Question 35 QMED01 - Junior Engineer

The Correct Answer is C ### Explanation for C (Superheated Steam) being Correct: Steam supplied to the main propulsion turbines (such as those used in large naval vessels, LNG carriers, or older steam-powered commercial vessels) is **superheated steam**. Superheating the steam—heating it above its saturation temperature at a constant pressure—provides several critical advantages: 1. **Increased Efficiency:** Superheated steam contains more thermal energy (enthalpy) per unit mass than saturated steam, allowing the turbines to extract more useful work, significantly improving the overall thermal efficiency of the power plant. 2. **Prevents Erosion:** When saturated or wet steam expands through the turbine stages, the slight cooling causes water droplets to form. These high-velocity water droplets cause severe erosion on the turbine blades, leading to damage, reduced efficiency, and costly maintenance. By using superheated steam, the steam remains entirely gaseous (dry) throughout most, if not all, of the expansion process, protecting the blades from physical damage. ### Explanation for why the Other Options are Incorrect: **A) Desuperheated steam:** Desuperheated steam is steam whose temperature has been reduced from its highly superheated state, usually by injecting water (attemperation). While desuperheated steam is commonly used for auxiliary services (like heating fuel tanks, operating steam pumps, or supplying saturated headers), it is *not* the primary steam supply for the main propulsion turbines, as using steam with lower energy content would sacrifice efficiency and still increase the risk of condensation later in the turbine stages compared to fully superheated steam. **B) Wet steam:** Wet steam is steam that contains suspended water droplets (i.e., its dryness fraction is less than 1). Using wet steam in high-speed main turbines would result in immediate and catastrophic erosion damage to the turbine blading and drastically reduce the plant's efficiency. Modern propulsion plants strictly avoid supplying wet steam to the main turbines. **D) Saturated steam:** Saturated steam is steam at the temperature corresponding to its pressure (dryness fraction of 1). While saturated steam is used extensively for auxiliary services, it is unsuitable for main propulsion turbines. As soon as saturated steam begins to expand and perform work in the initial turbine stages, its temperature drops, causing immediate condensation (it becomes wet steam). This resulting wetness leads to severe blade erosion and poor thermal efficiency, making it uneconomical and physically damaging for main propulsion.