Is it the compressor or the hole?
“The compressor is weak” is the most common complaint we hear from borewell crews, and it is a fair starting point, not a diagnosis. Slow penetration can come from the machine, the drill string, the hammer and bit, or the ground and water in the hole. Spending on an airend overhaul when the real cause is a worn bit or a deep water column wastes money and time.
The quickest way to separate the two is to test the compressor on its own, away from the hole, and then under load on the rig. If it reaches rated pressure and holds it with the engine at full speed when blowing through a known restriction, the machine is probably healthy and the cause lies downstream. If it cannot, the fault is in the compressor or engine.
| Points to the compressor | Points to the hole or tooling |
|---|---|
| Low pressure even with the hole dry and shallow | Pressure is normal near the surface but falls off below water |
| Engine does not reach rated speed on load | Engine and pressure normal, penetration still slow |
| Discharge temperature high or rising | Bit buttons flattened or gauge worn |
| Machine unloads below set pressure | Returns poor only in a larger bit size |
| Oil carry-over, smoke, unusual noise | Problem began after a change of hammer, rods or ground |
Pressure checks: what the gauge is telling you
Most borewell compressors work in the 200–400 psig band (about 14–28 bar), with special high-pressure units up to about 35 bar (507.6 psig). Know which band your machine is rated for and what its set pressure is, then check:
- Gauge accuracy. Panel gauges on portable machines take vibration and heat. Cross-check with a test gauge fitted at the service valve before trusting any reading.
- Unloaded pressure. With the service valve closed and the engine at full speed, the machine should rise to its set pressure and unload cleanly. If it will not reach set pressure even with no air demand, suspect the regulator, inlet valve, blow-down valve or a leak inside the package.
- Loaded pressure. With air flowing to the hammer, record pressure at the compressor and, if possible, at the rig. A large difference points to hose, swivel or string losses rather than the compressor.
- Pressure in relation to depth. Pressure that rises as the hole deepens and fills with water is normal back-pressure, roughly 1 bar (14.5 psi) per 10 m of water above the hammer. Our wet hole drilling page explains this.
Flow checks: is the compressor delivering its rated FAD?
Free air delivery (FAD) is the volume of air the compressor actually delivers, referred back to inlet conditions. A machine can make full pressure into a small restriction and still be short of flow, which shows up as poor hole cleaning and slow penetration on bigger bits.
On site, flow is usually judged by one of these methods:
- Test nozzle. A calibrated orifice or nozzle sized for the machine’s rating; if the compressor cannot hold the expected pressure through it at full engine speed, it is short of flow.
- Known hammer. A hammer whose air consumption at a given pressure is published by its maker. If the compressor holds less pressure on that hammer than the chart shows for its rated flow, flow is low.
- Workshop test. Where a site test is inconclusive, a proper flow test in the workshop removes the guesswork.
Common causes of low FAD include a choked intake filter, a partly closed inlet valve or faulty regulator, worn airend rotors or bearings, internal leakage through a minimum-pressure or blow-down valve, and low engine speed. Each has a different repair, which is why the test comes before the spanner.
Unloading and capacity control
Borewell compressors control output by throttling the inlet and reducing engine speed as demand falls. When the hammer is drilling, the machine should stay fully loaded. Watch for:
- Early unloading: the machine starts to throttle before reaching set pressure, cutting flow when you need it most. Usually a regulator, pressure-sensing line or control valve fault.
- Hunting: engine speed and pressure cycle up and down while drilling steadily. Often a sticking inlet valve, a leaking control line or a faulty speed control.
- No unloading: pressure rises past set point when the service valve is shut and the safety valve lifts. This is a safety concern; stop and investigate.
- Slow blow-down: long delay in venting on shutdown, risking hard restarts and oil carry-over.
Temperature: the early-warning signal
Normal discharge temperature on an oil-injected screw compressor is typically about 75–95 °C. Borewell machines run at high pressure and full load for long spells, often in 45 °C summer heat and heavy dust, so temperature margins are thin. A rising trend usually comes before a high-temperature shutdown.
- Dusty or oil-coated oil cooler and radiator fins block airflow; clean them from the fan side out.
- Low oil level, wrong oil grade or degraded oil reduce cooling.
- A sticking thermostatic valve can send oil past the cooler.
- A blocked oil filter or oil stop valve starves the airend.
- Recirculation of hot air around the machine when parked close to the rig or in a pit.
Never bypass the high discharge temperature shutdown. If it trips, find the cause; repeated overheating breaks down oil and shortens bearing life in the airend.
Engine power loss
On a diesel borewell compressor, the engine is half the machine. If it cannot hold rated speed on load, the airend turns slower and delivers less air, even if the airend itself is perfect. Signs include black smoke on load, speed drop when the hammer starts, hard starting and higher fuel use for the same work.
- Check engine speed with a tachometer, unloaded and loaded, against the rated speed.
- Inspect engine air filter restriction; dusty rig sites choke filters quickly.
- Check fuel filters, water separator and fuel quality; contaminated diesel is common on remote sites.
- Look at turbocharger condition, intake and exhaust leaks, and intercooler cleanliness.
- Review injector and fuel pump service history.
- Confirm low engine oil pressure and high coolant temperature shutdowns work.
Symptom table
| Symptom | Likely causes | First check |
|---|---|---|
| Does not reach set pressure | Regulator, inlet valve, blow-down valve, internal leak, low engine speed | Test gauge at service valve, engine rpm, closed-valve test |
| Full pressure but slow drilling | Low FAD, worn bit, deep water, string losses | Nozzle or hammer flow test, bit condition, water level |
| High discharge temperature trip | Dirty coolers, low or old oil, thermostatic valve, hot ambient | Cooler fins, oil level and condition, temperature trend |
| Oil in the air line | Separator element, scavenge line, overfilled oil, frequent hard stops | Oil level, scavenge orifice, separator differential |
| Engine black smoke on load | Air filter, injectors, turbo, overload | Filter restriction, rpm drop, injection service |
| Pressure and speed hunting | Control line leak, sticking inlet valve, speed control | Control lines and regulator, inlet valve movement |
| Safety valve lifting | Regulator or unloading fault, set point wrong | Stop; check control system before restart |
Building a useful service record
A diagnosis is much faster when the crew has written down what changed and when. Keep machine evidence and application evidence side by side: compressor hours, service history, alarms, pressure and temperature readings, and engine speed on one side; rig, hammer, bit, depth and water observations on the other. Note where each reading was taken. A short log over a few holes tells an engineer more than one reading taken on a bad day.
For a planned approach to servicing, see our breakdown diagnostics and fault triage and rotary screw service pages.
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 services borewell and DTH compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands as an independent multi-brand specialist. Our team of more than 20 service engineers and technicians works across Telangana and Andhra Pradesh from our office at Tarnaka, Secunderabad, and we supply spares across India.
- Pressure, flow, temperature and engine checks to separate a compressor fault from a hole or tooling problem.
- Repairs to regulators, inlet valves, cooling systems, separators and engines; airend overhaul where the evidence supports it. See borewell compressor service and repairs.
- Genuine or named aftermarket spares identified from the nameplate, serial number or old part, with fit confirmed before quoting: borewell compressor spare parts.
- A hire machine from our own fleet while yours is repaired, so the rig keeps drilling.
- Advice on whether repair, a used or reconditioned machine, or a new compressor makes better sense.
Call +91 99590 60718 or +91 92468 88812, or WhatsApp +91 99590 60718.
What to send us
- Make, model, serial number and a nameplate photo.
- Running hours and the date of the last service.
- Pressure, temperature and engine speed readings, loaded and unloaded, with where they were taken.
- Any alarms or shutdowns, and when they occur.
- Hammer and bit size, typical depth and water conditions.
- A short video of the gauge panel and the machine running, if possible.
- Site location in Telangana, Andhra Pradesh or elsewhere.
Frequently asked questions
How do I know if my borewell compressor is weak?
Fit a test gauge at the service valve and check that the machine reaches its set pressure and unloads cleanly with the engine at full speed. Then check it holds expected pressure through a test nozzle or a hammer with a known air consumption. If it cannot, the compressor or engine is short. If it passes and drilling is still slow, look at the bit, string losses and water in the hole.
Why does my borewell compressor pressure drop while drilling?
Some drop from set pressure under load is normal as air flows. A large drop usually means the compressor is not delivering its rated flow, because of a choked intake, faulty inlet valve, worn airend or low engine speed. Check pressure at the compressor and the rig to see whether hose and string losses are part of the cause. Rising pressure with depth is water back-pressure, not a fault.
What causes high temperature trips on a DTH compressor?
Typical causes are dusty or oil-coated coolers, low or degraded oil, a sticking thermostatic valve, a blocked oil filter and hot air recirculating around the machine. Borewell compressors run at full load and high pressure, often in 45 °C summer heat, so cooling margins are thin. Clean coolers regularly, keep oil at the right level and grade, and never bypass the high discharge temperature shutdown.
Why is there oil coming out with the air from my compressor?
Oil carry-over usually comes from a damaged or saturated separator element, a blocked scavenge line or orifice, overfilling the oil, or frequent hard stops that disturb the oil in the receiver. Excess oil in the air reaches the hammer and the hole, and the compressor runs low on oil. Check the oil level first, then the scavenge system and separator pressure difference, before replacing parts.
Can engine problems reduce compressor air output?
Yes. On a diesel borewell compressor the airend is driven directly by the engine, so if the engine cannot hold rated speed on load, air delivery falls in proportion. Black smoke, speed drop when the hammer starts and higher fuel use are common signs. Check engine speed with a tachometer, then air and fuel filters, fuel quality, turbocharger and injection service before suspecting the airend.
When does a borewell compressor need an airend overhaul?
An overhaul is justified when tests show low flow that cannot be explained by the intake, controls or engine speed, especially with rising discharge temperature, noise, vibration or bearing wear. Decide on evidence from pressure, flow, temperature and inspection rather than hours alone. Reddy Compressors can test the machine first so that you overhaul only when it is really needed.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.