SEE A
FUEL VENT FLOAT VALVE
SEE B
VENT STRINGER
FLOAT
FLOAT
VALVE FLAPPER
FUEL VENT FLOAT VALVE
B
TUBING
FLANGE

VENT TUBE
FLOAT ACTUATED VENT VALVE
A
Float Valves
Figure 2

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A

757
MAINTENANCE MANUAL

 B. Float-Actuated Vent Valve
 (1)  
Two float-actuated vent valves permit fuel tank venting while preventing fuel from entering the vent channels. The float-actuated vent valve opens and closes corresponding to the fuel level in the center fuel tank. As the fuel level rises to support the float during fueling, the float-actuated vent valves begin to close. The float-actuated vent valves close fully when fuel covers the float. As the fuel level lowers during defueling or normal operation the float is exposed and the float-actuated vent valves begin to open. The float-actuated vent valves open fully when no fuel supports the float.

 C.
Float Drain Valves (Fig. 3)

 (1)  
Two float drain valves bolt to the lower surface of each of the two vent channels in the center fuel tank. The float drain valves have a poppet seal and a float contained within a cover which snaps onto a mounting flange. The float drain valves permit fuel tank venting while preventing fuel from entering the vent channels. The float drain valves also supply drainage for all fuel which gets into the vent channels. The float drain valves open and close corresponding to the fuel level in the center fuel tank. As the fuel level rises to support the float during fueling, the float drain valves begin to close. The float drain valves close fully when fuel covers the float. As the fuel level lowers during defueling or normal operation the float is exposed and the float drain valves begin to open. The float drain valves open fully when no fuel supports the float.

 D.
Pressure Relief Valve

 (1)  
A pressure relief valve is attached to the most inboard surge tank access door of each wing. The pressure relief valve limits positive and negative pressure within the surge tank. A system of compressed springs, a lock, and a poppet are actuated by an actuation stem attached to a diaphragm. In the normally closed position, the poppet locks against the lower housing to seal the pressure relief valve and prevent leakage from the surge tank. When closed, the lock compresses the actuation spring. The spring is locked in this position by the actuation stem and three detent balls.

 (2)  
The pressure relief valve opens at a positive surge tank pressure of
　
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