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close the surge control valve. The valve moves between 10
degrees (open) and 90 degrees (closed). A linear variable
differential transformer (LVDT) supplies valve position feedback
to the ECU.
Air that flows through the surge control valve goes overboard
through the exhaust duct.
Training Information Point
The SCV has a visual position indicator. This indicator is on the
bottom of the SCV flow body.
APU BLEED AIR SYSTEM - SURGE CONTROL VALVE
EFFECTIVITY
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APU BLEED AIR SYSTEM - SURGE CONTROL VALVE
APU BLEED AIR SYSTEM - SURGE CONTROL VALVE
EFFECTIVITY
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General
The APU uses surge bleed to prevent load compressor surges.
If there is not sufficient flow through the load compressor, the
surge control valve (SCV) releases bleed air into the APU
exhaust. The bleed air flow into the exhaust is the surge bleed.
Corrected Air Flow and Surge Margin Setpoint
The ECU calculates the corrected airflow and the surge margin
setpoint.
The corrected airflow is the quantity of air that flows through the
load compressor. The ECU uses total pressure (PT) and
differential pressure (DP) to calculate the corrected airflow.
The surge margin setpoint is the minimum quantity of corrected
air that should flow through the load compressor to prevent load
compressor surge.
The ECU uses these inputs to calculate the surge margin set
point:
* Inlet temperature (T2)
* IGV position
* Bleed mode
* Air/ground.
If the corrected air flow is less than the surge margin set point,
the ECU opens the surge control valve. When the SCV is open,
the SCV releases air into the APU exhaust. This surge bleed
keeps the airflow through the load compressor at or more than
the minimum level.
SCV Actuator Control
The ECU sends control signals to the servo valve on the SCV
actuator to open or close the valve. The LVDT in the surge
control valve supplies valve position feedback to the ECU.
APU Start and Shutdown
When you start the APU, the surge control valve stays in the full
open position. The valve butterfly closes when the APU speed is
more than 95 percent speed and the APU bleed switch is in the
ON position. This helps prevent stall conditions during a start.
The surge control valve opens when the APU shuts down.
APU BLEED AIR SYSTEM - SURGE BLEED - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
49-50-00-009
APU BLEED AIR SYSTEM - SURGE BLEED - FUNCTIONAL DESCRIPTION
APU BLEED AIR SYSTEM - SURGE BLEED - FUNCTIONAL DESCRIPTION
EFFECTIVITY
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737-600/700/800/900 AIRCRAFT MAINTENANCE MANUAL
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Purpose
The APU bleed air system supplies air to the airplane
pneumatic system. The APU uses surge bleed to prevent load
compressor surges.
APU Bleed Air System
The ECU controls the APU bleed air system.
These are the components of the bleed air system:
* Load compressor
* Inlet guide vanes
* Inlet guide vane actuator
* Bleed air valve
* Pressure sensors
* Surge control valve.
The load compressor supplies airflow to the airplane pneumatic
system. The inlet guide vanes control the amount of air to the
load compressor. The inlet guide vane actuator operates the
inlet guide vanes.
The inlet guide vane actuator receives commands from the ECU
and uses fuel pressure for operation.
The ECU sends a open signal to the APU bleed air valve when
the APU is at 95 percent speed and the APU bleed switch is in
the ON position.
Electric power controls the bleed air valve and air pressure
operates it.
APU Surge Bleed
Load compressor surge protection is on during all APU
operations. The surge control valve gives this protection.
The ECU controls the surge control valve by a torque motor.
The ECU calculates the correct position of the surge control
valve with APU and airplane operating parameters.
APU BLEED AIR SYSTEM - FUNCTIONAL DESCRIPTION
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