Question 70 QMED01 - Junior Engineer

The Correct Answer is D. **Why Option D is Correct:** The purpose of the main steam stop bypass valve (often a smaller globe valve installed in parallel with the much larger main steam stop valve) is to allow a controlled, gradual flow of steam into the main steam piping system (the header or manifold) between the boiler and the prime mover (like a turbine). This process, known as warming up, is critical for safety and system integrity. By opening the small bypass valve first, the pressure and temperature of the cold piping are slowly raised, preventing thermal shock, rapid expansion, and resulting stresses that could damage the piping, flanges, or valve bodies. Only once the line is fully warmed and pressurized to near boiler pressure is the main steam stop valve fully opened. **Why the Other Options are Incorrect:** * **A) cross-connect two steaming boilers:** While steam systems use cross-connect valves (often called equalizing valves) to link steam lines from different boilers, this is a separate function. The main steam stop bypass valve is specifically associated with the boiler outlet/stop valve and is used for line warming, not for connecting two separate boiler headers. * **B) supply auxiliary steam when the main steam stop is closed:** The main steam stop valve controls the primary steam flow for power generation. Auxiliary steam (steam used for ship services, heating, or auxiliary machinery) is typically drawn from a dedicated auxiliary steam line or manifold, which often receives steam either directly from the boiler drum via a separate auxiliary steam stop valve, or via a pressure reducing station from the main steam line, not through the main steam stop bypass valve. * **C) isolate the main steam stop for repairs while steaming:** The main steam stop valve itself is the primary isolation point. To isolate it for repair, the boiler must generally be taken offline and depressurized (cold-isolated). The small bypass valve cannot isolate the main steam line; it is physically part of the main steam stop valve assembly and is opened *before* the main stop, not used as a backup isolation device.