Marine Compressed Air Systems

Background

Compressed air applications

Compressed air applications:

• Engine starting: 30 bar air injected through cylinder starting valves to rotate the engine for ignition.
• Pneumatic tools: hand-held drills, grinders, hammers used in maintenance.
• Cleaning and blow-down: high-pressure air for cleaning fouled equipment.
• Instrument and control: pneumatic transmitters, valves, and signalling.
• Cargo system: in some configurations, providing motive power for cargo system valves.
• Inert gas: in some configurations, dilution air for inert gas system.
• Whistle: air-driven ship whistles.
• Emergency systems: emergency engine starting, emergency steering hydraulic boost.

Major manufacturers

Marine compressor manufacturers:

• Atlas Copco (Sweden): leading supplier with extensive marine product range.
• Sauer Compressors (Germany): specialised marine compressor builder.
• Sperre (Norway): marine compressor specialist.
• Hatlapa (Germany, now part of MacGregor): compressor and deck machinery.
• Tanabe (Japan): Japanese marine compressor builder.
• Bauer (Germany): high-pressure compressors.

The market includes both general industrial compressor companies adapting for marine and dedicated marine compressor specialists.

Pressure levels and applications

Starting air (30 bar)

Starting air at 30 bar gauge:

• Main engine starting: large two-stroke crosshead engines using starting air valves on cylinders.
• Auxiliary engine starting: medium-speed four-stroke auxiliary engines.
• Storage: in dedicated starting air receivers (typically 2 to 6 receivers).
• Capacity: minimum 12 starts per SOLAS Chapter II-1 Regulation 27 for main engine.
• Compressor type: typically two-stage reciprocating with intercooling.

Working air (7-10 bar)

Working air at 7-10 bar gauge:

• Pneumatic tools: in engine room and other workshops.
• Cleaning operations: blow-down of equipment.
• General services: deck operations, accommodation cleaning.
• Storage: in working air receivers, separate from starting air.
• Compressor type: typically rotary screw or smaller reciprocating.

Control air (7 bar)

Control air at 7 bar gauge:

• Instrument and control: pneumatic transmitters, valve actuators.
• Drying required: dew point typically -20°C or lower.
• Filtration: typically 0.01 to 0.1 micron for instrument grade.
• Storage: dedicated control air receiver.
• Compressor type: typically smaller dedicated unit, or branched from working air with conditioning.

Emergency air

Emergency air for safety-critical applications:

• Emergency main engine starting: when normal starting air is depleted or unavailable.
• Emergency air compressor: typically electric or hand-pumped.
• Storage: dedicated emergency air receiver.

System components

Compressors

Marine air compressors:

• Reciprocating two-stage: traditional choice for starting air. 30 to 250 m³/h capacity at 30 bar.
• Rotary screw: increasingly common for working air. Higher continuous duty cycle.
• Centrifugal: rare on ships, used for very high capacity applications.
• Drive: electric motor most common; some smaller engines hydraulic or engine-driven.

Air receivers (pressure vessels)

Air receivers:

• Pressure vessel construction: per ASME BPVC or class society rules.
• Volume: typically 5 to 25 cubic metres for starting air.
• Multiple receivers: 2 to 6 starting air receivers, each capable of multiple starts.
• Drainage: low-point drains for water removal.
• Pressure relief valves: per pressure vessel codes.
• Hydrostatic testing: at construction and periodic class surveys.

Air dryers

Air dryers:

• Refrigerated dryer: cools air below dew point, condensing water for drainage.
• Desiccant dryer: passes air through desiccant material absorbing moisture.
• Membrane dryer: in smaller applications.
• Heat-of-compression dryer: using compressor heat for desiccant regeneration.

Dryer selection depends on dew point requirement and continuous duty cycle.

Filtration

Filtration:

• Pre-filter: removing particulates before drying.
• Coalescing filter: removing oil and water aerosols.
• Activated carbon: removing oil vapours for instrument quality.
• Particulate filter: final 0.01 to 0.1 micron for instrument air.

The filtration train ensures appropriate air quality for each application.

Piping and distribution

Piping and distribution:

• High-pressure piping: starting air piping rated for 30 bar plus margin.
• Low-pressure piping: working air piping rated for 7-10 bar.
• Branch lines: to each end-use point.
• Valves: shut-off valves at each branch, regulators where needed.
• Drain lines: at low points for water removal.
• Insulation: where cold-temperature operations might cause icing.

SOLAS requirements

Starting air capacity

SOLAS Chapter II-1 Regulation 27 requirements:

• Minimum 12 starting cycles of the main engine without recharging.
• Two starting air receivers of approximately equal capacity, each independently capable of multiple starts.
• Starting air arrangement must be functional after a fire or flooding affecting one receiver.
• Air compressor capacity sufficient to recharge from 50 percent to full pressure within 1 hour.

The 12-start requirement reflects the operational need to start, stop, and restart the main engine multiple times during manoeuvring without waiting for compressor recharge.

Auxiliary engine starting

Auxiliary engines:

• Sufficient air for at least 6 starts of one auxiliary engine per receiver.
• Multiple receivers: typically 2.
• Common arrangement with main engine starting air system on smaller ships.

Emergency starting

Emergency starting:

• Emergency air compressor: capable of providing starting air independently of main electrical supply.
• Hand-pumped option: for very small ships.
• Emergency air receiver: separate from main starting air system.

The emergency arrangement ensures engine restart capability even after total electrical failure.

Operational considerations

Startup procedure

Engine start procedure (with compressed air):

1. Verify air pressure in starting air receivers.
2. Open starting air master valve.
3. Engage barring gear if first start (rotating engine slowly to verify mechanical clearance).
4. Disengage barring gear before starting.
5. Activate starting valve: starting air enters cylinders sequentially, rotating engine.
6. Fuel injection commences as engine rotation reaches threshold.
7. Ignition: combustion begins.
8. Starting valve closes as engine takes over on its own combustion.
9. Air compressor restarts: refilling starting air receivers.

Air compressor operation

Air compressor operation:

• Auto-start: when receiver pressure drops below threshold.
• Auto-stop: when receiver pressure reaches target.
• Cycling: typical compressor operation is duty cycle of 30-50 percent.
• Cooling: compressor cooling typically by sea water or fresh water.
• Lubrication: oil-injected (less common in modern marine) or oil-free (preferred for instrument air).

Air quality management

Air quality management:

• Drainage: regular blowdown of receivers to remove condensate.
• Filter maintenance: periodic filter element changes.
• Dryer monitoring: dew point verification.
• Oil contamination: monitoring oil-injected compressor performance.
• Sampling: periodic air quality sampling for instrument air.

Cold weather operation

Cold weather operation:

• Receiver heating: preventing condensate freezing.
• Pipe heat tracing: in critical lines.
• Drainage discipline: preventing ice plugs in piping.
• Compressor cold-start: capability for low-temperature ambient.

Maintenance

Routine maintenance

Routine maintenance:

• Daily: visual inspection, drain valves operated, alarm panel review.
• Weekly: oil sample, filter pressure differential.
• Monthly: detailed inspection of accessible components.
• Quarterly: vibration analysis, valve maintenance.
• Annual: major overhaul of one compressor at a time.

Major overhaul intervals

Major overhaul intervals:

• Reciprocating compressor: 4,000 to 8,000 hours.
• Rotary screw compressor: 8,000 to 24,000 hours.
• Receiver hydrostatic test: every 5 to 10 years (class survey).
• Pressure relief valve test: annually.

Common operational issues

Common issues:

• Compressor capacity loss: from valve wear, ring wear, intercooler fouling.
• Receiver corrosion: from moisture accumulation.
• Pressure relief valve seat damage: from operation under abnormal conditions.
• Filter clogging: from inadequate maintenance.
• Dryer breakthrough: with moisture in downstream air.
• Oil contamination of instrument air: from compressor seals.

Class society oversight

Class society oversight:

• Initial certification: of compressor and receivers at delivery.
• Annual surveys: visual examination.
• Internal examination: of receivers at periodic intervals.
• Hydrostatic test: at major surveys (typically 5 years).
• Pressure relief valve test: annually with class witness.

Specific applications

Cargo ship compressed air

Cargo ship compressed air:

• 2 main compressors for starting air at 30 bar.
• 1-2 working air compressors at 7-10 bar.
• Control air: typically branched from working air with additional treatment.
• Total compressor capacity: 150-300 m³/h depending on ship size.

Tanker compressed air

Tanker:

• Standard cargo ship compressed air.
• Cargo system valve actuation: many cargo valves are pneumatic.
• Inert gas system: may use compressed air for dilution or scrubbing.
• Pump room ventilation: including air management.

Container ship compressed air

Container ship:

• Standard cargo ship compressed air.
• Reefer container hosing: high-pressure air for reefer cleaning.
• Container handling auxiliaries: hydraulic systems may use air supply.

Cruise ship compressed air

Cruise ship:

• Larger working air system: for accommodation and public space services.
• Higher quality control air: for HVAC and other automation.
• Multiple distributed compressors: providing redundancy.

Offshore vessel compressed air

Offshore vessels:

• DP-class compressors: with redundancy for DP-2/DP-3 classification.
• High working air demand: for cargo operations and equipment.

Future developments

Energy efficiency

Energy efficiency:

• VSD (Variable Speed Drive) compressors: matching output to demand.
• Heat recovery: from compressor cooling for water heating.
• Oil-free compressors: avoiding lubrication losses.
• Right-sizing: avoiding over-sized compressors operating in throttled conditions.

Decarbonisation

Decarbonisation impact:

• Electric drive: standard, with electricity from increasingly green auxiliary plant.
• Reduced air consumption: through better leak management and design.
• Energy efficiency: contributing to overall ship efficiency.

Smart monitoring

Smart monitoring:

• Continuous monitoring of compressor and receiver parameters.
• Predictive maintenance: based on operating data.
• Remote diagnostics: with manufacturer support.

See also

• Marine Diesel Engine
• Marine Auxiliary Engines and Generators
• Marine Boilers and Steam Systems
• SOLAS Chapter II-1: Construction, Subdivision, Stability, Machinery and Electrical Installations

References

• IMO SOLAS Chapter II-1 Regulation 27 (starting air).
• ASME Boiler and Pressure Vessel Code Section VIII.
• IACS Common Structural Rules.
• Class society marine compressor and receiver rules.
• Atlas Copco, Sauer, Sperre, Hatlapa technical documentation.