The reasons for blowing down the boiler ( both scum and bottom blow down) are to:

1. Reduce the density of the boiler water by reducing the water level and refilling it with distilled water.
2. Remove the sediments from boiler water.
3. Remove oil, grease etc. from top surface of boiler water.
4. Empty the boiler prior to entry.
5. Reduce the water level in an emergency.

The scum valve is used to remove floating, foam and oil from the surface of the water.

Open valves in order (1), (2), (3), and close in reverse. The blow down valve adjacent to the boiler (2) should be fully open to prevent cutting of the valve seating faces. The rate of blow down is controlled by the second blow down valve (3). Never leave the boiler unattended when blowing down and ensure all valves are fully closed after blowing down the boiler. A hot drain pipe indicates a leaking blow down valve. Never open the blow down valves before ship side valve. It result in unwanted pressure build up in the blow down line.

Reference

“Operation and Maintenance of Machinery in Motor Ships” by N.E. Chell