VALVES WITH PART NO. 792999-1 THRU -3 VALVES WITH PART NO. 792999-4 AND HIGHER
Pressure Regulating and Shutoff Valve Operation Figure 14
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 (3)  
The pressure control portion of the valve keeps the downstream pressure between 30 and 50 psi. The pressure control part of the valve is made up of these components:

 -
two control diaphragms

 -
two feedback diaphragms

 -a poppet control lever.

 (a)  
The control diaphragms control the movement of the poppet. The poppet lets servo pressure bleed to ambient or push against the piston. When downstream pressure pushes on the control diaphragm, the poppet opens, servo pressure bleeds to ambient, and the valve closes. The downstream pressure pushes against preset spring tension to close the poppet. This lets servo pressure push on the actuating piston which will move the valve to the open position.

 (b)  
The feedback diaphragms control the valve movement speed. As the valve closes, the valve shaft increases the feedback pressure to the pressure controller. This pushes on a diaphragm which applies closing forces on the controller poppet. These forces change in proportion to the speed of the valve. This decreases the speed of the valve. A second diaphragm supplied with the same feedback pressure through a restrictor, will balance the forces that close the valve.

 (4)  
The temperature control portion of the valve will begin to close the valve when the temperature gets to 428°F (220°C). When the temperature gets to 450°F (232°C) the valve will be nearly closed.

 (a)  
The control diapragms and a control lever with a poppet make up the PRSOV temperature control. The pressure on the diaphragms is controlled by a temperature sensor, crank, spring, and lever. The temperature sensor supplies a force on the lever through the crank and the spring. The change in the lever force is balanced by a change in the signal pressure on the lever nozzle.

 (b)  
The temperature is measured by a probe which extends into the valve duct. The temperature probe is protected by a thin metal tube which is easily damaged and will not support heavy loads.

 (5)  
A spring diaphragm and a ball poppet make up the reverse flow check part of the PRSOV. The upstream pressure is compared with the downstream pressure through the diaphragm. If downstream pressure exceeds upstream pressure by 0.13 psi, the poppet moves down and the servo pressure is sent to ambient. This will cause the valve to close.

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 (6)  An electrically operated reverse flow solenoid is used during engine start. When the engine start begins, a signal is sent to the solenoid. The solenoid then lets the valve use downstream air pressure to open the valve. This downstream air pressure will also operate the engine starter.
 11. Wing Isolation Valve (Fig. 15)
____________________
 A.  There are two wing isolation valves installed in the pneumatic manifold aft of the air-conditioning bay. The valves are normally in the open position.
 (1)  The valve is a 7-inch diameter butterfly type valve. The valve is electrically operated by a 115v ac actuator. The actuator causes the shaft to rotate. This will put the valve in one of two positions, fully open or fully closed. A lever, normally engaged with the motor drive, releases to let the butterfly valve be manually operated.
WING ISOLATION VALVES
SEE  A
MANUAL DRIVE SHAFT
ELECTRICAL
CONNECTOR

POSITION INDICATOR
ISOLATION VALVE (TYP)
A
Wing Isolation Valves Figure 15
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