The bleed air system consists of two
BLEED AIR SYSTEM
independent systems--the 14th-stage system and
the 5th- and 10th-stage system. The 5th- and 10th-
The bleed air system (fig. 3-17) on the
stage bleed air system unloads the compressor
model 104 and model 139 are nearly identical.
during starts. This reduces the possibility of
The major difference is found in the 14th-stage
compressor surge during the starting cycle. The
bleed air valve. A fast-acting 14th-stage bleed
14th-stage bleed air system extracts air from the
air valve is used on the model 139. This
compressor for the ship's bleed air system.
allows the engine to respond quickly to transient
Airflow up to 2.37 lb/sec at 55 to 60 psig may
load conditions brought about by failure of
be extracted. This is about 8 percent of
one of two GTGSs operated in parallel. This
compressor airflow.
GTGS loss is partially compensated for by
the quick closing (in about 150 to 200 milli-
Fourteenth-Stage Bleed Air
seconds) of the 14th-stage bleed air valve
on the surviving GTGS. The quick-closing
Fourteenth-stage compressor discharge air is
feature is controlled by the externally mounted
extracted from ports on the compressor diffuser.
turbine overtemperature protection system
A manifold surrounds the diffuser to collect the
(TOPS). This system is not covered in this
discharge air. This air is piped through a bleed
chapter.
Figure 3-17.--Allison 501-K17 bleed air system.
3-20
