How to deal with High Viscosity fuels?

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The answer to the above question depends on how high is the viscosity of the fuel.

Let us assume that the Bunker supplied has a Viscosity of 500 cSt instead of 380 cSt (assuming the grade ordered is RMG 380).

With regards to the above scenario:

  1. Please note – The fuel bunkered is Off-spec when compared with ISO 8217 – IFO 380 – RMG 380 specifications.
  2. Please send a sample to a reputed IMO approved lab for testing (for example: Viswa Lab) and once viscosity is confirmed to be high and off-spec, Kindly check the BDN and intimate the supplier with a notice for the Off-spec in Viscosity.

Important Points to Consider in dealing with High Viscosity Fuels:

  1. It is highly essential that the engine maker must have certified that the engine can burn such high Viscosity fuel.  The Engine Maker usually would have specified the safe operating range for Viscosity and all other basic parameters.
  2. Has the vessel earlier handled such fuels in the past? If so – please refer to the vessel’s operational History for handling such fuels.
  3. The Main parameter which has to be taken into consideration is Viscosity & Transfer Temperature.
  4. On board ship the viscosity result is used first to ensure that the fuel is correctly heated in storage and made ready for pumping.  Some smaller vessels have very limited tank heating and, therefore, use low viscosity residual fuels.  If the viscosity of the fuel delivered is significantly above ordered viscosity, these vessels would experience difficulties in pumping the fuel from the primary storage tanks to the service tanks.  This may be further aggravated if the fuel is stored in tanks which have sides in contact with the sea and when trading in cold areas.  The maximum viscosity for efficient pumping is considered to be around 600 cSt, however, lower viscosities should be maintained if long suction lines are installed.
  5. After storage and treatment, the fuel needs to be heated to achieve the correct viscosity for injection into the engine.  The desired injection viscosity is given by the engine manufacturer (typically in the range 10-15 cSt).  This process of heating for injection is usually automatically controlled, but if the fuel viscosity is significantly above that of the grade ordered, correct injection viscosity may not be achieved due to the limitations of the fuel heaters.  Before making a claim against a charterer, that the viscosity is too high for the vessel to handle, it should be remembered that only a few degrees extra heating may be needed to maintain the desired injection viscosity range.
  6. As an example, if the fuel normally received is 180 cSt at 50°C, and the viscosity required at injection is between 11 and 14 cSt, the normal temperature for injection would be between 116°C and 126°C.  When the fuel delivered has a viscosity of 250 cSt at 50°C the required temperature range for injection will be 123°C to 133°C.  In fact, it would not be necessary to raise the new fuel temperature to stay within the injection viscosity range even though the viscosity is 40% above the grade ordered.  Most certainly, viscosity differences of 20% between grade ordered and received should cause no operational problems to a modern motor vessel.
  7. However it’s good to ensure that the Viscosity at injection meets the Engine Maker’s recommended Range.
  8. Typical Values for 500 cSt Fuel – (though it can vary depending upon Bunker Port) – Temperature for injection viscosity of 13 cSt ranges from 135 Degree Celsius to 145 Degree Celsius.
  9. Usually such fuels are witnessed to have higher Density – Thus if It is an ALCAP type Purifier, this will be automatically taken care of. However conventional purifiers have a limitation in density of 991 kg/cm3.

The above points specified are generic – and it would be better to refer to the BDN or Oil supplier Quality Certificate to approximately know the Parameters and consider them based on Engine/Engine Room Operational conditions.

From the marine diesel engine operational point of view, injection viscosity that is, viscosity at which fuel is injected into the engine, is of critical importance as it will make a difference in the ignition and combustion in the diesel engine.  Usually engine makers stipulate a viscosity range of 10 to 15 cSt.  The temperature at which this viscosity will be reached will be anywhere around 135 C for a RMG 380 fuel.  A fair amount of difference in viscosity at 50 C will make a very small difference in the injection viscosity. For instance, if the viscosity at 50 C is 420 cSt instead of 380 cSt, the injection viscosity at 130 C is likely to be only 1 cSt more, which is acceptable.

Let us know if you have any further questions – voice out your comments below.

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