these pumps (such as the Edson 1^1 / 2 - in.,
40 mm diaphragm pump) have pumped an en-
tire shirt (left in the bilge) through the system
without clogging. An argument can be made
for not using strum boxes on pumps like this,
since the strum box could be blocked by, say,
this same shirt. In most instances, however, I
recommend a strum box on all bilge-pump
intakes.
ROTARY POSITIVE-DISPLACEMENT PUMPS
Rotary positive-displacement pumps may use
vanes, lobes, screws, or gears. The most com-
mon for use on boats are rotary-vane pumps
and flexible-impeller pumps (geared pumps
are for high pressure and high output, such
as fire pumps).
All positive-displacement pumps share
the following characteristics:- They are self-priming. They can lift
fluid to the suction/inlet port, or they
can have a negative head at the inlet
port—a suction head. - They act as check valves. Fluid will not
flow backward through the pump when
the pump is turned off.
In Figure 19-3, the flexible-impeller blades
at “a” rotating on the offset cam create a vac-
uum, causing the pump suction. At “b” the im-
peller continues to rotate, drawing in more liq-
uid. As the impeller rotates to the outlet port
“c,” the liquid is expelled. Flexible-impeller
pumps can usually handle some solids in the
flow, depending on the specific pump and im-
peller. Check with the manufacturer.
In rotary vane pumps (Figure 19-4), at
“a” the liquid is drawn in by the suction cre-
ated by the vane expanding as it comes off
the eccentric portion of the liner. These
vanes spring, or squeeze in and out, in their
slots in the rotor. Each successive vane “b”
draws more liquid into the pump. When the
vanes begin to compress again at the eccen-
tric portion of the liner “c,” the liquid is
expelled from the outlet port. Vane pumps
cannot tolerate solids in the fluid flow.Chapter 19:Selecting and Sizing Pumps
Figure 19-1. Diaphragm pump (Courtesy
Edson International)
Figure 19-2. Manual diaphragm pump
(Courtesy Edson International)
Figure 19-3.
Flexible-impeller
pump (Courtesy
Jabsco/ITT
Industries)