Earlier in March 2019, ClassNK had published the Guidance for onboard use of Compliant Fuel Oil with Sox regulation from 2020. The Risks associated and the measures with changing over to compliant fuels and five properties of residual fuel oils that should be further considered while using compliant fuel oil are explained here. Particular focus is given to “Compatibility” and “Cold flow properties” that are relevant to the changing over to compliant fuel oils.
The five properties of residual fuel oils
- Compatibility
- Low viscosity
- Cold flow properties
- Cat-fines (contents of Aluminium and Silicon)
- Ignition and combustion quality
Risks associated with the changing over to compliant fuel
1.1 Compatibility
Compatibility is an index of how likely sludge will be formed when mixing two different types of fuel oils. Such mixing of fuel oils can be a complex issue, and even two types of fuel oils which are independently stable may lead to sludge formation when they are mixed together.
The following issues may occur as a result of excessive sludge formation arising from fuel incompatibility.
Sludge precipitation in fuel tanks and pipes
Sludge formation may occur in storage tanks when different fuels are bunkered. Sludge formation may also occur in the settling tanks, service tanks or pipes when fuels for onboard machinery are changed to compliant fuels. In such cases, the presence of sludge can clog pipes and make fuel transfer impossible.
Plugging of fuel oil filters
When sludge precipitated in the fuel tanks is transferred or sludge is formed in the piping systems, filters may be plugged or blocked. In such cases, the fuel supply to the main engine may be reduced, which may even cause the main engine to stop in the worst case scenario. In addition, the fuel supply to auxiliary engines may also be stopped, which in turn may lead to blackouts.
Sludge accumulation in purifiers
When the plugging of filters is confirmed, it is indicative of a possible increase in the amount of sludge being captured by the purifiers. Excessive sludge sediment on the rotating parts of purifiers can generate an imbalance in such; therefore, the discharged of the sludge may lead to abnormal vibrations or result in damage to the purifiers.
1.2 Cold flow properties
In the case of compliant fuels with high pour points, wax crystal formation may start at low temperatures.
The following issues may occur as a result of wax formation.
Wax formation in fuel tanks
When wax formation occurs in storage tanks, fuel transfer from the storage tanks to the settling tank via fuel oil transfer pump may become difficult.
Choking of fuel pipes (particularly filters)
When wax formation occurs, it can lead to choking of fuel pipes, and the plugging or blocking of filters. In such cases, fuel supply to the main engine may be reduced which may even cause the main engine to stop in the worst case scenario. In addition, the fuel supply to the auxiliary engines may also be stopped, which in turn may lead to blackouts.
Wax accumulation in purifiers
When wax formation occurs and wax is accumulated in the purifier, the performance of purifier may be reduced which may result in an insufficient quality of fuel being provided to relevant machinery.
In addition, it can lead to excessive wax sediment on the rotating parts of purifiers which can generate an imbalance in such parts; therefore, the discharge of the wax may lead to abnormal vibrations or result in damage to the purifiers.
Since compliant fuels are said to have a lower viscosity than the conventional high-sulphur residual fuel oil (HSFO), fuels that do not need heating may be available on the market. If such fuels are stored in storage tanks not provided with heating equipment, problems may occur within cold areas or during winter seasons because the fuel may begin to form wax which may in turn start to solidify.
Although reheating the fuel to above its pour point will dissolve the waxy precipitates, reliquefying such fuel tends to take a considerable amount of time due to the generally high latent heat of waxy precipitates. Since it is fairly difficult to dissolve waxy precipitates by adding chemicals, etc., it is important to put measures in place to prevent wax formation in the first place.
Measures for changing over to compliant fuel oils
2.1 Measures for sludge formation due to mixing of fuel oils (Compatibility)
2.1.1 Measures
Measures to prevent sludge formation due to the mixing of fuel oils or to prevent problems related to sludge formation in machinery are as follows:
Changing over in fuel oil storage tanks
- Empty tanks as much as possible before bunkering a new batch of fuel.
- If mixing within storage tanks cannot be avoided, check compatibility by spot test (in accordance with ASTM D4740, and refer to pages 21 and 36 of this booklet) whenever possible at bunker time. If incompatibility is confirmed, then add sludge dispersant to the remaining fuel oil in the storage tank before bunkering.
Since compatibility may change over time and sludge may be generated after long periods of storage even when the results of initial spot testing were good, it may, therefore, be desirable to use sludge dispersant whenever different fuel oils are mixed.
Changing over in fuel oil pipelines
When changing over in fuel oil pipelines in order to change-over to compliant fuel oil that is stored in a storage tank, the different types of fuel oils are generally mixed together in settling or service tanks, as shown in figure 2.1.
Measures to be taken in this case are as follows:
- In order to avoid the mixing of different types of fuel oils in settling/service tanks, keep the amount of the remaining fuel oil in such tanks as minimal as possible, and then transfer compliant fuel oil from a storage tank to the settling/service tank.
- Carry out a spot test, and, add a sludge dispersant to the settling tank if incompatibility is confirmed.
- Use up the fuel oil which had been mixed together in tanks or pipelines as soon as possible.
- In the cases where sludge is frequently confirmed in fuel oil pipelines or where the differential pressure in the pipeline between before and after the filter rises sharply, take the following actions:
- Shorten the interval of backwashing;
- Keep the differential pressure of backwashing lower; and
- Clean the filter more frequently than usual.
5. In cases where sludge is frequently found in filters placed before purifiers, precipitation of sludge in the fuel oil purifier is also suspected. If a large amount of sludge generation is expected, take the following actions in order to avoid sludge accumulation on the separating disc of the purifier and to also improve separating efficiency:
- Reduce the flow rate. It is also recommended to operate two sets of purifiers in parallel and to treat half the amount of fuel oil by each purifier;
- Shorten the interval of discharge of sludge;
- Maintain a high temperature of oil flow in accordance with the purifier manufacturer’s manual; and
- Shorten the maintenance interval of the separating disc.
2.1.2 Cautionary notes
When changing over to compliant fuel oils
- In order to avoid serious accidents that may occur if the main engine fails, do not carry out the changing over to compliant fuel oil in fuel oil pipelines while the ship is operating in congested sea areas or narrow water channels.
- Changing over to compliant fuel oils in fuel oil pipelines should be carried out during the daytime, not at night, so as to ensure there are enough crew members readily available to solve potential problems; and
- Before changing over to a compliant fuel oil, clean the purifiers and filters. Keep purifiers and filters in a clean condition in order to identify whether any sludge, if confirmed, had been formed prior to the changing over or as the result of the changing over
Flushing by sludge solubilizer or MGO
- Before bunkering compliant fuel oils, ships may plan to undertake the flushing of storage tanks, settling/service tanks and fuel oil pipelines. In such cases, sludge that has been deposited on the bottom of such tanks or pipelines may flow out to pipelines in large quantities and clog purifiers and/or filters.
- In cases where large amounts of sludge actually flow out, the sludge should be removed by purifiers and/or filters, as it is difficult to deal with using chemicals. In such cases, take the actions prescribed in paragraphs 2.1.1-24) and 2.1.1-2(5).
2.2 Measures to prevent wax formation in fuel oils (Cold flow properties)
- As for paraffinic compliant fuel oils and MGOs, the risk of the wax formation can be mitigated by heating. Proper heating of fuel oils is, therefore, useful as a precautionary measure. In cases where the cold filter plugging point (CFPP: refer to page 11 of this booklet) of the fuel oil is known, heating the fuel oil above the CFPP is recommended. In cases where the CFPP is not known but the pour point (PP: refer to page 11 of this booklet) is known, heating the fuel oil at least 10°C above the PP is recommended.
- In cases where both the CFPP and the PP are not known, whether the fuel oil forms wax at a lower temperature can be confirmed by taking a sample of the fuel oil and keeping it in a refrigerator. It is desirable to set the temperature of the refrigerator to be the same as the temperature that the fuel oils are exposed to in the storage tank.
- Fuel oils having lower viscosities and higher pour points might be distributed; in such cases, attention should be paid when using such fuel oils. As it is essential to ensure that the viscosity of fuel oil at the entrance of the main engine is as recommended by the engine manufacture, the temperature of the fuel oil may need to be kept within a narrow range.
- If no heating equipment is provided in a fuel oil tank, it is recommended to use the fuel oil after confirming that the ambient temperature (seawater temperature and/or engine room temperature) is more than PP +10°C in consideration of the route and the climate. If it is not possible to ensure such a condition, using an additive to mitigate any wax formation may also be an option.
- If wax is frequently confirmed in filters placed before purifiers, then take the actions as prescribed in paragraphs 2.1.1-2(4) and 2.1.1-2(5).
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Source: ClassNK
