CHAPTER 4
PUMPS
Pumps are used for some essential services in
the Navy. Pumps supply water to the boilers, draw
condensation from the condensers, supply sea
water to the firemain, circulate cooling water for
coolers and condensers, pump out bilges, transfer
fuel, supply water to the distilling plants, and
serve many other purposes. Although the pumps
discussed in this chapter are used primarily in
hydraulic systems, the principles of operation
apply as well to the pumps used in other systems.
PURPOSE
The purpose of a hydraulic pump is to supply
a flow of fluid to a hydraulic system. The pump
does not create system pressure, since pressure can
be created only by a resistance to the flow. As the
pump provides flow, it transmits a force to the
fluid. As the fluid flow encounters resistance, this
force is changed into a pressure. Resistance to
flow is the result of a restriction or obstruction
in the path of the flow. This restriction is normally
the work accomplished by the hydraulic system,
but can also be restrictions of lines, fittings, and
valves within the system. Thus, the pressure is
controlled by the load imposed on the system or
the action of a pressure-regulating device.
OPERATION
A pump must have a continuous supply of
fluid available to the inlet port to supply fluid to
the system. As the pump forces fluid through the
outlet port, a partial vacuum or low-pressure area
is created at the inlet port. When the pressure at
the inlet port of the pump is lower than the local
atmospheric pressure, atmospheric pressure acting
on the fluid in the reservoir forces the fluid into
the pump’s inlet. If the pump is located at
a level lower than the reservoir, the force of
gravity supplements atmospheric pressure on the
reservoir. Aircraft and missiles that operate at
high altitudes are equipped with pressurized
hydraulic reservoirs to compensate for low
atmospheric pressure encountered at high
altitudes.
PERFORMANCE
Pumps are normally rated by their volumetric
output and pressure. Volumetric output is the
amount of fluid a pump can deliver to its outlet
port in a certain period of time at a given speed.
Volumetric output is usually expressed in gallons
per minute (gpm). Since changes in pump speed
affect volumetric output, some pumps are rated
by their displacement. Pump displacement is the
amount of fluid the pump can deliver per cycle.
Since most pumps use a rotary drive, displacement
is usually expressed in terms of cubic inches per
revolution.
As we stated previously, a pump does not
create pressure. However, the pressure developed
by the restrictions in the system is a factor that
affects the volumetric output of the pump. As the
system pressure increases, the volumetric output
decreases. This drop in volumetric output is the
result of an increase in the amount of internal
leakage from the outlet side to the inlet side of
the pump. This leakage is referred to as pump
slippage and is a factor that must be considered
in all pumps. This explains why most pumps are
rated in terms of volumetric output at a given
pressure.
CLASSIFICATION OF PUMPS
Many different methods are used to classify
pumps. Terms such as nonpositive displacement,
positive displacement, fixed displacement,
variable displacement, fixed delivery, variable
delivery, constant volume, and others are used to
describe pumps. The first two of these terms
describe the fundamental division of pumps; that
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