Deep-hole drilling air planning for DTH borewells

Why depth alone does not size a deep-hole compressor

Depth is the number everyone quotes, but it is not the number the compressor responds to. A DTH hammer needs a working pressure at the hammer, and the hole needs enough air volume moving up the annulus to lift cuttings and water. Both change as the hole gets deeper, as water enters, as the string grows heavier and longer, and as the bit and hammer wear.

Two holes of the same depth can need very different machines. A dry hole in hard granite with a small annulus may drill well on a mid-range package. A hole of the same depth that strikes heavy water early, or that is drilled with a larger bit through broken, clay-filled ground, may need far more pressure to unload the water column and far more volume to keep the annulus clean. Planning starts with the hole programme, not the brochure.

• Pressure must overcome the hammer’s working requirement, the losses in hoses, swivel and rods, and any standing water above the bit.
• Volume must satisfy the hammer’s consumption and still give an uphole velocity that clears cuttings from the largest annular section.
• Endurance matters because deep holes run long shifts at high load, often in 40–45 °C ambient heat across Telangana and Andhra Pradesh.

How do you build a stage plan for a deep borewell?

Break the hole into intervals where the configuration or the conditions change. A typical plan separates the overburden or casing section, the first hard-rock section, the interval where water is expected, and the final section to target depth. For each stage, record the same set of facts so they can be compared.

Keep planned values, measured values and unknowns visibly separate. A planned water strike at 90 m is an assumption until the driller reports it. When the measured condition differs, the stage plan should be revised rather than quietly stretched.

Pressure: where it goes between the compressor and the bit

A pressure budget avoids the most common planning mistake: counting the same loss twice, or forgetting one altogether. State each pressure at a defined point and in one unit system. Gauge pressure is normally used on site, and it helps to show both bar and psig (1 bar ≈ 14.5 psi).

1. Compressor discharge pressure – what the machine’s gauge shows at the service valve.
2. Hose and fitting losses – supply hose, any lubricator, the swivel and connections. Long or undersized hoses waste pressure.
3. Drill string losses – friction inside the rods, which grows with depth and flow.
4. Hammer working pressure – the pressure the hammer needs to cycle efficiently, from the hammer maker’s data.
5. Back-pressure at the bit – the head of any water and cuttings standing in the annulus above the bit.

The compressor must deliver the sum of these at the flow required. If the drilling team reports a hammer working pressure that already includes some of the line losses, note that clearly, so the losses are not added a second time.

Worked example: static water head

A column of fresh water exerts roughly 0.98 bar (14.2 psi) for every 10 m of height. If the bit is 120 m below a standing water level, the water alone adds about 11.8 bar (171.1 psi) of back-pressure that must be overcome before the hammer receives its working pressure. Add a hammer that works well at about 10 bar (145 psig) and two or three bar of line and string losses, and the duty rises to about 24–25 bar (348.1–362.6 psig), inside the upper borewell band of 300–400 psig (20.7–27.6 bar). This is an upper bound: the actual head depends on how much water stands above the bit at that moment, and on whether the air is already lifting it.

Volume: hammer consumption and uphole velocity

Free air delivery (FAD) is quoted at the compressor inlet conditions. Keep every flow figure on that same basis. A reference flow in normal cubic metres or a volume at hammer pressure cannot be compared directly with a FAD figure without conversion.

The air leaving the hammer has two jobs. It has already powered the piston, and it must now carry cuttings to the surface. Do not add the hammer consumption and the flushing requirement as if they were separate streams; it is the same air. The check is whether the air the hammer passes, plus any bypass, gives enough velocity in the annulus.

Worked example: annular velocity

Take a 152 mm (6 inch) bit with 114 mm rods. The annular area is π/4 × (0.152² − 0.114²), about 0.0079 m². A delivery of 900 cfm is about 25.5 m³/min of free air. At near-atmospheric pressure near the collar, that gives about 25.5 ÷ 0.0079 ≈ 3,200 m/min, or roughly 53 m/s. Lower down the hole, where the air is compressed by back-pressure, the actual volume and velocity are smaller. Many drilling references treat around 15–25 m/s as a practical minimum for lifting dry cuttings, with wet or sticky conditions needing more care.

Now repeat the calculation for a 203 mm (8 inch) section with the same rods. The annular area becomes about 0.022 m², and the same 25.5 m³/min gives only about 19 m/s at the collar. The larger section, not the deeper one, may be the volume-critical stage. Calculate geometry for every stage before arguing about velocity.

Water, formation and site conditions change the operating case

Treat a water strike as a new operating case, not a nuisance. Heavy inflow raises back-pressure, increases the mass that must be lifted and can slow penetration sharply. Stable air readings with poor returns often point to water or cuttings settling in a washout, a lost-circulation zone or a partly blocked annulus, not to a compressor fault.

• Rising pressure with falling progress – check for a choking annulus, a wet sticky formation, a blocked bit face or a hammer that is wearing.
• Falling pressure with normal progress – look for a leak in hose, swivel or rod joints, or a worn hammer passing air too freely.
• Normal readings, poor returns – suspect loss of returns into fractures or cavities, or an enlarged hole section.

Site conditions matter as much as the hole. At 45 °C ambient, intake air is about 8% less dense than at 20 °C, so the mass of air delivered falls even though the volume figure stays the same. Dust shortens air filter life, and long hot shifts test the cooler, the engine and the cooling package. A machine selected only on a short capacity check may run into high discharge temperature or high coolant temperature shutdowns in the afternoon.

How to select the compressor against the stage plan

Choose a machine whose rated pressure and FAD comfortably cover the most demanding stage, not the average. Then check that it can hold that point continuously, in your climate, for the length of shift you plan.

When one machine cannot cover every stage, there are two common routes. A booster raises the pressure of air from a primary compressor; it is a separate engineering decision with its own controls, safety valves and hose ratings. Temporary parallel supply from two compressors can add volume, but only with a defined manifold, matched pressures, non-return protection and a clear plan for which machine leads. Neither should be improvised at the rig.

Hoses, controls and safety at high pressure

At 20–35 bar, stored energy in hoses and receivers is serious. Use hoses and fittings rated above the maximum compressor pressure, with whip restraints on every connection. Isolate and vent before breaking any joint. Keep the swivel, check valve and rod joints in good condition, because a leaking joint wastes pressure and can fail suddenly.

Agree control transitions with the rig crew: how the compressor is loaded and unloaded during rod changes, how pressure is released before breaking out, and what the operator does when a safety shutdown trips. Typical shutdowns on drilling packages include high discharge temperature, low engine oil pressure and high coolant temperature. A trip is information about the operating case and should be recorded, not simply reset.

Wear, records and reassessment between stages

Hammers, bits, rods and compressor components wear during a deep hole. A hammer that consumed a certain flow when new may pass more air as it wears, reducing the pressure the compressor can hold. Record, at each stage boundary, the depth, pressure at the compressor, engine speed, penetration rate, returns and any trips. These few readings show whether a slowdown comes from the ground, the tooling or the air supply.

Separate drilling rate from shift productivity. A stage might drill at a good rate but lose hours to rod handling, water problems or a breakdown. Mixing those into one figure hides the real constraint. At handover, keep the stage plan and the actual readings together; the next hole in similar ground benefits directly.

How Reddy Compressors helps

We supply all types, all brands and all models of borewell compressors: new machines, used and refurbished machines, rentals, service and spare parts. As an independent multi-brand specialist, we work with ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and other makes, without being tied to one manufacturer’s range. We are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.

For deep-hole work, we start from your stage plan. We review the bits, hammers, rods, expected water and site conditions, then recommend a pressure and flow band that covers the hardest stage with margin. Our own rental fleet covers 50–2,000 cfm and 2–30 bar, including high-pressure diesel units, and for long-term projects we can supply brand-new compressors to site.

Our team of over 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India. On hire jobs we service machines every 300 running hours. Spare parts are identified from the machine’s make, model, serial and nameplate, and fit is confirmed before we quote. Our office is at Tarnaka, Secunderabad, and we have served industry since 2002.

What to send us

• Target depth and the planned casing points
• Bit diameters, hammer make and size, and rod outside diameter
• Expected water strike depth and static water level, if known
• Formation description from nearby holes
• Rig make and model, and your present compressor details (nameplate photo)
• Site location, shift pattern and whether you need sale, used or hire

Frequently asked questions

Can target depth alone tell me the compressor pressure I need?

No. Depth sets the length of the string and the possible water head, but the pressure you need also depends on the hammer’s working pressure, line and string losses and how much water stands above the bit. A dry deep hole may need less pressure than a shallower hole with heavy water. Build a stage-by-stage pressure budget and select against the hardest stage.

Does a deeper borewell always need a booster compressor?

Not always. Many deep holes are drilled with a single high-pressure package in the 300–400 psig (20.7–27.6 bar) class. A booster becomes worth considering when the water head and hammer pressure together exceed what the primary machine can hold continuously. It is a separate engineering decision with its own controls, pressure ratings and safety valves, and should be planned before mobilisation.

How much cfm do I need for a 6 inch DTH borewell?

It depends on the hammer’s consumption at its working pressure and on the annulus between bit and rods. For a 6 inch bit with common rod sizes, packages of 600–1,200 cfm are typical. Check the uphole velocity in the largest hole section, not only the final one, and allow for water inflow, wet cuttings and wear in the hammer.

Why is my compressor pressure high but the bit is not drilling?

High pressure with poor progress usually points to restriction downstream of the compressor: a choking annulus, sticky wet cuttings, a blocked bit face, a collapsed zone or a worn hammer. Check returns, flush the hole and inspect the bit and hammer before suspecting the compressor. Record the pressure and depth each time it happens, so the pattern can be diagnosed.

Does high summer temperature affect a borewell compressor?

Yes. At around 45 °C intake air is roughly 8% less dense than at 20 °C, so the mass of air delivered falls. The machine also works harder to reject heat, and dust loads the air filters faster. Clean coolers, correct oil, clean filters and good ventilation help it hold pressure and avoid high-temperature shutdowns through long afternoon shifts.

Can I join two compressors to drill a deep hole?

Yes, but only with a proper arrangement: a rated manifold, non-return valves on each machine, matched pressure settings, correctly rated hoses and a clear agreement on which unit leads. Two machines joined without this can fight each other, overload one unit or create unsafe pressure conditions. Plan it with the rig crew and the compressor supplier before drilling starts.

Related pages

• Borewell and DTH compressor library
• Drilling depth and compressor selection
• Booster-assisted drilling air
• Wet-hole drilling and water ingress
• Borewell compressor rentals and hire
• New borewell compressor sales