Question 38 MODE02 - Assistant MODU Engineer

The Correct Answer is B **Explanation for Option B (Superheated Steam):** Erosion in steam piping and machinery is primarily caused by two mechanisms: velocity-induced wear and, more significantly, **impingement by liquid water droplets**. When steam contains liquid water (as in wet steam or, to a lesser extent, saturated steam), these high-velocity droplets act like tiny hammers, rapidly eroding metal surfaces, especially at points of high velocity change (like valve seats, turbine blades, and bends in piping). **Superheated steam** is steam that has been heated above its saturation temperature for the given pressure. By definition, superheated steam contains **no liquid water droplets**. Because the primary erosive agent (liquid water) is eliminated, the wear and erosion on piping and machinery are dramatically minimized. Furthermore, superheated steam has a higher enthalpy (energy content), which improves efficiency in machinery like turbines, while keeping the physical surfaces dry. **Explanation of Incorrect Options:** * **A) Wet steam:** This option is the most incorrect. Wet steam is steam that contains a significant percentage of liquid water droplets (high moisture content). As explained above, these high-velocity water droplets are the primary cause of rapid erosion, making wet steam highly destructive to piping and machinery. * **C) Saturated steam:** Saturated steam is steam at the temperature corresponding exactly to its pressure. While better than wet steam, saturated steam is unstable; any slight heat loss immediately causes condensation, forming water droplets that lead to erosion. Saturated steam systems are difficult to manage without condensation, making them prone to water-related damage. * **D) Desuperheated vapor:** Desuperheated vapor is steam whose temperature has been reduced, often by injecting water, typically resulting in saturated or slightly wet steam. Because the process involves introducing or resulting in liquid water, it presents the same erosion risks as saturated or wet steam.