Hale DSD User Manual
Page 4
1. INTRODUCTION
OVERVIEW
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) to 2000 GPM, Hale pumps
offer the versatility, dependability, reliability, and ease of operation so necessary to effective fire
fighting. This section reviews the principles of operation of Hales single-stage and two-stage
midship pumps.
CENTRIFUGAL FORCE
A centrifugal pump operates on the principle that centrifugal force is heated 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 outward 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:
Speed (RPM) If the speed of rotation increases with flow held constant, the water
pressure increases.
Pressure. If pressure changes with speed held constant. The flow (measured in GPM)
will change inversely. That is, if pressure increases, flow decreases.
Flow. Flow is usually measured in the number of gallons of water per minute (GPM) 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 PSIG, and the inlet pressure is 45 PSIG, the pump must only
produce the difference in pressures of 75 PSIG. 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
Figure 1.1 Centrifugal Force From a Rotating Disc