There is a pressure relief valve in the inboard access door of each surge tank.
A check valve in the inboard rib of each surge tank lets any fuel in the surge tank drain back into the main tank.
Drain lines in the center dry bay connect to a drain mast on the lower skin of the airplane. This drain line contains a flame arrestor.
Functional Description
Vent channels and vent tubes keep the pressure of all the fuel tanks near the pressure in the surge tanks. The surge tanks are open to outside air through the vent scoops. The shape of the vent scoops keep positive pressure inside the surge tanks during flight.
Vent tube drain valves let fuel in the vent tubes drain back into the tank when the fuel level is below the level of the valve.
Fuel in the center tank vent channels drains through the float-actuated drain valves when the drain valves are not covered by fuel. Float-actuated drain valves close when the fuel level is above them. Thus, fuel does not get into the vent channels through the drain valves.
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FUEL STORAGE -TANK VENT SYSTEM

When the fuel level is higher than the outboard end of the main tank vent channels, the fuel vent float valves close to keep fuel out of the vent channels. When the fuel level is below these valves, the floats open them.
The pressure relief valve is normally closed. When it is closed, the valve is in line with the bottom of the wing. If a pressure difference opens the valve, it moves up as it opens. A spring holds the valve open until you close it. Pull the reset handle to move the valve back to the closed position.
Training Information Point
When you do an inspection, you must look at the pressure relief valves. Make sure they are closed. An open pressure relief valve is a symptom of a blocked vent scoop or flame arrestor. The pressure relief valve can also open to relieve air or fuel pressure if there is too much pressure during refueling.
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777 AIRCRAFT MAINTENANCE MANUAL

FUEL STORAGE -TANK VENT SYSTEM

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FUEL STORAGE -TANK ENTRY -TRAINING INFORMATION POINTS

Fuel Tank Entry
The fumes from fuel in a fuel tank are explosive and poisonous.
Before you go into a fuel tank, you must defuel the fuel tank and decrease the fuel fumes to a safe level. You do the purging and fuel tank entry procedure to remove fuel fumes.
A spark in the fuel tank can cause an explosion. When you are in a fuel tank you must do these things to prevent sparks:
*
Put on approved clothing

*
Obey the fuel fume concentration limits

*
Have good air flow through the tank

*
Obey instructions for use of tools.

Clothing
You must put on approved clothing, and rubber gloves. Nylon, rayon, silk, or wool clothing can cause sparks and are not approved.
Fuel Fume Concentration
The air in the fuel tank must be fire-safe or, health-safe. The limits for the fire-safe condition and the health-safe condition are in the purging and fuel tank entry procedure.
A fuel tank in the fire-safe condition is safe to work in if you use respirator equipment. A fuel tank in the health-safe condition is safe to work in without respirator equipment.
You use a combustible gas indicator to measure the fuel fume concentration in a fuel tank.
Air Flow
The fuel tank must have a good flow of air to keep it fire-safe or health-safe.
Install an air mover in an access door at one end of the tank. Remove an access door at the opposite end of the tank, so the air mover can pull a good flow of air through the fuel tank.
In the main tanks, make sure that the air mover pulls the air inboard. This lets a good flow of air go through the baffle rib.
Tools
Keep tools in non-static containers that have rounded corners while you do not use them.
Use only explosion-proof electrical equipment.
Make sure to remove all the equipment you bring into the fuel tank.
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FUEL STORAGE -TANK ENTRY -TRAINING INFORMATION POINTS

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PRESSURE REFUEL -INTRODUCTION

General
The pressure refuel system transfers fuel from the refuel adapters to the main and center tanks. You operate the pressure refuel system with an integrated refuel panel (IRP).
　
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