Why correct VLSFO temperature management is required to protect your assets

With the introduction of the IMO sulphur cap as of 1 January 2020, correct fuel management increased in importance. One of the challenges faced on-board is finding the optimal fuel temperature throughout the fuel system.

Very-low sulphur fuel oil (VLSFO) can quickly destabilise once on-board the ship, with some of these fuels having a shelf life of less than three months. This process of destabilisation accelerates when the fuel is heated. As a result, excessive sludging can be experienced, leading to issues such as clogging filters and separators.

VLSFOs are by nature, more paraffinic than “conventional” high-sulphur fuel oils. Should the fuel not be kept at a sufficient temperature, then wax formation can result, affecting the operation of separators, as well as clogging fuel systems, including filters. The historical advice of storing fuels at 10°C above the pour point can sometimes be misleading with regard to VLSFOs, as wax formation may start at temperatures higher than the pour point. VPS global statistics on VLSFO indicate that Wax Appearance Temperature (WAT) is about 20°C above pour point.

“VLSFO can quickly destabilise once on-board the ship”

Maintaining settling tank temperatures below the WAT can lead to wax precipitation in the tanks. This could then block the suction line and also reduce the heating efficiency if the wax covers the heating coils. Therefore, knowing the exact WAT is essential to ensure correct fuel management.

VPS introduced the WAT test method in 2019, helping numerous shipowners and operators to overcome this problem.

Keep temperature low

Fuel temperature must be managed on a case-by-case basis and is a fine balance between the viscosity and WAT. Heating the fuel in the storage tanks for an extended period is not recommended as it would significantly deteriorate the condition of the fuel and fasten the ageing process.

However, heating the fuel to achieve the appropriate purification temperature and injection viscosity cannot be avoided; since this is for a relatively short period assuming the fuel leaving the purifier is immediately consumed, the damage is limited.

In the past, a separation temperature of 98°C was carved in stone. With the introduction of VLSFOs in 2020, we have seen a significant reduction in viscosity which means lower separating temperatures at the purifiers.

The separator manufacturers generally recommend the storage and separation temperatures based on the viscosity of the fuel. Where the recommended separation temperature is below the WAT, this results in separator sludging or, in the worst case, complete blockage.

This problem can be more severe in cases where cat-fines are high and viscosity is low. The problem of heating a low viscosity fuel above the WAT is that the viscosity becomes so low that the fuel booster pumps of the vessel cannot produce sufficient pressure, introducing leakages and potentially leading to vapour lock. When a vessel cannot increase the temperature above the WAT, due to operational restrictions, the fuel is essentially unusable.

For this reason, understanding the correct WAT is critical when using VLSFO.