D. The pressure regulator is a variable output regulator which controls the pressure of the 13th-stage modulating and shutoff valve. The pressure in the bleed air ducts, just downstream of the precooler, is used as a reference for the regulator and this same pressure is also applied to the underside of the 13th-stage modulating valve. As the output pressure of the regulator is increased, relative to its reference pressure, the differential pressure across the 13th-stage valve is increased and the valve modulates towards the open position. The output pressure of the regulator is a direct function of the control pressure magnitude. The overall effect is that the actuator and 13th-stage modulating and shutoff valve operate in sequence. As the 8th-stage bleed pressure decreases the actuator opens to compensate until it reaches the limit of its travel. At this point the 13th-stage modulating and shutoff valve starts to open and supplies progressively more of the required airflow until it takes over completely and the 8th-stage check valves close. The reverse sequence takes place with increasing engine bleed pressure or decreasing system demand pressure.
E. Regardless of the system demand, or the control pressure level, the maximum pressure which the 13th-stage valve can deliver to the system is limited to approximately 45 psig above ambient. If the system pressure, (measured downstream of the precooler) exceeds this value, the 13th-stage valve will start to close under the action of its spring load. At the other end of the pressure range, the pressure regulator, in the absence of any control pressure signal, acts as a fixed reference regulator and controls the 13th-stage valve head pressure to a nominal 11 psig above ambient. This holds the 13th-stage valve open at engine idle power, when the 8th-stage pressure is too low to provide the normal control signal to the 13th-stage valve pressure regulator.
500
Feb 15/70 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 21-11-0 Page 9
H18529
Engine Bleed Air Compression Control System Schematic 501
21-11-0 Figure 6 May 01/99
Page 10
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
ENGINE BLEED AIR COMPRESSION CONTROL SYSTEM - TROUBLE SHOOTING
1. General
A. The compression control system is designed to provide an adequate supply of engine bleed air for starting engines and operating air conditioning at variable engine speeds without over pressurizing pneumatic ducts.
B. The regulating portion of this system consists of the 13-stage modulating and shutoff valve, a high pressure regulator and a pack valve. Other components which could cause trouble in the system are the 450°F thermostat and the engine bleed precooler. If all of the above units are functioning correctly but system pressure does not stay within limits of curves shown in Fig. 501 of Bleed Air Compression Control System - Adjustment/Test, the problem must exist in the interconnecting sense lines.
C. The following trouble shooting charts are provided to isolate problems in the system and utilize a test set to provide a go/no-go check of the modulating and shutoff valve, high pressure regulator and the pack valve.
D. We have eliminated the engine bleed air precooler as a possible fault since any leakage at the precooler massive enough to cause problems should be extremely obvious.
E. Trouble shooting charts, in part, utilize a test set and test set procedures for isolation of faults. The test set procedures follow trouble shooting charts.
2. Equipment and Materials
A. Portable Valve Actuating Test Set - AiResearch Part No. 290121-3
B. Electrical Cable, Special Purpose - AiResearch Part No. 290406-1-1
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