What to check on a Start Air Compressor.
Main and auxiliary diesel engines start by admitting high-pressure air directly into the cylinders to turn the crankshaft, which means the compressor supplying that air has to reach 25-30 bar, well above the 7-10 bar typical of a service or control air compressor. That pressure requirement pushes the design toward a multi-stage reciprocating machine rather than a screw compressor: screw compressors dominate low-pressure service and instrument air because they run continuously and quietly, but they do not reach starting-air pressure efficiently or reliably enough to be trusted for engine starting.
What sets a start air compressor apart
Main and auxiliary diesel engines start by admitting high-pressure air directly into the cylinders to turn the crankshaft, which means the compressor supplying that air has to reach 25-30 bar, well above the 7-10 bar typical of a service or control air compressor. That pressure requirement pushes the design toward a multi-stage reciprocating machine rather than a screw compressor: screw compressors dominate low-pressure service and instrument air because they run continuously and quietly, but they do not reach starting-air pressure efficiently or reliably enough to be trusted for engine starting.
Main components
Cylinders and stages
Typically two or three stages in a V or W arrangement, each stage raising pressure further, with intercoolers between stages to keep discharge temperature and compression work down.
Intercoolers and aftercooler
Water- or air-cooled heat exchangers between stages and after the final stage, dropping air temperature and condensing out moisture before it reaches the receiver.
Crankcase and drive
A crankshaft-driven arrangement similar in principle to a small diesel engine's bottom end, usually electric motor driven, with splash or forced lubrication.
Safety and relief valves
Fitted at each stage to protect against a blocked downstream line overpressurising an earlier, lower-rated stage.
Unloading and control system
Automatic start/stop or load/unload control tied to receiver pressure switches, so the compressor cuts in as starting air is consumed and cuts out once the receivers are topped up.
Selection and sizing
Sizing is driven by the number and size of starting air receivers to be recharged, the time allowed to recover full pressure after a set of engine starts, and the redundancy required. Most vessels carry at least two compressors so a single failure does not leave the ship unable to start its main engine. Capacity is usually expressed as free air delivery in Nm3/h or l/min at rated discharge pressure, matched to the receiver volume specified for the vessel's engine.
Regulations and class
Class rules require at least two independent means of charging the starting air receivers on vessels where main propulsion depends on compressed-air starting, and set minimum receiver capacity relative to the number of starts required without recharging. Safety valves on each compressor stage and on the receivers are subject to periodic survey and testing. IACS UR machinery requirements cover starting air system arrangement, including non-return valves and bursting discs to prevent a receiver explosion propagating back through the piping.
Typical faults
- Worn piston rings or valve plates: falling volumetric efficiency, so the compressor runs longer to reach set pressure, raising running hours and wear.
- Intercooler fouling on the water side: rising interstage temperature, higher risk of oil carbon deposits igniting in the receiver, a recognised cause of starting air line fires.
- Safety valve seat wear or seizure: either premature lifting and lost capacity, or failure to lift, risking overpressure downstream.
- Condensate drain valves left closed or blocked: moisture carries into the receivers, promoting internal corrosion and freezing of pilot lines in cold climates.
- Automatic unloader stuck: the compressor runs continuously against a closed valve and overheats.
What to look for in a supplier
- Class type approval for the discharge pressure and duty class specified for the vessel.
- Spare parts commonality with compressors already on board, where a sister-ship fleet standard exists.
- Documented interstage and final relief valve settings matched to the receiver design pressure.
- Service network able to supply piston rings, valve plates and cooler gaskets without a long lead time, since a starting air compressor failure is a departure-affecting defect.
Drain the intercoolers and receivers before every watch change, not just on the planned maintenance schedule - carbon-laden condensate sitting in a hot receiver is how starting air line fires start.