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 9.  Operation (Fig. 2 and 3)
_________
 A.  Functional Description
 (1)  Air Pressurization
 (a)  Bleed air pressure in the pneumatic crossover line varies between 30 and 50 psi. Pressurized air is directed from the pneumatic crossover duct, through the air filter, and check valve into the water storage tanks (Fig. 3). If the pressure from the pneumatic crossover duct drops to 30 psi or lower, the pressure switch closes to energize the compressor relay (Fig. 2). The energized relay closes contacts to direct ac power to energize the compressor motor. Air from the compressor passes through the unloader check valve before it enters the water tanks. When the compressor raises system pressure to approximately 40 psi, the pressure switch opens to de-energize the relay and thus de-energizes the compressor. The unloader check valve then prevents reverse flow of air. If the pressure switch fails to open at pressures above 40 psi, the compressor will continue to operate and then the pressure relief valve protects the system from too much pressure.
 B.  Control
 (1)  
With pneumatic system air available from normal engine operation, APU operation, or a ground pressure source, pneumatic system pressurization is automatic.

 (2)  
With pneumatic system air not available (all air sources shut down or with isolation valves closed) but with airplane power on and the water service panel access door closed, water system pressurization by the air compressor is automatic.

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 38-15-60
 ú ú 01 Page 5 ú Jun 10/94
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 WATER SYSTEM PRESSURIZATION - MAINTENANCE PRACTICES
___________________________________________________
 1.  General_______
 A.  This procedure contains two tasks. The first task removes the pressure from the potable water system components downstream from the overflow/depressurization valve. The second task pressurizes these components.
 B.  You must also remove the pressure from the pneumatic manifold (AMM 36-00-00/201) if it is necessary to release the pressure upstream from the overflow/depressurization valve. The removal of the pressure from the pneumatic manifold is necessary when you do maintenance on these components of the potable water pressurization system:
 (1)  
Overflow and depressurization valve

 (2)  
Check valve

 (3)  
In-line air filter

 TASK 38-15-60-842-008
 2.  Water System Depressurization_____________________________
 A.  Access
 (1)  Access Panel
 191FF Access Door, Water Service

 B.  Procedure
 S 862-001
 (1)  
Open the water service panel door.

 S 862-002

 (2)  
Open these circuit breakers and attach DO-NOT-CLOSE tags.

 (a)  
P414 Power Distribution Center - Left 1) 414E18 POTABLE WATER VALVES 2) 414R21 WATER SYS AIR COMPRESSOR

 (b)  
P180 DC Power Distribution Panel 1) 180G15 POTABLE WATER FILL AND OVERFLOW VALVES

 

 (3)  
Close the water service panel door.

 S 862-003
 TASK 38-15-60-862-004
 3.  Water System Pressurization___________________________
 A.  Reference
 (1)  AMM 24-22-00/201, Manual Control
 B.  Access
 (1)  Access Panel
 191FF Access Door, Water Service

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 38-15-60
 ú ú 01 Page 201 ú Jun 10/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 C. Procedure
 S 862-005
 (1)  
Supply electrical power (AMM 24-22-00/201).

 S 862-006

 (2)  
Remove the DO-NOT-CLOSE tags and close these circuit breakers.

 (a)  
P414 Power Distribution Center - Left
 1) 414E18 POTABLE WATER VALVES
 2) 414R21 WATER SYS AIR COMPRESSOR

 (b)  
　
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