Viscosity is a measure of a product’s fluidity at a given temperature and is defined as the resistance of a fuel to flow as reported by LinkedIn.
Viscosity of fuel
The viscosity of fuel decreases when the fuel temperature increases i.e. it becomes more fluid. The significance of viscosity for marine fuels is such that;
– It is used as an indicator of the grade
– It defines the heating temperatures for storage, transfer, fuel treatment and injection
– It is used for the calculation of CCAI (the index of the ignition quality of residual fuel oil)
Viscosity is not an indication of fuel quality, and it is a misunderstanding that residual fuels with a lower viscosity automatically mean that the product is lower in quality. In reality, viscosity only becomes a serious quality concern when the viscosity is below 2 cSt. Historically, viscosity was used to order a fuel by indicating the grade based on the viscosity of the product. Viscosity was also a key blending target for refineries and fuel suppliers where blending was usually done using a low-quality, heavy residual stream (e.g cycling oils containing considerable amounts of highly abrasive cat fines).
With the introduction of the global Sulphur cap of 0.50%, density and viscosity were no longer the main blending targets, but instead, it was replaced by sulphur as the main blending target. To produce 0.50% fuels, it is no longer possible to use large quantities of low quality and heavy residual streams in the blend. Instead, lighter blending components are being used such as higher quality refinery streams and even distillate fuels. These different blending components have inherently lower viscosity and density properties resulting in a lower density and viscosity product but with the higher overall quality. Therefore, low viscosity and low-density fuel products are generally an indication of a better quality product.
Lower viscosity fuels are in general;
– Higher energy content
– Lower in cat fines and other metals
– Lower in CCAI
– Requiring lower fuel temperatures so less thermal stress
– Easier to perform onboard fuel treatment (Stokes law)
Lubricity is the measure of the reduction of friction of a product. The misconception is that lower viscosity fuels might be more susceptible to lubricity issues. It is known that lubricity is not a critical parameter and is only applicable to ultra-low sulphur distillate fuels e.g. ISO 8217 includes lubricity for clear and bright distillate fuels only when sulphur is below 0.05 % m/m.
Lubricity was introduced as an ISO 8217 specification parameter with the release of the 2010 edition. The lubricity criterion was adopted from the automotive industry, where a variety of fuel pumps and fuel injection systems were damaged by low sulphur fuels with poor lubricity. The main reason behind poor lubricity is that the removal of sulphur during the refinery process also impacts (reduces) the lubricity capacity of fuel.
For marine, the impact is significantly less as marine fuels still contain sulphur and thereby normally maintain their lubricity capacity. For the marine industry, some engine manufacturers recommend testing for lubricity to be carried out only if sulphur is below 0.01% m/m.
What is often overlooked but what is recognized to be more critical for marine fuel systems and diesel engine fuel pumps is when the viscosity will be less than 2 cSt. The reason is that with a too low viscosity a satisfactory hydrodynamic oil film between the fuel pump plunger and barrel cannot be maintained, thereby causing the risk of fuel pump seizures and/or sticking.
So a very low-viscosity fuel oil challenges the function of the pump in three ways:
- Breakdown of hydrodynamic oil film (resulting in seizures),
- Insufficient injection pressure (resulting in difficulties during start and low-load operation), and
- Insufficient fuel index margin (resulting in limitation in acceleration and load).
To mitigate the risk of low viscosity a cooler or chiller might be needed to maintain the viscosity at the fuel pumps above the minimum of 2 cSt. This is due to an increase in fuel temperatures during operation onboard a vessel with a hot fuel system. What also should be noted is that fuel pumps with high running hours are more sensitive to the potential risks of using low viscosity fuels.
So in summary of the above, we can conclude that:
- A lower viscosity is not an indication of a lower quality product.
- Fuel viscosity only becomes an issue when;
– The vessel is not able to maintain the required injection viscosity at the engine inlet (lack of heating capacity)
– The vessel is not able to maintain the viscosity above 2 cSt at the engine inlet (lack of cooling capacity)
3. Lubricity is not a major concern for marine fuels and is only applicable to ultra-low sulphur “bright & clear” distillate fuels
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