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 6.  450°F Thermostat (Fig. 4)________________
 A.  The thermostat is located downstream of the precooler and precooler control valve sensor. The purpose of the thermostat is to control the PRSOV position above 450°F air temperature. The sensor is a bleed-off pneumatic thermostat which provides a pneumatic output signal that varies inversely with the sensed temperature. As sensed temperature increases, the hermetically sealed oil within the sensing tube expands and acts upon the bellows assembly. The resulting force against the bellows causes the output rod to move the metering valve off the seat and thereby produce flow through the control orifice and out the metering valve to ambient. The airflow from the constant supply pressure source develops a pressure drop across the control orifice. As the temperature increases, the metering valve opens up further, allowing more airflow through the control orifice with an associated larger pressure drop. The result is a decrease in output signal in response to an increase in sensed temperature. Similarly, when sensed temperature decreases, fluid contraction causes the output rod and metering valve to move in the closing direction. This reduces the pressure drop across the control orifice as sensed temperature decreases. Therefore, the metering valve is modulated to provide a discrete output signal pressure for each sensed temperature.
 7.  High Stage Valve (Fig. 5)________________
 A.  The high stage valve is a pneumatically actuated, spring-loaded closed, butterfly type valve used with the high stage regulator.
 B.  The high stage valve has two major components, a valve body assembly and a pneumatic actuator assembly.
 C.  The position indicator and manual override mechanism are mounted on the bottom of the valve body.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
9TH-STAGE
SHAFT HEX

High Stage Bleed Air Control System Schematic Figure 5
E02787
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 D.  A relief valve assembly is installed in the downstream port.
 E.  A control port is provided in the larger actuator piston cover for the application of external air pressure from the high stage regulator.
 F.  When regulated air pressure is applied to the control port, chamber C is pressurized. When chamber C pressure rises high enough, the spring force is overcome and the large piston rotates the valve toward the open position.
 G.  Air pressure, applied to the inlet of the valve body, flows thru the valve and pressurizes the downstream system sense port. Downstream pressure is ducted to chamber D.
 H.  As downstream pressure rises, chamber D pressure rises. When downstream pressure reaches a predetermined value, chamber D pressure, acting on the smaller piston, generates a closing force which, with spring force, overcomes opening force generated by chamber C pressure. The valve is moved toward the closed position, regulating downstream pressure.
 I.  Decreasing downstream pressure permits chamber C pressure to open the butterfly plate slightly and the butterfly plate is modulated about a position that will maintain a constant downstream pressure.
 J.  If chamber C pressure (control pressure) is reduced below a prescribed level, spring force and residual chamber D pressure will close the valve.
 K.  The valve may be manually locked in the closed position by loosening the position indicator/retaining screw and sliding the lock up into the recess in the cover plate. Retightening the position indicator/retaining screw will hold the lock in position.
 L.  The valve is unlocked by loosening the position indicator/retaining screw, sliding the lock down the shaft until contact is made with the top pin, and retightening the position indicator/retaining screw.
 M.  The relief valve assembly is provided to relieve excess downstream pressure caused by normal port leakage past the closed butterfly plate when no consumption of air exists downstream. The relief valve assembly may be removed to expose a test port.
　
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