The _____ modulates to regulate the bleed air pressure to about 45 psi?

The Pressure Regulating Shutoff Valve (PRSOV) is responsible for modulating the bleed air pressure to maintain it at a specific level, typically around 45 psi. This function is critical for ensuring that the pressurized air provided to the environmental control system and other components meets the necessary operational requirements without exceeding predefined limits. By effectively regulating the pressure, the PRSOV helps to prevent damage to the system and ensures optimal performance during different flight conditions.

In contrast, the other options have distinct roles within the bleed air system. The Bleed Air Control Valve mainly manages the flow of bleed air but does not specifically regulate the pressure to a set value like the PRSOV. The Flow Control Valve manages the rate of bleed air flow but is not focused on pressure regulation, while the Pressure Relief Valve serves as a safety device to prevent overpressure conditions in the system by venting excess pressure rather than modulating to maintain a specific pressure level.