Blow- Down Procedure for a Marine Boiler & Why its Done?

Boiler blowdown is of two types:

Scum Blowdown

Bottom Blowdown

Procedure for scum and bottom blowdown:
Below is the procedure for boiler blowdown using the blowdown valve located at the bottom of the boiler. In order to do scum blowdown, instead of bottom blowdown, the scum valve is to be opened.

Open the overboard or shipside valve 1 first.

Open the blowdown valve 2, this valve is a non-return valve.

The blowdown valve adjacent to the boiler 2 should be opened fully so as to preventing cutting of the valve seat.

The rate of blowdown is controlled by valve 3.

After blowdown close the valve in reverse order.

NOTE: A hot drain pipe even when all valves are closed indicates a leaking blowdown valve.

Boiler blowdown is done to remove sludge deposits and other impurities from the boiler water. Blowdown of the boiler is done to remove two types of impurities-scum and bottom deposits. This means that blowdown is done either for scum or for bottom blow down. Moreover, the reasons for boiler blowdown are:

To remove the precipitates formed as a result of chemical addition to the boiler water.

To remove solid particles, dirt, foam or oil molecules from the boiler water. This is mainly done by scum valve and the procedure is known as "scummimg".

To reduce the density of water by reducing the water level.

To remove excess water in case of emergency.

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