Question 27 QMED03 - Oiler

The Correct Answer is B **Explanation for Option B (fire-tubes and stay-tubes):** In a fire-tube auxiliary boiler, the tube sheets (the plates at the front and back of the boiler that hold the tubes) are subjected to immense pressure differential. The internal pressure of the steam/water mixture attempts to bow these plates outward. The primary structural components that resist this force and keep the tube sheets connected and held parallel are the tubes themselves. The tubes that carry the hot gases (fire-tubes) inherently act as stays. Additionally, some tubes are specifically designated as thicker, threaded, and expanded **stay-tubes** (or simply "stays") to provide extra rigidity and support, acting as tension members between the tube sheets. Therefore, the connection and structural support between the tube sheets are normally provided by the matrix of fire-tubes and specialized stay-tubes. **Explanation for Incorrect Options:** * **A) girder stays:** Girder stays are typically used to support flat surfaces like the crown sheet (top plate) of a combustion chamber, spanning between the crown sheet and the outer shell. They are not the primary means of connecting the main front and rear tube sheets in the boiler barrel section. * **C) external boiler plating:** The external boiler plating (the shell) is under tension due to internal pressure and primarily contains the steam and water. While the tube sheets are structurally welded or flanged to the shell, the external plating does not directly connect the *two tube sheets together* across the length of the boiler; that connection is achieved by the tubes running between them. * **D) separate crown sheets:** Crown sheets are the flat or curved plates forming the top boundary of a combustion chamber, usually associated with Scotch or locomotive-type boilers. They are components that *need* staying themselves, but they are not the mechanism that connects the front and rear tube sheets across the entire length of the boiler barrel.