POSITIVE-DISPLACEMENT-PUMPMAXIMUMSUC-
TIONLIFT The maximum theoretical lift (suc-
tion head or lift) to the suction/inlet port for
positive-displacement pumps is about 30 feet
(9 m). In practice, it is commonly around
24 feet (7.3 m). In most applications, you want
to keep the suction head to 4 feet (1.2 m) or
less if possible; however, it is sometimes neces-
sary to locate a pump far from the fluid source,
in which case greater suction heads may be
both unavoidable and acceptable. Be sure to
check with the pump manufacturer anytime
the suction head, particularly the dry suction
head, is over 4 feet (1.2 m). Dry suction head
is the head (see the discussion to follow) that
a pump must self-prime to when there’s no
water in the pump or intake line. The same
pump will lift from a greater suction head once
it’s wet, but in most boat installations, manu-
ally priming a pump is not practical.Kinetic Pumps
Kinetic pumps use the energy of motion to
move the fluid in the pump. For common use
on boats, kinetic pumps are essentially centrifu-
gal pumps. Water enters the pump inlet at the
center (“a”) and is spun by the pump’s vanes
out to the outside of the pump’s volute casing
(round housing) by centrifugal force “b,” where
the water is expelled at the discharge port “c.”
Since the water is spun outward radially, these
are radial pumps. Centrifugal or radial pumps
may have a single suction port on one side
(a single-suction pump) or a suction port on
both sides (a double-suction pump).
An axial-flow pump is basically a pro-
peller. In fact, boat propellers can be analyzed
as kinetic pumps. It is possible to have an
axial (propeller) pump that also uses centrifu-
gal force as part of its pumping action. This isPART SIX:PLUMBING SYSTEMS WITH NOTES ON FIRE SUPPRESSION
Figure 19-4.
Rotary vane pump
(Courtesy
Jabsco/ITT
Industries)
Figure 19-5. Centrifugal pump (Courtesy
Jabsco/ITT Industries)a combined axial-and-radial or mixed-flow
pump. Again, on the majority of boats, stan-
dard centrifugal pumps are most common.
Rotary kinetic pumps (centrifugal, axial,
or mixed-flow) all share the following
characteristics:- They are not self-priming. They cannot lift
fluid up to the suction/inlet port, or they
must have a positive head at the inlet port. - They will not act as check valves. Fluid
will flow backward through the pump
when the pump is turned off.
Centrifugal pumps are the quietest in operation.Pump Selection
With the preceding information, you can
select a type of pump suited to the intended
application. Ask yourself:- Does the pump need to be self-priming?
- Will backflow be a problem when the
pump is off? (A check valve can be
used if needed.) - Does the system require a high flow
rate or high pressure? - Does the pump need to handle solids
mixed with the fluid? - Will the pump be run dry?
We can arrange a selection chart for
pump types as in Table 19-1.