TM 5-3810-306-20
Automatic Drain Valve
The automatic drain valve is designed to eject moisture and contaminates from the air system reservoir upon a slight
reduction in reservoir pressure. It operates automatically from ascending and descending reservoir pressures, has only
one moving part, and requires no manual assistance of control lines from other sources. The valve has a die cast
aluminum body and cover.
With no air pressure in the system, the inlet and exhaust valves are closed. Upon charging the system, a slight pressure
opens the inlet valve which permits air and contaminants to collect in the sump. The inlet valve remains open when
pressure is ascending in the system until maximum (governor cutout) pressure is reached. The spring action of the valve
guide in the sump cavity closes the inlet valve. The valve and the exhaust valve are now closed.
When the reservoir pressure drops slightly (approximately 2 psi [13.8 kPa/.14 bar]), air pressure in the sump cavity opens
the exhaust valve and ejects moisture and contaminants from the sump cavity until pressure in the sump cavity drops
sufficiently to close the exhaust valve.
The length of time the exhaust valve remains open and the amount of moisture and contaminants ejected depends upon
the sump pressure and the reservoir pressure drop that occurs each time air is used from the system.
Manual draining from the valve can be accomplished by using a tool to move the wire in the exhaust port upwards, holding
it until draining is completed.
Single Check Valve
Inline single check valves are designed to allow air flow in one direction only, preventing the flow of air in the reverse
direction. Inline single check valves are used in the air system to prevent air pressure in the supply reservoir from bleeding
back into the dryer when the compressor is unloaded or shut down, or to prevent air pressure from bleeding back into the
supply reservoir in the event of a major malfunction of the air system. An arrow indicating the direction of air flow is cast
into the body of the check valve and should be pointed toward the reservoir supply tank.
Safety Valve
A safety valve is installed in the top of the supply reservoir to protect the air system against excessive air pressure buildup.
The valve consists of a spring loaded ball valve subjected to reservoir pressure which permits air to exhaust from the
reservoir to the atmosphere if the pressure rises above 150 psi (1034.3 kPa/10.3 bar). This pressure setting is
nonadjustable and is determined by the force of the spring.
When the reservoir pressure decreases to approximately 135 psi (930.8 kPa/9.30 bar), the spring force will seat the ball
check valve, sealing off reservoir pressure. The pressure setting of the safety valve is determined by the governor cut-out
pressure.
1-24
