Why borewell drilling needs high pressure and high flow

Why does a borewell compressor need both pressure and flow?

A down-the-hole (DTH) hammer is a pneumatic piston sitting just above the bit. Compressed air travels down the drill pipe, cycles the piston, and exhausts through the bit face. That same exhaust air then becomes the flushing medium that carries rock chips up the gap between the drill pipe and the bore wall, known as the annulus.

So the compressor is doing two jobs at once with one stream of air:

• Pressure sets the energy of each hammer blow. Higher supply pressure at the hammer means a harder blow and, within the hammer’s rated band, faster penetration in hard rock such as the granite and gneiss common across the Deccan plateau.
• Flow (free air delivery, FAD, in cfm or m³/min) sets how fast the air moves up the annulus. Too little velocity and the cuttings fall back, regrind at the bit and eventually pack around the hammer.

The two are linked but not interchangeable. Raising pressure on an undersized machine does not create more flow, and a high-flow machine at low pressure will clean the hole well but leave the hammer weak. The rig needs a compressor whose rated FAD is available at the working pressure the job actually needs.

How hammer pressure is decided

Each DTH hammer has a manufacturer’s rated pressure range and a consumption curve that shows how much air it passes at each pressure. Consumption rises with pressure, so a hammer run at a higher pressure also draws more cfm. That is why pressure and flow have to be chosen together, from the hammer’s own data.

The pressure the hammer sees is always lower than the pressure on the compressor gauge. Between the two sit:

• the compressor discharge valve, hoses and couplings at the rig;
• the rig swivel and any lubricator or inline fittings;
• the friction loss of air flowing through the full length of drill pipe, which grows with depth and with flow;
• check valves and the hammer’s own internal passages.

A practical rule is to state clearly where a pressure figure applies: compressor outlet, rig inlet or hammer supply. A quotation that says “350 psig” without saying where, or at what flow, cannot be checked against the hammer’s requirement. Gauge pressure (psig, bar g) is the normal convention on site; keep it consistent.

Annular velocity: the flow side of the equation

Annular velocity, sometimes called uphole velocity, is the speed of the return air in the annulus. It is the single best indicator of whether the hole is being cleaned. It depends on two things only: how much air is flowing and how large the annular gap is.

The annular area is the area of the bore minus the area of the drill pipe:

Annular area = π/4 × (bore diameter² − pipe outside diameter²)

Required flow is then annular area multiplied by the target velocity. For dry air drilling, common field practice aims for roughly 3,000–5,000 ft/min (about 15–25 m/s) of uphole velocity, with the upper end used for heavier, wetter or larger cuttings. Exact targets vary with formation and with hammer and bit maker guidance, so treat these as typical working values, not a guarantee.

Two consequences matter when choosing a compressor:

• Bigger bit, more air. Annular area grows with the square of diameter. Opening a hole from 6½ inch to 8 inch needs a substantially larger compressor for the same cleaning, even at the same depth.
• Thin pipe in a big hole wastes air. A small drill pipe in a large bore leaves a wide annulus that needs a great deal of flow. Matching pipe size to bit size is often cheaper than buying more cfm.

Our annular area and velocity calculation tool does this arithmetic for your own bit and pipe sizes.

Depth, water head and why deep wet holes need more pressure

In a dry hole the hammer exhausts against very little back pressure. Once the hole meets water, the picture changes. Water standing above the hammer forms a column that the exhaust air must push out before the hammer can cycle properly and before cuttings can be lifted.

A column of water exerts roughly 1 bar (14.5 psi) for every 10 metres of height. The useful figure is the height of water above the hammer, which is the drilled depth minus the static or standing water level. A hole drilled to 200 m with water standing at 50 m below ground has about 150 m of water above the bit when the air is first turned on.

In operation the return column becomes a mixture of air, water and cuttings, which is much lighter than solid water, so the running back pressure is lower than the full static head. But the compressor still has to be able to start the hammer and clear the column at the beginning of each rod, after every connection and after any stoppage. That is the point where under-pressured machines stall.

The practical effects of depth and water are:

• the pressure margin needed above the hammer’s working pressure grows with the water column;
• pipe friction losses grow with depth, so more of the compressor’s pressure is used up before reaching the hammer;
• heavy water inflow adds mass to the return column and demands higher annular velocity to keep it moving;
• in summer, high ambient temperature in Telangana and Andhra Pradesh reduces engine and cooling margins, so a machine working near its pressure limit is also working near its temperature limit.

This is why deep borewells in hard rock with significant water often move up from 200–250 psig machines to the 300–400 psig class, or add a booster to raise pressure on an existing compressor.

Why “750 cfm” is not a complete specification

A request for “a 750 cfm compressor” leaves the important questions open: 750 cfm at what pressure, at what depth, and with how much water in the hole? A useful way to state the duty is as a pressure budget at the operating flow:

Required discharge pressure = hammer working pressure + hose and fitting losses + drill string losses + water back pressure + working margin

A hammer that needs 20 bar at its operating point therefore does not mean a 20 bar compressor is enough. Note too that drilled depth is not the same as water head: what counts is the static and, while air-lifting, the dynamic water level relative to the hammer.

Typical pressure and flow bands for borewell and DTH work

The table below shows how duty bands are commonly matched to drilling conditions. It is a starting framework for discussion, not a selection; the hammer’s data and the site conditions decide the final choice.

These are the ranges we commonly work in for borewell and DTH compressors: 600–1,500 cfm at 200–400 psig, with high-pressure units up to about 35 bar.

A worked example (illustrative only)

The figures below are invented round numbers to show the method. They are not a recommendation for any real rig, hammer or site.

Assumed hole: 6½ inch (165 mm) bit, 4½ inch (114 mm) drill pipe, target depth 200 m, standing water at about 50 m below ground, hard granite.

1. Annular area. π/4 × (0.165² − 0.114²) ≈ 0.0112 m², or about 0.120 ft².
2. Flow for cleaning. At 3,000 ft/min: 0.120 × 3,000 ≈ 360 cfm. At 5,000 ft/min: 0.120 × 5,000 ≈ 600 cfm. With water inflow expected, aim towards the upper figure.
3. Hammer consumption. Check the hammer maker’s curve at the intended working pressure. Assume, for illustration, that the chosen hammer passes more air than the cleaning requirement at its working pressure; the larger of the two figures governs the flow.
4. Water head. About 150 m of water above the bit at full depth, roughly 15 bar (217.6 psi) of static head at start-up. The running back pressure will be lower once the column aerates.
5. Losses and margin. Add hose, swivel and pipe-friction losses and a working margin. In this illustration the duty clearly points to the 300–400 psig class rather than a 200 psig machine, with flow in the 800–1,200 cfm band to cover the hammer and the wet annulus.
6. Check derating. Confirm the selected machine delivers its FAD at that pressure in high ambient temperature and at the site’s altitude, not only at its catalogue rating.

The method, not the numbers, is the point: annulus sets a minimum flow, the hammer sets its own flow at pressure, the water column and losses set the pressure margin, and the compressor must meet all three at once.

How to choose pressure and flow for your rig

1. Collect the hammer model, its rated pressure range and its consumption curve.
2. Fix the bit size, drill pipe outside diameter and target depth.
3. Estimate the standing water level and likely inflow from nearby borewells or previous logs.
4. Calculate the flow for cleaning from the annulus and compare it with the hammer’s consumption; take the larger.
5. Estimate the pressure the hammer needs at full depth, including water column and losses.
6. Select a compressor whose rated FAD is available at that pressure, with a sensible margin, and check hoses and fittings are rated for it.
7. Decide whether to buy, hire or add a booster to an existing machine. See drilling depth and compressor selection for the depth side in more detail.

Symptoms of too little pressure or too little flow

If the machine itself is suspect, our borewell compressor performance diagnostics page covers how delivered flow and pressure are tested.

Hoses, fittings and safety at high pressure

At 300–400 psig, the stored energy in a hose is large. Every hose, coupling, whip check and valve between the compressor and the rig must be rated for the maximum pressure the compressor can produce, not just the working pressure. A coupling that looks the same as the old one, or has the same nominal size, is not necessarily rated the same.

• Use hoses and couplings rated for the compressor’s maximum pressure and discharge temperature.
• Fit whip checks or safety restraints across hose joints.
• Depressurise fully before breaking any connection.
• Keep the compressor’s safety shutdowns for high discharge temperature, low engine oil pressure and high coolant temperature in working order; never bypass them to finish a hole.

More detail is on our air hoses and rig connections page.

How Reddy Compressors helps

For borewell rig owners we supply all types, all brands and all models of borewell compressors: new sales, used and refurbished machines, rentals, service and spare parts.

Reddy Compressors, based at Tarnaka in Secunderabad and serving industry since 2002, is an independent multi-brand compressor specialist. We sell, rent and service high-pressure portable compressors of makes such as Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic and others, without being an authorised dealer of any brand.

• We work through the hammer, bit, pipe, depth and water data with you to set a pressure and flow duty before any machine is proposed.
• We supply new and used or refurbished borewell compressors, and rent from our own fleet of diesel portables and high-pressure units across our 50–2,000 cfm, 2–30 bar rental range.
• Our team of over 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh for service, annual maintenance contracts (AMC), breakdown repair and overhaul of drilling compressors.
• Spare parts are identified from the machine’s make, model, serial and nameplate, and fit is confirmed before we quote. Equipment, rentals and spares are supplied across India.

Our project team has supplied and commissioned high-pressure air on demanding sites before, including compressors rated 1,100 cfm at 300 psig for a natural-gas development project at Kakinada.

What to send us

• Rig make and model, and the hammer and bit model and sizes.
• Drill pipe outside diameter and length available.
• Target depth, expected standing water level and any known inflow.
• Formation (granite, basalt, weathered rock, overburden) and casing plans.
• Your current compressor’s make, model and nameplate photo, if you have one.
• Whether you want to buy, hire, add a booster, or service an existing machine, and the site location.

Frequently asked questions

What pressure compressor is needed for borewell drilling?

Most DTH borewell work uses compressors in the 200–400 psig range, about 14–28 bar. Shallow, dry holes can work at the lower end, while deep hard-rock holes with substantial water above the hammer usually need the 300–400 psig class or a booster. The right figure comes from the hammer’s rated pressure, the water column at full depth and the losses in hoses and pipe.

How many cfm do I need for a 6.5 inch borewell?

It depends on the drill pipe size and the hammer. With 4½ inch pipe, the annulus needs roughly 360–600 cfm just for cleaning at typical uphole velocities, and the hammer may need more than that at working pressure. In practice 6½ inch DTH rigs use machines of 600–1,500 cfm, depending on depth, hole size and water. Send us the hammer and pipe details to confirm.

Why does my DTH hammer stop working when the borewell hits water?

Water standing above the hammer creates back pressure, roughly 1 bar for every 10 metres of column. If the compressor’s pressure margin is too small, the hammer cannot exhaust and cycle, especially after a rod change. Higher pressure, a booster, or a staged flushing routine to aerate the column usually solves it. Check the machine is actually reaching its rated pressure under load.

Does a higher pressure compressor drill deeper?

Not by itself. Higher pressure gives a stronger hammer blow and more margin over the water column, which helps in deep wet holes. But depth also needs enough flow to lift cuttings, a hammer rated for that pressure, and a rig, pipe and hoses that can handle it. A high-pressure machine with too little flow will still stick in a dirty hole.

What is annular velocity in borewell drilling?

Annular velocity is the speed of the return air rising in the gap between the drill pipe and the bore wall. It decides whether cuttings are carried out or fall back to the bit. It is found by dividing air flow by annular area. Typical dry-air targets are around 3,000–5,000 ft/min, with higher values for wet holes and heavy cuttings.

Can I use a booster instead of buying a high-pressure compressor?

Often, yes. A booster takes air from an existing compressor and raises its pressure, which can extend a lower-pressure machine into deep-hole work. The booster must be matched to the compressor’s flow, and the combined package needs hoses and fittings rated for the higher pressure. Whether it is cheaper than a high-pressure unit depends on your fleet and hours.

Related pages

• Annular velocity in borewell drilling
• DTH hammer air requirement
• Booster-assisted drilling air
• Wet hole drilling and water ingress
• Borewell compressor rentals and hire
• Borewell and DTH compressor library