Some of the common cloth weaves available in the petroleum drilling industry are the plain square weave, the plain rectangular weave, and the modified rectangular weave. These are simple over/under weaves in both directions, which can be made from the same or different wire diameters. By making the spacing between the wires the same in both directions, a square weave is created. By making the spacing in one direction longer than the spacing in the other direction, a rectangular weave is made. Plain square and rectangular weaves are often referred to by the
number of wires (the same as openings) in each direction per linear inch. This is the mesh count. Mesh is determined by starting at one wire center and counting the number of openings along the screen grid to another wire center 1 linear inch away. For example, an 8 mesh screen has 8 openings per inch in two directions at right angles to each other. When counting mesh, a magnifying glass scale designed for the purpose is helpful.
Use of a single number for screen description implies square mesh. For example, ‘‘20 mesh’’ is usually understood to describe a screen having 20 openings per inch in either direction along the screen grid. Oblong mesh screens are generally labeled with two numbers. A ‘‘6020 mesh,’’ for example, is usually understood to have 60 openings per inch in one direction and 20 openings per inch in the perpendicular direction.
Referring to a 6020 mesh screen as an ‘‘oblong 80 mesh’’ is confusing and inaccurate. The actual separation that a screen makes is largely determined by the size of the openings in the screen. The opening size of a square mesh screen is the distance between wires measured along the screen grid, expressed in either fractions of an inch or microns. Screen opening size is most often stated in microns. One inch equals 25,400 microns. Specifying the mesh count does not specify the opening size! This is because both the number of wires per inch and the size of the wires determine the opening size. If the mesh count and wire diameter are known,the opening size can be calculated as follows:
D =25,400{(1/n)-d}
where
. D=opening size, in microns
. n=mesh count, in number of wires per inch (1 per inch)
. d=wire diameter, in inches
The preceding equation indicates that screens of the same mesh count may have different-size openings depending on the diameter of the wire used to weave the screen cloth. Smaller-diameter wire results in larger screen openings, and larger particles will pass through the screen. Such a screen will pass more drilling fluid than an equivalent mesh screen made of larger-diameter wires.
A specialty weave screen is available that consists of large-diameter wires in the long direction and multiple bunches of fine wires in the narrow direction. The long, narrow openings provide low flow resistance and remove spherical and chunky solids.
Layered screens were introduced into the industry in the late seventies. Layered screens are often chosen because they provide a high liquid throughput and a resistance to blinding by drilled solids lodging in the openings. A layered screen is the result of two or more wire cloths overlaying each other. Square and rectangular cloths can be layered.
Reducing the diameter of the wires increases liquid throughput. A large assortment of opening sizes and shapes are produced by the multiple screen layers and the particular screen wire diameter. Layered screens have a wide variety of opening shapes and sizes, and therefore a wide variety of sizes of particles pass through the screen.
In 1993 a three-dimensional surface screen was introduced into the industry. This screen surface is corrugated and supported by a rigid frame for use primarily on linear motion shale shakers. As drilling fluid flows down these screens, the solids are moved along in the valleys, and the vertical surfaces provide additional area for drilling fluid to pass. This increases the fluid capacity of a particular mesh size when compared with a flat surface screen.
In summary, specifying the mesh count of a screen does not indicate screen separation performance, since screen opening size, not mesh count, determines the sizes of particles separated by the screen. Because there are almost an infinite number of mesh counts and wire diameters, screen manufacturers have simplified the selection by offering several standard types of cloth series, such as MG, tensile bolting cloth (TBC), and extra-fine wire cloth (XF), as shown in Table 7.4. Notice in this table that an MG 80 cloth has an opening size of 181 microns, whereas
a TBC 80 has an opening size of 222 microns. The MG 80 cloth has a smaller opening size than the TBC 80 because the MG cloth’s heavier wires take up some of its opening space. As a result, an MG 80 cloth can remove smaller solids than a TBC 80. Furthermore, as a result of the larger wires, the MG 80 cloth will be more resistant to abrasion and will last longer. The major drawback of the MG 80 compared with the TBC 80 is that it will allow less than half the flow rate. That is (see Table 7.5), the screen conductance (ability to transmit fluid) for the
TBC 80 is 7.04 kilodarcy/mm, whereas for the MG 80 it is 2.91 kilodarcy/ mm. Similar comparisons can be made between the separation/fluid conductance of the TBCs relative to the XF cloths. For instance, a single layer of XF 180 screen cloth has almost the same opening size as a single layer of TBC 165. The XF 180 screen could pass 72% more flow!
The screen life of the XF 180 will most likely be shorter than the TBC 165, since the wire diameter of the XF 180 is 30.5 microns and that of the TBC 165 is 48.3 microns. Also, the larger openings would remove fewer drilled solids even though they would pass a larger quantity of fluid.
The National Bureau of Standards has a sieve series that is often used to describe screen opening sizes (Table 7.4). The opening size of this test series plots on uniform increments on semilog paper, making it ideal for use in plotting particle size distributions. Shaker screens used in the industry may be assigned an equivalent National Bureau of Standards sieve mesh count according to their opening sizes as shown in Table 7.4.
From the discussion above, it should be abundantly clear that mesh count alone does not specify the screen opening size. As a result, if mesh count is used, it must be accompanied by a designation of wire diameter, such asMG(mesh count) þ mesh count, TBC þ mesh count, or National Bureau of Standards Test Sieves equivalent mesh count. One other complicating factor enters with shale shaker screens: Layered screens do not have uniform opening sizes in either direction of the screen.
This is the reason that the API has developed a procedure to identify screens. Just as opening size has been used to measure separation performance, the percentage of open area of a single-layered screen has been used to indicate liquid throughput. The percentage of open area, or the portion.
Table 7.4
U.S. STANDARD SIEVE SERIES FOR WIRE CLOTH
d=16:84% of particles this size will pass through the screen;
d=50:50% of particles this size will pass through the screen;
d=84:16% of particles this size will pass through the screen.
of screen surface not block by wire, is calulated as follows:
P=(O)(o)(100)/(O+D)(o+d)
where
P=percentage of open area
O=length of opening in one direction along the screen grid (inches)
o=length of opening along screen grid perpendicular to the O direction (inches)
D=diameter of wire perpendicular to the O direction (inches)
d=diameter of wire perpendicular to the o direction (inches)
Although open area can be used to indicate the ability of a screen to transmit fluid, a better measure of the ability of a screen to pass fluid is the conductance (or equivalent permeability of the screen cloth). Conductance takes into account both the openings and the drag of the fluid on the wires. (Conductance is discussed later in this text.) For years there was confusion in screen designations. Mesh count and percentage of open area simply did not adequately quantify screen cloth performance. Deceptive marketing practices were common. Furthermore,
with the advent of the layered cloths, which have a range of hole sizes, there simply were no standards against which to compare screens.
