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Oil major and bunker supplier ExxonMobil has released its latest 'ExxonMobil Insights' video, which covers issues related to fuel compatibility and stability.

The main topics discussed are: avoiding fuel mixing, asphaltene precipitation and build-up, and fuel testing.

Sludge build-up

The new online video points out that cheaper residual fuel, or heavy fuel oil (HFO), can often contain a high level of asphaltenes. This can lead to problems if stored for a long time.

"If bulk fuel oil has been stored for long periods, it can become unstable. This can cause the asphaltenes to precipitate out, creating sludge, which has the potential to block pipes, pumps and filters. The higher the asphaltene content, the greater the sludge potential," the video says.

Related to this is the issue of fuel incompatibility and the problems it can cause. Michael Green, Global Technical Manager at Intertek, points out that the incompatibility of fuels can lead to significant sludge formation, which in turn can result in engine blackout.

"If fuels with greatly differing asphaltene content combine, these asphaltenes can coalesce into sludge, potentially blocking filters and pipes, leading to engine starvation and power loss," the video adds.

In order to mimimize compatibility issues, ExxonMobil suggests the following precautions are taken:

- Avoid mixing bunker fuels from different sources.

- Choose fuels with a similar viscosity and density.

- Do not mix HFO with marine gas oil (MGO).

Sample testing

When bunkering, ExxonMobil advises ship operators to always test a sample blend to ensure that the mix of the fuels will not trigger compatibility problems.

"Is the fuel going to cause any potential damage to the engine system when it is used? It's very, very important for the crew to know exactly what they have received, [so] that it can be treated appropriately before it is injected into the main engine," Intertek's Michael Green says.

The video goes on to explain the standard procedure for two basic test methods: sediment and spot testing.

The ISO 8217 total sediment potential test is generally carried out in fuel testing labs. During the test, the fuel is maintained at an elevated temperature for 24 hours and then vacuum-filtered to look at the residual weight of the sediment that is actually present within the fuel.

The ASTM D4740 spot test is a simple procedure that can be carried out on board a vessel. In this test, a blend of fuels is homogenized and heated before being dropped onto a test paper, which is then placed in an oven at 100C. After an hour, the paper is removed and the resulting spot compared to a reference chart. If the two fuels are compatible, one large spot should be visible without an asphaltenic ring in the middle; but if the two fuels are incompatible, a black ring will develop within the actual spot itself.

ExxonMobil products

To help reduce compatibility issues, ExxonMobil has developed premium ECA-category products that are designed to breach the compatibility gap that can exist between distillate and heavy fuel oil.

ExxonMobil Premium HDME 50 is a heavy distillate fuel that is said to be compatible with MGO and ECA-compliant. ExxonMobil Premium AFME 200, meanwhile, is said to be fully compatible with ExxonMobil Premium HDME 50 and MGO.

A link to the video has been provided below.

ExxonMobil Insights 4: Fuel compatibility and stability