Question 42 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is D **Explanation for Option D (completely drain the unit):** When securing any evaporator, including a flash-type evaporator, for an extended period (often referred to as layup), the primary goal is to prevent corrosion and damage. Evaporators typically use materials susceptible to corrosion when exposed to stagnant water, especially if that water contains dissolved salts, oxygen, or biological contaminants. By completely draining the unit, you remove the medium (water) necessary for electrochemical corrosion and scaling reactions to occur, thereby minimizing long-term deterioration. **(Note: While complete drainage is the standard first step, best practice for very long layups often involves additional steps like drying and/or nitrogen blanketing to completely exclude moisture and oxygen, but among the choices provided, completely draining is the essential and correct action.)** **Why the other options are incorrect:** * **A) tightly seal the unit to exclude air:** While excluding air (oxygen) is good practice, simply sealing a unit that still contains residual water or moisture is detrimental. Stagnant water, even with limited external oxygen ingress, can cause accelerated pitting and localized corrosion, especially under sealed, anaerobic conditions where microbial-influenced corrosion (MIC) can thrive. Draining is necessary before sealing. * **B) fill the unit with descaling compound:** Descaling compounds are typically strong acids or chemicals used for cleaning and removing existing scale. They are highly corrosive and are never used as a long-term storage or layup solution. Leaving them in the unit would severely damage the metal components. * **C) fill the unit with salt water:** Evaporators are designed to remove salt from water, but their internal surfaces are highly susceptible to corrosion when exposed to high-salinity water over extended periods. Salt water is extremely corrosive, and this action would ensure rapid and severe damage to the unit.