centrifugal pump driven by a^3 / 4 hp (560 watt)
electric motor, with^3 / 4 - inch (19 mm) inlet and
outlet and rated at 12 gpm (45.4 lpm). It can
also be a positive-displacement pump, how-
ever. A close match would be a Jabsco pump
3890-9011 (Figure 19-9), rated at 11.3 gpm
(42.7 lpm), with^3 / 4 - inch (DN 19 mm) IPS
ports and a^1 / 2 - inch-diameter (12.7 mm) mo-
tor shaft. This happens to be a flexible-
impeller pump.
Pumping Other Fluids
Calculations for other fluids, such as diesel and
lubricating oil, can be made the same way. Ap-
ply the fluid’s specific gravity to Formula 19-3.
The specific gravity for petroleum products is
lower than water, so static head also should be
reduced by multiplying it by the specific gravity.
Because diesel and lubricating oil are more
viscous than water, the friction head should be
increased. For diesel, multiply the friction head
by 1. 1. For lubricating oil, multiply by 1. 5.Pressure Drop
Another way to evaluate losses in a piping
system is by pressure drop. For instance, a
closed cooling system—such as a keel
cooler—may have a pump that is specified
as having an allowable pressure drop of no
more than, say, 14 psi, or 87 kPa, or 28. 5 in.
Hg (mercury). All are measures of pressure.
Refer to the conversion chart in Appendix C
to convert the allowable pressure drop to feet
or meter of water to head. The pressure drop
in such a system is the total friction resist-
ance we found earlier when determining the
friction head. In a closed-loop system, there
is no static head and the velocity into and out
of the pump is the same, so there is no
change in velocity and thus no velocity head.Chapter 19:Selecting and Sizing Pumps
Figure 19-9. Flexible-impeller pump
(Courtesy Jabsco/ITT Industries)