Slow Steaming of Ships: Optimization of Ship’s Main Engine

In the previous article, we discussed about several concerns regarding slow steaming in the minds of marine engineers.

As the low speed marine engines are not traditionally suited for prolonged slow steaming, a number of precautions need to be taken in case slow steaming operations are adopted without modification.

In this article we shall discuss the checks to be done, additional maintenance required and the precautions to be taken so that there are no long term damages to the machinery.

Optimization of ship’s main engine

Traditionally main engines are designed to run between 70 % to 85 % load range during continuous operation. The matching and designing of all the auxiliaries is based on this load range operation.

• The exhaust boiler size (surface area) is decided based on the exhaust temperature, volume of exhaust gas flow and the waste heat recovery in this range. Low load operation makes this waste heat recovery system ineffective and there is less production of steam, which increases the load on the oil fired boiler.
• The air cooler size (surface area) is selected based on the heat load of the air in this operating range. During low load operation the cooling water to the air cooler needs to be controlled by bypassing the cooler and throttling the water valves to maintain optimum scavenge air temperature. Too much throttling of the water valves reduces the flow velocity of the cooling water thereby increasing the deposit rates of the precipitants, leading to fouling and contamination of tubes.
• The turbocharger selection and matching to the main engine is based on the enthalpy of the exhaust gas that needs to be extracted. The other selection criteria is the quantity of the scavenge air that needs to be supplied to the cylinders for optimum combustion. The turbocharger is selected for the normal running load range of 70 to 85 %. Low load operations of the main engine lead to lower running RPM of the turbocharger and less generation of scavenge air. This leads to ineffective and incomplete combustion, increased fouling and also makes the cleaning measures like dry grit cleaning of the turbine ineffective.
• The propeller is designed to give maximum efficiency for the RPM in this range. Due to lower RPM the propeller efficiency may be affected.

• The Specific fuel oil Consumption (SFOC) is optimized for running in this range. Even though the fuel consumption is lower in totality, the SFOC is higher at part loads as injection and combustion is not proper.
• The fuel injectors and fuel pumps are designed for this range thus the atomization and penetration may be effected at low load operation.
• The operating parameters and their alarm and monitoring system is designed for this range.
• The hydrodynamic lubrication is RPM dependent and the grade of oil and its properties like oiliness are selected for this range.
• The shaft generators are designed and selected based on this range. Low load operation may make shaft generators unusable.

Thus running the main engine below its normal operating range of 70 to 85 % Maximum Continuous Rating (MCR), the whole system is not optimized.

Generally is known that if engine modifications and retrofitting is done on the main engine, then it is safe for slow steaming as well as ultra slow streaming. However we limit ourselves to slow steaming without any engine modifications in this article.

Slow steaming up to 50 to 55 % load can be done generally on most engines without harm in long terms if certain precautions are taken. That generally is the point above where the auxiliary blowers cut in. In the article “How to Test Ship’s Main Engine for Slow Steaming”, the testing method for the optimum eco speed by trial has been discussed.

About Author

Chief Engineer Mohit Sanguri is a Marine Chief Engineer (Class I Unlimited Power). He has 12+ years of experience as a Marine Engineer.

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