Page 198 - Air and gas Drilling Field Guide 3rd Edition

8.2 Minimum Volumetric Flow Rate and Compressor Selection 189

8.2.2 Engineering Practice
As shown in Figure 8-4, and the theoretical and experimental studies in the
pneumatic conveying literature and at the University of Tulsa and Pennsylvania
State University, it is clear that the higher the velocity and specific weight of
the gas in the vertical flow line, the greater the particle velocities approach the
average velocity of the gas flow.
For an air or gas drilling operation, this is also the desirable transport situation.
The choke situation occurs when the gas volumetric flow rate decreases and
cutting particles begin to slip in to the gas flow transporting them. This causes
the rock cutting particles to transition from the dilute phase of solids in the gas
flow (where the particles are spread out in the gas) to that of a dense phase of
solids (where the particles are clumped together) (see Figures 8-2 and 8-3).
Figure 8-4 can be used to further understand the choking condition and is a
schematic representation of empirical data. Line A–B in Figure 8-4 refers to zero
solids flow in the pipe (in our case the annulus). The family of curves in Figure 8-
4 shows increased solid flow rates as the weight rate of flow increases. At fixed

solids weight rate flow G 1 at a high gas velocity (at point C), the solids volumetric
concentration is low (well below 1%) and the particles are generally uniformly
dispersed and moving near the average velocity of the gas. This is dilute phase
solids flow.
As shown in Chapters 6 and 7, the pressure gradient in fully developed ver-
tical conveying is made up of two components: a wall frictional loss component
and a hydrostatic weight component. As the velocity of the gas is decreased for
a fixed solids flow rate, the solids volumetric concentration in the pipe
increases and the wall frictional loss decreases. Thus, line C–D shows an overall
pressure gradient decrease with the decrease in gas velocity. This line indicates
that the decrease in frictional loss component is significantly higher than the
increase in the hydrostatic component. As the gas velocity is further decreased
beyond point D, the hydrostatic component becomes more significant and the
resulting curve defines a minimum at point E. A further decrease in the gas
flow rate (and velocity) leads to an increase in the pressure gradient. This is
shown as the curve E–F and indicates the dominance of the hydrostatic compo-
nent. The solids concentration along curve E–F is high and is dense phase solids
flow.
In general, for air and gas drilling systems, the choking point can be defined
as the inflection point along the curve E–F. Fortunately, the choking condition
is rarely observed in actual vertical air or gas drilling operations. This is due to
two important air and gas drilling operational conditions. (1) As the drill bit
advances, the rotating drill string breaks up larger rock particles into smaller,
more easily transported particles when the larger particles collide with the drill
string surface. This breaking action occurs all along the drill string length,
but is likely very pronounced around and just above the drill collars. This mecha-
nism has been observed in vertical drilling operations where downhole