Question 17 QMED05 - Machinist-Pump Technician

The Correct Answer is B **Why option B ("The tank cooling system has malfunctioned.") is correct:** A low-pressure $\text{CO}_2$ fixed fire extinguishing system stores carbon dioxide in a refrigerated tank, typically maintained at around $-18^\circ\text{C}$ ($0^\circ\text{F}$), which corresponds to a pressure of approximately $20-22$ bar ($300-325$ psi). The cooling system (refrigeration unit) is essential for maintaining this low temperature and, consequently, the desired low storage pressure. If the tank cooling system malfunctions (stops working, loses power, or breaks down), the stored $\text{CO}_2$ will absorb ambient heat through the tank insulation. As the temperature of the $\text{CO}_2$ rises, its vapor pressure dramatically increases (governed by the phase diagram of $\text{CO}_2$). This rapid increase in pressure will exceed the normal operating range, activating the high-pressure alarm (and eventually, the pressure relief valve if the temperature continues to rise). Therefore, a cooling system malfunction is the most probable cause of a high-pressure alarm. **Why the other options are incorrect:** * **A) The pilot cylinder discharge valve is leaking:** The pilot cylinders are used to actuate the main tank discharge valves (or remote control valves) when the system is activated. They typically contain high-pressure nitrogen or $\text{CO}_2$ but are physically separate from the main storage tank pressure vessel. A leak in the pilot system might cause an unwanted system discharge, but it would not cause the pressure within the main low-pressure storage tank to rise. * **C) An air leak has developed in the tank:** An air leak (a leak *out* of the tank) would cause the low-pressure $\text{CO}_2$ to escape, leading to a *drop* in pressure (a low-pressure alarm), not a high-pressure alarm. If the leak were an air leak *into* the tank (which is unlikely as the tank is pressurized), the presence of non-condensable gas would raise the partial pressure slightly, but this is less probable and less dramatic than a temperature rise caused by a cooling failure. * **D) An excessive amount of insulation has been installed on the tank and piping:** Proper insulation is designed to *slow* heat transfer into the tank, helping the cooling system maintain the low temperature and pressure. Installing *more* insulation than necessary would further reduce heat gain, making it easier for the cooling system to operate and potentially causing the pressure to drop slightly (or maintain the current pressure), but it absolutely would not cause the pressure to increase to the point of activating a high-pressure alarm.