Shale shaker screens made of two or three layers of screen cloth of different mesh sizes present openings that cannot be easily characterized. A technique to describe these openings has been adopted by the API as the ‘‘Recommended Practice for Designations of Shale Shaker Screens,’’ API RP 13C, to be issued soon. This recommended practice supersedes the second edition (1985) of API RP 13E, which was valid for only single-layer screens.
The new designation system was chosen to convey information on screen opening size distribution and the ability of nonvibrating screens to pass fluid. Information for each of the following is legibly stamped on a tag attached to the screen panel in such a way as to be visible after the screen is installed on the shale shaker:
. Manufacturer’s designation
. API number
. Flow capacity
. Screen conductance
. Total nonblanked area
The screen manufacturer may name a particular screen in any manner it desires. This designation is used when ordering a shaker screen with particular characteristics.
Shaker screen designation has been complicated by the advent of multilayered screens. When two or three screens are layered together, the opening sizes are not uniform. Experience has shown that the flow rate through these layered screens is much higher than anticipated, and the solids-removal rate is maintained. Since the screens have different irregular shapes, the standard mesh equivalent cannot be used to describe the screens. API RP 13C was recently rewritten by a task group composed of most of the authors of this book. The task group selected a mechanical method of designating shale shaker screens and comparing them to equivalent square mesh opening sizes. This section describes this method for determining the API U.S. sieve number equivalent of a shaker screen using a laboratory sieve shaker,
U.S. Standard Test Sieves, and sized grit samples. Screens are rated on the U.S. sieve number scale by the separations they achieve in dry sieving standard grit samples and comparing these separations to the separations of the same standard grit samples with standard U.S. sieves.
For example, a shaker screen that separates the grit sample similar to a U.S. 100 mesh test sieve is designated an API 100. Standard U.S. Test Sieves applicable to this procedure are as follows:
. Hold the test screen securely between the top and bottom parts, which are designed to bolt together and to nest with regular 8-inch U.S. test sieves.
. Arrange the sieves in consecutive order with the coarsest on top and the finest on the bottom. Nest the sieve stack with the sieve pan on the bottom.
. Place the grit test sample on the top sieve, cover it, and shake it for about 5 minutes with the RoTap test sieve shaker. Determine the weight of the grit remaining on the test screen. The fraction of the weight sample retained on the test screen determines the API screen number. Calculate the cumulative weight percentage retained for each individual sieve (beginning with the coarsest) by summing up the results.
. Prepare a plot of cumulative weight percentage retained versus the U.S. sieve opening (in microns) using a linear plot from point to point.
. Sieve and size the test grit through square mesh ASTM (American Society for Testing and Materials) screens. Place equal quantities of five different sizes of the test grit on a test screen on a RoTap for 5 minutes. The quantity and sizes of solids presented to the test screen would be:
(1) no solids from an ASTM 80 mesh (180 microns);
(2) 10 g from an ASTM 100 mesh (150 microns);
(3) 10 g from an ASTM 120 mesh (125 microns);
(4) 10 g from an ASTM 140 mesh (106 microns);
(5) 10 g from an ASTM 170 mesh (90 microns);
(6) 10 g froman ASTM 200 mesh (75 microns).
Present the total sample of 50 g of solids to the test screen, shaken for 5 minutes on a RoTap, and weight the residue on the screen.
. Graphically determine the D100 separation, in microns, from the plot. The value of the D100 separation usually falls between two U.S. sieve openings.
. When the D100 separation falls at a point that is 0.5 or less of the difference between the openings of a finer and the next coarser consecutive U.S. sieve, rate the test screen as the finer U.S. test sieve.
When the D100 separation falls at a point that is more than 0.5 of the difference between the openings of a finer and the next coarser consecutive U.S. sieve, rate the test screen as the coarser U.S. test sieve. For example, if the D100 separation is between a U.S. 170 (90 microns) and a U.S. 200 (75 microns), the test screen is rated as an API 170 if the D100 separation is greater than 82.5 microns, and as an API 200 if the D100 separation is 82.5 microns or less. API numbers are assigned with the following D100 separations, in microns.
In the graph that follows, 33 weight percentage of the grit sample was captured on the test screen. This screen would have an API number of 140 and an opening size of 102 microns.
The designation would be an API number of 140, with the actual separation point of 102 microns in parentheses:
API 140 (102 microns)
The ‘‘mesh’’ designation is now called an API number so that the new designation will be more rig-user friendly. Rig crews recognize ‘‘mesh size’’ even though they may not actually know the definition. The change is necessary, since the API Recommended Practices are being converted to International Standards Organization (ISO) documents. ISO uses the metric system, consequently the number of openings per inch would need to be converted to the number of openings per centimeter or meter. This number would be meaningless to most rig crews.
The API designation number is specified to appear at least three times larger in physical appearance than any other letters or numbers on the screen tag. The ASTM 140 screen has openings of 106 microns, and an ASTM 170 screen has openings of 90 microns. The number in parentheses will indicate that the screen designation was actually measured and provide an indication of how close the openings are to standard screens.
Flow Capacity/Screen Conductance
A screen that makes an extra-fine separation is not useful in the drilling industry if it will not pass a high-volume flow rate. The amount of fluid that a screen will process is dependent on the screen construction as well as solids conveyance, solids loading, pool depth, deck motion and acceleration, drilling-fluid properties, and screen blinding. Although it is difficult to calculate the expected fluid processing capacity of a shaker, screens can be ranked according to their ability to transmit fluid.
Conductance is a measure of the ease with which fluid flows through a screen cloth. It is analogous to permeability per unit thickness of the screen, C¼k(darcy)/l(mm). To calculate the flow through a porous medium, Darcy’s law is used as follows:
V = K*Δρ/(μ*l)
Now conductance, C, can be calculated where Q=V*A as follows:
. C=conductance (darcy/cm)
. K=permeability (darcy)
. l=screen thickness (cm)
. V=velocity (cm/sec)
. μ=fluid viscosity (cP)
. Δρ=pressure drop across screen (atm)
. Q=volume flow rate (cm3/sec)
. A=screen area (cm2)
Higher conductances mean that for a given pressure drop across the screen, more fluid is able to pass through the screen.
To measure the conductance, a 50-gal container of motor oil is mounted above the test screen. A flow valve is adjusted so that some of the oil overflows the screen into catch pans outside the apparatus.
The oil that flows through the screen is captured in a container on a balance. The weight of the container and oil is observed and recorded. When the flow becomes steady and uniform, the weight of the oil flowing through the screen is measured as a function of time. The temperature of the oil is measured and is kept constant. The density and viscosity of the oil as a function of temperature is determined prior to the test.
From the height of overflow fluid above the test screen, the head can be measured. From the density/temperature charts, the pressure applied to the screen can be calculated. From the density/temperature charts and the weight measurements, the volume of motor oil flowing per unit of time can be calculated. Care is taken to ensure a low flow rate to prevent turbulence in the oil flowing through the screen. From these measurements and the equations described above, the permeability per unit thickness of the screen can be calculated. This is the conductance.
Total Nonblanked Area
Continuous cloth screens present all available screen area to the drilling fluid to remove solids. Panels are popular because screen tears are minimized and limited to only one small area of the screening surface. The screen panels, however, remove some of the screening area that would be available with continuous cloth screens. The nonblanked area allows an evaluation of the surface area available for liquid transmission through the screen.