Fuels for marine diesel engines are normally heavy fuel oils that are the direct residues of the distillation process and which are a grade before tar. Normally these fuels have high levels of naphtha compounds along with high waxy asphalts that form sludge when stored in tanks for a long period of time. This sludge has to be removed periodically from the tanks, which is done by adopting various tank cleaning methods.
Need For Marine Fuel Oil Tank Cleaning
On board the fuel oil are stored in the double bottom tanks, and they are being transferred from double bottom tank to settling tank for the gravity separation and it is purified in the purifier and stored in the service tank for the day to day consumption. So the fuel used for the marine diesel engines is heavy fuel oil which contains the high amount of asphalt residue which forms sludge when it is stored in the tank. A regular and periodic cleaning of tanks is necessary to prevent the sludge formation and deposits in the double bottom tanks as it create carbon residues which often reduce the heat transfer by the steam heating coils inside the tanks.
Normally the fuel oil storage tanks are maintained at 35’C – 45’C to prevent the fuel cloud formation and to reduce the viscosity to transfer the fuel. So if the tanks are not maintained they form sludge formation over the heating coils and finally reduces the heat transfer rate causing a serious problem.
A residue from fuel oil accumulates in the fuel oil storage tank and this must be regularly removed.The sludge blocks, the suction pipe lines and drain holes in the tank preventing the final draining of the fuel oil tank.Tank washing is a routine and is necessary in ships running with heavy fuel oil.
Sea Water Cleaning of Marine Fuel Tanks
In the early days, tank washing was done with the seawater. It used a rotating bronze nozzle through which heated sea water was sprayed. Sea water is unsuitable as a solvent for oily sludge, and static electricity generated during the washing process has caused numerous explosions. Every settling tank has a sludge valve or cock which is used to drain the sludge and oil-water mixture in to the sludge tanks. After the end of this process, the residue left contains a mixture of water and oily sludge, and as a result it creates a big problem in separating the oily water mixture.
Modern Heavy Fuel Oil Cleaning Method
As this was a failed method, a new method of cleaning was adopted that uses the same heavy fuel oil (as it is a good solvent for the hydrocarbon mixture). In this, a high pressure jet of heavy fuel oil or high speed diesel is sprayed on the tank side walls to remove the sludge formation in the tanks. For this purpose, a portion of the fuel oil is directed through piping to the tank cleaning nozzles, which are moved to clean the tank surroundings.
The suspended nozzles are controlled to give a spray pattern on the upper areas and then progressively moved further down as the lower surfaces have more sludge deposits. Washing is completed with the nozzle positioned on the tank bottom.
All the waste oil used for cleaning the tanks is drained to the sludge holding tanks. The effective washing can be carried out with the heavy fuel oil at the recommended tank heating temperature. The waxy and asphalt residues are readily dissolved in the crude oil of which they were previously a part and better results are obtained than with water washing.
The oil residues and sludge’s obtained from cleaning are pumped from sludge tanks to receiving facilities. Tank washing is necessary on a routine basis for preventing excessive accumulation of sludge. In general, tanks are washed at least every four months. Unless the sludge is regularly removed, drainage will be slow. The fuel oil tanks washing must be completed before the vessel delivers the discharge to the port and leaves the port.