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One refuel manifold drain valve is in each refuel manifold drain line. You get access to the drain valve through the first fuel tank access door.
One manifold vacuum relief valve is on the refuel manifold near the outboard refuel valve in each main tank. You get access to the drain valve through the eighteenth fuel tank access door (between ribs 24 and 25).
Refuel Manifold Drain Valve -Functional Description
When the manifold has pressure, the valve prevents fuel from flowing into the main tanks.
When the main tank is full, the drain valve prevents fuel from flowing from the tank into the manifold. When the main tank is not full, fuel drains from the refuel/jettison manifold into the main tank.
Refuel Manifold Vacuum Relief Valve -Functional Description
When fuel drains through a refuel manifold drain valve, the pressure in the refuel manifold becomes less than the pressure inside the fuel tank. This causes the refuel manifold vacuum relief valve(s) to open. The open valve permits air to enter the refuel manifold and increase the pressure. Then fuel can continue to drain from the refuel manifold.
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777 AIRCRAFT MAINTENANCE MANUAL

PRESSURE REFUEL -REFUEL MANIFOLD VALVES

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PRESSURE REFUEL -FUNCTIONAL DESCRIPTION -FUEL FLOW

General
Fuel flows from the refuel adapters into the refuel/jettison manifold. When the refuel valves open, fuel flows from the manifold into the fuel tanks. A flow tube at the end of each refuel valve decreases the exit force of the fuel. The flow tube also puts the fuel in different parts of the tank.
The refuel valves close to stop fuel flow. Fuel that is left in the refuel/jettison manifold goes through the manifold drain valves and into the main tanks. The manifold has two vacuum relief valves. These valves permit air to go into the manifold when the fuel goes out of the manifold drain valves.
If a refuel system failure prevents the refuel valves from closing, fuel goes into the surge tanks. If the fuel gets to the level of the surge tank float switches, the switches close. When one of the two switches close, the ELMS closes all refuel valves.
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PRESSURE REFUEL -FUNCTIONAL DESCRIPTION -FUEL FLOW

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PRESSURE REFUEL -FUNCTIONAL DESCRIPTION -POWER

General
Power for the refuel system comes from the ground handling bus or the hot battery bus. Power control relays for the refuel system are in the standby power management panel.
Ground Handling Power
When the refuel panel door opens, the refuel panel door relay energizes. This permits power to go from the ground handling bus to the IRP.
Battery Bus Power
If the ground handling bus has no power, you close the refuel power (POWER) switch. This energizes the refuel power transfer relay and permits power to go from the hot battery bus to the IRP and the overfill control relay.
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PRESSURE REFUEL -FUNCTIONAL DESCRIPTION -POWER

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PRESSURE REFUEL -FUNCTIONAL DESCRIPTION -CONTROL

General
During automatic refueling, you use the IRP to set the fuel load and to open the refuel valves. The FQPU closes the refuel valves.
During manual refueling, you use refuel valve control switches on the IRP to open and close the refuel valves
Refuel Valve Open Control
The refuel valves open when they get a refuel-valve-open command from the FQPU. The IRP sends refuel valve switch position signals to the FQPU on an ARINC 429 data bus. The FQPU sends refuel-valve-open commands on the systems ARINC 629 buses to the EEU in the P310 standby power management panel. The EEU supplies a ground to close each refuel valve control relay. This permits power to go to the related refuel valve solenoid. Fuel pressure opens the refuel valve.
Refuel Valve Close Control
Each refuel valve closes when the FQPU removes the related refuel-valve-open command.
When you put the refuel valve control switches in the close position, the FQPU removes the refuel-valve-open commands.

EFFECTIVITY
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The FQPU automatically removes the refuel-valve-open commands for these conditions:
　
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