Air Compressors in Shipbuilding and Marine Applications

Both shipyards and vessels at sea can't do without compressed air. In shipyards, compressed air drives sandblasting guns to clean hull rust, drives spray guns to complete coating work, plus pneumatic tools distributed throughout the hull. Ships themselves also have compressors, diesel main engine starting, automation system pneumatic valves, deck equipment drive, all depend on shipboard compressors for air supply. Marine environment salt spray corrosion and ship vibration and rolling during navigation make compressor selection and configuration in this field different from land-based industry.

Shipyard Compressed Air Consumption

Shipyard air usage is most concentrated in the sandblasting rust removal process. A ten-thousand-ton cargo vessel hull exterior plate area exceeds 10,000 square meters, doing full sandblasting treatment, air consumption is considerable. Sandblasting machine compressed air consumption is directly related to nozzle diameter, 8mm nozzle consumes about 6 cubic meters per minute, 10mm nozzle increases to about 9 cubic meters, 12mm nozzle exceeds 12 cubic meters. Large shipyards often have three to four sandblasting machines running simultaneously on hull rust removal, total consumption of 40-50 cubic meters per minute is normal.

Sandblasting

Hull Rust Removal

Mobile

Diesel Compressors

Coating

Spray Application

Shipyard sandblasting operation air supply method is interesting. Slipway and dock positions are relatively fixed, laying fixed piping is feasible; but sandblasting work positions move around the hull as construction progresses, piping end connects flexible hose dragged to work surface, longer hose means more pressure loss. Many shipyards just park several diesel mobile compressors at dockside, wherever sandblasting happens drive the machine there, saves the trouble of long-distance air delivery. Atlas Copco XATS series, Ingersoll Rand P series mobile machines have significant installed base at domestic shipyards. These machines range from 100kW to 300kW power, have their own diesel engine, don't depend on plant power grid, suitable for outdoor mobile operations.

Shipyard coating shop supply piping end typically has refrigerated dryer and three-stage filtration in series, pressure dew point pushed to 2-3°C, oil content controlled below 0.01ppm. Some high-end anti-corrosion coating processes even specify oil-free compressor air supply, avoiding any possibility of lubricant vapor contamination.

Pneumatic tool application density is high in hull construction. Steel plate joining, block assembly, outfitting work, grinders, air drills, riveters, wrenches, almost everyone has one. Pneumatic tools compared to electric tools have obvious advantages in shipyard environments: inside hull spaces are cramped and damp, dragging power cables to work is both inconvenient and has electrocution risk; pneumatic tools don't heat up, continuous grinding for hours won't burn your hand; coating areas occasionally have flammable gas concentrations exceeding limits, pneumatic tools don't produce electrical sparks. Of course pneumatic tools have weaknesses, same power level heavier than electric, compressed air transmission loses energy with low efficiency. Domestic shipyard common practice is pneumatic for welding and grinding, electric for assembly and fastening, each to their strengths.

Pipe and compartment pressure testing consumes not much compressed air, more sensitive to pressure stability and measurement precision. After pipe system welding is complete, pressurize and hold, observe pressure gauge reading changes, judge whether welds have pinholes or cracks based on this. This process supply pressure is determined by pipe system design pressure, some high pressure piping test pressure reaches several MPa, plant's normal 0.7MPa network pressure isn't enough, need separate high pressure source.

Shipyard air supply pattern is basically fixed station building plus mobile machines combined. Screw machine groups in station building handle shop floor and dock area network supply, dryer-filter combos or standalone refrigerated dryers for post-treatment. Slipway, dock and temporary work areas supplemented by mobile diesel machines. This configuration is quite common at domestic medium and large shipyards.

Shipboard Compressor Systems

Marine Systems

Shipboard Compressor Installation

Ship's own compressors divide into two pressure levels, functions completely different.

Starting air pressure is typically 25 to 30 bar, stored in dedicated high pressure air bottles. Classification societies have clear requirements for air bottle capacity, ensuring main engine can start forward and reverse several consecutive times each. Reciprocating compressors charge the high pressure bottles, discharge pressure 30 bar, flow from tens to over 100 cubic meters per hour, depending on air bottle capacity and specified charging time. Nanjing Compressor, Wuxi Compressor and other domestic manufacturers' marine high pressure machines have significant installed base on domestically built ships.

Working air system pressure level is much lower, 0.7 to 0.8 MPa, function is supplying air to whole ship's pneumatic instruments, control valves and miscellaneous air uses. Ship automation level is increasingly high, main engine, generator, boiler, steering gear regulation and control mostly use pneumatic actuators, once instrument air supply is cut ship control system immediately goes down. So instrument air continuity and quality standards are held very strictly. This system's compressors are mostly screw or small reciprocating, followed by refrigerated dryer and precision filters, dew point and oil content must meet standards.

Deck anchor windlass, mooring winches, some ship types use pneumatic drive, taking air from working air system. But hydraulic drive is more common on large vessels, pneumatic deck machinery is more seen on small and medium ships and workboats.

Marine compressor technical specs aren't much different from land-based industrial machines, trouble is in environmental adaptability.

Machine internal valve plates, cylinders, coolers and other parts contacting high humidity air, material selection and surface treatment can't be sloppy either.

Vibration and rolling is daily state for ship engine rooms. Main engine operation mechanical vibration superimposed on wave-induced hull rolling puts extra requirements on equipment installation. Compressor base needs vibration isolators, inlet/outlet piping needs flexible joints or expansion joints to absorb displacement, otherwise vibration fatigue easily damages pipe fittings and seals. Oil-lubricated machines also need to consider oil level detection and return path design under tilted conditions, when ship rolls and pitches oil surface sloshes, sensor false alarms or lube oil not getting where it should both cause problems.

Redundancy

Backup Systems

Certification

Classification Society

Ships sailing far from land, equipment breaks and there's nowhere to find repairs, parts can't be bought either. So marine equipment reliability requirements are a notch higher than land-based. Critical systems with redundant units is standard practice, instrument air compressors typically two-operating-one-standby or three-operating-one-standby, any single unit failure doesn't affect whole ship air supply. Ship owners also stock a batch of wear parts and spares based on route length and port call frequency, clutch plates, valve assemblies, seals, bearings, these consumables are stocked onboard.

Classification society certification is the threshold for marine equipment entering the market. CCS, DNV, Lloyd's, ABS, these major classification societies have complete standards for marine compressors, from design drawing review to factory type testing to post-installation ship inspection, every step must pass certification. Pressure vessels and safety accessories manufactured and inspected per classification society recognized pressure equipment standards. Compressor without classification society certificate, shipyard won't dare install, ship owner won't dare use, insurance company won't insure.

Offshore Platform Specifics

Offshore oil platform compressor applications overlap with ships, environmental conditions are harsher. Platform location far from shore, wind, waves and salt spray intensity greater than near-shore sailing vessels. Oil and gas production facilities have flammable gas leak risk, platform equipment explosion-proof rating is mandatory. Compressor motors, control cabinets, junction boxes all must meet hazardous area classification requirements, increased safety or flameproof motors are standard configuration.

Platform instrument air system controls entire production process flow, once interrupted, wellhead shutdown valves, process control valves all lose driving force, production is forced to stop. Instrument compressor configured with redundancy, drying and filtration treatment standards look toward pharmaceutical and electronics industries.

Breathing air compressors are typically oil-free models, post-treatment systems add activated carbon adsorption and catalytic converter devices, ensuring air reaches breathing grade.

Platform logistics supply is limited by weather and ship schedules, equipment reliability and spare parts stocking importance is no less than ships. Offshore engineering equipment selection also needs to consider lifting and replacement operability, platform space is compact, large equipment maintenance and replacement often requires crane vessels, cost and schedule are both big issues.