Service manual

G420F(FE) Service Manual 323 Appendix
Most liquid service valves have an internal
hydrostatic relief valve and are usually labeled
“LIQUID WITH INTERNAL RELIEF”. The
hydrostatic relief valve protects the fuel service line
between the tank and the lock off from over
pressurization. The internal hydrostatic relief valve
has a minimum opening pressure of 375 PSIG and a
maximum pressure of 500 PSIG. These type of relief
valves have an advantage over external relief valves
because the propane is returned to the tank in the
event of an over pressurization instead of venting
the propane to atmosphere.
Quick Disconnect Coupling
The liquid withdrawal or service valve on a DOT
tank has male threads and accepts the female
portion of a quick disconnect coupling (Figure 8).
The female portion is adapted to the liquid hose
going to the fuel system. Both halves are equipped
with 100% shutoffs, which open when coupled
together to allow fuel flow. The coupler has two
seals. One is an o-ring and the other is a flat washer.
The o-ring prevents leakage from the shaft on the
other coupling and the flat washer seals when the
coupler is fully connected.
NOTE: The flat seal and / or the o-ring will
sometimes pop off when disconnecting and slide up
the shaft of the mating connector, causing the valve
not to open when fully mated. The extra washer or
o-ring must be removed from the shaft and the
coupling reconnected.
Filler Valve
The liquid filler valve (Figure 9) has a male thread to
receive a fuel nozzle and typically has a plastic or
brass screw on cap that is retained with a small
chain or plastic band to keep debris out of the filler
valve. The filler valve is a one-way flow device that
uses two check valves to allow fuel to enter the tank
but prevent it from exiting. Both check valves are
backpressure type check valves, designed so that
backpressure from the tank assists the check valves
own spring pressure to close the valve. The first
valve uses a neoprene on metal seal and the
second valve uses a metal on metal seal.
A weakness ring is machined into the filler valve just
above the check valves and will allow the filler valve
to shear off in case of an accident. The valve will
break or shear off above the check valves so that
the tank will be sealed and no liquid propane can
escape.
Figure 8
Weakness
Ring
Figure 9