28-22-00
 ALL  ú ú 01 Page 2 ú Mar 15/88
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 4.  Fuel Boost Pump (Fig. 1)
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 A.  The fuel boost pumps deliver fuel under pressure from a tank to its respective engine or through the crossfeed valve to the other engine. Each pump is a centrifugal-type pump driven by an ac motor controlled by a separate switch on the overhead panel. Each pump is isolated from the fuel tank by being installed in a pump housing within a tank or on the forward side of the front spar.
 B.  Fuel enters the pump through a wire mesh screen at the suction tube inlet then flows through a manually operated pump removal valve. The pump removal valve allows removal of a pump without draining the tank. A portion of the fuel being pumped is circulated through the pump to act as a lubricant for the bearings and a coolant for the motor. Fuel is discharged from the pump through the boost pump check valve. A small vent valve vents the pump casing to the fuel tank. Boost pump reprime lines prime center tank boost pumps whenever center tank is refueled.
 C.  The low pressure lights for the boost pumps will come on if the boost pump pressure is low and the pump switch is ON. For the boost pumps in the No. 1 tank and No. 2 tank, the low pressure lights will come on if the boost pump switch is OFF. For the boost pumps in the center tank, the low pressure lights are off if the boost pump switch is OFF. Thus, no low pressure lights are on during the usual part of the flight when the center tank is empty and the boost pumps in the center tank do not operate.
 5.  Fuel Boost Pump Check Valve
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 A.  The fuel boost pump check valve closes the boost pump outlet line when the pressure drops below a predetermined amount, thus preventing reverse fuel flow to the boost pump. Main tank boost pump check valves differ from center tank boost pump check valves as the cracking pressure for main tank boost pump check valves is approximately 12 psi and cracking pressure for center wing tank boost check valves is approximately 1.3 psi. The valve consists of a spring-loaded-closed flapper valve mounted in a housing. The housing has three ports: fuel in, fuel out and a pressure sensing port. The pressure sensing port is located upstream of the flapper valve and is connected to the boost pump low pressure switch. The valves are located inside the fuel tank on the wall of the boost pump housing.
 6.  Fuel Boost Pump Removal Valve
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 A.  The fuel boost pump removal valve permits removing the fuel boost pump without defueling the tank. The valves are manually operated slide shutoff valves and are located in the inlet line to each fuel boost pump. The valve control shaft passes through the wall of the boost pump housing so that the valve may be operated from the outside of the fuel tank.
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 28-22-00
 ALL  ú ú 01 Page 3 ú Nov 12/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 7.  Fuel Boost Pump Bypass Valve
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 A.  A boost pump bypass valve allows the engines to draw fuel from tank No. 1 or 2 by suction in case of complete boost pump failure in that tank. The valve also allows defueling the tanks by suction through the manually-operated defueling valve.
 B.  Boost pump bypass valves are located at the inboard end of tanks No. 1 and No. 2, installed in-line between the suction inlet and the crossfeed manifold. Valves are not installed in the center tank.
 C.  With boost pump pressure applied, the bypass valve will be closed, preventing fuel in the fuel manifold from re-entering the fuel tank. With suction applied, the bypass valve will be open, allowing the engine to draw fuel from the tank.
 8.  APU Fuel Shutoff Valve (Fig. 1)
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 A.  The APU fuel valve is mounted on the left wing rear spar and controls delivery of fuel from tank No. 1 to the APU. The valve is an electric motor-driven rotary gate valve with a manual override handle. The manual override handle provides a visual check of the valve position and a manual method of positioning the valve when the electric motor is not energized. To limit pressure buildup in the fuel lines, the valve housing incorporates a thermal relief valve. The valve is operated by 28 volt dc power and is controlled by the APU master switch.
　
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