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Impeller Pumps
Unlike centrifugal pumps, impeller pumps often get cranky and need
some tending, especially if they’ve been run dry for more than a
couple of minutes. We put up with them because of their self-priming ability:
Impeller pumps can draw fluids into themselves from a remote supply, such
as a through-hull fitting. Virtually every inboard engine depends on an
impeller pump to feed raw water into its cooling system. They’re
also commonly used as washdown and baitwell pumps, in oil-change systems,
as cooling pumps for air-conditioning systems–anywhere fluid is drawn
from a reservoir by a remote pump and moved somewhere else.
An impeller pump works
by creating suction with its flexible impeller, which is generally made
of neoprene and turns inside a close-fitting housing. (That’s why
lubrication is important.) A cam in the housing deflects the vanes of
the impeller as it spins; this creates the suction that draws fluid into
the housing. The function of an impeller pump depends on the vanes being
intact and not excessively worn. Also, the housing and cover plate must
fit snugly against the impeller to preserve the suction.
Impellers can be as
fragile as actors’ egos, especially if allowed to run without fluid
to lubricate and cool them. If your engine overheats because a plastic
bag in the seawater strainer has cut off the supply of water, it’s
a good idea to check the impeller before you start up again. Remove the
faceplate from the pump housing–it’s usually held on by four
or six screws–and open the pump carefully (sometimes you have to
slip a knife blade under the cover to persuade it to pop off). The impeller
inside will look like a paddlewheel, with the vanes pushed out of shape
on one side by the cam. Even if the impeller appears okay, inspect each
vane carefully to ensure it’s healthy and not torn across its base.
When in doubt, take it out: If a vane comes loose and is pumped into the
cooling system, it can block the cooling-water flow and cook a manifold.
Replacing an impeller
is easy. If you are lucky, it fits onto a splined shaft with no setscrew
holding it in place, and you can pull it out with a pair of slip-joint
pliers; their widely adjustable, offset jaws are ideal for reaching into
the pump. If the impeller won’t come out, there is probably a setscrew
through the body and shaft. Remove one of the hoses, whichever one is
easier, and bump the starter until you can see the setscrew through the
unoccupied nipple. Remove the screw with a screwdriver, and the impeller
will slide right out.
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