Question 62 AEL01 - Assistant Engineer - Limited

The Correct Answer is A **Explanation for Option A (Correct):** A photoelectric cell (often called a flame scanner or flame eye) in a boiler burner management system (BMS) is specifically designed to monitor the presence of the flame. Its primary safety function is to confirm the flame is lit. If the photoelectric cell stops sensing the intense light/radiation characteristic of a successful flame combustion (i.e., it senses a flame failure), it immediately signals the control system. This signal triggers the safety sequence, which includes opening (breaking) the main burner fuel valve circuit, thereby shutting off the fuel supply and preventing the accumulation of unburned fuel in the furnace, which could lead to a dangerous explosion. **Explanation for Other Options (Incorrect):** * **B) Detects a flame failure by monitoring radiant heat from glowing refractory:** While the photoelectric cell monitors radiant energy, it specifically monitors the visible or UV light spectrum emitted by the *flame itself*. Glowing refractory (the heat-resistant lining of the boiler walls) continues to glow even after the flame has gone out. Relying solely on refractory heat would lead to a dangerous delay in detecting the actual flame failure, defeating the purpose of the safety device. * **C) Must be bypassed at low firing rates:** Safety devices like flame scanners are critical at *all* firing rates. Bypassing them at any time would violate safety regulations and introduce a significant hazard. The BMS is designed to manage the flame signal (which may be weaker at low fire) without requiring the safety device to be disabled. * **D) Requires mechanical linkage to secure the burner fuel supply:** The photoelectric cell is an electronic sensing device. It communicates its status to the control system (BMS) electrically. The BMS then uses electrical signals (relays, solenoids) to actuate the fuel valves. There is no direct mechanical linkage between the light sensor and the physical fuel valve mechanism.