21-51-00
ALL ú ú 07 Page 9 ú Mar 20/89
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
(k)
The reference pressure regulator
1) The reference pressure regulator has a regulator diaphragm, a poppet valve, a poppet valve guide, and the adjustment M. The regulator diaphragm divides the reference pressure regulator into two chambers. The ajustment chamber bleeds to ambient. The poppet valve chamber bleeds to the bleed nozzle port and to the sloenoid valve assembly B. A spring in the adjustment chamber pushes against the regulator diaphragm.
(l)
The preset adjustable orifices (E, F, and G)
1) Each adjustable orifice has two bleed ports and a bleed control pin. The bleed control pin can be adjusted to open or close the orifice.
2) The adjustable orifice E bleeds to the inlet-pressure-sense probe and to chamber C of the zero-differential-pressure valve.
3) The adjustable orifice F bleeds to the inlet-pressure-sense probe and to the solenoid valve assembly A. 4) The adjustable orifice G bleeds to the electro-magnetic-control valve and to the solenoid valve assembly A.
(5)
The function of the flow-control-and shutoff valve:
(a)
The position of the flow-control-and-shutoff valve is controlled by the pneumatic actuator. When the pressure of air in chamber N of the pneumatic actuator becomes great enough, the flow-control-and-shutoff valve opens. The pressure of air in chamber N comes from the inlet-pressure-sensing probe. The amount of air that flows from the inlet-pressure-sensing probe to chamber N controls the position of the flow-control-and-shutoff valve.
(b)
The flow-control-and-shutoff valve has two air flow modes: the normal flow mode and the high flow mode. The air flow rate can also change because of changes in the altitude, changes in the electro-magnetic-control-valve current, and manual operations. Here is a description of how the flow-control-and-shutoff valve controls all the air flow rates.
(c)
The normal flow mode
1) The flow-control-and-shutoff valve will open to the normal-flow-mode position when solenoid B is open and solenoid A is closed. Solenoid B opens when it is momentarily energized or it is manually opened (see the manual mode paragraph). When solenoid B is energized, the manual control rod pushes the ball in the ball valve assembly. This lets air flow through the ball valve assembly of solenoid B.
2) The electrical current necessary to energize solenoid A and solenoid B is supplied by the applicable (left or right) flow control card. The flow control card supplies 28v dc to the electromagnetic coil of the solenoid.
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21-51-00
ALL ú ú 08 Page 10 ú Mar 20/89
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757 MAINTENANCE MANUAL
3) Solenoid A is closed when it is not energized. When solenoid A is closed, air can flow through the ball valve assembly and into chamber C of the zero-differential-pressure valve.
4) With solenoid B open, air from the solenoid-B-inlet connection can flow to the mechanical-manual-override valve. If the mechanical-manual-override valve is open, the air will flow into chamber N of the pneumatic actuator. The pressure of air in chamber N causes the butterfly plate to open.
5) The pressure difference in the zero-differential-pressure valve controls how far the butterfly plate will open. When the pressure in chamber C is greater than the pressure in chamber M, the diaphragm will move off of the plate valve. This lets the air from chamber C flow into chamber D. The pressure in chamber D pushes against the actuator lever assembly in the slave servo valve. This causes the lever to move off of the bleed nozzle port. Thus, air from the solenoid-B-inlet connection can flow through the bleed nozzle port to chamber Q. Chamber Q lets the air bleed to ambient. This means that there is less air pressure to flow through solenoid B and into the pneumatic actuator. Thus, the butterfly plate will not open as much.
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