*
Turbine bearing compartment.

Scavenge
Scavenge pump elements in the lube cluster send the oil from the midframe and turbine bearing compartments back to the reservoir. Other scavenge pump elements send the scavenge oil from the APU generator through the scavenge filter and back to the gearbox reservoir.
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APU AND GENERATOR LUBRICATION SYSTEM -FUNCTIONAL DESCRIPTION

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APU FUEL SYSTEM -INTRODUCTION

Purpose
The APU fuel system supplies pressurized and metered fuel to the APU combustion chamber. It also supplies pressurized fuel to actuators for the inlet guide vanes and the surge control valve.
Interfaces
Fuel system ac boost pumps or the APU dc fuel pump in the wing supply fuel from the airplane fuel system for APU operation.
The APU controller (APUC) controls the APU fuel system.
Components
Most fuel system components are on a common fuel cluster on the forward side of the APU gearbox.
Other fuel system components are the fuel manifolds and the fuel nozzles.
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777 AIRCRAFT MAINTENANCE MANUAL

APU FUEL SYSTEM -INTRODUCTION

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APU FUEL SYSTEM -FUEL CLUSTER -INTRODUCTION

General
The fuel cluster contains the APU fuel control and other fuel system components. The fuel cluster is attached to the lube cluster which turns the fuel system pumps.
Training Information Point
The entire fuel cluster is an LRU. These components are also LRUs:
*
Fuel filter

*
Fuel filter differential pressure switch

*
Fuel temperature sensor.

These fuel cluster components are not LRUs:
*
Fuel shutoff solenoid

*
Fuel metering valve

*
Fuel distribution manifold.

The fuel cluster also contains the fuel pumps and some internal pressure regulating valves.
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APU FUEL SYSTEM -FUEL CLUSTER -INTRODUCTION

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APU FUEL SYSTEM -FUEL CLUSTER -FUNCTIONAL DESCRIPTION

General
Fuel for the APU fuel cluster comes from the airplane fuel system. The APU fuel cluster supplies pressurized and metered fuel for combustion and to operate the IGV and SCV actuators.
The fuel cluster includes these components:
*
Fuel boost and gear pumps

*
Fuel filter

*
Fuel differential pressure switch

*
Gear pump check valve

*
Pressure relief valve

*
Fuel temperature sensor

*
Mode select valve

*
Pressure regulator

*
Bypass valve

*
Fuel metering valve

*
Pressurizing shutoff valve

*
Fuel shutoff solenoid valve

*
Shutoff relief valve

*
Shutoff restrictor

*
Flow divider.

Fuel Boost and Gear Pumps
A shaft from the lube cluster turns the fuel boost and gear pumps. The first stage boost pump pressurizes fuel for the gear pump. The second stage gear pump sends high pressure fuel for use in the fuel cluster. The fuel line from the boost pump to the gear pump is a return line in the fuel cluster.
Fuel Filter and Differential Pressure Switch

There is a 35 micron filter between the boost pump and the gear pump. The fuel filter has a differential pressure switch that sends a signal to the APUC when the pressure difference across the filter is 11 psi or more.
Gear Pump Check Valve
The gear pump check valve permits flow from the filter to the gear pump. The check valve prevents pressurized fuel to flow back to the filter during reverse windmill operation of the APU.
Pressure Relief Valve
The spring-loaded pressure relief valve keeps the maximum pressure in the fuel system to 1050 psi more than the boost pump pressure.
Fuel Temperature Sensor
The fuel temperature sensor supplies the fuel temperature signal to the APUC. The APUC uses this signal to correct for fuel viscosity changes during APU start and operation.
Mode Select Valve
The mode select valve has a wash filter to supply clean fuel to dirt-sensitive components. The mode select valve sends high pressure fuel to the pressure regulator, the bypass valve, and the fuel shutoff solenoid valve.
　
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