The fluid circulation across the bit face must be designed to remove the cuttings efficiently and also to cool the bit face. These requirements may be satisfied by increasing the fluid flowrate and/or the design of the water courses that run across the face of the bit. This increased fluid flow may however cause excessive erosion of the face and premature bit failure. More than three jets are generally used on a PDC bit.
Drilling fluid passes from the drillstring and out through nozzles in the bit. As it passes across the face of the bit it carries the drilled cutting from the cones and into the annulus. The original design for rock bits only allowed the drilling mud to be ejected from the middle of the bit (Figure 12). This was not very efficient and led to a build up of cuttings on the face of the bit (bit balling) and cone erosion. A more efficient method of cleaning the face of the bit was therefore introduced. The fluid is now generally ejected through three jet nozzles around the outside of the bit body (Figure 13). The turbulence created by the jet streams is enough to clean the cutters and allow efficient drilling to continue.
Jet nozzles (Figure 15) are small rings of tungsten carbide and are available in many sizes. The outside diameter of the ring is standard so that the nozzle can fit into any bit size. The size of the nozzle refers to the inner diameter of the ring. Nozzles are available in many sizes although diameters of less than 7/32″ are not recommended, since they are easily plugged. The nozzles are easily replaced and are fitted with an “O” ring seal (Figures 17). Extended nozzles (Figure 16) may also be used to improve the cleaning action . The nozzles are made of tungsten carbide to prevent fluid erosion.