Question 10

The Correct Answer is A. **Explanation for Option A (Correct Answer):** A **cross-compound** steam turbine design is characterized by having multiple turbine casings or shafts arranged side-by-side that drive separate generators (or a common gearbox). In this configuration, the steam flow is split sequentially across these separate units. Typically, the steam first enters a **high-pressure (HP) turbine**, exhausts into a **crossover pipe** (or receiver), and then enters one or more **low-pressure (LP) turbines** where it completes its expansion. Therefore, the essential components defining the steam path of a cross-compound unit are the HP turbine, the crossover piping, and the LP turbine(s). **Explanation of Incorrect Options:** * **B) consists of one Curtis stage and reaction blading:** This description refers to a common staging arrangement within a single turbine cylinder (often the HP section of a large utility turbine), where a two-row velocity-compounded Curtis stage is used for the first stage, followed by pressure-compounded reaction stages. This describes internal blading design, not the overall mechanical arrangement of a cross-compound unit. * **C) consists of reaction stages and a dummy piston:** Reaction staging refers to the blade type (generating thrust and utilizing both pressure and velocity drop), while a dummy piston is a balance mechanism specifically required in pure reaction turbines (like the original Parsons design) to counteract the large axial thrust generated by the blading. While cross-compound units may use reaction stages, this option describes specific internal components and a balancing feature, not the fundamental multi-casing design of a cross-compound unit. * **D) is made up of a varied assortment of impulse and reaction staging:** This description refers to the general thermodynamic design of almost all modern large utility turbines (which are typically impulse-reaction or reaction-impulse combinations) and does not specifically define the structural arrangement distinguishing a cross-compound unit from a tandem-compound unit (which also uses varied staging).