Question 43 MODE02 - Assistant MODU Engineer

The Correct Answer is C **Why option C ("drain off condensation") is correct:** Casing drains on the gas passage side of a waste heat boiler are primarily installed to allow for the removal of liquids that may accumulate within the boiler casing. The most common source of such liquid, especially during startup, shutdown, or when the boiler is non-operational, is **condensation**. Flue gases contain water vapor, and when the metal surfaces of the boiler casing or heat transfer tubes cool below the dew point (particularly when using fuels containing sulfur, which lowers the acid dew point), this water vapor condenses. If this condensate is not drained, it can lead to corrosion (especially acid corrosion) and damage to the boiler structure and insulation. **Why the other options are incorrect:** * **A) prevent an accumulation of boiler water entering gas passages as a result of a pinhole tube leak:** While a tube leak (pinhole or larger) would introduce water/steam into the gas passage, casing drains are typically too small and are not the primary means of dealing with the large volume of liquid associated with a major tube failure. Furthermore, the goal is often to prevent accumulation of normal condensation, not just leak detection/mitigation. Tube leaks are usually detected by monitoring feedwater loss, pressure drops, or gas temperature changes. * **B) sample gases for testing:** Gas sampling is performed using specialized probes and ports located within the gas stream, not through low-point casing drains. Casing drains are designed for liquid removal. * **D) release excess pressure:** Boilers are designed to handle internal pressure, and the gas passage side is usually near atmospheric pressure or slightly negative (draft). Excess pressure on the gas side would be relieved through safety devices like explosion doors or pressure relief dampers, not small casing drains, which are intended for liquid management.