Annular velocity in air and DTH drilling

What annular velocity means in air drilling

In conventional direct-circulation air drilling, compressed air travels down the drill pipe, powers the DTH hammer, leaves through the bit and returns up the annulus carrying cuttings to the surface. The speed of that returning air is the uphole or annular velocity.

If the velocity is too low, cuttings are not lifted cleanly. They regrind at the bit, fall back when air is shut off, pack around the hammer and can cause stuck pipe. If it is much higher than needed, the compressor is larger and burns more fuel than the hole requires, and erosion of the hole wall and the bit increases.

A velocity figure is only meaningful if you know how it was calculated. The same hole can give very different numbers depending on whether the airflow used is free air from the data sheet or the actual compressed volume at a point down the hole.

The basic equation and its units

For a circular hole around circular drill pipe:

• Annular area: A = π × (D² − d²) ÷ 4
• Annular velocity: V = Q ÷ A

D is the hole diameter, d is the outside diameter of the drill pipe, and Q is the airflow on the stated basis.

For field use with diameters in inches and flow in cfm, the equation becomes:

V (ft/min) = 183.35 × Q ÷ (D² − d²)

The factor 183.35 combines the π/4 area term with the conversion from square inches to square feet (144 × 4 ÷ π). Using 144 alone would leave out the circular geometry and overstate the area. Remember to convert millimetres to metres before squaring when working in SI units.

The result is an average across the section. It does not describe the velocity distribution around an off-centre pipe or the local changes at tool joints, stabilisers or the hammer body.

Typical velocity guidance

For dry air drilling, a widely used rule of thumb is an uphole velocity of at least about 3,000 ft/min (about 15 m/s), calculated on free air delivery. Higher velocities are commonly used for wet holes, heavy or coarse cuttings, and when foam or mist is not being used. Some drillers work to 4,000–5,000 ft/min (about 20–25 m/s) in difficult ground.

These are practical guidance values, not guarantees. The hammer and bit maker’s recommendations, the formation and the drilling method take priority. Treat any target as a starting point to be checked against how the hole actually cleans.

Worked example: one hole, two different velocities

Take an 8 inch (203 mm) hole drilled with 4½ inch (114 mm) drill pipe, and a compressor delivering 900 cfm (about 25.5 m³/min) of free air.

1. Annular area in SI: π ÷ 4 × (0.203² − 0.114²) = 0.0222 m².
2. Velocity on free-air basis: 25.5 ÷ 0.0222 = about 1,150 m/min, or about 19 m/s.
3. Check in field units: 183.35 × 900 ÷ (8.0² − 4.5²) = 183.35 × 900 ÷ 43.75 = about 3,770 ft/min.

This is the nominal figure drillers compare with guidance values. It is not the actual gas velocity at the bottom of the hole.

Suppose the air at a point in the annulus is at 3 bar absolute rather than about 1 bar absolute at the surface, at the same temperature. For the same mass flow, the actual volume is about one-third, so the actual local velocity is about one-third as well: roughly 6 m/s. Near the surface, as pressure falls, the air expands and speeds up again.

Both numbers are correct; they answer different questions. Guidance values such as 3,000 ft/min are normally expressed on the free-air basis, so compare like with like.

Why depth and water change the picture

As a hole gets deeper, the pressure at the bottom rises because of friction in the annulus, the weight of cuttings and, above all, any water column standing in the hole. Every 10 m of fresh water adds roughly 1 bar (14.5 psi) of static head. Higher bottom-hole pressure compresses the return air, reducing its actual velocity just where cuttings need lifting.

A wet hole also means the annulus is not filled with air alone. Water and cuttings occupy part of the passage, so a gas-only calculation does not describe how the mixture moves. Foam or mist injection, higher air volume and higher pressure are the usual responses. This is why borewell compressors in India commonly run at 200–400 psig (13.8–27.6 bar) with 600–1,500 cfm, and why deeper or wetter holes need both more pressure and more volume.

Using the result for compressor selection

Annular velocity sets a minimum volume; hammer consumption sets another. The compressor must satisfy both, at the pressure the hammer needs plus the losses down the string and the backpressure in the hole.

1. Fix the hole diameter for each section and the drill-pipe outside diameter.
2. Calculate the airflow needed for the target uphole velocity on free-air basis.
3. Find the hammer’s air consumption at its working pressure from the maker’s chart.
4. Take the larger of the two flows, then add allowance for leakage and wear.
5. Estimate the pressure needed at the compressor: hammer pressure plus string losses plus expected water head and annular backpressure.
6. Select a machine rated for that flow at that pressure, with margin for site heat and altitude.

If the hole needs more velocity than one compressor can give at the required pressure, the options are a larger machine, two compressors combined, a booster, or a larger pipe size to reduce annular area.

Make the calculation reviewable

A velocity figure on an enquiry or a drilling report should carry its basis, so someone else can check it:

• Hole diameter (bit size) and drill-pipe outside diameter, in the same units.
• The flow figure used, and whether it is free air delivery or an actual volume.
• For actual volume: the location, pressure and temperature assumed.
• Whether the hole is dry, damp or making water, and any foam or mist used.
• Whether the result is a nominal average or an estimate for a specific depth.

Field checks that confirm the calculation

Use the calculation to plan, then confirm it at the rig. Watch the returns at the discharge: a steady stream of cuttings of consistent size suggests the hole is cleaning. Note the compressor pressure while drilling and when the bit is lifted off bottom; a rising difference can indicate cuttings building up in the annulus. Record penetration rate, water strikes and any changes in returns against depth, so the next hole in the same area can be planned on real evidence.

Common mistakes

• Using the bit diameter but the tool-joint diameter instead of the pipe-body diameter, or the reverse, without saying so.
• Mixing free-air and actual-volume figures in the same comparison.
• Using the compressor’s nameplate flow without allowing for wear, hot weather or a dirty intake filter.
• Assuming a gas-only calculation describes a hole full of water and cuttings.
• Treating a satisfactory number as proof that the hole will clean, without watching returns at the surface.

How Reddy Compressors helps

We sell new and used/refurbished borewell and DTH compressors, rent them from our own fleet, and service and supply spares for all types, all brands and all models, including ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and Doosan. We are an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands.

Send us your hole sizes, pipe size, hammer and depth, and we will check the annular velocity and hammer demand together and suggest a compressor with the right flow at the right pressure. If an existing machine is struggling to clean the hole, our service engineers check whether it is still delivering its rated flow and pressure. Our teams cover the whole of Telangana and Andhra Pradesh from Tarnaka, Secunderabad, and we supply compressors and spares across India.

What to send us

• Bit or hole diameter for each section, and drill-pipe outside diameter.
• Hammer make and model, and the bit size.
• Expected depth and water level, if known.
• Your compressor’s make, model and rated flow and pressure, if you already have one.
• What you are seeing: slow drilling, poor returns, regrinding or stuck pipe.

Frequently asked questions

Is annular velocity the same as cuttings velocity?

No. Annular velocity is the average speed of the air in the annulus. Cuttings travel more slowly than the air because they have weight and slip back through the airstream; larger, heavier chips slip more. That is why practical guidance sets the air velocity well above the speed needed to keep a single chip suspended. Water in the hole slows the cuttings further.

Can I use the cfm printed on the compressor?

You can use it for a nominal free-air calculation, which is how most guidance values are expressed. Do not treat the result as the actual velocity at depth, because the air is compressed there. Also check that the compressor still delivers its rated flow: wear, hot weather and dirty filters reduce real delivery, so a measured or conservative figure is safer.

What annular velocity is needed for DTH drilling?

A common rule of thumb for dry air drilling is at least about 3,000 ft/min (about 15 m/s) on free-air basis. Wet holes, coarse cuttings and difficult formations often need more, and foam or mist may be added. The hammer and bit maker’s guidance and the conditions in your hole come first; use the figure as a starting point and watch the returns.

How do I increase annular velocity without a bigger compressor?

Reduce the annular area or reduce losses. Larger outside-diameter drill pipe reduces the annulus and raises velocity for the same flow. Fixing leaks at couplings and swivels, servicing the compressor so it delivers full flow, and using foam to help lift cuttings are other options. Each has trade-offs, so check pipe weight and hammer pressure before changing tooling.

Does a good velocity figure prove the hole will clean properly?

No. The calculation is an average for an ideal round hole with dry air. Real holes are irregular, the pipe is off-centre, and water and cuttings change the flow. A good figure is a necessary starting point, but the real test is steady returns at the surface, consistent penetration and no packing around the hammer.

Related pages

• Annular area and velocity calculation tool
• Bore diameter vs air requirement
• DTH hammer air requirement
• Groundwater effects on drilling air
• Borewell and DTH compressor library
• New borewell compressor sales