I'll do my best to
explain the dark secrects of this system. It's not difficult but PFM to
most flight crew.
What does it do
? / System Variations The Antisurge beed system senses N1 inlet (Pt2) pressure and N1 outlet
(Ps3) pressure to schedule the opening and closing of the antisurge bleed
valves to minimise the possibility of compressor surge or instability.
High compressor discharge air is bleed overboard. It's a basic arrangement
of 8th and 13th stage scheduling of these valves. There are two types
of system Pre and Post SB 5425. There are two configuration types on the
Pre SB 5425
All the bleed
valves are controlled by the PRBC. There is no start bleed control
Two 13th stage
bleed valves installed
The two Pre
5425 configurations vary in that in earlier versions there is no
8th stage bleed valve.
have the 8th stage bleed valve (all engines built after Jan 1976
installed from new)
Post SB 5425
One 13th stage
bleed valve is controlled by the SBCV
Three 8th stage
antisurge bleed valves controlled by the PRBC
SB 4597 offers an 8th stage bleed valve to those earlier engines that
did not have it.
(Post SB 5425): As the engine RPM increases during engine start, Ps4 pressure opens
the start bleed control valve and closes the 13th stage bleed valve. The
13th remains closed until the engine is shut down. When the engine is
shut down, Ps4 pressure decreases to close the start bleed control valve
and open the 13th stage bleed valve.
(Pre SB 5425) Engine Start and Idle. During this period compressor air pushes
the bleed valves open when the muscle valve in the PRBC is (1) closed
to the flow of Ps4 and (2) opened to atmospheric vent. Above Idle. As the engine is accelerated above idle, the increasing
Ps3 overcomes the combined force of Pt2 and spring. This forces the poppet
valves to reverse position, allowing Ps4 to reposition the muscle valve.
The repositioned muscle valve allows Ps4 muscle pressure to flow to the
compressor bleed valves to force them closed. Engine Shutdown. With the engine shutdown there are no active forces
on the compressor bleed valves. Since they open at idle power they would
remain in the open position after engine shutdown.
consists of the following components:
Eighth stage antisurge
bleed valves which are mounted on the compressor case. Thirteenth stage
bleed valves are on the diffuser case.
Pt2 (inlet pressure)
probe mounted on the inlet case at the 5:30 position.
Bleed Control (PRBC) is on the diffuser outer fan duct at at the 4:30
Start Bleed Control
Valve (SBCV) is on the diffuser outer fan duct at flange H. Normally
located around the 3:00 - 3:30 position. This is only installed on SB
Ps3 strainer (filter)
is on the diffuser outer fan duct at the 11:00 position.
The routing of external
tubing and some component locations differ on different aircraft models
( the JT8D is fitted to various aircraft types)
The following components
are line replaceable units, Pt2 Probe, Pressure Ratio Bleed Control, Start
Valve Bleed Control, Ps3 Strainer, External plumbing , tubes and fittings.
The eighth and thirteenth stage antisurge bleed valves are not line replaceable.