Booster-assisted drilling air for deep DTH holes

What a booster-assisted drilling system is

A booster is a second compressor that takes already compressed air as its inlet and raises it to a higher final pressure. In drilling, the arrangement separates two requirements that a single portable compressor may not meet together:

1. The primary compressor provides the airflow, typically a high-pressure diesel screw machine.
2. The booster raises the pressure to what the hammer and hole conditions need.
3. The drill string carries the high-pressure air to the DTH hammer.
4. The hammer converts the pneumatic energy into impact at the bit.
5. The annulus returns the air and cuttings to the surface.

The approach is used in deep water-well and borewell drilling, exploration drilling, hard-rock holes and wells with high water inflow, where the pressure needed at the hammer rises with depth and water head beyond what the primary machine can deliver efficiently on its own.

Why drilling pressure rises with depth

The compressor must supply the hammer’s working pressure plus everything that resists the air on its way down and back up:

• Pressure loss in hoses, swivel, drill pipe and tool joints.
• Backpressure in the annulus from friction and the weight of cuttings.
• Static head of any water column standing above the hammer.

Water head is often the largest term. A static estimate is P = ρgh: for 100 m of fresh water at 1,000 kg/m³, that is 981,000 Pa, or about 9.8 bar (142.1 psi). Roughly, every 10 m of water adds 1 bar. Here h is the vertical height of water above the hammer, not total drilled depth. Aeration of the water column, inflow, cuttings and changing conditions all make the real figure differ from the static estimate, so treat it as a guide for planning.

Pressure alone is not enough

Raising pressure without enough volume does not clean the hole. The air must still return up the annulus fast enough to lift cuttings, and the hammer must receive its rated air consumption at the higher pressure. Because the booster compresses the same mass of air, the actual volume leaving it is smaller than the volume entering it. What matters for hole cleaning is the mass of air delivered, usually expressed as free air delivery, and that is set by the primary compressor.

A booster with a large rating fed by a small primary compressor simply runs short of inlet air. The primary compressor’s free air delivery limits the whole system.

Pressure ratio must use absolute pressures

Booster performance depends on its pressure ratio, calculated with absolute pressures (gauge plus atmospheric).

Example: primary compressor delivering 20 bar gauge (about 290 psig) to the booster inlet, booster discharging at 34 bar gauge (about 493 psig), with about 1 bar atmospheric pressure. The ratio is (34 + 1) ÷ (20 + 1) = 35 ÷ 21 = about 1.67. Dividing the gauge pressures (34 ÷ 20 = 1.7) gives a different and wrong figure. Use the actual site atmospheric pressure for a real calculation.

For dry ideal gas with the same mass flow at both points, actual volume changes as Q₂ ÷ Q₁ = (P₁ ÷ P₂) × (T₂ ÷ T₁), with absolute pressures and temperatures in kelvin. This shows how the volume shrinks through the booster; it does not set the booster’s capacity, which comes from the manufacturer’s performance data at the stated inlet conditions.

Typical booster configurations

Within the range of high-pressure portable screw compressors, typically up to about 35 bar (507.6 psig) for special units, a single machine is usually simpler. A booster becomes worth considering where the duty exceeds that, or where an existing fleet of compressors can be upgraded for occasional deep work.

Temperature is a critical design variable

Compression heats air. Air from the primary compressor should be aftercooled and its condensate removed before the booster, because hot, wet inlet air raises booster discharge temperatures and carries water into the cylinders. The booster’s own intercoolers and aftercooler must cope with Indian summer ambient temperatures of 40–45 °C, and above 45 °C on open sites.

Watch for:

• Booster inlet temperature above the maker’s limit.
• Stage discharge temperatures rising over a shift.
• Dirty or blocked coolers on dusty drilling sites.
• Condensate drains not working, causing water carry-over.

The primary compressor’s own protections still apply: oil-injected screw discharge normally runs at about 75–95 °C, and the machine should trip on high discharge temperature, low engine oil pressure and high coolant temperature.

When is a booster worth considering?

• The primary compressor delivers enough free air but not enough pressure for the hammer at depth.
• Water inflow raises the backpressure beyond the primary machine’s rating.
• Only part of a project, such as the deepest holes, needs very high pressure.
• A contractor already owns suitable primary compressors and wants to extend their range.

Signs that the existing system is reaching its limit: pressure falling as depth increases, the compressor at full load with no reserve, penetration slowing sharply in wet zones, and weak returns despite a correct hole and pipe size.

A booster is not the answer if the primary compressor is short of flow, if the problem is a worn or poorly serviced machine, or if a single high-pressure compressor within its rating would do the job more simply.

Sizing workflow

1. Define the hammer’s working pressure and air consumption from the maker’s data.
2. Estimate string losses, annular backpressure and water head for the deepest planned section.
3. Add these to set the pressure needed at the booster outlet, with margin.
4. Calculate the free air needed for both the hammer and hole cleaning; take the larger.
5. Choose a primary compressor that delivers that free air at the booster’s inlet pressure.
6. Confirm the booster’s performance at that inlet pressure and flow from its data.
7. Check the pressure rating of every hose, swivel, valve and pipe downstream of the booster.
8. Plan cooling, condensate removal, controls and start-up and shutdown sequence.

Common mistakes

• Selecting by pressure only and ignoring flow.
• Selecting by depth only and ignoring the actual water level.
• Ignoring pressure drop in the drill pipe and tool joints.
• Ignoring annular velocity in the larger hole sections.
• Mixing free-air and actual-volume figures.
• Oversizing the booster but undersizing the primary compressor.
• Using gauge pressures for the pressure ratio.

Safety and high-pressure integrity

High-pressure air stores a large amount of energy. A hose or fitting failure at 30 bar and above is dangerous. Every component downstream of the booster must be rated for the maximum pressure the booster can produce, not just the normal working pressure. Fit relief valves on each stage and at the discharge, whip checks on every hose connection, and a clear exclusion zone around high-pressure lines. Depressurise fully before breaking any connection. Follow the equipment makers’ instructions and the site’s own safety procedures.

Basic instrumentation helps both safety and diagnosis: pressure gauges at the primary outlet, booster inlet and booster outlet, temperature indication at the booster discharge and stages, and running-hour meters for service tracking.

Unloading pressure versus running pressure in a wet hole

The pressure a deep wet hole demands is not constant. It peaks when air first has to push a full column of water out of the hole, then falls once the column is aerated and flowing. This peak usually decides whether a booster is needed.

Worked example (illustrative). The hammer is at 250 m and the static water level is 90 m below ground, so the hammer is 160 m below the water surface.

1. Static water head = 1,000 kg/m³ × 9.81 × 160 m = 1,569,600 Pa = 15.7 bar.
2. Assume the hammer needs 10 bar across it to cycle properly, and that hoses, swivel and drill string lose 1.5 bar.
3. Pressure to unload the hole after a rod change or a stop: 15.7 + 10 + 1.5 = 27.2 bar(g).
4. Once air is flowing, the column becomes a mixture of air, water and cuttings. With an average density of 300–400 kg/m³ (an assumption; actual values depend on air flow and water inflow), the head falls to 300 × 9.81 × 160 ÷ 100,000 = 4.7 bar up to 400 × 9.81 × 160 ÷ 100,000 = 6.3 bar.
5. Running pressure: 4.7 to 6.3 + 10 + 1.5 = about 16 to 18 bar(g).

A 24 bar(g) compressor can drill this hole once it is flowing, but cannot reliably restart it after each rod change. The 27.2 bar(g) unloading pressure is above its rating. That is the classic case for a booster. It provides the pressure margin for unloading, while the primary compressor provides the air volume for running. It also explains why the gauge peaks after each rod change and then settles. A non-return valve in the hammer that holds water out of the string during rod changes reduces the unloading peak, so check its condition before deciding on a booster.

Matching the primary compressor to the booster inlet

A booster cannot create air. Every kilogram it delivers must come from the primary compressor. Problems arise because the two machines are often rated in different units.

• The primary compressor is rated in free air delivery (FAD), meaning volume referred to inlet conditions near atmospheric.
• Some booster data quotes FAD at a stated inlet pressure. Other data quotes actual inlet volume, measured at the booster’s inlet pressure and temperature. The two numbers differ by a factor of about 20 at drilling pressures.

Worked example. A primary compressor delivers 1,100 cfm FAD at 20 bar(g). The air reaches the booster at 50 °C.

1. FAD = 1,100 ÷ 35.315 = 31.15 m³/min.
2. Actual volume at the booster inlet = 31.15 × (1.013 ÷ 21.013) × (323.15 ÷ 293.15) = 31.15 × 0.0482 × 1.102 = 1.66 m³/min, which is about 58 cfm of actual inlet volume.
3. A booster quoted as “60 cfm at 20 bar inlet” in actual volume is therefore matched to this compressor. A booster quoted as “1,100 cfm” may be the same machine, using FAD.

Ask which basis the booster rating uses before comparing machines. The table shows what happens when the two flows do not match.

Booster sizing for plant applications is covered on the booster compressors page. On a drilling site, the matching must also hold across the full range of primary engine speeds and pressure settings the driller will use.

Booster discharge temperature at 45 °C ambient

On a summer site in Telangana or Andhra Pradesh, the primary compressor’s aftercooler typically delivers air 10–15 °C above ambient, so the booster takes in air at 55–60 °C rather than the 30–40 °C assumed in many data sheets. The discharge temperature of a single compression stage is estimated as T2 = T1 + T1 × (r to the power 0.286 − 1) ÷ η, with temperatures in kelvin, r the absolute pressure ratio and η an assumed stage efficiency of 0.80.

Check the middle row: T1 = 328.15 K. 1.714 to the power 0.286 = 1.167, so the ideal rise is 328.15 × 0.167 = 54.7 K. Dividing by 0.80 gives 68.3 K, and 55 + 68.3 = 123 °C.

A hot day and a starved inlet together push the discharge temperature about 50 °C higher than mild conditions with correct feed. Practical consequences:

• Clean the primary aftercooler and the booster coolers before the summer season. Dust-blocked fins on a borewell site can add 5–10 °C at the booster inlet.
• Set the booster’s high-temperature trip, and keep it working. Do not bypass it to finish a hole.
• Position both machines so that neither draws in the other’s cooling exhaust or engine exhaust.

Fuel per metre: one high-pressure unit or compressor plus booster (illustrative)

The thermodynamic minimum work to compress a given mass of air from atmosphere to 35 bar(g) is the same whether it happens in one machine or two. For 1,100 cfm, the isothermal power is 101,325 Pa × 0.519 m³/s × ln(36.01 ÷ 1.013) = 188 kW. For the primary’s share to 24 bar(g) it is 169 kW, and for the booster’s share from 24 to 35 bar(g) it is 19 kW. Real differences come from machine efficiency and from how many metres actually need the higher pressure.

The table uses these assumptions. The single high-pressure unit has an isothermal efficiency of 47% at 35 bar(g) and 45% when run at 24 bar(g). The 24 bar(g) primary has 50%. The booster needs 30 kW shaft power for the 24 to 35 bar(g) step (adiabatic 22.6 kW at 55 °C inlet, divided by 0.75). Diesel use is 0.25 litre per kWh, from about 210 g/kWh divided by 840 g/litre, rising to 0.30 litre per kWh for the lightly loaded booster engine. Penetration rates are assumed for illustration.

The two correctly equipped set-ups differ by about 6–10% per metre. Under these assumptions, they are much closer than the last row, where pressure is short and penetration falls. The decision between them usually turns on whether a suitable primary is already owned, how much of the drilling programme needs the higher pressure, and the cost of moving and servicing two machines. Fuel efficiency alone rarely decides it. Measure your own litres per hour and metres per hour to replace these figures; see drilling productivity and cost per metre.

Stored energy and relief valve capacity at 35 bar

Thirty metres of 2 in hose at 35 bar(g) holds about 350 kJ, released by adiabatic expansion if a joint fails. That is enough to lift a one-tonne load about 36 m, and roughly half as much again as the same hose at 24 bar(g). Add the booster’s own receiver or pulsation bottle and the interconnecting line, and the stored energy on a boosted rig is far higher than on a standard one. That is why the bleed-down and exclusion-zone rules above matter more on a boosted rig. Before first start, also check the relief valves themselves:

1. Confirm the booster’s final relief valve is set no higher than the lowest-rated component downstream: hose, coupling, swivel, gauge or manifold.
2. Confirm that the relief valve can pass the booster’s full flow without the pressure rising more than about 10% above its set point. A small valve that lifts but cannot pass the flow gives no protection.
3. Protect the interconnecting line between the machines with its own relief valve, set no higher than that line’s rating, and a non-return valve at the booster inlet. If a booster suction valve fails, or the booster stops with pressure in its discharge, air at up to 35 bar(g) can flow back into a line rated only for the primary’s pressure.
4. Confirm the relief valves are tested or replaced at the interval their maker or your site rules set, and that no valve has been plugged, wired shut or reset higher to stop it lifting during a hard section.

How Reddy Compressors helps

We supply all types, all brands and all models of borewell and drilling compressors: new sales, used/refurbished machines, rentals, service and spare parts, for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan, Sullair and others. We are an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. Our rental fleet covers 50–2,000 cfm and 2–30 bar, including high-pressure units.

For deep or high-backpressure drilling we review the hammer, depth, water level and existing equipment, and advise whether a single high-pressure compressor, combined machines or a booster arrangement fits the duty. Our team has supplied and commissioned high-pressure air packages before, including compressors rated 1,100 cfm at 300 psig and 1,000 cfm at 275 psig for a natural-gas development at Kakinada. Our 20+ service engineers and technicians serve all of Telangana and Andhra Pradesh from Tarnaka, Secunderabad, and we supply equipment and spares across India.

What to send us

• Hammer make and model, bit size and drill-pipe size.
• Planned depth, expected water level and known wet zones.
• Your existing compressor’s make, model, rated flow and pressure.
• Pressure rating of the rig’s hoses, swivel and pipe.
• Site location, season of work and duration.
• Whether you want to buy, hire or upgrade an existing set-up.

Frequently asked questions

Does a booster create additional air?

No. A booster compresses air already delivered by the primary compressor to a higher pressure. The mass of air, and therefore the free air delivery available for the hammer and for lifting cuttings, is set by the primary compressor, less any condensate and leakage. The actual volume leaving the booster is smaller because the air is at higher pressure. If you need more air, you need a larger primary compressor.

When do I need a booster for borewell drilling?

Consider one when your compressor has enough flow but cannot give the pressure the hammer needs at depth, usually because of high water head or very deep holes. If a single high-pressure compressor, typically up to about 35 bar (507.6 psig) for special units, can meet the duty, it is often simpler. If flow is the problem, a booster will not help.

Can bore depth alone decide the booster pressure?

No. The water level matters more than total depth, because it is the water column above the hammer that adds backpressure, roughly 1 bar per 10 m. Hammer pressure, drill-string losses and cuttings load also contribute. Two holes of the same depth with different water levels can need very different pressures, so send us the expected water level as well as depth.

Can I keep my existing hoses with a booster?

Only if every hose, coupling, swivel and pipe downstream of the booster is rated for the booster’s maximum possible pressure, not just its normal setting. Hoses used with a lower-pressure compressor often are not. Check the pressure markings, replace anything under-rated, and fit whip checks at every connection. The hoses upstream of the booster only need to suit the primary compressor’s pressure.

Should the pressure ratio use gauge or absolute pressure?

Always absolute pressure. Add atmospheric pressure, about 1 bar at low altitude, to each gauge reading before dividing. For 20 bar gauge in and 34 bar gauge out, the ratio is 35 ÷ 21, about 1.67, not 34 ÷ 20. Using gauge pressures gives the wrong ratio and misleading estimates of booster temperature and volume.

Is a booster better than one high-pressure compressor?

Neither is better in general. A single high-pressure machine is simpler to transport, operate and service, and suits most borewell work within its rating. A booster suits duties above that rating, occasional deep sections, or contractors who want to extend existing compressors. Compare the full package, including fuel, maintenance, transport and the skills needed on site.

Why does my rig need much more pressure after a rod change in a wet hole?

While drilling, the column in the hole is a light mixture of air, water and cuttings. When air stops for a rod change, water fills the hole above the hammer, and restarting means pushing out a solid water column. Every 10 m of water above the hammer adds about 0.98 bar. The unloading pressure can therefore be 8–12 bar higher than the running pressure on a deep hole with a high water level. A booster, or a well-maintained hammer non-return valve, provides the margin for this peak.

Does a booster use less diesel than one 35 bar drilling compressor?

Not by much when both are correctly sized. In the illustrative comparison above, a primary plus booster used about 7.8 litres per metre in the deep section against 8.3 litres per metre for one 35 bar unit, a difference of about 6%. The larger saving comes from switching the booster off in the shallow section, where only 24 bar(g) is needed. The worst case is a primary that is short of pressure: penetration falls and fuel per metre can rise by half. Measure your own litres per hour and metres per hour before deciding.

How do I match a booster to my existing drilling compressor?

Start with your compressor’s free air delivery at the pressure it will actually feed the booster, not its brochure maximum. Find out whether the booster’s rating is in free air or actual inlet volume. For example, 1,100 cfm of free air at 20 bar(g) and 50 °C is only about 58 cfm of actual volume at the booster inlet. Match the two flows, check that the interconnecting hose does not drop the inlet pressure, and confirm the booster’s final pressure against the hammer and water-level requirement.

How hot does a drilling booster run in Indian summer conditions?

With 45 °C ambient, a booster typically takes in air at 55–60 °C from the primary compressor’s aftercooler. Compressing from 20 to 35 bar(g) then gives a discharge temperature of roughly 123–129 °C, assuming a stage efficiency of 80%. If the booster is starved and its inlet pressure falls to about 16 bar(g), the discharge temperature rises to about 150–158 °C. Clean coolers, correct inlet feed, working temperature trips and good machine spacing keep it within limits.

Related pages

• Booster compressors
• High-pressure borewell air
• 35 bar / 507.6 psi portable air
• Deep hole drilling air planning
• Borewell and DTH compressor library
• Borewell compressor rentals and hire