The technical update by the Viswa Groups explains the analytical tests for stability and compatibility of two fuels.
Fuel Stability
Stability of residual fuel is defined by its resistance to breakdown and precipitate asphaltenic sludge, despite being subjected to forces, such as thermal, physical or aging stresses, while handled and stored under normal operating conditions.
Instability can happen if the fuel is blended such that the asphaltenes are not well suspended and if the rest of the fuel does not have enough aromatic components to keep asphaltenes in suspension.
Asphaltenes need other aromatic components in order to stay in suspension, whereas if they are blended with high amounts of paraffinic cutter stocks, the risk of instability is large.
In case of asphaltene precipitation:
The result of an unstable fuel might be excessive sludge formation in tanks or further down in the fuel system, which then can block filters and separators.
Once the asphaltenes have precipitated and formed sludge, there is a very low possibility that the process can be reversed and the asphaltenes can be re-suspended in the fuel again, neither by physical nor by chemical means.
Make sure that:
- The fuel must be stable and homogenous at delivery.
- The fuel shall also be stable in typical ship fuel systems and in the engine high-pressure fuel injection system.
- It is the responsibility of the fuel blenders and suppliers to ensure fuel stability.
Fuel compatibility
Compatibility is defined as the ability of two or more fuels to be mixed at a defined ratio without evidence of material separation, resulting in the formation of multiple phases e.g. flocculation or separation of asphaltenes, precipitating sludge in the fuel system.
The process of the asphaltenes dropping-out can be very quick and is IRREVERSIBLE.
The resulting issues are the same as for an unstable fuel;
Problems in the fuel cleaning system with sludge in tanks and further in the fuel system and thereby blocked filters and separators.
Make sure that:
It is important to always remember that different fuel batches may not be compatible.
Challenge in case of VLSFO preparation:
The challenge is that the 0.50% S VLSFO’s will be made from a diverse range of various blending stocks varying from highly paraffinic to highly aromatic.
When mixing two fuels, one paraffinic and one aromatic, which are stable by themselves, the resulting mixture might become unstable.
- The result will be an excessive amount of sludge because the asphaltenes in the aromatic fuel will drop out of the suspension.
Test Analysis for checking Stability & Compatibility of Two Fuels:
1. Spot Test/ASTM D4740
All these days Spot test analysis had been done to evaluate whether two blended fuels are compatible. Since there are a lot of new fuels coming into the market. Many of them have very low asphaltene content.
If asphaltene is not there, the characteristic ring formation will not be seen particularly when we are looking at compatibility (spot test).
There is also another confusion caused by paraffinic fuels with no asphaltenes forming rings based on the light and heavy wax content in the fuel. Thus, the spot might be a foolproof test analysis in due course especially for VLSFO.
2. Sediment testing
- Total Sediment Existent (TSE)/ISO 10307-1
ISO 10307-1 (Existent Total Sediment, TSE) TSE measures the amount of sediment present in fuel at a moment by filtration and weighing the amount of sediment on the filter. TSE includes organic and inorganic sediment.
- Accelerated Total Sediment (TSA)/ISO 10307-2 (Chemical Aging)
ISO 10307-2 (Accelerated Total Sediment, TSA) TSA is the total sediment after dilution of a sample of residual fuel with a paraffinic solvent (hexadecane) under carefully controlled conditions, followed by storage for 1h at 100°C i.e. the amount of sediment after stressing the fuel chemically and storage at 100°C for 1 hr.
This method is expected to show the maximum amount of sediment that is likely to form when applying chemical stress. TSA takes 2 hours.
While TSA is a listed test in the standards, it is also recognized that, very often, TSA can give wrong results and it is easy to make mistakes.
Thus, the industry practice is to go for TSP if time is available or else carry out TSA first and wherever there is doubt, confirm it with TSP. However, TSP has its own limitations.
- Total Sediment Potential (TSP)/ ISO 10307-2 (Thermal aging)
ISO 10307-2 (Potential Total Sediment, TSP) TSP is the total sediment after aging a sample of residual fuel for 24h at 100°C under prescribed conditions i.e. the amount of sediment after stressing the fuel through heating.
This method is expected to show the maximum amount of sediment that is likely to form when applying thermal stress. The max permitted variation in the TSP result is as much as 0.05%. This means for a targeted result of 0.10%, we will never be sure if a result of 0.08% is really passing the test or failing.
Only if the TSP values are below 0.06%, then the results are within spec. A typical TSP takes 26 hours.
Though TSP is the deterministic test, it is not foolproof. Hence, alternate tests such as PORLA, ROFA and Zematra are introduced in PAS, CIMAC and Concawe document.
Publicly Available Specification (PAS) document was published in September 2019 and they recommended to use the referee method (TSP) for predicting stability and compatibility of fuels.
They stated that the TSA test is not applicable for all of the VLSFOs since it has been shown in different studies that in some cases the TSA is less than 0.1% while TSP is greater than 0.1%. Therefore, it is recommended to use the TSP to evaluate the stability of the
fuels.
3. ROFA, PORLA, and ZEMATRA methods
In addition, three ASTM methods were introduced (PAS document and Concawe article) for stability and compatibility prediction:
- ASTM 7157- ROFA- “S” VALUE- ASPH > 0.5% –
- ASTM 7112- PORLA- “P” VALUE- ASPH > 0.05% –
- ASTM 7060- ZEMATRA- “P RATIO”- ASPH > 1% –
These three methods can provide further insight into the compatibility of the fuel by investigating the asphaltene flocculation point. Not all of these methods are applicable to the low asphaltene content samples.
Also, you need to have the equipment in the lab and specialized operator to evaluate the results. Some outliers will be present in each of the methods. Based on Cancawe article:
“The study shows that ASTM D7157, D7112, and D7060 can be used to obtain information on the degree of compatibility, though in some cases inconsistencies have been observed.”
Below table shows the difference between three ASTM methods:
Below table shows the summary of the available methods, their pros, and cons:
Recommendations for storage, handling and managing the stability and compatibility of fuels:
1. Avoid mixing different fuel batches.
- Always keep different batches of fuels in separate tanks.
- It is recommended to have dedicated tanks for different types of fuels.
- Fuels should not be mixed until it is ensured that the fuels are compatible with each other.
2. Empty tanks as much as possible before bunkering a new fuel batch.
- Install more tanks in the ship, so that different fuel batches can be bunkered and stored in different empty tanks.
- There are two different types of systems that can be used to store and manage the incompatibility between different fuels.
- The first system is the flexible fuel system which has a separate fuel line which makes it easier to handle different fuel types.
- The second type is a simple fuel system that uses shared fuel lines. In this type more attention has to be paid to the fuel system handling when changing fuel.
3. It is possible to fix partitions in existing fuel tanks to convert one tank into two tanks without any hot work.
4. If mixing of the fuels is unavoidable:
- Reduce the amount of fuel in the tank as much as possible before bunkering the new fuel.
- Fuels with similar viscosity, density and pour point often show acceptable compatibility, as they are possibly of the same or similar type.
- Check the compatibility of the fuel in the tank and the new fuel (on-board testing: spot test).
5. Establish good fuel switch-over procedures in advance.
6. Establish procedures for fuel tank cleaning in advance, both for normal operation, but also for when excessive sludging occurs in tank, fuel lines, purifier, and filters.
Did you subscribe to our daily newsletter?
It’s Free! Click here to Subscribe!
Source: The Viswa Group
