When does the APU surge valve extract air from the APU?

The surge valve in an Auxiliary Power Unit (APU) is designed to manage airflow and maintain optimal performance by regulating the air pressure. The surge valve is activated in response to the conditions of the load control valve. Specifically, when the load control valve is not open, the system needs to ensure that there is sufficient air available to prevent potential surges or fluctuations in pressure.

When the load control valve is closed or not fully open, this indicates that there is either a need for less air or that the demand has decreased. In this scenario, the surge valve must extract air from the APU to balance the pressure and ensure stability in the system. Extracting air under these conditions helps to maintain the overall efficiency and reliability of the APU during operation.

Therefore, the mechanism of the surge valve functioning when the load control valve is not open is crucial for maintaining proper air pressure and preventing any adverse effects that might arise from imbalances in the system. Understanding this relationship between the load control valve and the surge valve is fundamental in managing the APU effectively.