APU Check Valve, APU Pressure Relief Valve and APU Bleed Air (Shutoff) Valve
Figure 7
C96723
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A 737-300/400/500MAINTENANCE MANUAL
C. With the isolation valve switch in AUTO, engine bleed switches ON and pack switches in AUTO or HIGH, the isolation valve will stay closed. If either of the four switches is put to the OFF position, power is transferred to turn open the isolation valve. This operation must be manually performed as the circuit passes through the switch, not the affected valve.
D. The other two positions, OPEN and CLOSE, function as a conventional switch regardless of pack and bleed switch positions.
14. Dual Bleed Light (Fig. 9)
________________
A. The DUAL BLEED light on the P5 pilot's overhead panel indicates that the pneumatic system is pressurized by engine(s) and APU at the same time.
B. The DUAL BLEED light circuit is completed by a combination of Engine No. 1 bleed switch ON and the APU bleed valve open. A combination of Engine No. 2 bleed switch ON, isolation valve open, and APU bleed valve open will also complete the circuit.
15. Operation
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A. Operation Using the APU
(1) The pneumatic manifold can be pressurized when the APU is operating and the APU bleed air valve is open. Pressures of 30 to 50 psig can be obtained in the manifold without systems operating. There is no means of regulating the pressure.
TO DUAL BLEED LIGHT CIRCUIT
FORWARD OVERHEAD PANEL (P5) CONDITION: ISOLATION VALVE SWITCH "AUTO" Isolation Valve Control Circuit Figure 8
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B. Operation Using a Pneumatic Ground Source
(1) A pneumatic ground source may be connected to the pneumatic ground service connector. Ground source should have pressure regulating equipment.
C. Operation with Engines Running
(1) With electrical power switched to the bleed air regulator and with the engines operating, each Bleed Air Control System (BACS) automatically selects the engine bleed air supply from either the low-stage or high-stage bleed ports and regulates the pressure and temperature supplied to the aircraft air using systems. The specific subsystems are discussed in subsequent paragraphs.
(a) Engine Bleed Air Stage Selection
1) Each BACS receives air from either the engine low-stage bleed port or the high-stage bleed port. This selection of the bleed supply is controlled by the high-stage valve and the high-stage regulator setting.
2) During takeoff, climb, and most cruise and hold conditions, the pressure available from the low stage is above the switchover pressure. Air is extracted from the low-stage engine port and is ducted through the bleed air check valve to the pressure regulator and shutoff valve and precooler. The high-stage valve remains closed during these operating modes.
P6 PANEL
Dual Bleed Indicating System Circuit
Figure 9
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737-300/400/500MAINTENANCE MANUAL
3) The high stage valve and regulator use a simple bleed pressure sensing control that will use high stage bleed air when 5th stage pressure is below a range of 25-33 psig . As 5th stage pressure increases toward this range, the high stage valve will modulate toward close. When 5th stage pressure reaches this range, the high stage valve will be closed. The reverse of this is true on decreasing pressure. To prevent high temperature and pressure air entering the high stage regulator when 9th stage bleed is not required, the high stage pneumatic shutoff will be in the closed position. The shutoff is actuated when 9th stage pressure is 105-115 psig.
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