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Air starting system for marine diesel engine
Air at a pressure of 20 to 30 bar is required for starting main and auxiliary diesel engines in motorships and for the auxiliary diesels of steamships. Control air at a lower pressure is required for ships of both categories and whether derived from high pressure compressors through reducing valves or from special control air compressors, it must be clean, dry and oil free.
A starting air system for main diesels (Figure below) normally has two air compressors and two reservoirs with sufficient capacity for twelve main engine starts (six if a non-reversible engine). The receivers must store sufficient air for the starts without the need for top up from the compressors. Safety valves are normally fitted to the air receivers but in some installations the reservoirs are protected against overpressure by those of the compressors. There is a requirement that if the safety valves can be isolated from the reservoirs, the latter must have fusible plugs fitted to release the air in the event of fire. Reservoirs are designed, built and tested under similar regulations to those for boilers.
Figure : Starting air system for main diesel
Explosions can and do occur in diesel engine starting air systems. Also air start valves and other parts are sometimes burned away without explosion. These problems have been caused by cylinder air start valves which have leaked or not closed after operation and have allowed access from the cylinder to the air start system of the flame from combustion. Carbon deposits from burning fuel and oily deposits from compressors are available as substances which may be ignited and produce an explosion in the air start system.
If no explosion occurs, the flame from the cylinder and high temperature air from compression can cause carbon deposits in the system to burn. Careful maintenance of air start valves, distributors and other parts is vital as is regular cleaning of air start system components to remove deposits.
The lubrication of components is limited as excess lubrication could cause the air start valves to be stuck by grease which has become hardened by the heat, and oil could accumulate in the pipes from this source. The draining of compressor coolers and air receivers is important. Drains on air start systems are also checked. Flame traps or bursting caps are fitted at each air start valve but it should be noted that protection of the latter type failed to prevent a serious explosion with the loss of seven lives in I960.
Starting air for auxiliary engines may be taken directly from the main engine air start receivers or from a small auxiliary receiver which can be kept at full pressure. The low pressure control air system receiver is supplied ideally from a low pressure, oil free compressor. The supply may be obtained from the main air start reservoirs through reducing valves or pressure regulators, driers, oil traps and filters.
For steamships, the starting air arrangements for diesel auxiliary engines are similar to those in motorships. The low pressure air for instrumentation and remote control devices is likely to be supplied from a low pressure, oil free compressor.
Summarized below some of the basic procedure of machinery service systems and equipment :
- Marine air compressor
A single stage compressor used to provide air at the high pressures required for diesel engine starting, would unfortunately generate compression temperatures of a level similar to those in a diesel. Such heat would be sufficient to ignite vaporized oil in the same way as in a compression-ignition engine. The heat produced in a single stage of compression would also be wasteful of energy.....
- Air starting system
Air at a pressure of 20 to 30 bar is required for starting main and auxiliary diesel engines in motorships and for the auxiliary diesels of steamships. Control air at a lower pressure is required for ships of both categories and whether derived from high pressure compressors through reducing valves or from special control air compressors, it must be clean, dry and oil free.....
- automatic-operation-air-compressor
Before the general introduction of control equipment, air compressors were stopped and started by engine room staff, as necessary, to maintain air receiver pressure. In port or at sea, this usually meant operating one compressor for about half an hour daily unless air was being used for the whistle (during fog), for work on deck or for other purposes. ....
- Compressed air systems for steamships
A compressed air system is necessary to supply air for boiler soot-blower air motors, hose connections throughout the ship and possibly diesel generator starting. A general service air compressor would supply air at 8 bar but greater pressure (as for diesel ships) would be necessary for diesel starting.....
- Two stage starting air compressor
Hamworthy 2TM6 type which was designed for free air deliveries ranging from 183 m3 per hour at a discharge pressure of 14 bar to 367m3 per hour at 42 bar. The crankcase is a rigid casting which supports a spheroidal graphite cast iron crankshaft in three bearings.....
- Fuel handling
Fuels and lubricating oils are obtained from crude primarily by heating the crude oil, so that vapours are boiled off and then condensed at different temperatures. The constituents or fractions are collected separately in a
distillation process.....
- Fuel transfer and fire risk
The oil fuel system provides the means for delivering fuel from the receiving stations at upper deck level, port and starboard, to double-bottom or deep bunker tanks. Sampling cocks are fitted at the deck connections to obtain a representative specimen for (a) shore analysis; (b) on board testing; and (c) retention on the ship.....
- High density fuel treatment
The density of a fuel tested at 15 deg C may approach, be equal to or greater than that of water. With high density fuels, the reduction in density differential between fuel and water can cause a problem with separation but not with the usual solid impurities.....
- Viscosity controller
A continuous sample of the fuel is pumped at a constant rate through a fine capillary tube. As the flow through the tube is laminar, pressure drop across the tube is proportional to viscosity. In this unit an electric motor drives the gear pump through a reduction gear, at a speed of 40 rpm......
- Fuel blenders
Conventionally, the lower cost residual fuels are used for large slow speed diesel main engines and generators are operated on the lighter more expensive distillate fuel. The addition of a small amount of diesel oil to heavy fuel considerably reduces its viscosity and if heating is used to further bring the viscosity down then the blend can be used in generators with resultant savings.....
- Fuel heaters
The system which delivers residual fuel from the daily service tank to the diesel or boiler, must bring it to the correct viscosity by heating.For burning heavy fuel oil in a boiler furnace, or a compression-ignition engine, it is necessary to pre-heat it....
- Homogenizer
The homogenizer provides an alternative solution to the problem of water in high density fuels. It can be used to emulsify a small percentage for injection into the engine with the fuel. This is in contradiction to the normal aim of removing all water, which in the free state, can cause gassing of fuel pumps, corrosion and other problems......
- Package boiler combustion system
The elementary automatic combustion system based on a two flame burner is used for many auxiliary boilers. The burner is drawn oversize to show detail. Various different control systems are employed for the arrangement.....
- Lubricating oil treatment
Mineral oils for lubrication are, like fuel, derived from crude during refinery processes. Basic stocks are blended to make lubricants with the desired properties and correct viscosity for particular duties. ....
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