What Are the Functions of Marine Boiler Water Circulation on Ships

The most vitalpart of any marine boiler is preserving the accurate water level. As the boiler fires, steam is liberated from the surface of the water and as a result as the steam gets consumed, there is subsequent drop in water level available inside the boiler. Steam gets generated only when there is an appropriate circulation of water within the boiler.

For the tank type boilers, circulation is not distinct and the water that circulates within the tank is done by the boiler itself. For water tube boilers, circulation is vital within each and every tube and headers. This occurs naturally if both the steam and water drums are linked with tubes. But there is some basic design reason which has to be emphasized on.

How is natural Circulation within a Water Tube Boiler done?

The steam drum and water drum may or may not get separated depending on the boiler design. To fully understand the boiler water circulation principle, we will assume the following parts to be separate. A steam drum at the top is linked to the water drum at the bottom through down comers outside the boiler shell and risers inside the boiler shell. Cold water is then fed to the top drum/steam drum, by water feed pump via a screw down non-return valve and a check valve.

As we already know, hot water stays on the top and relatively cold water (being denser) moves to the bottom of the drum. The steam/water drum has down comers connecting to the water ring at the bottom of the boiler. The down comers are found outside the boiler shell and are large in diameter when compared to the water tubes. As the cold water travels the down comers gets to the water ring in the bottom, the circulation begins within the tubes and drums.

As the water particles approach the water tubes, which are found inside the boiler furnace, they begin to heat up and get wet steam with some bubbles. As they are less dense, they all of a sudden rise up to the steam drum and thus are continuously being replaced by relatively cold water from down comers. Thus boiler water circulation occurs naturally inside a water tube boiler.

Design Part

If the circulation has to occur in a natural way, then there are some design considerations to be given attention. Ratio between the amount of steam leaving the risers & the amount of water entering the down comers is of the order 4:1.

It is vital to have more water than steam on the risers. If the riser at some point gets completely full of steam, the tube would over heat and “BURN OUT”.

“BURN OUT” is a process where the tube metal over heats can no longer resist the pressure forces within the tube and as such ruptures.

Why External Down comers are Vital?

It is well known that as the temperature and pressuregoes up, the density of water decreases correspondingly. Thus the density difference between water and the steam lowers & thus at higher pressures, the natural circulation of water-steam gets impaired. Thus to preserve the natural circulation, external, unheated down comers are thus fitted. These external down comers are some large bore tubes linking the steam drum with the existiing water drum.