-receive an overheat signal from the overheat sensing elements (the signal shows that there is a leak in the bleed air ducts),
-find a fault in the sensing elements.
When the ECB receives the APU bleed signal it transmits an open
signal to the APU load control valve.
The ECB also transmits a position signal to the IGV actuator. The
IGVs opening angle refers to the momentarily operation of the
pneumatic user systems.
When the load control valve is in the closed position, the IGVs are
also in the closed position.
(2)
Bleed Air Duct The bleed air flows from the scroll of the APU load compressor to the tee-duct which is installed on the right side of the APU. The APU load control valve is installed at the forward end and the surge control valve at the aft end of the tee-duct. The forward end of the APU load control valve is connected to the bleed air duct-elbow. When the APU load control valve is in the open position, the APU bleed air flows through the bleed air duct-elbow to the pneumatic system of the aircraft.
C. Surge Air A static pressure sensor and a total pressure pressure sensor measure the air pressure in the tee-duct and transfer it the total pressure and the delta pressure transducer. The transducers translate the pressure input into an electrical signal, which is transmitted to the ECB. The ECB commands the surge control valve in the necessary position.
(1) Surge Control Valve The surge control valve bleeds the surge air from the bleed air to avoid a load compressor surge. The surge air flows out through the APU exhaust system. The surge control valve is a shutoff (butterfly) valve which is normally spring loaded open. The valve is operated pneumatically. You can see the valve position on the visual position indicator on the shaft end of the valve. The ECB calculates the ratio of the differential and the total pressure during the APU operation. It compares this value to the value for the calculated bleed air flow which it has stored of the opening angle of the IGVs. Each difference between the momentarily allowed value stored in the ECB and the calculated value causes a control signal the ECB to the surge control valve. The surge control valve corrects the surge airflow. When the bleed air flow decreases suddenly, a variable volume chamber and an orifice help the surge control valve to operate quickly enough to prevent a surge condition.
4. Interface
_________
The bleed and surge air system has an interface with:
-
the APU Bleed Air Supply and Crossbleed System (Ref. 36-12-00),
-
the APU Load Compressor (Ref. 49-23-00),
-
the Control and Monitoring System (Ref. 49-61-00).
5. Component Description
_____________________
A. Surge Control Valve (SCV) (Ref. Fig. 001) The SCV includes a valve flow section, a pneumatic actuator, a servo section and a quick-dump element.
(1)
The valve flow section has a valve body, a butterfly shaft and a butterfly plate. The valve body is a cylindrical air-flow duct, flanged at each end, for in-line installation with the air ducts. Flanges at the top and bottom of the valve body each have an axially-aligned bore for the butterfly shaft. The butterfly shaft is bearing-mounted across the valve body and extends upward, through the top of the actuator. The butterfly plate, attached to the butterfly
shaft, operates as a spring-loaded air-flow flap inside the valve body.
(2)
The pneumatic actuator has a housing and cover, two diaphragms (A and B), a diaphragm retainer and an actuation linkage. Together these parts operate the butterfly. Diaghragm 'A' is the larger of the two diaphragms and operates between the actuator housing and the cover. It is supported on the diaphragm retainer and is spring-loaded in the butterfly open direction. Diaphragm B is installed between the diaphragm retainer and the cylindrical flange inside the actuator housing. The diaphragm retainer acts as a piston inside the cylindrical flange when the diaphragm moves. The two diaphragms (A and B) divide the actuator into three chambers (D, E and F). The butterfly actuation linkage connects the center of the diaphragm retainer to the butterfly shaft. It is made so that movement of the diaphragm retainer causes an equivalent movement of the butterfly valve. Visual indication of the position of the butterfly valve is given on top of the actuator housing. A groove in the end of the butterfly shaft aligns with an OPEN/CLOSED indicator mark.
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