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Fuel Feed
Both engines are normally pressure fed from the center tank until the center tank quantity decreases to near zero. The engines are normally then pressure fed from their respective main tanks. Check valves are located throughout the fuel system to ensure the proper direction of fuel flow and to prevent transfer of fuel between tanks.
Fuel Pumps
Each fuel tank uses two AC powered fuel pumps which are cooled and lubricated by fuel passing through the pump. Center tank pumps produce higher pressure than main tank pumps. This ensures that center tank fuel is used before main tank fuel, even though all fuel pumps are operating. Individual pressure sensors monitor the output pressure of each pump.
YD651 - YD655 Each center tank pump will automatically shut off, after a short delay, when that pump’s sensor detects low output pressure.
Note: Fuel pump LOW PRESSURE lights may flicker when tank quantity is low
and the airplane is in a climb, descent, or on the ground with a nose-down
attitude.
Suction Feed
When main tank fuel pump pressure is low, each engine can draw fuel from its corresponding main tank through a suction feed line that bypasses the pumps. As the airplane climbs, dissolved air is released from the fuel in the tank due to the decrease in air pressure. This air may collect in the suction feed line and restrict fuel flow. At high altitude, thrust deterioration or engine flameout may occur as a result of the fuel flow reduction.
The dissolved air in the fuel tank will eventually deplete after reaching cruise altitude. The depletion time is dependent upon airplane altitude, fuel temperature, and type of fuel. Once the dissolved air is depleted, the engine may be capable of suction feed operation at cruise power.
Copyright . The Boeing Company. See title page for details.
Fuel -System Description 737 Flight Crew Operations Manual

The main tank bypass valves may also be used for suction defueling.
Fuel Crossfeed
The engine fuel manifolds are interconnected by use of the crossfeed valve. The valve is DC motor operated from the battery bus.
Fuel pressure can be provided from a main tank with operating fuel pumps to both engines by opening the fuel crossfeed valve. Continued crossfeed use will result in a progressive fuel imbalance.
Fuel Shutoff Valves
Spar fuel shutoff valves are located at the engine–mounting wing stations. The valves are DC motor operated from the hot battery bus. The engine fuel shutoff valves are fuel actuated, solenoid controlled valves powered from the battery bus. Both the spar fuel shutoff valve and the engine fuel shutoff valve close whenever their respective engine fire warning switch is pulled or engine start lever is placed to CUTOFF.
Center Tank Fuel Scavenge Jet Pump
With the main tank fuel pump No. 1 FWD Switch ON, the center tank fuel scavenge jet pump operates automatically to transfer any remaining center tank fuel to main tank No. 1. Fuel transfer begins when main tank No. 1 quantity is about one-half. Once the fuel scavenge process begins, it continues for the remainder of the flight.
Fuel Temperature
The FUEL TEMP indicator located on the fuel control panel displays fuel temperature. A sensor in main tank No. 1 allows monitoring of fuel temperature. The temperature indicating system uses AC electrical power.
APU Fuel Feed
YA613 - YA722, YC153 - YK104 When AC fuel pumps are operating, fuel for the APU is supplied from the left side of the fuel manifold. If the AC fuel pumps are not operating, fuel is suction fed from main tank No. 1.
YA801 - YA809 A DC operated APU fuel boost pump is installed to ensure positive fuel pressure to the APU fuel control unit. During APU start and operation, the pump operates automatically when the APU fuel control unit senses low fuel pressure. The pump shuts off automatically when an AC fuel pump pressurizes the fuel manifold.
Copyright . The Boeing Company. See title page for details.
12.20.2  March 28, 2005

Fuel -System Description 737 Flight Crew Operations Manual
Fuel Quantity Indication
The fuel quantity indication system calculates the usable fuel quantity in each tank. The fuel quantity in each tank is displayed on the upper display unit and on the fueling station panel.
Fueling/Defueling/Ground Transfer
Rapid fueling and defueling is accomplished at the single–point pressure fueling station in the right wing. The fueling station is also used for the ground transfer of fuel between tanks.
The manual defueling valve, located outboard of engine No. 2, interconnects the engine feed system and the fueling station. It is opened for defueling and tank to tank transfer operations.
A shutoff system is used during fueling to automatically close the fueling valve in each fuel tank when the tank is full.
Fuel Tank Location and Capacities (Usable Fuel)
　
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