Question 21 QMED04 - Boiler Technician-Watertender

The Correct Answer is D. **Explanation for Option D (Correct Answer):** The purpose of the main steam stop bypass valve is to facilitate the controlled warming and pressurization of the steam piping system between the boiler's main steam stop valve and the turbine or engine throttle valve (or main steam header). When starting up a boiler or bringing a cold steam line online, the large, main steam stop valve is kept closed. Opening the small bypass valve allows a limited flow of steam into the cold piping. This **gradually increases the pressure and temperature** of the main steam piping, preventing thermal shock, excessive stress, and water hammer that would occur if the cold line were suddenly exposed to full boiler pressure and temperature by opening the large main stop valve immediately. **Explanation of Incorrect Options:** * **A) isolate the main steam stop for repairs while steaming:** This is incorrect. The main steam stop valve is the final valve on the boiler and its purpose is to isolate the boiler from the steam system. It cannot be repaired while the boiler is steaming unless redundant, downstream isolation valves exist, which is a separate function. The bypass valve does not aid in isolating the main stop valve itself. * **B) supply auxiliary steam when the main steam stop is closed:** This is incorrect. The bypass valve only supplies steam to the main steam line leading to the propulsion unit (or main header). Auxiliary steam systems are typically supplied through separate, smaller steam lines and dedicated auxiliary stop valves, drawing steam either directly from the boiler drum or from a dedicated auxiliary steam header. * **C) cross-connect two steaming boilers:** This is incorrect. Connecting two steaming boilers (cross-connecting) is done via the main steam header system, not through the small bypass line around a single boiler's main steam stop valve. The main steam stop bypass valve's function is localized to the piping immediately downstream of its corresponding main stop valve.