Question 40 MODE02 - Assistant MODU Engineer

The Correct Answer is C ### Explanation of Why Option C is Correct: In an automatically fired auxiliary boiler, the primary purpose of the control system is to maintain the steam pressure within predefined limits. The **high limit pressure control** (often simply called the operating limit control or high limit control) is set to the desired maximum operating pressure. When the boiler reaches this desired steam pressure, the pressure switch associated with the high limit control opens its contacts. Opening these contacts **secures (cuts off) power to the entire oil firing system** (including the burner motor, fuel pump, and ignition transformer), causing the burner to shut down immediately. This action prevents the boiler pressure from exceeding the safe or desired limit. ### Explanation of Why Other Options are Incorrect: * **A) A temperature sensing device opens the circuit breaker in the burner motor.** * **Incorrect.** Auxiliary boilers are primarily controlled by **pressure**, not temperature, when modulating the firing rate or shutting down based on setpoint attainment. Furthermore, a safety device operating on excessive temperature (like a high-limit aquastat) would trip the boiler, but the initial shutdown when reaching the *desired* operating pressure is handled by the pressure control switch, not a temperature sensor. The main circuit breaker is typically opened manually or by a major fault, not by a normal operating control action. * **B) The stack relay actuates the low limit control which breaks the ignition circuit.** * **Incorrect.** The stack relay (or flame safeguard control) is a safety device that monitors the flame status. It is involved in starting and stopping the burner, but it does not actuate the low limit control. The low limit control is responsible for ensuring minimum safe operation (e.g., minimum water level or minimum pressure for restarting). When the *desired* steam pressure is obtained, the **high limit control** stops the burner, not the low limit control, and the stack relay is responding to the flame loss that follows the shutdown, not initiating the pressure-based shutdown. * **D) The stack relay secures power to the high voltage side of the ignition transformer.** * **Incorrect.** As explained above, the stack relay monitors the flame. When the desired pressure is reached, the **high limit pressure control** secures power to the main burner components. The stack relay will eventually de-energize the ignition transformer, but this action is consequential to the main shutdown command initiated by the pressure control, and the shutdown command affects the entire firing system, not just the ignition transformer high voltage side. Furthermore, the stack relay is primarily concerned with proving the flame, not controlling the firing rate based on set pressure.