G01762
Pneumatic Actuator Installation  502 
49-51-32Page 402   Figure 401 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 15/79 

APU BLEED AIR SYSTEM - DESCRIPTION AND OPERATION
1.  General
A.  The APU bleed air system provides compressed air for the air conditioning system, the engine starting system, and for other pneumatically-operated components. The system consists of a bleed air duct, bleed air valve, differential air pressure regulator, APU control thermostat, bleed air switch, and a pneumatic switching system consisting of a normally open, three-way thermostat selector solenoid valve (Fig. 1). In addition, overtemperature protection in the load control mode is provided by an overtemperature control switch in conjunction with an EGT immersion thermocouple. A surge bleed valve is attached to the turbine plenum and is used to reduce compressor surge at altitude without a bleed load on the APU. The bleed air duct is found in the APU compartment between the upper shroud and the bleed air valve. The bleed air duct is attached to the upper shroud and the bleed air valve with support clamps. The pneumatic line filter for the cooling air shutoff valve and the check valve for the fuel sensing flow control are installed on the bleed air duct.
B.  When the APU is operating at or above 95% service speed, bleed air can be obtained by positioning the bleed air switch to ON. The amount of bleed air extracted from the APU is automatically regulated by the APU control thermostat. This is to provide the maximum amount of bleed air without overloading the unit.
2.  Bleed Air Valve
A.  The bleed air valve is a shutoff and modulating valve used to control the bleed airflow from the APU turbine plenum into the airplane pneumatic ducts. The valve is located in the bleed air duct on the aft side of the APU compartment forward bulkhead (Fig. 1). The bleed air valve consists of the main valve, actuator, rate control valve, and the switcher valve (Fig. 2). The main valve is a normally closed butterfly valve enclosed in a housing. The housing contains a port for a control air connection upstream of the butterfly. The actuator is attached to the main valve housing and consists of a spring-loaded diaphragm and an actuating linkage contained in a housing which also encloses the switcher valve. The switcher valve is a solenoid-operated two-ball selector valve that directs control air to either side of the actuator diaphragm. One of the switcher ball valves is the control air pressure check valve; the other is the switcher check valve. The rate control valve controls the opening rate of the main valve. It consists of a spring-loaded, diaphragm-operated poppet valve contained in a housing.
502 
Feb 01/96  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  49-52-02 Page 1 

APU Bleed Air System Component Location  500 
49-52-02  Figure 1 (Sheet 1)  May 01/75 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

G01976
501  APU Bleed Air System component location 
May 01/75  Figure 1 (Sheet 2)  49-52-02 
Page 3 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

B.  The solenoid in the bleed air valve is armed when the APU is operating at or above 95% service speed. While the bleed air valve solenoid is not energized, the switcher valve directs the constant pressure control air to the underside of the actuator diaphragm to keep the main valve closed (Fig. 2). Control air from upstream of the main valve is directed to both sides of the rate control valve diaphragm and, since the pressure is the same on both sides of the diaphragm, the poppet valve remains closed. When the solenoid is energized by positioning the bleed air switch to ON, the switcher valve constant pressure control air ball check valve opens and the switcher ball check valve closes. This transfers constant pressure control air from the bottom to the top of the actuator diaphragm, and the main valve opens. When the main valve opens, the pressure sensed by the rate control valve will drop in proportion to the rate at which air bleeds from the APU. Since the adjustable orifice restricts the bleed off from the top of the rate control diaphragm, the diaphragm will move down and open the poppet valve to bleed some of the constant pressure control air from the actuator to atmosphere. Reducing the air pressure in the actuator slows down the opening speed of the main valve and the amount it will open, thus preventing a momentary overload of the unit. As the pressure continues to bleed off from the top of the rate control diaphragm, the poppet valve will gradually close. As the poppet valve closes, the control air pressure in the actuator will increase and open the main valve further. When the poppet valve is fully closed, the main valve is fully open. The adjustable orifice is normally set so that the main valve will open in between 12 and 14 seconds.
　
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