Common Screen Cloth Weaves

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.

SHAKER SCREEN CLOTHS

Shale shakers remove solids by processing solids-laden drilling fluid over
the surface of a vibrating shaker screen. Particles smaller than the shaker screen openings pass through the screen along with the liquid phase of the drilling fluid.

 

Larger particles are separated into the shaker overflow for discard.
The shaker screen acts as a ‘go no-go’ gauge. That is, particles larger than
the screen openings remain on the screen and are discarded. Particles
finer than the screen openings go through the screen with the drilling
fluid. The criterion for early shale shaker screens was a long screen
life. This demand for screen life was consistent with the shaker designs
and solids-removal philosophies of the time period. Early shale shakers
could remove only large solids from the drilling fluid. The sand trap,
reserve and settling pits, and downstream hydrocyclones (if utilized)
removed the bulk of drilled solids. Today’s shale shakers are capable
of utilizing finer screens that remove more solids. Desirable characteristics
for a shaker screen are:
1. Economical drilled-solids removal
2. Large liquid flow rate capacity
3. Plugging and blinding resistance
4. Acceptable service life
5. Easy identification
For any particular shale shaker, the size and shape of the shaker screen openings have a great effect on solids removal. This means that the performance of any shaker is largely controlled by the screen cloth used.
The first four items in the preceding list are largely controlled by
choice of screen cloth and by the screen panel technology. Large gains
in shale shaker performance are a direct result of improved screen
cloth and panel fabrication. shaker Screens used on shale shakers have evolved into complex opening patterns.

 

SHALE SHAKER USER’S GUIDE

Every solids-removal system should have enough shale shakers to process 100% of the drilling-fluid circulating rate. In all cases, consult the owner’s manual for correct installation, operation, and maintenance 154 Drilling Fluids Processing Handbook procedures. If an owner’s manual is not available, the following general guidelines may be helpful in observing proper procedures.

Continue reading “SHALE SHAKER USER’S GUIDE”

DRYER SHAKERS

The dryer shaker, or dryer, is a linear motion shaker used to minimize the volume of liquid associated with drilled cuttings discharged from the main rig shakers and hydrocyclones. The liquid removed by the dryers is returned to the active system.

Why use dryer shaker

Dryers were introduced with the closed-loop mud systems and environmental efforts to reduce liquid-waste haul-off. Two methods, chemical and mechanical, are available to minimize liquid discharge. The chemical method uses a system called a dewatering unit, while the mechanical method takes place through linear motion shakers. These systems may be used separately or together.

The main function of drying shaker

The dryer shaker deliquifies drilled cuttings initially separated by another piece of solids-separation equipment. These drilled solids can be discharged from the main shaker or a bank of hydrocyclones. Dryers recover liquid discharged with solids in normal liquid/solids separation that would have been previously discarded from the mud system. This liquid contains colloidal solids, and the effect on drilling-fluid properties must be considered since dewatering systems are frequently needed to flocculate, coagulate, and remove these solids.

The dryer family incorporates pieces of equipment long used as independent units: the main linear motion shaker, the desander, and the desilter, which are combined in several configurations to discharge their discard across the fine screens (e.g., API 200) of a linear motion shaker to capture the associated liquid. These units, formerly used as mud cleaners, are mounted on the mud tanks, usually in line with the main linear motion shaker. They can be tied into the flowline to assist with fine screening when not being used as dryers. Their pumps take suction from the same compartments as desanders and desilters and discharge their overflow (effluent) into the proper downstream compartments.

How to use it

A linear motion dryer may be used to remove the excess liquid from the main shaker discharge. The flow rate across a linear motion dryer is substantially smaller than the flow rate across the main shaker. The lower flow rate permits the removal of the excess fluid by the linear motion dryer by using a finer screen. The dryer is usually mounted at a lower level than the other solids-separation equipment to use gravity to transport solids to it. Whether by slide or by conveyor, the cuttings dump into a large hopper, located above the screen, that replaces the back tank, or possum belly. As the cuttings convey along with the screen, they are again liquefied. This excess fluid, with the fine solids that passed through the screens, is collected in a shallow tank that takes the place of a normal sump. The liquid is pumped to a catch tank that acts as the feed for a centrifuge or back to the active system.

A dryer unit can be used to remove the excess fluid from the underflow of a bank of hydrocyclones (desanders or desilters). This arrangement resembles a mud cleaner system. In this configuration, the dryer unit may be used on either a weighted or an unweighted mud system. The liquid recovered by the linear motion shaker under the hydrocyclones can be processed by a centrifuge, as previously described.

How to select a proper one for your mud solution

The perfection of the linear motion shaker for drilling-fluid use, coupled with advanced fine-screen manufacturing technology, has made these dryers very efficient. In most configurations, the dryers use the same style of screens, motors, and/or motor/vibration combinations as do other linear motion shakers by the same manufacturer.

Depending on the fluid, saving previously discarded liquid may be financially advantageous. The dryer discard is relatively dry and can be handled by backhoe and dump truck rather than by vacuum truck.

Drilling-fluid properties must be monitored properly when the recovered liquid is returned to the active system. Large quantities of colloidal solids may be recovered with the liquid. This could affect the PV, YP, and gel strengths of a drilling fluid.