Air hoses and rig connections for borewell compressors

Why the connection between compressor and rig matters

A borewell compressor is rated at its outlet valve. The hammer only sees what is left after the air has passed through the discharge valve, the hose, any manifold or lubricator, the rotary head swivel, the drill pipe and finally the hammer’s own valving. Every one of those elements takes a share of pressure, and the share grows quickly as flow rises.

On a rig running 1,000–1,200 cfm at 250–300 psig (17.2–20.7 bar), the hose is not a minor accessory. It is the last link in the energy chain before the drill string. A hose one size too small, a crushed section where a tractor wheel has run over it, or a reducing coupling at the rig inlet can cost more pressure than the operator realises, and the drilling crew will usually blame the compressor first.

The same connection is also the most dangerous part of the package. A high-pressure hose carries a large amount of stored energy. If a coupling pulls out without a restraint, the hose end can whip with enough force to cause fatal injury. Good practice on hoses is therefore both a performance issue and a safety issue.

What makes up the air path from compressor to hammer?

It helps to think of the connection as a chain of named parts, each with its own rating and wear pattern.

When a drilling crew reports low hammer performance, we walk this chain from end to end before assuming the compressor is weak.

How to choose hose size and pressure rating

Hose selection starts from the duty, not from what happens to be on the truck. Three questions settle most cases.

1. What is the maximum working pressure? The hose and every fitting must be rated above the compressor’s maximum pressure, including the relief valve setting, with the manufacturer’s safety factor intact. A hose rated for 10 bar construction duty must never be used on a 24 bar borewell compressor.
2. What flow must it carry? Pressure loss in a hose rises roughly with the square of flow velocity. For the same flow, a larger bore gives a lower velocity and a much smaller loss. Large borewell packages commonly use 2 inch (50 mm) or larger hose; smaller machines may run 1.5 inch. The right size depends on flow, length and pressure, so it should be checked rather than guessed.
3. What will the hose face on site? Heat from the compressor discharge, sharp rock, vehicle crossings, oil and sun exposure all shorten hose life. A hose rated for high temperature and abrasion lasts far longer on an Indian summer site, where ground temperatures in Telangana and Andhra Pradesh can be extreme.

Length matters as much as bore. Each extra metre adds friction, so the compressor should be placed as close to the rig as safety and access allow, with the hose laid in smooth curves rather than tight loops.

Where does pressure get lost between compressor and rig?

Pressure loss is the difference between the gauge at the compressor and the pressure available at the rig inlet while drilling. It is caused by friction in the hose, sudden changes in bore at fittings, valves that are not fully open, and restrictions inside lubricators, swivels and manifolds.

The useful working relationship from drilling practice is simple: the compressor must supply the pressure the hammer needs plus every loss along the way, including the hose, the drill string and the back-pressure of the water column in a wet hole. If the hose and fittings alone take a large share, there is less margin left for depth and water.

• Reducing couplings at the rig inlet are a common hidden restriction. A 2 inch hose that ends in a smaller bore fitting behaves like a smaller hose at that point.
• A long hose coiled on the ground adds loss and traps condensate.
• Internal hose collapse or a delaminated liner may not be visible from outside.
• Leaks at couplings and swivels reduce the flow that reaches the hammer, which shows up as lower pressure under load.

The practical test is to fit a reliable gauge at the rig inlet and compare it with the compressor gauge under the same drilling condition. Readings taken at different times or with the hammer on and off bottom should not be compared as if they were the same.

Couplings, whip checks and safe hose practice

Most serious hose accidents on drilling sites come from a coupling separating under pressure. The risk is managed by the right hardware and a consistent routine.

• Use couplings designed and rated for the pressure, with full bore where possible, and keep coupling types consistent across the package.
• Fit a whip check or safety cable across every hose-to-hose and hose-to-equipment joint, correctly sized for the hose and pressure, with the loops pulled snug.
• Secure claw-type couplings with safety pins or clips as the manufacturer intends.
• Never disconnect, tighten or move a hose while it is pressurised. Close the service valve and bleed the line first.
• Keep crew out of the line of a pressurised hose wherever practical, and protect hoses that cross access roads.
• Remove from service any hose with exposed reinforcement, bulges, cuts deep into the cover, or damaged fittings.

A short pre-start walk of the hose route each shift takes minutes and catches most problems before they become failures.

Manifolds, gauges and multi-compressor connections

For deeper or wetter holes, contractors sometimes combine two compressors, or a compressor and a booster, into one rig supply. A manifold then joins the air streams. Its internal bore must be large enough for the combined flow, each inlet needs a non-return valve so one machine cannot back-feed the other, and the manifold should carry a pressure gauge and a safe bleed-down valve.

When two machines are combined, their pressure settings and control behaviour must be compatible. A mismatched pair can leave one compressor running unloaded while the other does all the work. The manifold is also the natural point to measure delivered pressure, so it is worth fitting a good gauge there.

Inspection and replacement routine

1. At each shift start, walk the full hose route and check couplings, pins and whip checks.
2. During drilling, note the compressor and rig-inlet pressures at a stable drilling condition.
3. Weekly, inspect hoses closely for abrasion, cuts, kinks, heat damage and soft spots.
4. Replace worn coupling seals and gaskets, and do not mix coupling types from different systems.
5. Record hose assemblies by size, pressure rating and age, so replacement is planned rather than forced by a failure.

What a hire package needs between the compressor and the work

A hired compressor only delivers useful air if everything between its service valve and the tools or rig is correctly specified. When you request compressor hire, list every item in this table and confirm in writing who supplies each one. The page on hoses, manifolds and air distribution on hire covers the wider site layout.

Small 2–10 hp piston compressors on hire come with their air receiver, because the receiver is part of the unit. For larger machines, treat the receiver as an engineering question rather than an accessory. A DTH rig draws almost steady flow and rarely needs one. A site where several breakers start and stop together may need one. The size follows from V = t × (peak demand − compressor output) × atmospheric pressure ÷ Δp. Here V is in m³, t is the duration of the peak in minutes, the flows are free air in m³/min, atmospheric pressure is 1.013 bar(a) and Δp is the pressure dip you can accept, in bar. See air receiver tanks for sizing in detail.

Hose pressure drop by bore and length at rig flows

The table gives the calculated pressure drop along 30 m of straight hose for typical drilling flows. It assumes a smooth rubber inner tube and air at 50 °C. Each additional 30 m adds the same drop again. Couplings, reducers and swivels add to these figures, so treat them as the minimum loss for a line, not the whole of it.

Two patterns are worth remembering. Pressure drop rises with roughly the square of flow and falls with roughly the fifth power of bore, so one size up in bore usually solves a hose loss problem. At the same free-air flow, higher pressure means denser air moving more slowly, so a given hose loses less pressure at 24 bar(g) than at 17 bar(g).

Worked example: 1,000 cfm at 17 bar(g) through 30 m of 2 in hose.

1. Free air flow: 1,000 ÷ 35.315 = 28.32 m³/min = 0.472 m³/s. Mass flow at 1.204 kg/m³ (free air at 20 °C) = 0.568 kg/s.
2. Air density in the hose at 18.01 bar(a) and 50 °C: 1,801,000 ÷ (287 × 323) = 19.4 kg/m³.
3. Bore area: π × 0.0508² ÷ 4 = 0.002027 m². Velocity = 0.568 ÷ (19.4 × 0.002027) = 14.4 m/s.
4. Reynolds number is about 730,000, which gives a friction factor of about 0.0166 for a smooth hose.
5. Pressure drop = f × (L ÷ D) × ρv² ÷ 2 = 0.0166 × (30 ÷ 0.0508) × 19.4 × 14.4² ÷ 2 = 0.0166 × 590.6 × 2,024 Pa = about 19,900 Pa = 0.20 bar.

With the same flow, a 1½ in hose loses about 0.90 bar, and a 2½ in hose about 0.06 bar. Doubling the 2 in hose to 60 m doubles the loss to 0.40 bar. If the gauge at the rig reads more than about 0.5 bar below the compressor gauge on a short line, look for a restriction rather than blaming the hose length.

Pressure rating, safety factor and hose construction

Compressed air stores far more energy than water at the same pressure, which is why rating margins matter. Thirty metres of 2 in hose at 24 bar(g) holds about 228 kJ, released by adiabatic expansion if a joint separates. That is enough energy to lift a one-tonne vehicle about 23 m.

• Maximum working pressure must be at least the highest pressure the compressor can produce. That means the safety valve setting, not the normal working pressure. For example, for a 24 bar(g) (348 psig) machine whose safety valve is set at 26 bar(g), the hose and every fitting must be rated at 26 bar(g) or more.
• Burst pressure of compressed-air hose is commonly specified at four times its maximum working pressure or more. Use the working pressure, burst pressure and safety factor that the hose maker states for air service. A hose rated for water or hydraulic oil with a lower ratio is not suitable for air even if its working pressure figure looks adequate.
• Construction. Standard textile-reinforced industrial air hose (for example to ISO 2398, whose highest type is rated at 25 bar) is commonly rated up to about 25 bar. DTH duty needs hose rated above the compressor’s maximum pressure, with the safety factor the hose maker states. For most high-pressure DTH and 25–35 bar boosted lines that means wire-braided or spiral-reinforced drilling hose with permanently swaged or crimped end fittings.
• Temperature. Hose ratings fall at high temperature, and textile air hose to ISO 2398 is limited to 70 °C. On a 45 °C day, air leaving a portable compressor can reach 55–65 °C and the hose cover in direct sun can be hotter still. Check the hose’s air-temperature limit, and keep hoses off hot exhaust parts.
• Markings. A drilling hose should carry its maximum working pressure, bore and date of manufacture. Unmarked hose should not be used above ordinary plant pressures.

Coupling types for high-pressure DTH lines

The coupling is usually the weakest point of a hose line. The type must be rated at least as high as the hose, and must be designed for compressed air, which can drive a separated joint violently.

Never use hose clamps (jubilee clips) on any drilling line. Whip checks must match the hose bore and carry a rating at least equal to the line pressure. Fit each loop behind the collar or ferrule on either side of the joint, with the cable pulled straight and no slack.

Manifold bore for several users

When one compressor feeds several tools or rigs, the manifold body must not become the restriction. A practical rule is that the manifold bore area should be at least equal to the combined area of the outlets in use at the same time.

Worked example (illustrative). Four 25 mm outlets have a combined area of 4 × (π × 25² ÷ 4) = 4 × 491 = 1,963 mm². A single bore with the same area has a diameter of √(4 × 1,963 ÷ π) = 50 mm. Choose 50 mm as the minimum and 65 mm if more outlets may be added later. Three 50 mm outlets, by the same rule, need √(3) × 50 = 87 mm, so an 80 mm body would be too small and a 100 mm body is the safe choice.

Check the velocity as well. The full 1,000 cfm of the hose example above, at 17 bar(g) and 50 °C, moves at about 14 m/s through a 51 mm bore, inside the 15–20 m/s range often used as an upper guide for air mains. The same flow through a 38 mm bore would move at about 26 m/s, and the loss at the manifold would show on the rig gauge.

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.

We sell, hire and service borewell and DTH compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes as an independent multi-brand specialist. From our office at Tarnaka, Secunderabad, our service teams cover the whole of Telangana and Andhra Pradesh, and we supply machines and spares across India.

On a hose or connection problem we help in practical ways: we review the hose size, length and rating against your compressor’s flow and pressure, check for restrictive fittings, compare compressor and rig-inlet pressures, and advise on whip checks and safe connection practice. Where a rental compressor goes to site, we agree in advance who supplies the main hose and fittings. We also supply replacement hoses, couplings and compressor spares after identifying them from your machine details.

What to send us

• Compressor make, model and nameplate photo, with rated cfm and pressure
• Hose size, length, pressure rating and a photo of the couplings at each end
• Typical bore diameter, depth and whether the hole makes water
• Compressor and rig-inlet pressure readings while drilling, if available
• Any manifold, lubricator or booster in the line

Frequently asked questions

What size air hose is used for a borewell compressor?

Large borewell compressors commonly use 2 inch (50 mm) or larger hose, while smaller machines may use 1.5 inch. The right size depends on flow, working pressure and length, because loss rises steeply when air velocity in the hose is high. Every hose and fitting must also be rated above the compressor’s maximum pressure, so the bore and the pressure rating should both be checked against the actual machine.

How much pressure is lost in a compressor hose?

It depends on hose bore, length, flow and fittings, so there is no single figure. A correctly sized, short hose with full-bore couplings loses little, while an undersized or long hose with reducing fittings can lose a significant share of the available pressure. The reliable way to know is to compare the compressor gauge with a gauge at the rig inlet during stable drilling.

What is a whip check and why is it needed?

A whip check is a steel cable restraint fitted across a hose coupling. If the coupling separates under pressure, it stops the hose end from flailing violently and injuring people nearby. On high-pressure borewell compressors the stored energy is large, so a correctly sized whip check should be fitted across every hose joint and kept snug, alongside the coupling’s own safety pins.

Why does my rig show lower pressure than the compressor gauge?

The difference is the pressure lost between the two gauges. Common causes are an undersized or long hose, reducing couplings, a partly open service valve, a restrictive lubricator or manifold, worn swivel seals and leaks. Some difference is normal, but a large or growing gap points to a restriction or leak in the connection rather than a weak compressor, and is worth checking point by point.

Can I use a construction air hose on a high-pressure DTH compressor?

Only if its working pressure rating, including the fittings, is above the compressor’s maximum pressure with the manufacturer’s safety factor intact. Many general construction hoses are rated for around 10 bar duty and are not suitable for borewell machines working at 14–24 bar. Using an under-rated hose is a serious burst and whipping hazard, so check the marking on the hose itself.

When should a drilling air hose be replaced?

Replace a hose when its reinforcement is exposed, the cover has deep cuts or abrasion, there are bulges, soft spots, kinks or heat damage, or the fittings are damaged or slipping. Hoses that have been crushed by vehicles may be damaged inside even if the outside looks acceptable. Recording each hose’s age and duty helps you replace it in a planned way rather than after a failure.

Does compressor hire include hoses, a receiver and a manifold?

Small 2–10 hp piston compressors on hire come with their air receiver as part of the unit. For larger hire compressors, who supplies the main hose and fittings is agreed in advance, and a receiver or manifold should not be assumed. List everything you need in the hire brief: hose length and bore, the maximum pressure it must withstand, the coupling type at the tool or rig, the number of manifold outlets, and whether peak demand needs a receiver. The scope is then confirmed in writing before the machine leaves for site, so it does not arrive with no means of connecting it to your equipment.

What pressure rating should a compressor hose have?

The hose’s maximum working pressure must be at least the highest pressure the compressor can reach, which is its safety valve setting, not its normal operating pressure. For example, a 24 bar(g) drilling compressor with its safety valve at 26 bar(g) needs hose, couplings and whip checks all rated at 26 bar(g) or more. Compressed-air hose is also commonly specified with a burst pressure of four times its working pressure or more. Standard textile-reinforced industrial air hose is commonly rated up to about 25 bar. Above that, use wire-reinforced drilling hose rated above the compressor’s maximum pressure, with the safety factor its maker states.

What size manifold do I need for several tools on one compressor?

Make the manifold bore area at least equal to the combined area of the outlets that will be open at the same time. Four 25 mm outlets add up to about 1,963 mm², which matches a 50 mm bore, so 50 mm is the minimum and 65 mm leaves room for another outlet. Then check the velocity through the body at the full compressor flow and pressure; keeping it below about 15–20 m/s avoids a noticeable loss. Rate the manifold, its valves and its gauge above the compressor’s safety valve setting.

Are claw couplings safe on a high-pressure drilling compressor?

Generally not. Claw couplings are designed for construction tools and plant air, and are usually rated for working pressures of around 10 bar(g). A DTH compressor at 17–35 bar(g) needs threaded fittings with crimped ferrules, hammer unions or flanged joints. Each must be marked with a rating above the compressor’s safety valve setting. If a claw coupling is used on a lower-pressure line, its locking pin must be fitted and a correctly rated whip check must cross the joint.

Related pages

• Borewell and DTH compressor library
• Drill string and air path review
• Hoses, manifolds and air distribution on hire
• Borewell compressor rentals and hire
• Borewell compressor spare parts
• Booster-assisted drilling air