Page 250 - Air and Gas Drilling Manual

5-92 Air and Gas Drilling Manual
Using this same injection calculation procedure, the injection pressures for
drilling at depths less than 1,200 ft can be obtained.
Figure 5-21 shows the derated fixed pressure output of the compressor and the
injection pressure to the drill string as a function of drilling time (or drilling depth).
Compressor output pressures that are different from the actual injection pressures is a
unique characteristic of rotary compressor systems. The fixed internal design of the
rotary compressor dictates a fixed pressure output from the compressor regardless of
the back pressure resistance (assuming the back pressure is less than the fixed
pressure output). In this case the back pressure resistance is the injection pressure.
Therefore, as air exits the compressor it decompresses when it passes into the surge
tank at the exit of the compressor to match the injection pressure resistance in the
flow line to the dual wall pipe annulus.

Figure 5-21: Compressor output pressure and dual wall pipe annulus injection pressure
for Illustrative Example 5.6c.