Fuel oil can be contaminated as a result of seawater being delivered from a barge at the time of loading. In some cases ballast water or incorrect valve operations can contribute to high levels of seawater contamination.
A high concentration of saltwater, which not only contains a range of metals, but also sodium, will contribute to a higher ash content than fuels containing non-seawater.
It is well documented that sodium compounds present in fuel oils, can combine with the vanadium in fuels to cause corrosion of exhaust valves. Vanadium and sodium oxidise during combustion to form semi-liquid low melting point salts, which adhere to exhaust valves. A ratio of vanadium to sodium of 3: 1 is to be avoided.
A seawater contamination where sodium is diluted in the water is easily remedied by the vessels’ operations. Settling and centrifuging should reduce the amount of seawater present in the fuel, and hence the sodium content.
If the salts present in seawater are not removed they will contribute to ash deposits on turbochargers and blades. The performance of the turbocharger would therefore be impaired resulting in poor combustion and increased fouling.
Desalting – the refinery process
Crude oil desalting is one of the most critical steps in the refining process. It removes salt and other contaminants and breaks re-emulsified water from crude feedstocks.
Most crude oil desalters are capable of removing up to 90% of entrained salt from a crude.
There are many desalting processes. For instance crude can be heated to 300ºF, mixed with 5% of fresh water and then fed to the desalters.
For electrostatic desalting operations, crude is heated to decrease the viscosity. The water in oil dispersion is then introduced into a pressurised vessel where a high voltage electrical field accelerates separation of the water laden with salt and other contaminants from the crude. Distributors feed the emulsified crude and water into an intense electrical field; the oil emulsion stream flowing outward from the distributor encounters optimal conditions for water coalescence.
Clean oil, free from contaminants continuously rises to the top of the vessel and flows out while accumulated water and sediment mixture is automatically withdrawn from the bottom of the vessel for disposal.
A cause of high Sodium
If a desalting process used by a refinery is not operating at optimum efficiency, the sodium in the crude oil is concentrated in the residual fuel. This will result in a much higher sodium value than for fuels processed when desalters are working efficiently.
For instance the average sodium value for residual bunker fuels is in the order of 20 mg/kg. At this level sodium would not be a concern for the end user although there is no limit specified in ISO 8217:1996.
However, if a refinery experiences desalter problems, the amount of sodium in residual fuels would escalate enormously.