Overview of A Ship’s Steam System
A boiler, or a waste heat recovery or both jointly, generate the steam on board the ship. The consumption of fuel by the boilers in the world fleet is about the 6% of the total consumption; therefore, it is very important to operate the boilers at optimum condition and low consumption. Further, use the maximum quantity of heat of the exhaust gases and this way saving an important quantity of fuel and reducing emissions.
All systems that use steam, like turbines, heat exchange, water generators, etc. must have a high-level cleanness and maintenance to get the higher efficiency.
The hot well temperature of the feed water of boiler must be maintained between 80-85 ºC to avoid a higher consumption of fuel.
After each chemical analysis of the boiler water, it is necessary to do the corresponding surface or bottom water extractions to eliminate the salts and solids contented in the water.
The combustion in the boiler must be made with the air excess index fixed by the manufacturer. An air excess index upper to the specified will provoke loss of heat to outside and lower efficiency.
The optimum air excess index can be determined in accordance concentration of the O2 or the CO2, if the concentration of O2 is high and the CO2 low, it is an indication that the combustion is been made with a high air excess index.
The efficiency of the boilers depends of the workload; the maximum efficiency of boilers is when they are working with a workload of approximately 80% of the nominal power. In the case that the ship has two auxiliary boilers and the workload demand is low, should only one boiler work with all demanded workload.
In the distributions system of steam all kinds of measures must be taken to avoid steam loss in the by-pass valves, filters, etc. It is convenient to maintain a very good insulation of the pipes and equipment. The heating of the cargo in the ships tanks should be made through a planning to know the best condition of heating and maintenance of the temperature of the cargo, and avoid an excess consumption of fuel. This increases energy efficiency and reduces emissions (MariEMS 2017).
Boilers and Steam System Introduction. The steam system plays a major role in energy efficiency of certain ship types (such as steam driven LNG ships) and a medium role in ships such as oil tankers carrying liquid cargo that require cargo heating or there is a need for cargo transfer using steam driven pumps but also need to generate Inert Gas for cargo tank cleaning, purging or tank top ups. Figure 26 below shows typical level of fuel use in boilers as compared to main and auxiliary engines for a VLCC vessel (MariEMS 2017).
Figure 26: Overall annual fuel consumption and boiler share (MariEMS 2017).
In some ship types, boilers and steam are using significant amount of energy for either propulsion turbines or for other auxiliary services such as operation of cargo and ballast pumps, cargo heating, fuel oil treatment and conditions and so on.
Figure 27 below also reveals the level of energy use in marine boilers for the whole of international fleet.
Figure 27: Annual shipping fuel consumption per ship type and combustion system (MariEMS 2017).
It can generally be stated that an overall average number of up to a maximum of 6% of shipping fuel consumption could be attributed to the use of boilers. As stated, for steam turbine propulsion ships such as steam LNG ships, more than 80% of energy use is due to boilers (MariEMS 2017).