Scale forms where water is in direct contact with heated surfaces. In steam boilers, that location is below the water line, where dissolved minerals (calcium, magnesium, silica) precipitate and adhere to heat-transfer surfaces as scale. This reduces efficiency and can cause overheating of boiler tubes.

B. below the water line (Correct): Primary area where water meets heat-transfer surfaces; mineral deposits form here.

A. huddling chamber: This is part of a safety relief valve, not a surface for scale buildup.

C. weld above the water line: There is no continuous water exposure above the water line, so scale does not accumulate there.

D. blowdown valve: This is used to remove sludge/sediment, not a typical location for scale accumulation.