Question 195Steam Plants - 1st Asst/Chief

The correct answer is C) calcium sulfate. In a water-tube boiler, waterside scale formation is caused by the precipitation of dissolved minerals, primarily calcium sulfate (CaSO4), also known as gypsum. As the boiler water is heated, the solubility of calcium sulfate decreases, leading to its deposition on the boiler's internal surfaces, forming a hard, insulating scale. The other options are incorrect because: A) Sodium hydroxide (NaOH) is a caustic substance used to control the pH in boiler water, not a cause of scale formation. B) Sodium phosphate is a water treatment chemical used to prevent scale, not a cause of it. D) Magnesium phosphate is also a water treatment chemical, not a contributor to scale formation.