilodarcies/mm)
- Non-blanked area (ft²)
- Manufacturer’ Designation / Part Number
Optional but recommended information include：
- Manufacturer’s name
- Country of manufacture or assembly
This description is sufficient for single-layered screens, but with the advent of multiple-layered screens and bonded screens, this designation falls short in describing the effective cut point of a screen, the ability to pass fluid and the actual screen area available to pass fluid.
A new description is recommended to be attached o all screen panels and will call for,
- cut point at D-50 and D-95
- percentage of total screen area available for screening.
A D-50 cut point is defined as the point measured in microns where 50 volume % of the solids are larger than the size specified and 50% are smaller than then specified micron size. D-95 indicates that 95% are smaller and 5% are larger than the micron size specified. See figure 1 for some typical solids distribution curves and the indicated D-50 points.
Conductance is defined as the permeability of the screen cloth divided by the thickness of the cloth, and is usually given in kilodarcys per millimeter (kd/mm). Permeability of the screen is a function of the opening size and the wire geometry. Basically, it is a measure of the ease with which fluid will flow through the screen. No units will be indicated on screen conductance designation, as this will be a relative measurement.
Commonly manufactured screen cloths come in square mesh and rectangular mesh. These are used to build screens in many combinations from single layers, for the coarser meshes, to every conceivable combination of square and rectangular meshes stacked together. The square mesh removes more randomly shaped solids than a rectangular mesh having the same minimum aperture dimension. For the same minimum aperture on one dimension, the rectangular screen has a higher fluid capacity since the percent open area is greater. Also, the rectangular screen can be woven of heavier wires, thus offering a potential for longer screen life. This longer life and increased throughput versus lower solids removal potential are typical tradeoffs to be considered when selecting a screen.
Table 1 lists U.S. Test Sieve Numbers. These sieve numbers each have a standard opening size given in microns. The sieve number refers to the mesh count of the given screen. Cut points of screens will be referred to commonly either using the sieve number or the micron cut point.
|Test sieve NO.||Opening μm||Test sieve NO.||Opening μm|
Table ASTM Sieve Designation³
The number usually given in screen designations can be misleading. It can refer to micron opening size, number of openings per inch, U.S. Test Sieve equivalent opening size, sum of wires in each direction, or average cut point related to U.S. Test Sieve screens.
Two common screen cloths used for the manufacture of screens are market grade (MG) and tensil bolting cloth (TBC). These terms refer to wire sizes used in the manufacturing of the screens, market grade being a coarser wire than tensil bolting cloth. Some manufacturers use wire diameters finer than tensil bolting cloth to achieve a greater open area on the screen while maintaining the mesh count. This results in a weaker wire that will pass larger solid particles.