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Operation
10 3A1334F
Operation
Thermal Induced Pressure
Relief (Oil pumps only)
Thermal Relief Theory of Operation
The pump operation is similar to most double acting
reciprocating pumps. The pump only comes to a com-
plete stop on the up stroke. The pump will stroke
through on the down stroke due to the inlet check relief
passage.
Excess pressure due to downstream thermal expansion
causes the air motor/pump to run backwards (down-
ward). Fluid pressure is relieved through the inlet check
relief passage as the pump piston moves downward. Air
pressure is simultaneously relieved through the air inlet
passage. The pump piston moves downward exposing
the pump cylinder relief passage at the bottom of the
stroke. Additional excess pressure is then relieved
through both the inlet check relief passage and the
pump cylinder relief passage.
Pressure is relieved at the pump through the pump inlet
relief passage. The pump will not change over on the
bottom of the stroke as a result of relieving excess pres-
sure due to the positioning of the pump cylinder groove
with respect to the air motor pilot valve.
NOTE:
• The air supply does not have to be turned off to
enable thermal relief activation.
• The Thermal Induced Pressure Relief design fea-
ture of the pump will cause the pump to dive on the
downstroke when a dispensing valve is closed.
THERMAL EXPANSION HAZARD
Fluids subjected to heat in confined spaces, including
hoses, can create a rise in pressure due to the
thermal expansion. Over-pressurization can result in
equipment rupture and serious injury.
Oil pumps have a built-in, thermal induced, pressure
relief as shown in F
IG. 4. To ensure system pressure
will automatically bleed excess thermal induced
system pressure through the pump and back to the
oil reservoir:
• Always use a relieving air pressure regulator to
allow the pump/motor to run backwards. The regu-
lator bleeds excess air pressure.
• There should not be any obstructions between air
inlet of pump and regulator.
• There should not be any obstructions between the
downstream plumbing and pump outlet such as
closed ball valves or check valves.
• There should not be any obstructions between the
pump inlet and fluid container such as closed ball
valves or check valves.
• Only use Graco designed suction tubes for in-drum
or in-tank applications.
• Only use Graco designed suction tubes with a
built-in relief device for wall mount applications.
• Always use an inlet screen to prevent debris from
plugging relief passages from entering the pump.
- Do not operate the pump with out an inlet screen.
• Periodically inspect the inlet screen for plugging.
Graco recommends inspecting the inlet screen
every time the container is changed out.
FIG. 4 a = Pump cylinder relief passage
b = Inlet check relief passage
a
b