Figure 3-11 shows two typical air hammer bits that would be used with direct circulation air hammers. These air hammers drill boreholes with diameters from 7 7/8 inches to 33 inches. Reverse circulation air hammers are available in larger outside housing diameters (6 inches to 24 inches). The reverse circulation air hammer bits are designed with two large orifices in the bit face that allow the return air flow with entrained rock cuttings to flow to the inside of the drill string and then to the surface. These unique air hammers allow air pressure in the annulus to actuate the hammer via ports in the outside housing of the hammer. There are also reverse circulation air hammers. These air hammers drill boreholes with diameters from 3 5/8 inches to 17 1/2 inches. Direct circulation air hammers are available in a wide variety of outside housing diameters (3 inches to 16 inches). As the air passes into the annulus, the flow entrains the rock cuttings and carries the cuttings to the surface in the annulus. In direct circulation operations, air flow passes through the hammer section, through the drill bit channel and orifices to the annulus. This rotation allows a different position on the rock face to receive the impact load as the upper end of the hammer bit is struck by the hammer. The rotation of the drill string allows the inserts (i.e., tungsten carbide studs) on the bit face to move to a different location on the rock face surface. The air hammer drill string must be rotated just like a drill string that utilizes tri-cone or single cone drill bits. The air hammer is made up to the bottom of the drill string and at the bottom of the air hammer is the air hammer bit. Therefore, air hammer drilling operations require far less WOB as comparable drilling operations using tri-cone or single cone drill bits. But in this situation, the crushing action is dynamic and is more effective than the quasi-static crushing action of tri-cone and single cone drill bits. This impact load creates a crushing action on the rock face very similar to that discussed above at the beginning of Section 3.2.2. The drill bit in turn transfers this impact load to the rock face of the bit. In the downward stroke, the hammer strikes the bottom of the upper end of the drill bit shaft (via a coupling shaft) and imparts an impact load to the drill bit. The internal piston moves up and down in a chamber under the action of air pressure applied either below or above the piston through ports in the inside of the air hammer. The air hammer utilizes an internal piston (or hammer) that is actuated by the compressed air (or other gas) flow inside the drill string. The use of percussion air hammers (or down-the-hole air hammers) is an acceptable option to using rotating tri-cone or single cone drill bits for air and gas drilling operations. Percussion air hammers have a distinct advantage over roller cutter bits in drilling abrasive, hard rock formations. In the past decade, however, the percussion air hammers have seen increasing use in drilling deep oil and natural gas wells. These shallow operations have been directed at the drilling of water wells, monitoring wells, geotechnical boreholes, and mining boreholes. Percussion air hammers have been used for decades in shallow air drilling operations.
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