Question 27 QMED02 - Electrician-Refrigerating Engineer

The Correct Answer is C **Explanation for Option C (Correct):** Option C states: "The fuel should be secured to the boiler and the furnace purged of combustible vapors with the forced draft fan before another light off attempt is made." This is the standard, mandatory safety procedure following a failed light off attempt on a boiler. When a light off fails (meaning the burner fuel was admitted but ignition did not occur), unburned fuel is likely present within the furnace. This accumulated fuel vapor, if mixed with air, creates an explosive mixture. To prevent a furnace explosion upon the next introduction of an ignition source (or if an unexpected source ignites the vapor), two steps are critical: 1. **Secure the Fuel:** Stop all flow of fuel to the boiler immediately. 2. **Purge the Furnace:** Run the forced draft (FD) fan for a specified amount of time (a "purge cycle," usually 5 to 8 volume changes of the furnace and gas passages) to sweep all potentially combustible fuel vapors out of the furnace and uptakes. Only after a complete and proper purge cycle can the operator safely attempt a relight, ensuring the furnace is clear of explosive mixtures. **Explanation for Incorrect Options:** **A) The fuel should be secured to the boiler and the boiler allowed to remain idle for a few minutes before another light off attempt is made.** * While securing the fuel is correct, simply waiting ("remaining idle") does not remove the dangerous fuel-air mixture already present in the furnace. The accumulated combustible vapors must be actively purged out using the forced draft fan before any subsequent light off attempt is made. Waiting without purging poses a high risk of explosion. **B) Another light off attempt (with no limit) can be undertaken before any additional precautions need to be observed.** * This is extremely dangerous and fundamentally violates all boiler safety codes. Allowing unlimited light off attempts without purging the furnace guarantees the continuous accumulation of fuel vapors, leading almost certainly to a catastrophic furnace explosion. **D) Another light off attempt (just one) can be undertaken before any additional precautions need to be observed.** * Attempting even one additional light off without purging is highly unsafe. If the first attempt failed, there are unburned fuel vapors present. Igniting those vapors on the second attempt risks an explosion (a "re-flash" or furnace puff) because the required safety step (the mandatory purge) was skipped.