C.
APU Fuel Line and Shroud (Fig. 1 and 3A)

 (1)  
The APU fuel line connects from the DC fuel pump to the APU shutoff valve, and aft under the passenger compartment floor to the APU. The APU fuel line carries fuel from the left main fuel tank to the APU.

 (2)  
A shroud surrounds the APU fuel line, between the dry side of the fuel tanks and the APU firewall. The shroud is to prevent leakage of fuel vapor.

APU Fuel Line and Drain Mast
Figure 3A

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757
MAINTENANCE MANUAL

 (3)  
The APU fuel line drain mast is on the right lower side of the tail cone. The drain line to the drain mast connects to the APU fuel line just forward of the APU firewall.

 D.
APU Flame Arrestor (Fig. 4)

 (1)  
Two APU flame arrestors mount in the APU fuel line shroud drain tubing. One flame arrestor is installed in the drain tubing just above the APU fuel sump drain valve between the main gear wheel wells. The other flame arrestor mounts in the drain tubing just above the center fuel tank.

 (2)  
The flame arrestors prevent external flame from entering the airplane through the APU fuel sump drain valve. Each flame arrestor acts as a heat sink to keep fuel vapor below the ignition point.

 E.
APU Sump Drain Valve.

 (1)  
The APU sump drain valve is located between the main wheel well doors and mounts on the inside of the keel beam.

 (2)  
The sump drain valve is a manually operated drain valve. It consists of two spring loaded poppets, a primary poppet and a secondary poppet.

 (3)  
Pushing upward on the drain valve primary poppet opens the drain valve and allows fuel to drain from the drain lines. The secondary poppet closes to seal the valve during replacement of the primary poppet.

 F.
APU Fuel Check Valve

 (1)  
The APU fuel check valve, a one way flapper valve, allows the engine fuel feed system to supply the APU with fuel. The valve prevents fuel flow to the engine feed manifold when the DC fuel pump operates.

 G.
Pressure Switches

 (1)  
Two identical pressure switches, a DC fuel pump pressure switch and an APU fuel pump shutoff pressure switch, provide APU fuel feed system indication and shutoff control, respectively. Both pressure switches are cylindrical in shape with a pressure port on one end and an electrical connector on the other end. The switches open at 4 psig.

 3. Operation (Fig. 5)
　
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