Crust or scale formation in the surfaces of pressure parts steam boiler such as water wall tubes, header, water drum and economizer are the result of minerals which are collected to form crust or scale. These minerals are hardness ions like magnesium (Mg) and calcium (Ca). Crust or scale formation can be also caused by water evaporation and heating boiler. The types of crust or scale are common in the steam boiler is calcium sulfate, compounds of carbonate and silicate. Silica is precipitated along with magnesium and calcium so as to make hard crust or scale. These substances can form a hard and dense crust or scale. If this condition is long left without maintenance, the crust or scale will be difficult to remove.
Crust or scale that surrounds the steam boiler surfaces affects the heat transfer surfaces. There two main consequences because of slag formation. They are heat transfer reduction from hot combustion in the furnace to water and decrease efficiency of steam boiler.
To reduce the occurrence of scaling or crust in the steam boiler is perform prevention action as follows:
- Reduce the amount of minerals with softener equipment
- Conducting regular blowdown
- Providing anti-crust or scale chemicals
Suspended and dissolved substance found in natural water can be eliminated or reduced on the pretreatment which proved economical. Crust or scale existing countermeasures can be done by:
- Mechanical cleaning with scrubs, chisel, brush, etc.
- Off-line cleaning or acid cleaning which dissolves the old crusts or scales with special acid but steam boiler must be shut down.
- On-line cleaning which softening old crust or scale with chemical injection during operation of steam boiler