Heavy fuel oils are normally purchased on the basis of a limiting viscosity due to:
- Handling or
- Engine-related restrictions
Viscosity is a measure of a fuel’s resistance to flow. The higher the viscosity, generally the higher the specific gravity. The viscosity of any petroleum oil increases when the oil is cooled and decreases when it is heated. For this reason, the viscosity value of an oil must always be accompanied by the temperature at which the viscosity was determined. The viscosity value by itself is meaningless.
Viscosity is used principally to give information about the handling, treatment and atomization of the fuel. However, it also is a rough indicator of its carbon and asphalt content.
The lower the viscosity, the easier it is to settle or to separate entrained water and solid particles.
Although high viscosity fuels require proper preheating for good separator operation and heating before injection for good atomization, this characteristic usually can be handled without any problems.
By preheating fuel separation in a centrifuge is improved, but at the same time one need to be cautious that the temperature does not exceed 98 degrees centigrade. The reason is if it exceeds it can cause flashing of water in the separator resulting in loss of the centrifuge water seal.
Caution must be exercised when heating the fuel to temperatures above 135 degrees Centigrade. If the temperature exceeds the limit it may result in cracking, gases may be given off and water may vaporize forming steam pockets in the fuel line.
On the other hand, insufficiently heated fuel can result in poor atomization and delayed burning which may lead to other issues like higher thermal loading, scuffing problems, possible piston and piston ring failure and to an increase in fuel consumption.
In addition to heating prior to injection, an increase in fuel injection pressure may also be necessary to maintain design atomization spray patterns depending on fuel used. In case of high viscosity fuels an increase in primary fuel pump pressure may be necessary to prevent vaporization of the fuel’s more volatile components, due to the heating prior to injection.
It is for this reason that the residual marine fuels are classified based on Viscosity – typically as 80 cSt, 180 cSt, 380 cSt, 500 cSt and 700 cSt fuels.