Location.
The APU, load compressor, and bleed air valve are in the APU compartment.
The ECU is on the E6 rack in the aft cargo compartment.
The P5-10 panel control module is in the flight compartment.
The APU check valve is in the keel beam, between the forward air conditio- ning bays.
Operational Displays.
The APU bleed air valve has a visual position indicator on the valve assembly.
These indications show valve operation : - P5-10 panel duct pressure gage,
- P5-10 panel DUAL BLEED indication light.
The apu bleed air system supplies compressed air to the pneumatic manifold:
- On the ground,
- In the air (up to 17,000 feet).
The APU can supply power to these systems : - Engine start systems,
- ECS systems.
General Description.
The APU load compressor is the source of the APU bleed air. The load compressor inlet guide vanes and surge control systems control the pressure and flow of the APU bleed air.
The APU bleed air valve (BAV) isolates the APU load compressor from the pneumatic manifold.
The M1709 APU electronic control unit (ECU) controls the APU bleed air valve.
The ECU receives bleed control signals from the P5-10 air conditioning control panel.
The APU bleed air duct connects the APU bleed air valve to the pneumatic manifold.
An APU check valve is in the APU duct. This valve protects the APU load com- pressor from engine bleed air flow.
B737NG/36/301 Pneumatic System
Training manual
APU BLEED AIR VALVE (ROTATED FOR CLARITY)
APU CHECK VALVE
APU BLEED SYSTEM
6.1. aPu bleed air valve.
This schematic shows the relationship between the APU bleed air valve and its interfaces :
- APU electronic control unit (ECU), - APU load compressor,
- APU pneumatic duct,
- DUAL BLEED indication circuit.
General Description.
When the P5-10 module APU BLEED switch is in the ON position, it supplies a ground discrete to the APU ECU. A control signal from the APU ECU energi- zes the APU bleed air valve solenoid. The energized solenoid sends pressure from a tap upstream of the valve butterfly plate to the valve actuator.
The pressure opens the valve against the valve spring.
This moves the valve but- terfly plate to the wide open position.
Both of these conditions are necessary for the apu bleed air valve to to open:
- A control signal from the apu ecu to energize the apu solenoid valve, - Upstream duct pressure.
When the APU ECU removes the control signal from the APU bleed air valve control solenoid, the solenoid deenergizes. The de-energized solenoid bleeds the pressure from the valve actuator. The valve’s spring then pushes the valve closed.
A position limit switch on the valve does these things :
Airplane logic inputs to the APU ECU also have an effect on the control signals to the APU bleed air valve. These are the logic inputs :
- P8-1 APU FIRE switch (UP causes an APU shutdown and the bleed air valve to close),
- M279 fire detection module (FIRE causes APU shutdown and bleed air valve to close),
- APU speed sensors (UNDERSPEED causes the APU bleed air valve to close),
- ARINC 429 altitude (altitude above 17,000 ft causes the APU bleed air valve to close).
See the APU chapter for more information on the APU bleed air valve inter- faces (ATA 49).
B737NG/36/301 Pneumatic System
Training manual
APU BLEED AIR VALVE
6.2. aPu Pressure relief valve.
The APU pressure relief valve protects the pneumatic ducts from too much pressure. This can occur if the pressure regulating function of the APU bleed system fails. Too much pressure can rupture the ducts.
Location.
The APU pressure relief valve is on the APU bleed air duct. It is in the section 48 stabilizer compartment under the catwalk.
Functional Description.
The APU pressure relief valve is a spring-loaded, air operated, pressure relief device.
The spring holds the poppet closed at pressures less than 75 psi (nominal).
Duct pressure more than 75 psi pushes the poppet off its seat. This opens the valve. The open valve relieves duct overpressure.
When duct pressure is less than 65 psi, the spring returns the poppet to its seat. This closes the valve.
Training Information Point.
Remove and replace the old valve packing ring and replace it with a new packing ring when you install a new APU pressure relief valve.
Before you install a new APU pressure relief valve, apply a thin layer of
antiseize compound to the mounting threads of the valve. The combination of the mounting thread forces and high duct temperatures can cause uncoated valve mounting threads to seize in their mounting boss.
B737NG/36/301 Pneumatic System
Training manual
APU PRESSURE RELIEF VALVE
NOTE : The duct leak overheat protection system uses 115v ac power.
Without power, the system will not operate.
See the wing and body overheat detection section for more information (ATA 26).
Some duct flange clamps are in areas that contain control cables or other mechanical/electrical devices. Special care must be taken so that the clamps and these components do not touch.
WARNING : DO NOT REMOVE CLAMPS ON PRESSURIZED DUCTS. PERSONAL INJURY OR EQUIPMENT DAMAGE MAY OCCUR.
Keep the pneumatic ducting clean and free from these types of contamination :
- Oil,
- Hydraulic fluid, - Fuel,
- Other hydrocarbon compounds.