Figure 29-9. A bit severely eroded by the mudstream. 



erosion of the nozzle-retaining ring and the head-section material adjacent to 

 the exit end of the nozzle. If conditions that cause this erosion persist, the sup- 

 port for the ring will be eroded away, and the nozzle will be blown out of the 

 bit. Failures of this kind are associated with high drilling rates and inadequate 

 jet velocities that promote balling-up. The mud stream is deflected by the 

 balled-up formation around the cutters so that it strikes the face of the nozzle 

 and causes rapid erosion. Failures of this kind can be prevented by adjusting 

 the operating conditions. 



The cones on the bit in Figure 29-9 have been eroded severely by the mud 

 stream from the conventional drilled water courses. In a bit having conventional 

 drilled water courses, the nozzles are positioned so that the drilling fluid is 

 directed onto the cones for cleaning purposes, usually at velocities of 100 feet 



649 



