A screen that enables an extra-fine separation is useless 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.
Screen conductance is a measure of the ease that fluid flows through a screen cloth. It is analogous to permeability per unit thickness of the screen, C = k(darcy)/l(mm). To calculate the flow capacity through a porous medium, Darcy's law is used as follows:
Spherical diameter, microns
FIGURE 6-7
Spherical diameter, microns
FIGURE 6-7
Conductance, C, can be calculated where Q = V x A as follows:
where C = Conductance (darcy/cm) K = Permeability (darcy) 1 = Screen thickness (cm) V = Velocity (cm/sec) ^ = Fluid viscosity (centipoise) Ap = Pressure drop across screen (atm) Q = Volume flow rate (cm3/sec) A = Screen area (cm2)
Higher conductances indicates that for a given pressure drop across the screen, more fluid is able to pass through the screen. Conductance for a screen can be calculated by the equations reported in API RP 13E, 3rd edition, May 1, 1995, Section 1.1. These equations are based on experimental data using a water-glycerol mixture. Conductance is measured experimentally using the following equation, where 0.014375 is required for unit compatibility:
where Q = Volume flow rate (gpm)
^ = Fluid viscosity (centipoise) A = Screen area (in.2) Ap = Pressure drop across screen (lb/in.2)
Most screens used by the drilling industry consist of one or two fine screening cloths in combination with a coarse backing cloth. The conductance of these screens can be calculated as follows:
where C, = Conductance of first screen (kilodarcy/mm) C2 = Conductance of second screen
(kilodarcy/mm) C3 = Conductance of third screen (kilodarcy/mm)
The conductance of commonly used oil field screens and backing cloths is shown in Table 6-8. The conductance of common oil field screens in the later section on Screen Panel Technology may be calculated using the above equation in conjunction with Table 6-8.
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