Eductors, or jet pumps as they are sometimes called, use a high pressure stream of fluid to pump a larger
volume of fluid at a lower pressure. An eductor consists of three basic parts: the nozzle, the suction
chamber, and the diffuser. The high pressure fluid is directed through a nozzle to increase its velocity. The
high velocity creates a low pressure area that causes the low pressure fluid to be drawn into the suction
chamber. The low pressure fluid is then mixed with the high velocity fluid as it flows through the diffuser,
and the velocity energy of the mixture is converted into pressure at the discharge. Eductors are commonly
used in powerplants and dams to dewater sumps below the inlet of the sump pumps.
By far, the most common type of dynamic pump is the centrifugal pump. The impeller of a centrifugal
pump, the rotating component of the pump which imparts the necessary energy to the fluid to provide flow
and pressure, is classified according to the direction of flow in reference to the axis of rotation of the
impeller. The three major classes of centrifugal impellers are:
Impellers may be further classified by their construction. The impeller construction may be:
An open impeller consists of vanes attached to a central hub. A semi-open impeller has a single shroud
supporting the vanes, usually on the back of the impeller. The closed impeller incorporates shrouds on both
sides of the vanes. The shrouds totally enclose the impeller's waterways and support the impeller vanes.
Centrifugal pumps are also classified by the means in which the velocity energy imparted to the fluid by the
impeller is converted to pressure. Volute pumps use a spiral or volute shaped casing to change velocity
energy to pressure energy. Pumps which use a set of stationary diffuser vanes to change velocity to
pressure are called diffuser pumps. The most common diffuser type pumps are vertical turbine pumps and
single stage, low head, propeller pumps. Large volute pumps may also have diffuser vanes, but while these
vanes direct the water flow, their main purpose is structural and not energy conversion.
Centrifugal pumps are further classified as either horizontal or vertical, referring to the orientation of the
pump suction can be more easily positioned below the water surface to eliminate the need for priming, and
the pump motor can be located above the water surface to prevent damage in the event of flooding.
Vertical pumps can be either dry-pit or wet-pit. Dry-pit pumps are surrounded by air, while wet-pit pumps
are either fully or partially submerged. The dry-pit pumps are commonly used in medium to high head,
large capacity pumping plants. These large dry-pit pumps are generally volute pumps with closed, radial
There are a variety of wet-pit pump designs for differing applications. One of the most common types is
the vertical turbine pump. The vertical turbine pump is a diffuser pump with either closed or semi-open,
radial-flow or mixed-flow impellers. Vertical turbine pumps, while most commonly used for deep well