Page 140 - Drilling Technology in Nontechnical Language

Chapter 5 – RIG SELECTION AND RIG EQUIPMENT 131

As mud moves round the inside of the cone top, it eventually comes
back to the inlet pipe position, where more mud is coming in. This forces the
mud stream downwards, into the cone. As the cone gets narrower, the fluid

speed has to increase to accommodate the flow rate. Very high centrifugal

forces are exerted on the fluid stream, so the heavier solids particles will

move towards the outside of the fluid stream—moving towards the cone.

As the fluid stream nears the bottom, pressure builds up to the point where

the fluid changes direction and starts back upwards, spiralling up inside
the descending mud, which stays close to the cone inside surface. The
solids particles, being heavier, cannot change direction so readily and are
ejected at the bottom of the cone. The cleaned mud stream exits at the top

of the cone, out of the overflow opening.
All of this happens very quickly—it will take only about 1/6 of a second

from the mud entering the cone at the inlet to it exiting at the overflow.
Larger cones process larger volumes of mud and remove larger
particles. Smaller cones process smaller volumes (per cone) but can remove

finer particles. Most rigs will have a set of 3 or 4 large cones, around
12" diameter at the top. These are called desanders because they remove
particles of sand grain size. Most rigs will also have a set of perhaps 16 or
20 small cones, 4" diameter at the top. These are called desilters because

they remove particles of silt grain size (fig. 5–17).

Fig. 5–17. A bank of 4" desilter hydrocyclones

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