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Fuel Vent Float Valves (Fig. 1)
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 A.  A fuel vent float valve allows fuel tank venting while preventing fuel from entering the vent channels/tubing. The valve opens and closes corresponding to the fuel level in the tanks and airplane attitude changes. As the fuel level rises to support the float the valves begin to close. The valves close fully when fuel covers the float. As the fuel level lowers during defueling or normal airplane attitude change, the float is exposed and the valve begins to open. The valve opens fully when no fuel supports the float. A float stop supports the float in its fully open position.
 B.  Tank No. 1 and 2 float valves are located in the outboard channel of the wing tanks. The center tank float valve is in the vent tubing in cavity No. 3.
Fuel Vent Scoop (Fig. 1)
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 A.  The fuel vent scoops, located in the outboard half of the vent surge tanks, are the overboard vents for the vent system. Each scoop consists of a recessed ram air scoop mounted in a lower wing surface access panel, and a five inch standpipe assembly. The diverging walls of the standpipe act as a plenum chamber ensuring that ram air will be applied to the vent system during all flight attitudes of the airplane. A screen is installed at the top of the standpipe to prevent the entrance of large insects, birds, or foreign matter. The design of the scoop is such that icing conditions have minimum effect on ram air pressure.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
RELIEVES TRAPPEDAIR DURING FUELING VENT DUCT
(UPPER WING
STRINGER) HINGE
VALVE STOP CHECKVALVE
FLOAT
CHECK
VALVES
(TYP 2
PLACES)

FLOAT VALVE CHECK VALVE
(NORMAL VENT)
(TYP 2 PLS)  UPPER SURFACE
(SEALED)

VALVE STOPSTRINGER 11
VENT CLIMB LINE VENTS FWDFLOAT
VENT DRAINSCOOP SURGE TANK
HOLE
INBD

O-RING DRAIN LINEGRAVITY CROSS-
OVER
TUBE OVERBOARD
VENT & RAM
PRESSURE
INLET
(LWR SURF) FLOAT VALVE(NORMAL VENTS)DEFLECTOR
CENTER TANK
UPPER SURFACE
TANK 1  (SEALED) TANK 2 STRINGER 9
SURGE TANK  SURGE TANK
CHECK
VALVE

Fuel Vent System
Figure 1

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 4.  Surge Tank Flame Arrester Pressure Relief Valve (if installed)
_______________________________________________
 A.  A positive and negative pressure relief valve is used to bypass the fuel vent scoop for fuel system venting if the surge tank flame arrester core cells are plugged such as from severe icing conditions. The relief valve is installed on the access panel for the inboard half of the surge tank.
 B.  A spring-loaded poppet, on the relief valve, with outer surface flush with the wing lower skin is triggered open through a latching mechanism by a pressure or vacuum force acting on a diaphragm. The upper chamber of the diaphragm is vented to the atmosphere through three ambient air holes in the lower outer surface of the valve while the lower chamber is vented to the interior of the surge tank. When the poppet opens, it is raised about 1 inch above the wing lower skin surface which causes a manual reset knob and stem to protrude about the same distance downward out of the center of the poppet. Once actuated open, the poppet must be mnaually reset to the closed position at the conclusion of the flight. Manual reset is accomplished by pulling reset knob downward until the poppet is relatched in the closed position. The relief valve is designed to open at 2.0 to 2.7 psi increasing positive pressure and -4 to -10 inches of water increasing negative pressure or vacuum.
 5.  Shroud Drain Line
_________________
 A.  A shroud drain line is provided to prevent an accumulation of fluid from condensation or leakage in the APU fuel line shroud. The drain line contains a flame arrester and terminates at a drain mast located in the left wing-body fillet fairing (Fig. 2). A drain cock is provided in the wheel well to check for and drain any fluid while on ground. The drain cock must be closed for flight.
　
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