Overhauling Air Compressor on Ships

Different types of air compressors are used onboard for various purposes. The main air compressors are used to supply high-pressure air for starting the main engine and auxiliary engines. Control air compressors are used to supply control air, while topping air compressors are used to top up and take account of leakages. On some ships, deck air compressors are also available to provide air for use on decks.

Typically, two pressures of air are used: 30 bars high-pressure air for starting engines and 7 bars low-pressure air for control and general service air. When selecting an air compressor for applications requiring air above 7 bars, reciprocating air compressors are usually preferred due to their positive sealing. Other types of compressors such as rotary vane, centrifugal, and rotary screw compressors are not as effective in this regard.

In reciprocating compressors, the compression ratio is limited to 1:7 due to the rise in temperature. To achieve a pressure of 35 bars, a double-stage compressor with inter-cooling is required. Most marine compressors used are two-stage reciprocating compressors with inter-cooling and after-cooling.

On ships compressed air is used for a variety of uses some of which have been mentioned below:

• Onboard, compressed air is used for various automation and control purposes, including the main engines and auxiliary engines. It is also used for starting the main engines, auxiliary engines, emergency generators, emergency fire pumps, and for heaving lifeboats up using an air motor.
• Additionally, compressed air is utilized for ship horns and whistles, as well as pneumatic tools such as grinders, chisels, drills, spanners, and for painting operations. It is also used for general cleaning and for soot blowing the boilers.
• Compressed air is also used in pneumatic pumps for oil transfer and as general service air. In short, compressed air is a versatile utility that is essential for the smooth operation of a ship.

Safety Features of Marine Compressors

Marine compressors are equipped with various safety features and cutouts to prevent them from running in an unsafe mode. While they are generally reliable machines, they require proper maintenance and regular inspections to ensure their continued safe and efficient operation. In the past, there have been incidents where cylinder heads have burst, resulting in fatalities. Therefore, it is essential to have safety devices installed in marine compressors, including low oil pressure and high cooling water temperature trips, motor overload and overheating trips, high-pressure air relief valves, suction air filter trips, inter-stage relief valves, discharge air temperature and pressure trips, vibration trips, cylinder high temperature trips, low suction pressure trips, and emergency stop buttons.

These safety features must be inspected regularly and any damaged or malfunctioning parts should be repaired or replaced promptly to prevent accidents. It is vital to follow proper procedures and adhere to safety guidelines when operating marine compressors to prevent accidents and ensure the safety of crew members and passengers.

Operating Procedure of a Reciprocating Compressor

Overhauling Air Compressor on Ships – Marine Engineering

Before overhauling an air compressor, certain precautions need to be taken to ensure safety and efficiency. Firstly, ensure that all the necessary spare parts, including joints, gaskets, major components such as pistons, piston rings, bearings, etc., are available on board.

The compressor should be electrically and mechanically isolated and locked out, with clear ‘MEN AT WORK’ notices posted. Drain the oil and cooling water from the compressor and strip it down by removing the cylinder head, valves, piston, and connecting rod. Using the appropriate tools, remove the circlips, gudgeon pin, and top end bearing from the piston. Remove the piston rings as well. Clean the parts for survey purposes, but do not remove any parts that show no signs of wear or damage. Gauge the parts, such as ring grooves, piston pin diameter, and bearing clearances, and record the readings.

Finally, ensure that all safety devices are checked, and use a hydraulic tester to test the opening pressure of the relief valves. These precautions are crucial to ensure the safety of the engineers working on the compressor and to maintain the efficiency of the compressor itself.

Air Compressor Assembly

During the overhaul of the air compressor, several major parts such as piston, liner, and bearings must be thoroughly inspected and replaced if necessary. It is also important to renew the piston rings, gudgeon pin, and top end bearing, and check the butt and axial clearances before fitting the new piston rings. Proper tools should be used to guide the rings into the cylinder bore, and all nuts must be torque tightened to the correct settings with new locking devices.

Suction and discharge valves should also be renewed or overhauled, and bumping clearance must be checked after a major overhaul. A normal value of bumping clearance for a two-stage main air compressor is about 0.5 mm, and modern measuring tools such as ‘squeezable plastic’ or ‘lead wire ball’ can be used to measure it. After fitting the cylinder cover, the flywheel should be turned by hand to ensure that it is free to turn.

It is also important to clean or renew the crankcase oil suction filter and refill the crankcase with new oil. Lube oil filters and air filters must be renewed, and cooling water valves should be opened and checked for leakages. The flywheel should be continuously turned by hand to prime the lube oil pump. An initial running of the compressor should be conducted in an unloaded condition, and motor amperes, noise, etc. should be checked. The compressor can then be loaded and checked for bearing overheating. Once everything is found to be satisfactory, the compressor can be test run, and the time taken to fill the air bottle from empty to full should be measured.

During the operation of an air compressor, there are several common issues that may arise. These include:

1. Motor overload: This can occur when the motor is working too hard and becomes overheated. It may be caused by a variety of factors such as dirty air filters, worn belts, or low voltage.
2. Oil and water in compressed air: This problem can occur when the compressor’s oil and water separator is not functioning properly. It can be caused by excess moisture in the air or a clogged separator.
3. Oil flow suction valve: This issue can occur when the compressor’s oil flow suction valve is not functioning properly. It can lead to low oil pressure, which can cause damage to the compressor’s components.
4. Overcurrent: This issue can occur when the compressor is drawing too much current, which can lead to overheating and damage to the motor.
5. Failure to reach set pressure: This problem can occur when there is a leak in the compressed air system or when the compressor is not capable of producing enough air pressure to reach the set level.
6. Failure to start: This issue can be caused by a variety of factors, including a faulty motor, low voltage, or a tripped circuit breaker.
7. Low pressure in compressed air system: This problem can be caused by leaks in the system, a clogged air filter, or a malfunctioning pressure regulator.

We, HAI HA M&S company have done a lot of works on repairing compressors in some ports in Vietnam such as:

• Hai Phong Port
• Vung Tau Port
• Ha Long Anchorage
• Campha Anchorage
• Nghi Son port
• Hochiminh Ports alongside