Compressed Air Line On Ships

Compressed air onboard ships is utilized for many different purposes; however, this high-pressure air at 30 bar is mainly utilized for the start-up of the main engine. This high-pressure air is reduced to lower working pressures through pressure reduction valves and utilized for other vital purposes. Service air at reduced pressures of 7-8 bar is used for a number of applications, including air starts for the auxiliary engines and emergency generators, and freshwater and drinking water hydrophores. The fog horn is also blown using the reserve, which provides spring air for exhaust valves of the main engine. Dry washing of the main engine turbochargers, sewage plants aerobic sewage treatment, boilers soot blowing, and operation of pneumatic pumps for oil transfers are also done using it. Also included are cleaning, painting operations chipping and operating pneumatics tools like grinders and chisels.

Another vital branch of 7-8 bar compressed air is used as control air. This control air is filtered air of service air, without any moisture and oil carry-over in it. The pneumatic controllers and other onboard machinery require this kind of air for their operation. That shows the importance of the air line system in ships. The article outlines the pneumatic air line system fitted on board a ship, its major components, and their functionality. The whole pneumatic air line system comprises the main high-pressure air line, the service air line, and the control air line.

Main Air Compressor

The main air compressor is the heart of the pneumatic air line system on board. It compresses air by reducing its volume to increase pressure. Compressors in use are of the following types:

1. Centrifugal Compressors
2. Rotary Vane Compressors
3. Rotary Screw Compressors
4. Reciprocating Air Compressors

Most modern merchant vessels have multi-stage reciprocating air compressors with intercoolers and aftercoolers, and are fitted with automatic draining and unloading gear arrangements. The main air compressor capacity is given as Free Air Delivery (FAD) in cubic meters per hour. FAD is the volume of air which is actually discharged by the compressor in one hour expanded down to atmospheric pressure and cooled to atmospheric temperature.

The discharge of the main air compressor is sent to a main air bottle or reservoir, which stores the pressurized air at a maximum of 30-32 bar. Depending on each compressor capacity, depending on the volume of air required to start the main engine, depending on the overall air demand on that particular ship, a ship may have two or three main air compressors. Some ships also make use of higher capacity air compressors for soot blowing of marine boilers and exhaust gas economizers.

Main Air Reservoir

Each ship has two air reservoirs installed onboard, either in a vertical or horizontal configuration. Hydraulic testing on an air reservoir is normally carried out to 1.5 times of its working pressure. According to the regulations of SOLAS, the total capacity of air reservoirs should be such that at least 12 consecutive main engine starts are guaranteed in the case of a reversible engine, and at least six consecutive starts for a non-reversible engine without refilling the reservoirs. To be more specific, every vessel shall be fitted with two identical main air receivers and one emergency air bottle.

Air Reservoir Mountings

Each air reservoir shall be fitted with the following mountings:

1. Fusible Plug: Composition – Bismuth 50%, Tin 30%, Lead 20%. Melting Point: 220°F (104.4°C). Fitted at the bottom of the reservoir or on the side in case a relief valve is not directly fitted on the reservoir, it discharges compressed air in case the air temperature becomes abnormally high.
2. Atmospheric Relief Valve: This valve is intended for overpressure protection and as a duplicate of the fusible plug. It shall be opened before CO2 flooding of the engine room in case of an engine room fire.
3. Spring Loaded Safety Valve: Set at 32 bar on a working pressure of 30 bar, with an allowance for a 10% or greater pressure rise. It can be fitted directly or with an extension.
4. Compensation Ring: The provision of a hole in a pressure vessel results in increased stresses in the material immediately around it. Compensation rings fitted around such openings reduce these stresses to a minimum, thus ensuring structural integrity.
5. Hand or Automatic Drain Valve.
6. Pressure Gauges.
7. Access Doors.
8. Main Starting Valve, Auxiliary Starting Air Valve, Fitting Valve, Service Air or Whistle Air Valve.

Air Bottle Inspection

Air receivers should be inspected in line with the PMS and checked for corrosion evidences. The presence of moisture in the air receivers will promote corrosion. Properly operating compressor cooler drains can allow a surprising amount to collect, especially under conditions of high humidity. Regular testing of air reservoir drains is desirable as a means of determining the amount of liquid that may be present. Under severe conditions, drains must be actuated 2 or 3 times daily to discard built-up emulsion. Corrosion is generally concentrated around and near the bottom of the air receiver drain. This can be followed by the measurement of thickness by using an ultrasonic thickness gauge following proper and adequate visual inspection. In instances where the air receiver thickness has been compromised, the air pressure needs to be reduced accordingly.

Starting Air Bottles

The reduction can be accommodated by setting the cut-in and cut-out of air compressor for that particular receiver and readjusting the settings for relief valves. Alternatively, the air receiver can be taken to a no-load condition and kept stand-by, filled manually with caution. All internal welds or minor changes in cross-section shall undergo careful examination. If the air bottle is too small for manual entry, internal inspection can be done using probe camera.

Internal Surface Coating:

The coating shall be of graphite suspension in water, linseed oil, Copal varnish, or epoxy and shall be anti-corrosive, nontoxic, and resistant to oxidation.