Hale MG - Manual (Page 8)

Muscle Pumps
1-1
1. INTRODUCTION
A. Description
Hale single -stage and two-stage midship pumps are
favorites of firefighters throughout the world.
Covering a range of capacities from 750 Gallons
Per Minute (GPM) (2,838 Liters Per Minute, LPM)
to 2,250 GPM (8,516 LPM), Hale pumps offer the
versatility, dependability, reliability, and ease of
operations necessary for effective fire fighting.
This section reviews the principles of operation of
Hale’s single -stage and two-stage midship pumps.
B. Principles of Operation
Centrifugal Force
A centrifugal pump operates on the principle that
centrifugal force is created by a rapidly spinning
disk. Figure 1-1 shows that an amount of water has
been placed at the center of a disk. The disk is
rotated at some speed, and the water is thrown
from the center toward the outer circumference of
the disk. The distance that the water travels from
the center directly relates to the diameter of the
disk and the speed of rotation. When water is
confined in a closed container (such as the pump
body), its pressure rises to a level that depends on
the speed of rotation. There are three
interrelated
factors that regulate the performance of a
centrifugal pump:
q SPEED (RPM). If the speed of rotation
increases with flow held constant, the water
pressure increases.
q PRESSURE. Pressure is usually measured in
Pounds Per Square Inch (PSI) (BAR). If
pressure changes with speed held constant, the
flow (measured in GPM) (LPM) will change
inversely, that is, if pressure increases, flow
decreases.
q FLOW. Flow is usually measured in the
number of gallons of water per minute (GPM)
(LPM) that a pump can deliver when supplied
from draft. If the pressure is held constant, the
flow will increase with an increase in the speed
of rotation.
The centrifugal pump is preferred by the fire
protection service due to its ability to fully utilize
any positive suction inlet pressure, reducing the
amount of work done by the pump. For example, if
the required discharge pressure is 120 PSI (8
BAR), and the inlet pressure is 45 PSI (3 BAR),
the pump must only produce the difference in
pressures of 75 PSI (5 BAR). This contributes to
low engine and pump speeds with reduced
maintenance. Decreased maintenance is aided by
the fact a centrifugal pump has basically only two
moving parts the impeller and the shaft.
Pump Stages
The number of impellers on a common shaft
determines the number of pump stages. The Hale
series of single -stage pumps provides the same
normal operating and rating test pressures as the
Hale series of two-stage pumps. The two-stage
pump provides an additional level of operating
pressures if required, but adds some operating
complexity.
Figure 1-1. Centrifugal Force From a Rotating Disk