BAKER HUGHES - Drilling Fluids Reference Manual

MECHANICAL SOLIDS CONTROL
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 (ft 2 )
• Manufacturer’ Designation / Part Number
Optional but recommended information includes:
• 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 to all screen panels and will call for,
• cut point at D-50 and D-95
• conductance
• 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 the specified micron size. D-95 indicates
that 95% are smaller and 5% are larger than the micron size specified. See Figure 10-4 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 10-3 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.
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 10-9
REFERENCE MANUAL