7 signs that a compressor unit needs a major overhaul
A compressor rarely fails without warning. In almost every case the failure is preceded by months of signals visible in the operating log, in the oil analysis and simply by ear. Here is how to tell which symptoms mean wear that requires opening the unit, and which are resolved by adjustment and are not worth the downtime.
1. Running hours have been reached
The most formal indicator, and also the most reliable. The manufacturer defines the overhaul interval in running hours. It differs by an order of magnitude between unit types, so the reference is the operating manual for your specific model, not general figures found online.
Approaching the limit is grounds for a planned overhaul even if the unit runs smoothly. Scheduled repair is always cheaper than emergency repair: you choose the shutdown window, you have time to order genuine parts, and you do not pay for express logistics.
2. Rising vibration
Vibration is the main quantitative indicator of rotating equipment condition. Assessment follows ISO 20816 (formerly ISO 10816), which defines acceptance zones of RMS vibration velocity for each machine class.
What matters is not the absolute level so much as the trend. If vibration velocity has increased by half again or doubled over a quarter under an unchanged operating regime, that is wear, not measurement error. Spectral analysis then indicates the source: rotor unbalance, misalignment, a bearing defect or rubbing.
What can be corrected without opening the unit: shaft misalignment, loose mounting to the frame, unbalance caused by deposits on the impellers.
What requires an overhaul: worn bearing assemblies, a deformed or cracked rotor, worn seating surfaces.
3. Loss of capacity at the same operating point
If the unit now consumes more energy or takes longer to reach the set discharge pressure, internal leakage is increasing. On reciprocating machines this means wear of the cylinder-piston group and leaking valves; on centrifugal machines, widening labyrinth seal clearances and impeller erosion.
Before planning a repair it is worth ruling out the simple causes: a clogged suction filter, a fault in the capacity control system, or an instrument reading error.
4. Metal in the oil and changes in its properties
Regular oil analysis is the cheapest way to look inside the unit without disassembly. What the results indicate:
- iron and bronze particles — wear of journal bearings and liners
- silicon — dust ingress through a leaking suction path
- water — a fault in the cooling system or condensate in the crankcase
- rising acid number and viscosity — thermal ageing of the oil, often a consequence of overheating
A single spike is a reason to retest. A steady rise in metal concentration from sample to sample is a reason to prepare the unit for opening.
5. Elevated discharge temperature
A sustained rise in stage outlet temperature under unchanged suction conditions means part of the compression work is turning into heat instead of useful pressure. The causes are the same internal leakage, plus fouled heat exchangers and reduced cooling system performance.
Overheating is doubly harmful: it accelerates oil ageing, and degraded oil in turn accelerates bearing wear. That loop closes quickly.
6. Leakage through seals
Oil or gas appearing where the shaft exits the casing indicates worn mechanical or labyrinth seals. On its own this is not yet an overhaul: replacing the seal kit is often enough.
But if seals fail repeatedly at short intervals, the root cause usually lies deeper — shaft runout, a worn seating surface, excessive vibration. Replacing the seal in that situation is pointless; it will fail again.
7. Unusual noise and knocking
Knocking in time with rotation on a reciprocating machine typically means excessive clearances in the connecting rod and piston assembly, or loose mounting. Whistling or howling on a centrifugal machine may indicate rotor rubbing against the casing, or surge.
This is the one sign that cannot be deferred. Unlike a slow rise in vibration, the appearance of knocking means destruction is already under way, and time to failure is measured in days rather than months.
How the repair decision is made
No single sign is conclusive on its own. The decision is based on the combination: running hours, vibration trend, oil analysis results, operating parameters. In practice we proceed as follows:
- record vibration characteristics and operating parameters on the running unit
- take an oil sample and compare it against previous analyses
- carry out fault assessment, with partial disassembly where required
- issue a condition report with remaining service life and a scope of works
The report shows what is actually needed: adjustment, replacement of a seal kit, or full restoration with component replacement. The same report serves as the basis for ordering genuine spare parts — before the unit is stopped, not after.
Unsure about the condition of your unit?
Describe the compressor model and the symptoms — a QANSAT ENERGY engineer will advise whether an on-site inspection is needed or whether checking the log parameters will do.
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