Bendix Commercial Vehicle Systems ST-3 SAFETY VALVE - Manual

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DESCRIPTION
The safety valve protects the air brake system against
excessive air pressure build-up. The valve consists of a spring
loaded ball valve subjected to reservoir pressure which will
permit air to exhaust reservoir pressure to atmosphere if
reservoir pressure rises above the valves pressure setting,
which is determined by the force of the spring.
OPERATION
To illustrate the operation of the safety valve, we shall assume
that the governor cutout pressure is set at 125 psi. A safety
valve with a setting of 150 psi could then be used. Should
system pressure rise to approximately 150 psi air pressure
would force the ball valve off its seat, and allow reservoir
pressure to vent to atmosphere through the exhaust port in
the spring cage.
When reservoir pressure decreases sufficiently, the spring
force will seat the ball check valve, sealing off reservoir
pressure. This would occur at approximately 135 psi for the
150 psi valve. It is important to note that the desired pressure
setting of the safety valve is determined by the governor
cutout pressure. The opening and closing pressures of the
safety valve should always be in excess of governor cutout
pressure setting. The pressure setting is stamped on the
lower wrench flat of the valve.
Normally, the safety valve remains inoperative and only
functions if for any reason reservoir pressure rises above the
setting of the valve. Constant “popping off” or exhausting of
the safety valve can be caused by a faulty safety valve, faulty
governor, faulty compressor unloading mechanism, or a
combination of any of the preceding.
PREVENTIVE MAINTENANCE
Important: Review the Bendix Warranty Policy before
performing any intrusive maintenance procedures. A warranty
may be voided if intrusive maintenance is performed during
the warranty period.
No two vehicles operate under identical conditions, as a
result, maintenance intervals may vary. Experience is a
valuable guide in determining the best maintenance interval
for air brake system components. At a minimum, the safety
valve should be inspected every 6 months or 1500 operating
hours, whichever comes first, for proper operation. Should
the safety valve not meet the elements of the operational
tests noted in this document, further investigation and service
of the valve may be required.
OPERATING AND LEAKAGE CHECKS
OPERATING TEST: With air pressure in the system, pull
the exposed end of the valve stem removing the spring load
from the ball check valve. Air should exhaust from the valve’s
exhaust port. Release the stem, the air flow should stop.
Failure of valve to pass operating test would indicate the
valve should be disassembled, cleaned and rebuilt. (See
“Disassembly and Assembly” section). If adjustment is
necessary, see “Adjustment” section.
LEAKAGE CHECK: Coat the exhaust port with soap
solution. A leakage of a 1” bubble in 5 seconds is permitted.
Excessive leakage indicates dirt in valve, faulty ball valve or
seat. Valve should be disassembled, cleaned and rebuilt.
(See “Disassembly and Assembly” section).
REMOVING AND INSTALLING
REMOVING
1. Block wheels or otherwise secure vehicle and drain
reservoirs.
2. Using wrench flat closest to reservoir, unscrew valve from
reservoir.
INSTALLING
Safety valve should be installed in same reservoir that
compressor discharge line is connected to. Install in a
convenient location in a top port of the reservoir. If safety
valve is installed horizontally, exhaust port should point down,
stem of the valve should face rear of vehicle.
PRESSURE
SETTING
150 PSI
EXHAUST
PORT
ST-3 ™
VALVE
ST-1 ™
VALVE
SD-03-1901
Bendix ® ST-1 ™ & ST-3 ™ Safety Valve