Question 44 MODE02 - Assistant MODU Engineer

The Correct Answer is C **Explanation for Option C (Correct Answer):** Scale formation in a boiler occurs when dissolved minerals precipitate out of the boiler water and deposit onto the heat transfer surfaces (like tubes and drums). The primary substances responsible for this precipitation are the salts of calcium and magnesium, collectively known as hardness minerals. Calcium sulfate ($\text{CaSO}_4$) is particularly notorious because its solubility decreases significantly as the temperature and pressure within the boiler increase. When boiler water evaporates to produce steam, the dissolved solids, including $\text{CaSO}_4$, become increasingly concentrated. Once the concentration exceeds the saturation limit, the $\text{CaSO}_4$ precipitates directly onto the hot surfaces, forming a hard, dense, and highly insulating scale. Therefore, high concentrations of calcium sulfate (or other hardness minerals) in the boiler water are the direct and most common cause of mineral scale formation. **Explanation of Incorrect Options:** * **A) Improper treatment of the feedwater with calcium sulfate:** Calcium sulfate is a contaminant that must be removed or stabilized, not a chemical used for *treatment*. While improper treatment can lead to scale, the immediate cause is the *presence and concentration* of the scaling substance ($\text{CaSO}_4$) itself within the boiler water, not the treatment method. * **B) Fuel oil in the feedwater:** Fuel oil contamination (oil carryover) primarily causes foaming, surging, or localized overheating and failure (due to deposit formation, which traps heat). While oil deposits can sometimes mix with scale, they cause sludging or deposits of an organic nature, not the common, hard, mineral scale primarily associated with boiler hardness, which is caused by calcium sulfate. * **D) Excessive feedwater alkalinity:** Excessive alkalinity (high pH) typically contributes to problems like caustic corrosion (embrittlement) or foaming/carryover. While alkalinity control is crucial, the primary mineral scale (calcium sulfate or silicate scale) is caused by the precipitation of hardness minerals, which is independent of excessive alkalinity.