Fluid Circulation
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.
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Figure 12 Fluid circulation through water courses |
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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.
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Bit Worn But Not Underguage
Hard Facing Figure 14 Hard facing for gauge protection |
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Figure 15 Jet nozzles |
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Figure 16 Extended nozzles |
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Figure 17 Nozzle wrench for installing nozzle and "O" ring |
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The five major components of PDC bit design are
• Cutting Material
• Bit Body Material
• Cutter Rake
• Bit Profile
• Cutter Density
• Cutter Exposure
• Fluid Circulation
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