Maintaining the proper water level is the most important aspect of any marine boiler. As the boiler fires, steam is liberated from the surface of the water, and as the steam is consumed, the water level inside the boiler falls.
Steam is produced only when the water within the boiler. is effectively circulated. For tank type boilers, circulation is not distinct, and water circulates within the boiler’s tank. Circulation is required within each tube and header in water tube boilers.
This occurs naturally if the steam and water drums are linked by tubes. However, there are some fundamental design reasons that must be emphasized.
Natural Circulation Within Water Tube Marine Boilers
Depending on the design of the boiler, the steam and water drums may or may not be separated.
To comprehend the boiler water circulation principle, we will assume that the following components are distinct. Through down comers outside the boiler shell and risers inside the boiler shell, a steam drum at the top is connected to a water drum at the bottom.
The water feed pump delivers cold water to the top drum/steam drum via a screw down non-return valve and a check valve. As we all know, hot water rises to the top of the drum, while cold water sinks to the bottom due to its density.
The down comers connect the steam/water drum to the water ring at the bottom of the boiler. These are situated outside the boiler shell and have a larger diameter than the water tubes.
The circulation within the tubes and drums begins when the cold water from the down comers reaches the water ring at the bottom. As the water particles enter the water tubes inside the boiler furnace, they begin to heat up and turn into wet steam with bubbles.
Since these particles are less dense, they rise quickly to the steam drum and are thus constantly replaced by relatively cold water from downcomers. As a result, boiler water circulation occurs naturally within a water tube boiler.
Design Aspect of Marine Boilers
If the circulation must take place naturally, some design considerations must be taken into account. The ratio of steam leaving the risers to water entering the down comers is approximately 4:1.
It is critical that the risers contain more water than steam. The tube would overheat and “BURN OUT” if the riser became completely full of steam at some point. “BURN OUT” occurs when the tube metal overheats and becomes unable to withstand the pressure forces within the tube, causing it to rupture.
Causes of External Downcomers
The density of water decreases with increasing pressure and temperature. As a result of the decreasing density difference between water and steam, the natural circulation of water-steam is hampered at higher pressures.
External, unheated down comers are thus fitted to maintain natural circulation. External downcomers are large bore tubes that connect the steam and water drums.