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VPS: Shipowners face ‘tricky situation’ to balance VLSFO shelf life and wax appearance temperature

29 Sep 2020

The following article is part of pre-event coverage for the upcoming Singapore International Bunkering Conference and Exhibition (SIBCON) 2020; where Manifold Times is an official media partner:

Shipowners using Very Low Sulphur Fuel Oils (VLSFOs) as bunker fuel to meet IMO 2020 requirements have entered into a “tricky situation” of finding the perfect temperature for fuel storage, says the Group Managing Director of international fuel testing and inspection firm Veritas Petroleum Services (VPS).

“Generally, you have to heat VLSFOs to prevent waxing, but the more you heat them the lower the shelf life is going to be,” Dr Malcolm Cooper told Singapore bunkering publication Manifold Times.

He shared some common situations that shipowners are now facing with VLSFOs and offered advice for stakeholders who have already measured the Wax Appearance Temperature (WAT), Wax Disappearance Temperature (WDT) and Total Sediment Potential (TSP) of their product:

VLSFOs with high TSP and high WAT

  • Characteristics – Heating of these fuels in the storage tank to keep the temperature above WAT to prevent cold flow issues can cause TSP to rise and increase stability issues. By the time the vessel starts to burn the fuel, the TSP could become off-spec and thick sludge is formed. Typically, the fuel forms sludge at the purifiers and increasing the temperature further (from storage to separation temperature) has generally worsened the situation.
  • Recommendation – Limitation of VLSFO bunker storage time on board to 3 months.

VLSFOs with low viscosity (<10) and high WAT (WAT>40°C and WDT>75°C)

  • Characteristics – The product’s separation temperatures may be lower than WAT and for that reason the vessel cannot at any time heat the fuel above WAT. This causes a problem with sludge formation in the storage tank and transfer pipeline, and results in filters clogging with wax, together with serious handling and transfer issues.
  • Recommendation – Controlled use of additives in some cases can improve the situation just enough to use the remaining fuel.

VLSFOs where WDT is above 75°C

  • Suggest carrying out an exact flashpoint test to make sure that the fuel is not being heated above the flashpoint.

However, the measurement of the WAT and WDT of VLSFOs have always been a challenge as the product, typically a blend of 80% heavy fuel oil (HFO) and 20% marine gas oil (MGO), is not transparent, according to Dr Cooper.

“Current test methods to measure cloud point can only be applied to petroleum products that are transparent in layers of 40mm in thickness. For this reason, the CIMAC recommendation is to keep the fuel at 10°C above the pour point,” he says.

Though the occurrence of extremely high WAT measurements has significantly decreased over 2020, attention needs to be given towards an ongoing development where VPS detected an increase in the number of VLSFOs with very low pour points over the past few months.

The development could also mean a shipowner receiving VLSFO with a very low pour point with a high WAT (for example >PP + 200C or more) and having his bunkers turn into wax/sludge if following the advisory of CIMAC to store marine fuel at 10°C above the pour point. This is because measurement of pour point is an unreliable indicator of WAT/cloud point.

“The VPS recommendation is to measure WAT to be absolutely sure of the cloud point and to proactively maintain the fuel condition for efficient storage and handling of the fuel,” suggests Dr Cooper.

VPS graph showing range of WAT/Cloud Point of VLSFOs during 2020

A bespoke VPS method to measure WAT/Cloud Point of VLSFOs

Fortunately, VPS has already conducted a significant amount of research to develop a reliable method for the measurement of cloud point of VLSFOs during Q2/Q3 2019, shared Dr Cooper.

The output was a White Paper (available from the VPS website) outlining an automatic test method for WAT which is the cloud point of VLSFOs.

The method was validated by correlating with current methodology used for distillates, while its repeatability has been confirmed across the range of 10°C to 65°C.

“This bespoke VPS method allows the WAT/cloud point of VLSFOs to be measured. The method also enables the measurement of WDT which is the temperature at which the last wax solids are melted back into liquid when the oil is heated,” he notes.

“VPS has now measured the WAT/cloud point of thousands of VLSFO samples. As may be expected, we have seen a direct correlation between the WAT and the likelihood of operational issues arising from waxing/sludging of the fuel.”

Related: VPS develops new method to measure Wax Appearance Temperature of VLSFOs


Photo credit: Manifold Times
Published: 29 September, 2020

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