Engineering:Compressed air requirements in down-the-hole drilling

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Compressed air requirements in down-the-hole drilling refer to the pressure and air-flow conditions required to operate a down-the-hole (DTH) hammer and remove rock cuttings from a borehole.

In DTH drilling, compressed air performs two main functions. It supplies energy to the hammer piston, which repeatedly strikes the drill bit, and it acts as the flushing medium that carries broken rock from the bottom of the hole to the surface. Correct air-system sizing therefore affects penetration rate, hammer performance, hole cleaning, fuel consumption, and drilling reliability.

The main parameters used when sizing the air supply are working pressure, free air delivery (FAD), hammer size, hole diameter, drill-pipe diameter, drilling depth, hose losses, altitude, and ambient temperature.[1]

Role of compressed air in DTH drilling

A DTH hammer is positioned directly behind the drill bit. Compressed air travels from the compressor through the hose, rotary head and drill string before entering the hammer.

The supplied air performs several functions:

  • drives the hammer piston;
  • exhausts through passages in the drill bit;
  • cleans broken rock from the bit face;
  • transports cuttings upward through the annular space around the drill pipe;
  • helps cool and clean the hammer and drill bit.

Pressure and air flow perform different functions. Insufficient pressure can reduce hammer impact performance, while insufficient flow can cause poor hole cleaning even when the hammer is still operating.

Main sizing parameters

The compressed-air requirement of a DTH drilling system depends on several interacting parameters:

  • Hammer size – Larger DTH hammers normally require greater air consumption.
  • Hole diameter – A larger borehole creates a larger annular area and generally requires more flushing air.
  • Working pressure – Determines the pressure available to operate the hammer.
  • Free air delivery (FAD) – Represents the usable air output of the compressor under defined reference conditions.
  • Drill-pipe diameter – Together with hole diameter, determines the annular area through which cuttings return.
  • Drilling depth – Longer air paths increase pressure losses and can reduce flushing efficiency.
  • Altitude and temperature – Changes in air density can affect compressor capacity.
  • Hose and pipe resistance – Long or undersized flow passages reduce the pressure available at the hammer.

Pressure and flow should therefore be evaluated together rather than as independent specifications.

Pressure requirement

The required operating pressure is primarily determined by the design of the DTH hammer and the drilling conditions.

Increasing pressure can increase the energy available to the hammer and may improve penetration in suitable formations. However, increasing pressure without sufficient air flow does not necessarily improve hole cleaning.

Modern DTH drilling equipment may operate at different pressure and flow settings. For example, Sandvik specifies flushing-air performance for the Leopard DI560 DTH drill rig at both 24 bar and 30 bar operating conditions.[2]

The pressure shown at the compressor outlet is also not necessarily the pressure received by the hammer. Pressure is lost as air passes through hoses, valves, fittings, drill pipes and other restrictions. Compressor selection therefore normally includes a margin for these losses.

Air-flow requirement and hole cleaning

Air flow is especially important for removing cuttings from the borehole.

After leaving the hammer, exhaust air moves upward through the annular space between the borehole wall and the outside of the drill pipe. The approximate annular air velocity can be expressed as:

v=QA

where:

  • v is the average annular air velocity;
  • Q is the volumetric air-flow rate;
  • A is the annular cross-sectional area.

For a circular hole and drill pipe, the annular area can be approximated by:

A=π4(Dh2−Dp2)

where Dh is the borehole diameter and Dp is the outside diameter of the drill pipe.

If hole diameter increases while air flow remains unchanged, the annular area becomes larger and return-air velocity decreases. This can make cuttings removal less effective.

Actual flushing requirements also depend on rock density, particle size, moisture, hole inclination, depth, leakage and the hammer manufacturer's recommendations.

Free air delivery

Free air delivery (FAD) is commonly used to describe the delivered capacity of an air compressor. It expresses compressor output in relation to defined inlet or reference conditions and is useful for comparing compressor capacity.[3]

For DTH applications, FAD is generally more useful than theoretical compressor displacement.

A compressor can have sufficient pressure but insufficient FAD. In such a case, the hammer may operate while the borehole is poorly cleaned. Conversely, high air flow with insufficient pressure may not provide the hammer with its required operating condition.

Pressure losses in hoses and drill pipes

Pressure loss increases as air moves through the delivery system. Important factors include:

  • hose diameter;
  • hose length;
  • drill-pipe internal diameter;
  • air-flow rate;
  • bends and fittings;
  • valves and restrictions;
  • internal surface condition.

High flow through an undersized hose can produce significant pressure loss. This may cause the compressor pressure gauge to show an acceptable value while the actual pressure at the hammer is lower.

Larger flow passages and shorter hose arrangements are therefore generally preferred when high air delivery is required.

Altitude and ambient temperature

Compressor performance also depends on atmospheric conditions.

At higher altitude, atmospheric pressure and air density decrease. The compressor therefore draws in less air mass for the same inlet volume. High ambient temperature also reduces air density and increases the thermal load on the compressor cooling system.

A compressor selected for sea-level operation may consequently provide less effective capacity at a high-altitude drilling site. Manufacturer derating data should be considered when selecting equipment for extreme altitude or temperature.

Practical sizing procedure

A practical compressed-air sizing process for DTH drilling is:

  1. identify the DTH hammer and bit diameter;
  2. obtain the hammer manufacturer's recommended pressure and air consumption;
  3. determine the borehole and drill-pipe diameters;
  4. evaluate the annular area and flushing requirement;
  5. consider drilling depth and expected hole conditions;
  6. estimate pressure losses through hoses and drill pipes;
  7. correct for altitude and ambient temperature where necessary;
  8. select compressor working pressure and FAD with an appropriate operating margin.

Selecting the compressor only by engine power, maximum pressure, or nominal flow can lead to poor drilling performance. The complete air path from compressor inlet to borehole exhaust should be treated as one system.

See also

References

  1. ↑ Atlas Copco: Understanding air compressor measurements – work, power and flow.
  2. ↑ Sandvik Mining and Rock Solutions: Leopard DI560 down-the-hole drill rig.
  3. ↑ Atlas Copco: Understanding air compressor measurements – work, power and flow.




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