included in this classification, among which are
the gear pump, the screw pump, and the moving
vane pump. Unlike the centrifugal pump, which
we have discussed, the rotary pump is a positive-
displacement pump. This means that for each
revolution of the pump, a fixed volume of fluid
is moved regardless of the resistance against which
the pump is pushing. As you can see, any blockage
in the system could quickly cause damage to the
pump or a rupture of the system. You, as the
pump operator, must always be sure the system
is properly aligned so a complete flow path
exists for fluid flow. Also, because of their
positive displacement feature, rotary pumps
require a relief valve to protect the pump and
piping system. The relief valve lifts at a preset
pressure and returns the system liquid either to
the suction side of the pump or back to the supply
tank or sump.
Rotary pumps also differ from centrifugal
pumps in that they are essentially self-priming.
As we saw in our discussion of centrifugal pumps,
Insufficient suction pressure may cause vibration,
the pump is usually located below the liquid being
as well as noisy operation and fluctuating
pumped; gravity creates a static pressure head
discharge pressure.
which keeps the pump primed. A rotary pump can
operate within limits with the pump located above
the liquid being pumped.
Rotary Pumps
A good example of the principle that makes
Another type of pump you find aboard ship
rotary pumps self-priming is the simple drinking
is the rotary pump. A number of types are
Figure 7-8.--Gear pump located above the tank.
7-8
