Question 41 OSE02 - Assistant Engineer - OSV

The Correct Answer is C **Explanation for Option C (Correct):** Option C accurately describes the typical characteristics of the jacket water cooling system (freshwater circuit) found on marine diesel engines, particularly those used in offshore supply vessels. 1. **Vented System:** Marine engine cooling systems often employ a "vented" or open expansion tank (header tank) positioned above the engine. This tank serves multiple purposes: providing a reserve volume, accommodating thermal expansion, and allowing air and steam to vent to the atmosphere, thereby maintaining the system pressure close to atmospheric (non-pressurized, or "vented"). 2. **Stationary/Marine Type 3-Way Thermostatic Control Valve:** Temperature regulation in large marine diesel engine cooling systems is achieved using a **3-way diverting or mixing valve**. This valve continuously modulates the flow of freshwater, directing a portion of the hot water either back into the engine (bypassing the cooler) or through the heat exchanger (cooler). The 3-way design is standard for precise temperature control in stationary and marine applications, unlike the simpler 2-way valves often found in automotive systems. **Explanation of Incorrect Options:** * **A) The freshwater circuit is a vented system using an automotive type 2-way thermostatic control valve for temperature control.** * *Incorrect because:* While the system is typically vented, marine/stationary systems use a 3-way valve for temperature control, not a 2-way automotive type. * **B) The freshwater circuit is a pressurized system using a stationary/marine type 3-way thermostatic control valve for temperature control.** * *Incorrect because:* While the 3-way valve is correct, the system is generally **vented** (open expansion tank) rather than operating under high pressure maintained by a pressure cap or regulating valve, which defines a "pressurized system." * **D) The freshwater circuit is a pressurized system using an automotive type 2-way thermostatic valve for temperature control.** * *Incorrect because:* The system is typically vented, and a 3-way valve (not a 2-way valve) is used for robust temperature control.