Question 58 QMED05 - Machinist-Pump Technician

The Correct Answer is D. **Why option D is correct:** Blowback (or furnace explosion) occurs when unburned fuel accumulates in the furnace and ignites violently. To prevent this when lighting off an idle boiler: 1. **"The boiler fuel oil supply header temperature must be maintained at the temperature necessary to obtain proper atomization of the fuel"**: Proper atomization creates a fine mist, allowing the fuel to mix effectively with air and ignite instantly and smoothly. If the oil is too cold, atomization is poor, leading to large droplets that fall to the floor or evaporate slowly, potentially accumulating explosive vapors. 2. **"The furnace floor should be free of oil"**: Any standing oil on the furnace floor will vaporize when heat is applied, creating highly explosive gas concentrations which are a primary cause of blowback upon ignition. 3. **"And the furnace should be purged"**: Purging (running forced draft fans to introduce fresh air and remove exhaust gases) is the crucial step to ensure all potentially explosive or combustible gas mixtures (unburned fuel vapors, residual flue gas) are cleared from the furnace and exhaust passages before introducing the ignition source and new fuel. These three conditions represent standard safe operating procedures for boiler light-off. **Why the other options are incorrect:** * **A) Incorrect:** This option correctly identifies the need for proper atomization temperature and a clean floor, but the final condition, "there should be sufficient combustible gases in the furnace," is fundamentally incorrect and dangerous. The entire purpose of the light-off procedure is to *remove* all combustible gases (via purging) before introducing the controlled combustion process. Sufficient combustible gases guarantee a blowback/explosion upon ignition. * **B) Incorrect:** While the oil temperature should be above the flash point, maintaining it *at* the temperature necessary for proper atomization (often significantly higher than the flash point, especially for heavy fuel oil) is the requirement for safe and efficient combustion. More critically, like Option A, this option states "there should be sufficient combustible gases in the furnace," which is a hazardous condition that promotes blowback. * **C) Incorrect:** Maintaining the temperature **below the pour point** of the fuel would mean the fuel is too viscous, possibly solidified, and certainly too cold for atomization. This would prevent flow and proper firing, leading to immediate shutdown or accumulation of unburned, poorly atomized fuel—a risk factor for explosion. Furthermore, while it correctly calls for purging and a clean floor, the temperature requirement makes the procedure impossible or unsafe.