Low oil pressure is one of the most typical alerts in a screw air compressor. Failure to observe this can lead to rapid overheating, bearing damage and in the worst case to seizure of the air end. By understanding how the lubrication system works and why the oil pressure lowers, you can promptly diagnose the problem and save costly downtime.
In this guide: How the oil circuit works · The 11 most common causes · A step-by-step troubleshooting procedure · What happens if you ignore it · Prevention · FAQ
How the Lubrication System of a Screw Compressor Works
In an oil-injected (oil-flooded) rotary screw compressor, the lubricating oil performs four functions simultaneously. It lubricates the bearings and rotors, it seals the clearances between the rotors and the housing, it cools the compressed air and it decreases noise.
Most screw compressors do not utilize a mechanical oil pump, unlike piston compressors. The oil circulation is created by the pressure difference between the oil separator tank and the air end input. An example of an oil circuit is:
Compressing element (air end) -oil is fed into the compression chamber and mixed with the air.
Receiver tank (oil separator tank) - The air/oil mixture is sent into the tank where centrifugal force and gravity separate most of the oil. The oil circulation is driven by the pressure in the tanks.
Mini Pressure Valve(MPV) -maintains the tank pressure above a certain mini (typically 4–4.5 bar) so that oil can flow even at low system demand.
Temperature control valve (thermostatic valve) - sends oil back to the air end (cold), or via the oil cooler (hot).
Oil cooler - heat exchanger which cools the oil, either air or water cooled.
Oil filter - eliminates solid particles prior to the oil’s return to the air end.
Oil separator element - fine filtering that eliminates the last oil mist from the compressed air and returns the collected oil to the air end via the scavenge line.
Oil piping, hoses and fittings, - connect all the dots.
Oil Pressure Sensor/Gauge and Temperature Sensor - monitor the system and raise alarms.
Some large industrial screw compressors and oil less screw compressors have a separate gear type oil pump but following troubleshooting logic still works.
When this guide doesn’t apply. It is written for oil flooded (oil injected) rotary screw machines. Large machines with a separate gear pump work differently than the oil free screw compressor which has no oil circuit to lose pressure from. Also, the number one cause of “low oil pressure” readings is a sensor problem, not low oil at all – therefore check the reading before you touch the machine.
Low oil pressure Because circulation depends on pressure difference, any reduction in tank pressure, restriction in the oil flow or oil leak will create low oil pressure.
The 11 Common Causes of Low Oil Pressure
To summarize, the most common causes are: - Faulty pressure sensor - Low oil level - Clogged oil filter - Failed thermostatic valve - Blocked oil cooler - Faulty mini pressure valve - Running low system pressure - Damaged separator element - Oil leaks - Degraded or incorrect oil - Worn air end bearings Here is an explanation of each, from the simplest to the most serious.
1. Faulty Oil Pressure Sensor or Gauge
Check the reading before you assume there is a true problem. Pressure sensors drift with age, connectors corrode and mechanical gages can stick . A wiring failure or a faulty connector on the sensor can potentially trigger a false low-pressure alarm.
How to verify: Compare reading with a calibrated test gage installed at the same site. Check the sensor wires and connector.
Solution: Sensor, gage recalibration or replacement.
2. Oil level is low
The oil pickup can draw in air when the oil level in the separator tank is below the mini line, and the circulation volume is not sufficient to sustain the pressure. Slow decreases in oil level are due to normal carry-over. A sudden decrease in oil level is generally indicative of a leak or damaged separator element.
How to verify: When the compressor has shut down, been depressurized and the oil has had a few minutes to settle, read the sight glass.
Answer: Add the correct oil to the recommended level If the level continues to fall, locate and repair the source of oil loss.
3. Oil Filter Clog
This is the most common reason, number one. As the filter becomes clogged with impurities, the pressure loss across the filter increases and the pressure to the air end decreases. Many compressors have a bypass valve in the filter head and if it opens, unfiltered oil gets to the bearings and causes accelerated wear.
How to check : Check the controller for a filter differential pressure indicator or alarm. Check the running hours since last change.
Solution: Replace the oil filter element with an original or genuine quality part. Never re-use a spin-on filter after cleaning. Reduce change interval in dusty situations.
Not sure what filter your machine has? Give us the compressor model and current hours and we'll validate the correct part number and change interval - before you buy the wrong one.
4. Failure of Thermostatic Valve
The thermostatic valve controls the oil flow between the cooler and the bypass line. If the valve element is stuck or leaking, the oil cannot flow properly and severe temperature and pressure variations can occur. A partially open valve will limit the total oil flow.
How to check: Check oil temperature before and after the cooler. Usually, abnormal temperature behavior is a symptom of the thermostatic valve.
Answer: Replace the thermostatic element or the entire valve.
5. Oil cooler blocked up
Internally it can be clogged by sludge, varnish and degraded oil. Externally it can be clogged by dust on the fins (air cooled) or scale in the tubes (water cooled). A small cooler increases the flow resistance and reduces the oil pressure downstream.
Find out how: Check pressure drop across the cooler and check visually the fins or tubes.
Answer: Clean outside of cooler using compressed air or water. If internal fouling is suspected, flush the cooler with an appropriate cleaning fluid or replace it.
6. Mini Pressure Valve Malfunction
Mini pressure valve keeps the required tank pressure to circulate oil. If the seat is worn or it is jammed open, tank pressure will be too low, especially during start-up or mild load if its spring is weak. The result is not enough oil pressure, and often not enough oil separation.
How to check?
Monitor tank pressure during start-up and on unload. It should promptly attain and hold the mini preset pressure (typically 4–4.5 bar).
Answer: Repair or replace the mini pressure valve and set the pressure according to the instructions in the manual.
7. Operation at Low System Pressure or Unload for Extended Periods
If the compressor works constantly at very low discharge pressure, for example due to a big leak in the plant air system or an inaccurate pressure setting, the driving pressure for oil circulation is similarly lowered. The same is true during long periods of operation with no load.
How to verify: Compare the actual discharge pressure to the design value and the mini pressure valve setting Check the air network for leaks.
Solution: Adjust pressure setting, correct air leaks, and do not run unloaded for extended periods.
8. Oil Separator Element Blocked or Damaged
The blocked separator device causes an increase of pressure loss between the tank and the outflow. When an element bursts, substantial amounts of oil can be transferred into the air system, and the oil level will drop rapidly. Both conditions affect the oil and pressure equilibrium.
How to appear: Separator differential pressure, oil usage and oil content in the supplied air
Solution: Change the separator element at the appropriate interval or when the differential pressure exceeds the limit (usually 0.8 – 1.0 bar).
9. Oil leaks in circuit
Loose fittings, damaged hoses, worn O-rings, a leaky shaft seal on the air end, or a leaking oil cooler all decrease the amount of oil and pressure in the system. Internal leaks such as a broken oil stop valve or leaking scavenging line check valve may be more difficult to identify.
How to check: Inspect all pipes, hoses and joints for wet spots and oil build-up. Inspect the drain pan and around the shaft seal on the air end.
Answer: Secure fittings; replace worn hoses, seals, and gaskets.
10. Lubricating Oil, Improper or Bad
Oil that’s been used too long, contaminated with water or dust, or blended with another type of oil might lose viscosity, foam or create sludge and varnish. Thin or foaming oil will not hold pressure. Sludge clogs filters, coolers. Using oil with the improper viscosity grade for the ambient temperature can have the same effect.
How to check it
Check the oil for color and smell, and also check for milky appearance (water contamination) or froth in the sight glass. If in doubt, send a sample for oil analysis.
Answer: No. Drain, cleanse and replace with the compressor oil specified by the manufacturer. Don't combine different types or brands of oil.
11. Worn Air End Bearings/Internal Clearances
As the bearings and seals inside the air end wear, internal clearances rise, allowing more oil to move through them with less resistance, causing a drop in system oil pressure. This is frequently accompanied by increased vibration, loudness and increased discharge temperature .
How to check it: Conduct a vibration analysis and verify the bearing status. Compare operating hours to the suggested air end overhaul interval.
Solution: Rebuild or replace the air end. This is the most expensive cause and should only be checked after all other causes have been eliminated.
Step-by-Step Troubleshooting Procedure
When a low oil pressure alarm appears, work through the causes from the simplest to the most complex:
Step Check Possible Cause
| Step |
Check |
Possible Cause |
| 1 |
Verify the reading with a test gauge; inspect sensor wiring |
Faulty sensor or gauge |
| 2 |
Check the oil level in the sight glass |
Low oil level, leak |
| 3 |
Check filter and separator differential pressure and service hours |
Clogged oil filter or separator element |
| 4 |
Check tank pressure at start-up and unload |
Mini pressure valve fault, low system pressure |
| 5 |
Check oil temperature before and after the cooler |
Thermostatic valve or cooler blockage |
| 6 |
Inspect pipes, hoses, fittings, and shaft seal for leaks |
External or internal oil leaks |
| 7 |
Check oil condition and specification |
Degraded or wrong oil |
| 8 |
Check vibration, noise, and discharge temperature |
Air end wear |
Safety note: Always stop the compressor, isolate the power supply, and release all internal pressure before opening any part of the oil circuit.
Effects of Ignoring Low Oil Pressure
Overheat:
●less oil, less cooling. Discharge temperature increases, compressor trips on high temperature.
Failure of the bearing:
●Lack of lubrication causes metal-to-metal contact and quick bearing wear.
Rub and seizing of rotor:
●In extreme circumstances, the rotors touch one other or the housing and wreck the air end.
Degradation of oil:
●Higher temperatures reduce oil life and cause varnish, creating a vicious cycle.
An air end is often a large component of the cost of a new compressor therefore it is always worthwhile to respond quickly to a low oil pressure indicator.
Preventive Maintenance To Prevent Low Oil Pressure
●Check oil level daily and add as necessary.
●Replace the oil filter, separator element and oil at the manufacturer’s suggested intervals — or sooner in severe service.
●Use only the kind and viscosity of oil recommended for your compressor. Do not mix oils.
●Clean oil cooler fins periodically, especially in dusty conditions. ●Keep the compressor room clean, well ventilated and within the recommended ambient temperature range.
● Check hoses, fittings and seals for leaks during every routine servicing.
●Monitor oil pressure, temperature and differential pressure trends on the controller to detect problems early.
●Avoid extended idle operation and repair air leaks in the plant network.
Frequently Asked Questions
What is the oil pressure for screw air compressor ?
Most oil injected screw compressors run at an oil pressure of 2-5 bar depending on the model and the load. Always consult the specification of your machine from the manufacturer.
Running a screw compressor with low oil pressure can you?
No. Running low oil pressure might destroy air end bearings in a short time. Shut down the compressor and find the cause before restarting.
How frequently should you replace the oil filter on a screw compressor?
Typically every 2,000–4,000 operating hours or sooner if operating in dusty conditions or when the differential pressure indication activates.
Why does my compressor only show low oil pressure at startup?
This generally indicates a problem with the thermostatic valve, cold and viscous oil or a bad mini pressure valve. If the alarm clears within a few minutes of running then examine these three things first.
Does low oil pressure create high temperature？
Yes. Less circulation of oil means less cooling and so a low oil pressure alarm is typically followed by a high discharge temperature signal.
Is low oil pressure the same as low oil level?
No. Low oil level can create low oil pressure . It can also happen with a full tank if the filter , cooler or valves are impeding flow .
Low oil pressure on a screw air compressor is seldom a mystery. It is usually caused by a blocked filter, low oil level, defective thermostatic or mini pressure valve, blocked cooler or a leak – all of which can be detected by a systematic examination. The air end, the most important component of the machine, will be protected and most of these problems may be avoided with regular maintenance of the machine using genuine filters and the suitable lubricant oil.
If your compressor is still giving low oil pressure alarms after the basic checks above, tell us what the symptom is, or send us a short video of the machine running and the readings on its display, and an engineer will help you narrow it down to the likely cause before you spend anything on parts or a service call. No obligation. Free.
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