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Fuel handling & treatment for motorships

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.

Crude oil contains gaseous fuels, gasoline (petrol), kerosene (paraffin), gas oils, distillate diesel fuels and lubricating oils which can be collected from the fractionating tower (Figure 1 below) where they condense out at the different levels maintained at appropriate temperatures. The crude is heated in a furnace, as shown at the left of the sketch.

The boiling process produces a residue which is very dense as the result of having lost the lighter parts. This high-density remainder has much the same hydrocarbon make up as the lighter fractions and is used as a fuel. Unfortunately the initial refining process not only concentrates the liquid but also the impurities.

Oil refinery process
Figure 1: Oil refinery process

Vacuum distillation, a second process, removes more of the lighter fractions, to leave an even heavier residue. As can be seen from Figure , the refinery can have additional conversion equipment. Vis-breaking or thermal cracking is one process using heat and pressure to split heavy molecules into lighter components giving a very dense residue.

Catalytic cracking is a process that uses a powdered silica-alumina based catalyst with heating, to obtain lighter fractions, with, however, an increasingly heavy residue. In the latter process catalyst powder is continuously circulated through the reactor then to a regenerator where carbon picked up during the conversion reaction is burnt off. Unfortunately, some of the catalyst powder can remain in the residue which may be used for blending bunker fuel oils. The very abrasive silica-alumina catalytic fines have caused severe engine wear when notdetected and removed by slow purification in the ship's fuel treatment system.

Fuel testing

Bunkers are classified as Gas Oil, Light and Marine Diesel Oil, Intermediate Fuel Oil and Marine or Bunker (C) Fuel Oil. The delivery note specifies the type of fuel, amount, viscosity, specific gravity, flash point and water content. Trouble frequently results from inferior fuels and there can be insuffient information to give warning. Fuel grading schemes and more detailed delivery notes are being used. Some of the Classification Societies and specialist firms provide testing services and on-board testing equipment is available.

A representative sample is needed to give an accurate test result and this is difficult to obtain unless a properly situated test cock is fitted in the bunker manifold where flow Is turbulent. The sample is taken after flushing the test cock. Because of the variation in heavy fuel, small quantities are taken into the test container over the period of bunkering, to give a representative sample.

A full analysis can be given by the shore laboratory, On-board tests are limited to those which give reliable results and kits for specific gravity, viscosity, pour point, water content and compatibility are on the market. Flash point is found with a Pensky—Martin closed cup apparatus which has been carried on some ships

Summarized below some of the basic procedure of machinery service systems and equipment :
  1. Marine air compressor

  2. 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.....

  3. Air starting system

  4. 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.....

  5. automatic-operation-air-compressor

  6. 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. ....

  7. Compressed air systems for steamships

  8. 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.....

  9. Two stage starting air compressor

  10. 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.....

  11. Fuel handling

  12. 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.....

  13. Fuel transfer and fire risk

  14. 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.....

  15. High density fuel treatment

  16. 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.....

  17. Viscosity controller

  18. 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......

  19. Fuel blenders

  20. 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.....

  21. Fuel heaters

  22. 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....

  23. Homogenizer

  24. 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......

  25. Package boiler combustion system

  26. 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.....

  27. Lubricating oil treatment

  28. 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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