Page 81 - Air and gas Drilling Field Guide 3rd Edition

72 CHAPTER 4 Downhole Equipment

FIGURE 4-11. Two typical air hammer bits with a concave face (courtesy of Rock Bit
International Incorporated).

Figure 4-11 shows two typical air hammer 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
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bit. In Figure 4-11, the larger bit (standing on its shank end) is an 8 8 = -in
(219 mm)-diameter concave bit and the smaller bit (on its side) is a 6-in (152 mm)-
diameter concave bit.
The drill string with an air hammer must be rotated just like a drill string that
utilizes tricone or single cone drill bits. 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. This rotation allows a different position on the rock face
to receive the impact load transferred from the piston through the hammer
shank. In direct circulation operations, air flow passes through the hammer sec-
tion, through the drill bit channel and orifices to the annulus. As the air passes
into the annulus, the flow entrains the rock cuttings and carries the cuttings to
the surface in the annulus. Direct circulation air hammers are available in a wide
variety of outside housing diameters from 3 in (76 mm)to16in(406 mm).
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These air hammers drill boreholes with diameters from 3 8 = in (92 mm)to
1
17 2 = in (445 mm).
There are also reverse circulation air hammers. These unique air hammers
allow air pressure in the annulus to actuate the hammer via ports in the outside
housing of the hammer. The reverse circulation air hammer bits are designed with