Question 20 QMED03 - Oiler

The Correct Answer is C ### Why Option C is Correct Option C states: "auxiliary stop valve fully open and the auxiliary stop-check valve used to regulate the amount of flow." 1. **Role of the Auxiliary System:** When the main feed valve malfunctions (as specified in the EMERGENCY conditions), the auxiliary feedwater system must be used. 2. **Role of the Auxiliary Stop Valve:** The primary purpose of the stop valve (whether main or auxiliary) is to provide positive, manual isolation. When feeding the boiler, this valve is typically left **fully open** to minimize pressure drop and ensure maximum flow capability. It is not designed for throttling or flow regulation. 3. **Role of the Auxiliary Stop-Check Valve:** This valve combines two functions: * **Check function:** Prevents backflow of hot water and steam from the boiler into the cooler feedwater line. * **Stop/Regulation function:** The valve spindle, when engaged, allows the operator to control the degree to which the valve opens, thereby **throttling the flow** and regulating the amount of feedwater entering the boiler. 4. **Conclusion:** Under manual, emergency control, the most reliable and correct procedure is to fully open the isolation (stop) valve and use the dedicated regulating mechanism (the stop-check valve's throttling capability) to manually control the flow required to maintain water level. ### Why Other Options Are Incorrect **A) auxiliary stop-check valve fully open and the auxiliary stop valve regulated by the feedwater regulator** * **Incorrect Regulation Method:** The auxiliary stop valve is designed for isolation, not throttling. Throttling a stop valve causes rapid wear (wire-drawing) and makes precise regulation difficult. * **Incorrect Automation:** The scenario implies an emergency manual control situation. While the feedwater regulator might exist, the operator is typically responsible for *manually* regulating the flow via the appropriate valve mechanism, not using the stop valve as the regulated component. **B) auxiliary stop and stop-check valves fully open and the feed pump speed used to regulate the amount of flow** * **Impractical Control:** Leaving both valves fully open results in a massive surge of water into the boiler, potentially causing carryover and thermal shock (a severe emergency). * **Unreliable Regulation:** While pump speed *can* influence flow, fine-tuning the required flow (which is often small in emergency conditions, especially if steaming is reduced) solely by pump speed is impractical and provides less immediate, precise control than throttling a valve directly at the boiler inlet. Manual valve regulation is the standard method for precise control in this situation. **D) auxiliary stop-check valve fully open and the auxiliary stop valve used to regulate the amount of flow** * **Incorrect Valve Function:** Similar to Option A, this uses the auxiliary stop valve for throttling. The stop valve is designed for on/off isolation and should not be used for flow regulation, while the stop-check valve (which is fully open here) is the component specifically designed for both backflow prevention and manual regulation.