What to check on a Starting Air Compressor.
82 models from Atlas Copco Marine in Starting Air Compressor. Below: the full model list.
What defines this type
A starting air compressor is a reciprocating machine, usually two or three-stage, raising atmospheric air to the working pressure an engine's starting air system needs, commonly in the range used for medium and slow-speed diesels. That sets it apart from a service or control air compressor, which runs continuously at a much lower pressure — a starting air compressor runs intermittently, charging a receiver rather than supplying a steady demand, and is sized for high pressure rather than continuous flow. Many modern units use an oil-free final stage specifically so the delivered air stays clean enough for the starting air lines and the engine's own starting valves.
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Main components
- Cylinders per stage — with intercoolers between stages and an aftercooler on the final discharge.
- Crankcase and running gear — crankshaft, connecting rods and pistons driving the compression stages.
- Suction and discharge valves per stage — the components most exposed to wear and the most common source of falling delivery pressure.
- Safety valves per stage — protect each stage against overpressure if a downstream valve or line is blocked.
- Oil and water separator — removes carried-over oil and condensed moisture from the discharge before it reaches the receiver.
- Unloading arrangement — allows the compressor to start and stop against receiver pressure without excessive load.
Selection / sizing
Receiver capacity is set first, driven by the minimum number of engine starts class rules require to be available without recharging; compressor capacity, expressed as free air delivery, then follows from how quickly those receivers must be recharged. Discharge pressure must match the design pressure of the engine's starting air system. Because a single compressor failure cannot be allowed to remove the ship's ability to start its engines, at least two independent compressors are fitted, each capable on its own of recharging the receivers within the required time.
Regulations / Class
Class rules require a minimum of two starting air compressors, each independently capable of charging the receivers, together with receivers holding enough air for the minimum number of starts the rules specify without the compressor running. Safety valves on each stage are set below the design pressure of the equipment downstream and are checked at periodic survey, alongside internal or hydrostatic examination of the receivers on the class survey cycle.
Typical faults
- Worn piston rings or valves — falling delivery pressure and longer recharge times, eventually threatening the number of available starts.
- Oil carryover past a fouled separator — deposits build up in the starting air line and in the engine's own starting valves, and in the worst case create a fire risk inside the air line itself.
- Intercooler fouling — raises stage discharge temperature, accelerating deposit formation and reducing efficiency.
- Safety valve set-point drift — either nuisance lifting under normal operation or, in the other direction, failure to lift and overpressure of a stage.
- Moisture carryover — corrosion inside the receivers and inside the engine's starting air valves.
What to look for in a supplier
- Free air delivery and stage pressures matched to the actual receiver volume and engine starting demand, not a generic model selection.
- Spare valve and piston ring kits genuinely stocked, since these are the parts that wear fastest.
- Documented safety valve certification for each stage.
- Reference installations on the same compressor family from other vessels of comparable engine size.
Keep starting air scrupulously oil- and moisture-free — a fouled separator does not just cost efficiency, it puts oil straight into a high-pressure line feeding the engine's own starting valves.