Fuel Oil System:
Very low-grade fuels are used on Marine Diesel Engines (which are available at low cost) to reduce the fuel cost. These Fuels contain impurities in the form of water and sediments and hence, they are to be purified before spending to the engines. The fuels used are of very high viscosity up to 700 cst at 50-degree celsius.
As already explained earlier, three types of fuel are used on board large cargo vessels where two-stroke slow speed engines are fitted.
- Heavy fuel oil (up to viscosity 700 cst at 50 – degree celsius).
- Light diesel oil (Of viscosity 12 – 14 cst).
- High-speed diesel oil (with low sulphur content and high flash point > 60-degree celsius).
Initially, the low-grade fuel oil (HFO) is bunkered into the large storage tanks in the double bottom. Since the fuel is of very high viscosity and in a semi-solid form, to reduce the viscosity heating coils are provided in the Double bottom tanks and the oil is heated (using steam as the heating medium) initially to abut 40 to 50 – degree celsius. This oil is pumped to a settling tank by a transfer pump. The sediments and suspended impurities get settled down in the settling tank. The water content also settles down at the bottom. These impurities are removed to a sludge tank.
From the settling tank, the oil is taken to the oil purifiers. The purifiers purify the oil (by removing the water content and sediments by centrifuging) and the purified oil is pumped to the day tank ( also called service tank) from where oil is used for the diesel engines. Each purifier contains an oil heater where the oil is heated before sending to the purifier (for effective separation of oil and water).
Two separate systems are provided fo HFO and LDO and a common purifier to work as standby for both HFO and LDO.
2 – day tanks shall have a capacity suitable for 8 to 12 hours operation of diesel engines.
In the next post, we shall talk about the fuel oil service system which forms a crucial part of the fuel oil system.