Bendix Commercial Vehicle Systems DRO UNLOADER VALVE User Manual
®
Bendix® DRO Unloader Valve
4
CONTROL
1
SUPPLY
21
DELIVERY
SD-08-203
ATTACHES
TO THE
EXHAUST OF
THE DRO VALVE
3
EXHAUST
FIGURE 1 - DRO UNLOADER VALVE
DESCRIPTION
The Bendix® DRO unloader valve allows an air dryer
with a turbo-cutoff valve to be used as a discharge line
unloaded dryer. The DRO valve allows discharge air
from the compressor to be exhausted to the atmosphere
in the unloaded mode of operation. Some compressors
are designed for discharge line unloading instead of head
unloading. When the DRO valve is placed in the discharge
line it allows both the compressor and the dryer, with turbocutoff, to function properly. The DRO valve can be used to
convert a head unloaded compressor to a discharge line
unloaded compressor.
OPERATION (SEE FIGURE 1)
The DRO unloader valve discharges air in the compressor
discharge line to the atmosphere. During the charging
operation of the compressor, air travels into the supply port
(1) of the DRO valve. When the air brake system reaches
the cut-out pressure, a pneumatic signal from the governor
is sent to the control port (4) of DRO
pneumatic signal opens the DRO valve exhaust port (3)
allowing air pressure in the compressor discharge line to
unloader valve. The
FIGURE 2 - SILENCER
exhaust to atmosphere. Any air pressure created by the
compressor during the unload operations fl ows out the
DRO unloader valve to atmosphere.
The exhaust (3) can be directed away from the valve using
a 3/4” rubber hose. The hose should be securely fastened
to the frame and should not run uphill or contain any kinks
or sharp bends.
An optional silencer, as shown in Figure 2, is available
to reduce noise level during the unloaded operation. The
silencer snaps into the bottom of the exhaust port (3).
REMOVAL
1. Disconnect the supply and delivery line fi ttings.
2. Remove the mounting hardware that secures the
DRO unloader valve to the frame rail or mounting
surface.
1
INSTALLATION
The DRO valve can be installed in two confi gurations,
series or parallel.
Series Confi guration
The series confi guration, shown in Figure 3, allows all the
compressor discharge air to pass through the valve and
into the dryer or reservoir.
3. For series installation, install a discharge line from the
compressor to the DRO valve, as shown in Figure 3.
For parallel installation add another discharge line
into the existing compressor discharge line using a
tee fi tting. Connect the opposite end of this new line
to the DRO unloader valve as shown in Figure 4. The
valve must be at the same height or lower than the
discharge line.
TO THE
CONTROL
GOVERNOR
DRO
UNLOADER
COMPRESSOR
FIGURE 3 - SERIES CONFIGURATION
VALVE
PORT OF THE
AIR DRYER
FROM THE
RESERVOIR
TO THE SUPPLY
PORT OF THE
AIR DRYER
Parallel Confi guration
The parallel confi guration, shown in Figure 4, allows the
DRO unloader valve to be mounted using a branch circuit.
This branch circuit provides the fl exibility of installing the
unloader valve in a more convenient location. The branch
line should not run uphill or have kinks or sharp bends that
may trap condensed liquids.
TO THE SUPPLY
PORT OF THE AIR
DRYER
TO THE
GOVERNOR
CONTROL
PORT OF THE
AIR DRYER
4. For a series confi guration, the delivery line out of the
DRO should be connected to the air brake dryer or
reservoir tank.
5. For a parallel confi guration, the delivery port out of the
DRO must be plugged.
6. The supply and delivery ports are 22 mm metric
threads. The control port is 12 mm metric threads.
Consult a fi tting supplier for conversion fi ttings.
7. Make sure there is a line from the governor unloader
port to the DRO valve control port.
8. Test all connections for leaks.
9. Start the engine and allow the system to reach cutout pressure. The valve should open allowing air to
exit the exhaust port. A silencer, shown in Figure 2, is
recommended to reduce the noise level out of the DRO
valve when exhausting. Bendix provides a silencer that
snaps onto the exhaust port the DRO.
10. Fan the brakes until system cut-in is reached. The
DRO valve exhaust should shut allowing the air brake
system pressure to increase until cut-out pressure is
reached.
Note: The operating range of the DRO unloader valve is
from -40°F to 300°F. The DRO valve must be installed with
suffi cient discharge line length to prevent the air dryer inlet
temperature from exceeding 160°F.
FROM THE
PORT 21 MUST
DRO
UNLOADER
VALVE
FIGURE 4 - PARALLEL CONFIGURATION
BE PLUGGED
RESERVOIR
1. Locate a position in the discharge line of the
compressor where the DRO valve can be mounted.
The valve must be mounted at the same height or
below the compressor outlet.
2. Secure the DRO valve using 5/16” grade 5 or higher
bolts. It is not recommended to install the valve directly
on to the engine or compressor.
2
To prevent failure or freeze-ups, the control air into the
DRO valve must be free of liquid water and fl uids and the
maximum inlet pressure to the DRO valve is 174 PSI.
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