An elephant may be huge and strong as marine diesel engines are. Whereas an ant can bring the elephant down as it enters the elephant’s ears. Same is the story with cat-fines. As long as the cat-fines are kept away from the main fuel injection system and combustion chamber, your engine stays well protected. The problem starts if cat-fines enter those sensitive areas.
Viswa Lab has seen several cases of engine breakdowns resulting from catalytic fines/sludge having accumulated in the tanks and entered the fuel system during operation. International legislation requiring the use of fuels with a limited sulphur content has led to a steady increase in the levels of catalytic fines (cat fines) found in fuels. The majority of the operators prefer to test the bunker fuels as per ISO 8217:2005 which has an Al+Si limit of 80 mg/Kg. However, there is a complete discrepancy amongst the ISO 8217 specification limits and engine makers’ requirements. Most engine makers recommend that the fuel at the engine inlet should have an Al+Si at levels 10 to 15 ppm.
At one end, the bunker fuel supplied are having high Al+Si content and on the other end the engine makers require a much lower limit. Thus the only aspect which bridges this huge gap is the onboard fuel treatment plant and its effectiveness. It is, therefore, important that the fuel treatment plant is managed and operated in the most efficient way in order to reduce the levels of bunker cat fines to less than 15 ppm at the engine inlet and hence prevent serious damage to main engine components.
As a Generic Precaution – before Bunkering, please ensure:
- Ensure that there are sufficient empty tanks available to store the newly purchased fuel.
- Be aware of the fuel quality analysis statistics of the port of supply, especially if there are relevant warnings issued by Viswa Lab, P&I Clubs and the maritime press.
- Ensure the vessel has sufficient fuel on board to enable the testing of new bunkers prior to usage. The vessel should avoid using newly purchased fuel without obtaining and acting on the results of fuel analysis.
Checkpoints To Prevent Engine Damage due to Cat-fines:
- Purifiers
- Fuel storage, settling and service tanks
- Equipment maintenance
1. Purifier:
- Where possible, run two purifiers in parallel with a minimum flow and keep the HFO inlet temperature at the optimal temperature of 98ºC to ensure efficient purification.
- If the vessel is equipped with a conventional purification plant, always ensure that the purifiers are fitted with the optimal gravity disc corresponding to the actual fuel and the fuel test result.
- Purifier capacity should be sufficient to cope with daily fuel consumption plus 10% in order to enable some recirculation of fuel in the settling tank to occur. Purifier efficiency tests should be carried out regularly by fuel specialist bodies.
- Regular checks of the purifiers should be made by the manufacturer’s service engineers to enhance system efficiency. Fuel system filters should be regularly inspected and cleaned – not only when high differential alarms are activated. However, where a ship has bunkered “dirty” fuels a few times, tank cleaning at shorter intervals should be considered. Furthermore, it is recommended that these tanks should have sloping bottoms to prevent the build-up of sediments and cat fines in the bottom.
2. Fuel storage, settling and service tanks
- New bunkers should preferably be placed in empty tanks, and blending of different fuels avoided. If blending is deemed necessary ensure that an adequate compatibility test has been performed.
- Frequent (daily) draining of water and settled bottom sediments from fuel storage, settling and service tanks should take place. During calm weather, the heavy components in the Heavy Fuel Oil (HFO), such as cat fines, will settle in the tank bottom, and in heavy weather these abrasive particulates can be stirred up and fed into the purifiers in concentrations exceeding the maximum acceptable levels. If unchecked, this can impede the efficiency of the treatment system, leading to large quantities of cat fines at the engine inlet.
- Drained oil from automatic fuel oil backwash filters should not be reintroduced into the fuel treatment system.
- Clean settling and service tanks during dry docking in order to deal with any long-term build-up of cat fines and sediment in the bottom of the fuel oil storage tanks.
3. Equipment maintenance:
- Fuel treatment heaters should be opened and cleaned regularly to ensure that the optimal temperature of 98ºC for purification is reached.
- Purifiers should be opened for cleaning at the scheduled intervals recommended by the manufacturers, or more often if poor fuel quality is suspected. Vessels should maintain the necessary spare parts on board.
If a problem is found:
If engine damage is thought to be due to cat fines, experts should be instructed to confirm the presence of cat fines. Such confirmation can only be obtained by replica testing of the affected cylinder liners and piston rings carried out by the technicians at Viswa Lab (or) by the Engine Makers.
If the presences of cat fines are confirmed, all tasks necessary to eradicate them from the fuel should be carried out immediately. This should include:
- Cleaning of all relevant fuel tanks including fuel system components,
- Replacement or machining of all affected engine components.
A Simple Math:
Let us assume that the Bunker fuel of grade RMG 380 has an Al+Si content of 80 ppm. This is the maximum limit as per ISO 8217:2005 specifications. The below table will show an estimate of purifier efficiency and cat-fines at engine inlet considering all other factors as negligible.
|
S.No |
Cat-fines In bunker fuel |
% Purifier Efficiency |
Cat-fines at Engine Inlet |
Remarks |
|
1 |
80 ppm |
10 |
72 ppm |
High |
|
2 |
80 ppm |
20 |
64 ppm |
High |
|
3 |
80 ppm |
30 |
56 ppm |
High |
|
4 |
80 ppm |
40 |
48 ppm |
High |
|
5 |
80 ppm |
50 |
40 ppm |
High |
|
6 |
80 ppm |
60 |
32 ppm |
High |
|
7 |
80 ppm |
70 |
24 ppm |
High |
|
8 |
80 ppm |
80 |
16 ppm |
Close to Engine Makers’ Recommendation. |
Thus from the above table, it is evident that even if the purifiers on board operate at 80% efficiency, the catfines are marginally higher than the required 10 to 15 ppm as per Engine Maker.
Viswa Lab reports that the average purifier efficiency which was observed over the past years reveals that the average efficiency falls in the range of 50 to 60%.
In the next technical write-up, we will throw lights on various parts of fuel system and how the cat-fines can be best addressed to meet the recommended limit of 10 to 15 ppm.
Source: Viswa Lab & Ship’s Bunker Guide.
A very well presented write up, highlighting key issues – Congratulations!
I will share here our observation based on addressing similar problems for ship managers.
This has particular reference to the table above under “A Simple Math” in the write up:
1 You have rightly highlighted in the article the Cumulative Effect of “Bunker Tanks – Settling Tanks – Purifier – Service Tanks” in lowering “Catfines” to the acceptable limit of 10 – 15 mg/kg at engine entry.
2 A typical bunker fuel (as per ISO 8217:2005) with Al+Si at 80 mg/kg, if receives adequate treatment (tender love & care!) in “Bunker Tanks & Settling Tank , the Al+Si content at the “Purifier” entry point should be down to 56 – 57 mg/kg.
3 Bunker Tank & Settling Tank play a highly crucial role here to reduce Al+Si by about 30%, if well maintained and operate at optimum efficiency. So, that the “Purifier / Service Tank/ Filters” can take it up at this point to do the rest of the job.
(Operating engineers need to understand this fundamental marine engineering!)
4 With reference to table above under “A Simple Math”, I think we need to factor in the cumulative effect of “Bunker Tanks – Settling Tanks – Purifier – Service Tanks” in reducing Al+Si at engine entry point to recommended level.
Thank you once again for your efforts in keeping us all up-to-date!