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Power Transfer Unit

 Power Transfer Unit

the Power Transfer Unit(PTU) 
The Power transfer unit (PTU )is a safety system to ensure hydraulic pressure is available, especially at takeoff, even if one or two hydraulic circuits have failed and works with the combination of a motor/pump

The PTU can restore pressure in a deficient circuit without actually transferring fluid from the working circuit to the deficient one. This prevents throwing hydraulic fluid from the working circuit overboard if the deficient circuit is leaking.
The PTU is always activated but is actually triggered based on a pressure differential where PTU is connected to two independent hydraulic systems: The first to drive a hydraulic motor (or turbine) mechanically connected to a pump. The second hydraulic system is pressurized by the pump.
PTU can be reversible (each hydraulic system can drive the motor or be pressurized by the pump) or not (the same system always drives the motor).
On Airbus A320 as example series, the PTU is actually the second source of pressure for the yellow and green hydraulic systems, after the EDP. It's always ON, ready for the pump activation if there is a pressure drop on the yellow or green hydraulic system and the differential pressure is more than 500 psi.

CREDIT BY Andrew naidu
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