Question 40 OSE01 - Chief Engineer - OSV

The Correct Answer is C **Why Option C (Scale control and suppression) is correct:** Organic phosphates (often referred to as phosphonates) are a widely used class of chemical treatment in industrial cooling water systems, including those used for offshore vessel diesel-generators. Their primary function in this context is as *scale inhibitors*. They perform this function through several mechanisms: 1. **Threshold Inhibition:** They prevent scale-forming mineral ions (like calcium carbonate, magnesium silicate, etc.) from precipitating out of the water even when the concentration exceeds the saturation point. 2. **Crystal Modification:** They distort the shape of any mineral crystals that do form, preventing them from adhering tightly to the heat transfer surfaces (like the engine jacket water passages), making them easy to flush away. 3. **Dispersion:** They help keep microscopic particles suspended in the water, preventing them from agglomerating and settling as sludge or scale. Scaling on heat transfer surfaces severely reduces the efficiency of the cooling system and can lead to localized overheating and engine damage, making scale control a critical priority for cooling system maintenance. **Why the other options are incorrect:** * **A) Freezing point depression:** Freezing point depression is primarily achieved by adding large amounts of glycol (ethylene or propylene glycol) to the cooling water. Organic phosphates are used in very low concentrations (PPM range) and do not significantly affect the freezing point. * **B) Corrosion inhibition:** While some phosphonates can offer secondary corrosion protection by forming protective films (often used in conjunction with molybdates, nitrites, or silicates), specialized corrosion inhibitors (like those based on nitrite/molybdate chemistries) are typically the primary components dedicated to corrosion control in diesel cooling systems. Organic phosphates' *primary* role is scale control. * **D) Biological growth inhibition:** Biological growth (algae, bacteria, fungi) is controlled by biocides (like oxidizing chlorine/bromine compounds or non-oxidizing organic nitrogen/sulfur compounds). Organic phosphates have negligible or no biocide properties and are not used for microbiological control.