Several ships today are equipped with modern electronically controlled marine engines, which are customised to operate at both normal and slow speeds to reduce fuel consumption. This is because the cost of bunker oil accounts for more than 60% of the freight cost, which means that operating vessels as energy-efficiently as possible is an obvious solution.
In spite of the rapid advancement of technology in the maritime industry, there are still ships sailing in international waters, having conventional old marine engines with minimal automation. Shipowners are under great pressure to operate their ships at slow steaming to keep themselves above the profit margin and also to comply with stringent environmental regulations.
It is observed that slow steaming of old marine engines deteriorates the engine parts and its efficiency, as low speed marine engines are not traditionally suited for prolonged slow steaming. A number of precautions need to be considered in case slow steaming operations are to be adopted without modification of the engine. (However, those alone wouldn’t solve the problem as there are several other technical issues related to slow steaming of ships.)
When operating an old marine propulsion engine for slow steaming, following precautions and measures must be taken for safe operation:
1. The main components, which are involved in slow steaming operations, are engine fuel and fuel system equipment (Pumps, injectors etc.). It is necessary to ensure that the fuel is properly treated and heated to achieve required viscosity for the engine.
2. The fuel injectors must be in proper condition for good atomisation. If the engine needs to be in prolong slow steaming operation, the injector needle can be modified after consulting with the engine makers.
3. The fuel booster pressure must be regulated through pressure regulating valve and not by adjusting the by-pass safety valve for maintaining fuel pressure and recirculation of fuel in the system at all times.
4. The jacket and piston cooling water (most of the old engines have water cooled piston) temperatures must be maintained high to ensure that the combustion of air in the cylinder does not cool down.
5. In main engines with central cooling system, the air cooler’s fresh water-cooling inlet must be throttled so that a higher air temperature is maintained.
6. For system with sea water cooler, the sea water (S.W) inlet temperature must be kept at higher side to keep the combustion air and in-turn the exhaust temperatures higher.
7. Old Marine engines are provided with mechanical drive cylinder oil pump block. These block pumps are driven by a “driving arm” connected to the cam shaft. The fulcrum of the driving needs to be adjusted (reduced) which will in-turn adjust the amount of cylinder oil delivery in the engine cylinder.
8. With engines fitted with load dependent cylinder oil feed system, it is important to adjust the feed rate manually in addition to the automatic adjustment, especially on variable pitch propeller installation as the engine will operate at the same RPM even when the pitch has been reduced.
9. During slow steaming, the engineers must ensure that the exhaust gas temperatures after each cylinder should not go below 200 Deg C. If the exhaust gas temperature is falling below this limit, the engine power must be increased till the temperature reaches 275 deg C. The engine should be continued with this power for at least twice in a period of 24 hours before reducing again to slow steaming to ensure the exhaust receiver trunk is dried out.
10. An additional retro-fit steam heater in the scavange air system is of additional advantage for slow steaming operation as it will increase the exhaust gas temperature and reduce the risk of turbocharger problems.
Enabling old marine engines for slow steaming is not an easy task as several factors need to be taken into consideration, taking into account various long-term effects on the ship’s machinery systems and other feasibility factors.
Higher efficiency is achieved mainly due to the vessels spending more of their time sailing at optimum speed, which is defined as being the most efficient speed for each individual vessel and voyage during the current fuel and freight market conditions.
As a result of combining traditional energy efficiency methods and reduced bunker fuel consumption with increased efficiency, the relationship between freight and bunker fuel can be maximised from a more commercial and operational perspective.
Over to you..
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