Which device is used to shut off hydraulic power to heavy users if system pressure drops below a predetermined value?

The priority valve is the device that is specifically designed to manage hydraulic power distribution within the aircraft's hydraulic systems. When system pressure falls below a predetermined threshold, the priority valve automatically activates to restrict hydraulic flow to less essential systems and ensures that critical users, like flight control surfaces and landing gear, receive the necessary hydraulic power to operate effectively. This prioritization helps maintain essential functions of the aircraft, ensuring safety and operability even in situations where hydraulic pressure may be compromised.

In contrast, the other options serve different functions: the ENG FIRE switch is used to interrupt fuel flow to the engine in the event of a fire, the LAF accumulator primarily stores hydraulic fluid under pressure, which can be used during certain operations, and the PTU (Power Transfer Unit) assists in transferring hydraulic pressure from one system to another, rather than managing priority based on system pressure levels.