Compressed Air Line on Ships – A General Overview
Compressed air on ships has a wide range of uses. High-pressure air (30 bar) is primarily used for main engine operation. This high-pressure air is reduced to a lower operating pressure by a pressure-reducing valve and used for other important purposes. Low-pressure air (7-8 bar) is used as service air in many applications.
These uses include starting auxiliary engines, emergency generators, charging fresh water and potable gas, blowing fog horns, blowing spring air for main engine exhaust valves, dry cleaning main engine turbochargers, addressing airborne sewage problems in sewage treatment plants, blowing off boiler soot, and transporting oil for pneumatic pumps. There are many other applications, such as service air for cleaning, painting, cutting, and operating pneumatic tools (such as grinders and chisels).
Another important branch of compressed control air is 7-8 bar air. Control air is a filtered branch of service air that is free of any moisture or oil residue. This controlled air is used for pneumatic control devices and is essential for the operation of shipboard machinery.
This makes shipboard air piping a critical part of ship operations. This article explores the general characteristics of a ship’s air line, its key components, and its operating principles.
A complete shipboard air line consists of the main high-pressure line, service air lines, and control air lines.
Main Air Compressor
The main air compressor is the heart of a ship’s air line. Air compressors compress air by reducing its volume and increasing its pressure. There are several types of air compressors, including:
- Centrifugal Compressor
- Rotary Vane Compressor
- Screw Compressor
- Reciprocating Air Compressor
Most modern commercial ships use multi-stage reciprocating air compressors equipped with intercoolers and aftercoolers, as well as automatic drain and unloading systems.
The capacity of a main air compressor is measured in “free air distribution” (FAD), expressed in cubic meters per hour.
“Free air distribution” refers to the amount of air actually discharged by the compressor in one hour, after the compressor has expanded to atmospheric pressure and cooled to ambient temperature. The main air compressor discharges air into the main air tank, which stores compressed air at a maximum pressure of 30 to 32 bar.
A vessel may be equipped with two or three main air compressors, depending on factors such as the capacity of each compressor, the amount of air required to operate the main engines, and the ship’s air consumption.
Due to the emissions from marine boilers and exhaust gas economizers on some ships, larger capacity air compressors may be used during the design phase, depending on the shipowner.
According to the requirements of the International Convention for the Safety of Life at Sea (SOLAS), a ship’s main air compressor must be able to charge the air tanks from zero pressure to the maximum pressure (30 bar) within one hour.
Main Air Tanks
Each vessel should be equipped with a set of two air tanks, either vertical or horizontal.
The air tanks should be hydraulically tested to a pressure of up to 1.5 times the operating pressure.
According to the International Convention for the Safety of Life at Sea (SOLAS),
The total capacity of the air tank must be sufficient to operate a reversible engine at least 12 times, or an irreversible engine at least 6 times, without recharging.
Each vessel must be equipped with two identical main air tanks and one emergency air bottle.
Air Tank Accessories
Each air tank must be equipped with the following accessories:
1. Fusible Plug
Composition: 50% bismuth, 30% tin, 20% lead
Melting Point: 104.4°C (220°F). Installed at the bottom of the tank or, if not directly connected to a safety valve, at the ship’s side.
Used to release compressed air when the compressed air temperature is abnormally high.
2. Air Pressure Relief Valve
This valve is used to prevent overpressure and replaces the fusible plug.
If a fire in the engine room requires carbon dioxide flooding, this valve must be opened before evacuating the engine room.
(The air pressure relief valve can be opened from outside the engine room through the ship’s funnel or from inside the engine room. In this case, the number of CO2 cylinders required for engine room fire suppression will be calculated accordingly, and the required additional CO2 will be considered during the ship design process.)
3. Spring-Loaded Safety Valve
Can be installed directly with a set pressure of 32 bar (for a working pressure of 30 bar) or through an expansion joint with a pressure buildup of 10% or greater.
4. Compensating Rings
When a hole is cut or machined in a pressure vessel, the material surrounding the hole is subjected to increased stress. To reduce these stresses, a compensating ring is installed.
A compensating ring is a flange typically used to mount valves or fittings. Compensating rings ensure the structural integrity of the pressure vessel.
5. Manual or automatic pressure relief valve
6. Pressure gauge
7. Service door
8. Main start valve, auxiliary air start valve, accessory valve, service air valve, or whistle air valve
Large cylindrical gas cylinders typically use a single longitudinal weld. Both longitudinal and circumferential welds are mechanically welded with full penetration.
Welding details are subject to gas pressure and must be stored in accordance with classification society regulations.
All welded air storage tanks must be stress relieved or annealed at approximately 600°C, and for safety reasons, the welds must be X-rayed.
Gas receivers must undergo statutory inspections and regular hydraulic testing at 1.5 times the operating pressure, with larger tanks requiring this test every 10 years.
Gas Receiver Inspection
Gas receivers should be inspected according to maintenance management (PMS) standards and checked for any signs of corrosion. Moisture in the gas receiver can cause corrosion, and even if the compressor refrigerant discharge line is operating normally, significant amounts of liquid can accumulate, especially in humid environments.
It is recommended to regularly inspect the gas receiver discharge line to assess the amount of liquid within. In severe conditions, the discharge line may need to be accessed two to three times daily to remove any emulsion buildup. Corrosion typically occurs near the gas receiver discharge line.
After a careful visual inspection, the thickness can be measured using an ultrasonic thickness gauge, if necessary. If the gas receiver thickness decreases, the gas pressure in that tank must be reduced.
Operating Cylinders
After careful calculation, this can be achieved by changing the air compressor’s on/off settings and resetting the pressure relief valve settings while the gas receiver is in use. The air tank can also be completely isolated and placed in standby mode, allowing for careful manual refilling when necessary.
All internal welds or minor variations in cross-section should be carefully inspected.
If the cylinder is too small for manual insertion, a camera with a probe can be used for internal inspection.
Internal Surface Coating
Graphite, linseed oil, copal varnish, or epoxy coating suspended in water is preferred, taking into account its primary properties such as corrosion resistance, toxicity, and oxidation resistance.
Safety Devices in the Master Cylinder
- 1. Fuse plug
- Pressure relief valve
- Air pressure relief valve
- Low-pressure alarm
- Automatic or remote moisture control valve
Control Air System
The control air line is supplied to the branch air line via a pressure-reducing valve.
Pneumatic control equipment is very sensitive to possible contaminants in the compressed air. Viscous oil-water emulsions can cause seizure of moving parts in control equipment and control valves, leading to general damage to diaphragms, rollers, and other rubber components.
Water causes rust, which can also cause components to stick or be damaged by rust particles. Metal corrosion and other small particles can also cause corrosion damage.
Any solids mixed with the oil-water emulsion can bind and clog small openings. Therefore, clean, dry control air is essential for the smooth operation of the control air system.
When the control air source and equipment are the main air compressor and the main air tank itself, special equipment is required to ensure high air quality.
The pressure-reducing valve, which is used to raise the main air pressure to the 7 or 8 bar required by the control air system, can be affected by emulsion migration and requires frequent cleaning to prevent air contamination.
Automatic drain valves can be installed in the control air system, but many require the crew to drain the air daily.
Large amounts of free water and oil emulsion accumulated in the air can be removed by special control air membrane filters installed in the control airline.
Control Air compressors
A typical control air filtration system consists of a filter to collect oil and water and a membrane filter to dry the air. These membrane filters process air, filtering and drying it, removing virtually all traces of oil, moisture, and airborne contaminants.
A simple, inline Air compressor features a small plastic float and an automatic drain system.
If the container is exposed to very humid environments and requires frequent draining, manual draining is also possible.
The filter-drying unit consists of a primary filter,
Control the Air compressor
A typical control air filtration system consists of an oil-water collection filter and a membrane Air compressor-dryer. Air passes through these membrane filters, where it is filtered and dried, removing virtually all oil, water, and impurities.
The system includes a simple Air compressor with an integrated small plastic float and an automatic drain system.
If the vessel is exposed to high humidity, frequent draining is also possible.
The filter-dryer assembly consists of a primary filter, a secondary filter, and hollow fiber membrane elements.
Airflow through the Control Air Dryer
Control air enters the drying chamber through an integrated filter located at the bottom of the unit. Inside the unit, the primary filter removes coarse rust particles, dust, and larger impurities.
The secondary filter acts as a collection unit, separating water droplets and oil mist down to 0.3 microns. A differential pressure gauge indicates the status of the primary and secondary filters.
A high differential pressure indicates dirty membrane filters. Membrane elements should be replenished based on the vessel’s top dead center (TDC).
Piping
The high-pressure air piping from the air compressor to the receiver should be as smooth as possible, without any bends, to ensure free, unrestricted air flow to the receiver. Bends can cause backpressure in the piping if moisture or oil emulsions accumulate in the piping.
Emergency Air Compressor and Emergency Air Cylinder
The emergency air compressor is a small, self-contained compressor that can be driven by an independent main engine (such as an electric motor) or by the emergency switchboard.
It is used to fill emergency air cylinders, which contain enough air to operate the ship’s auxiliary engines in the event of a malfunction.
The ship’s control air is also used to operate the emergency shut-off valve system.
A QCV emergency air cylinder with a pressure of 7 bar is used to operate all quick-close valves, fire dampers, and pressure relief doors.
The system consists of a 7-bar QCV emergency air cylinder equipped with a QCV shut-off valve, which is always operational.
If an uncontrollable fire occurs in the engine room, the quick-acting shut-off valve will be activated to provide control air to the dedicated shut-off valves of all fuel and lubricating oil tanks, as well as the sewage room funnel and sewage door, cutting off the fuel and air supply.
