(4)
The solenoid valve assembly is divided into two chambers. The solenoid is installed in one chamber and the ball-valve in the other. The ball-valve chamber has three ports, an orificed vent port, an inlet port and an actuator port. The inlet port is pneumatically connected to the regulated pressure line from the surge control valve. The vent port vents to ambient, and the actuator port is pneumatically connected to chamber G of the pneumatic actuator. The solenoid operated ball-valve pneumatically connects chamber G of the pneumatic actuator to the inlet port, or to the vent port.
C. Compressor Inlet Pressure Transducer (P2) The P2 transducer is installed on the left side of the air intake assembly. The signal from this transducer gives pressure signals to the ECB. The ECB uses this signal to control the surge control valve, the IGVs and the fuel control schedules.
D. Pneumatic Sensor Assy The pneumatic sensor assy is installed on the right side of the APU. It contains:
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the total pressure transducer,
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the differential pressure transducer,
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the variable volume chamber,
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the total pressure sensor.
(1)
Total Pressure Transducer (PT) The total pressure transducer translates the total pressure from the total pressure sensor into an electrical signal. This signal gives total pressure information to the ECB. The ECB uses this signal to control the surge control valve and the IGVs.
(2)
Differential Pressure Transducer (Delta P) The differential pressure transducer translates the pressure from the total pressure sensor and the static pressure sensor (via a variable volume chamber) into an electrical signal. This signal gives differential pressure information to the ECB. The ECB uses this signal to control the surge control valve and the IGVs.
(3)
Variable Volume Chamber The variable volume chamber has two chambers divided by a diaphram. It absorbs pressure surges which might tend to damage either of the transducers.
(4)
Total Pressure Sensor The total pressure sensor samples the air pressure in the bleed air duct. A dampening orifice within the total pressure sensor decreases pressure spikes that could damage the transducer. The total pressure from the sensor is applied to the variable volume chamber and the total pressure transducer.
E. Static Pressure Sensor The static pressure sensor samples the air pressure in the bleed air duct. It is located between the load compressor outlet and the bleed air duct on the right side of the APU. Small holes drilled in the interior wall of the sensing ring lead to common passage and to a single disharge port. The average pressure from this port is applied to the variable volume chamber and the differential pressure transducer.
6. Operation/Control and Indication
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(Ref. Fig. 002, 003)
A. Start Procedure of APU Bleed-Air Supply The start sequence of the APU is complete when the APU acquires 95% speed (Ref. AMM 49-42-00). Above the acquired 95% speed the APU is obtainable for the supply of bleed-air and electrical power. An ECB logic permits an APU speed decrease to 88% during normal APU operation before it automatically stops the bleed air. Information on the ECAM and the overhead panel shows that the APU is available: - on the APU page of the lower ECAM display unit, the green AVAIL
indication comes on, - on the APU overhead panel 25VU, the green AVAIL legend on the APU START
pushbutton switch comes on, When the APU is available you can push the APU BLEED pushbutton switch to start the APU bleed-air supply. When you push the APU BLEED pushbutton switch:
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the blue ON legend in the APU BLEED pushbutton switch comes on,
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the BMC starts a test of the sensing elements on the APU bleed-air duct and the left wing bleed-air ducts. If the test is correct the BMC transmits a signal to the ECB. Then the ECB opens the APU load control valve.
NOTE : If the BMC 1 finds a leak in the APU bleed-air ducts, while there
____ is a Main Engine Start (MES) signal from the engines, it ignores the leak signal and tells the ECB to open the APU bleed load valve.
(1) Stop of the APU Bleed-Air Supply To stop the APU bleed-air supply you release the APU BLEED pushbutton switch:
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the blue ON legend on the APU BLEED pushbutton switch goes off,
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the BMCs removes the 'demand' signal to the ECB,
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the APU load control valve closes and stops the bleed air supply,
- on the BLEED and APU pages of the lower ECAM display unit, the APU bleed-valve symbol is shown in the closed position.
NOTE : If an APU emergency shutdown or an APU automatic shutdown
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