applications, have a wide variety of uses, including irrigation pumping plants and sumps in powerplants
impeller and its casing called a bowl. The main advantage of this type of construction is that system
pressure can be varied by simply adding or reducing the number of stages of the pump. The use of vertical
propeller pumps is normally limited to low head, high capacity use.
Horizontal pumps are classified according to the location of the suction pipe. The suction can be
from the end, side, top, or bottom. Also common in horizontal pumps is the use of double suction
impellers. In a double suction impeller pump, water flows symmetrically from both sides into the
impeller which helps to reduce the axial thrust load
pumps are normally used for fire water and cooling water applications.
Positive Displacement Pumps.
Positive displacement pumps enclose the fluid through the use of gears,
pistons, or other devices and push or "displace" the fluid out through the discharge line. Displacement
pumps are divided into two groups: reciprocating, such as piston and diaphragm pumps and rotary, such as
gear, screw, and vane pumps. Since positive displacement pumps do "displace" the fluid being pumped,
are required in the discharge line ahead of any shutoff valve or any
device that could
conceivably act as a flow restriction.
Reciprocating piston or plunger pumps are suitable where a constant capacity is required over a variety of
pressures. Piston and plunger pumps are capable of developing very high pressures, although capacities
are somewhat limited. These pumps provide a pulsating output which, depending on the application, may
be objectionable. The use of reciprocating pumps in hydroelectric powerplants is limited.
Rotary positive displacement pumps are used in a variety of applications, one of the most common being
hydraulic systems. Some of the most common rotary pumps used in hydraulic systems are gear, vane,
radial piston, and axial piston pumps
multiple screws, are most commonly used for fluid transfer although they are sometimes used in hydraulic
system applications such as governor oil pumps.
Gear pumps are relatively simple in design relying on the meshing of the mating gears and the fit of the
gears in the pump casing to pump the fluid. External gear pumps utilize two meshing gears, usually spur
or herringbone types, in a close fitting casing. The fluid is pumped as it is trapped between the rotating
gears and the casing and moved from the suction of the pump to the discharge. An internal gear pump
utilizes an external gear rotating eccentrically within and driving an internal gear to pump the fluid.
Vane pumps consist of a case and a single eccentric rotor with multiple vanes sliding in slots in the rotor.
Centrifugal force keeps the vanes in contact with the interior of the pump casing. As the rotor rotates, the
fluid is drawn into the pump by the gradually increasing volume between the vanes and is pushed out
through the discharge as the volume gradually decreases.
The radial piston pump is similar in construction to the vane pump in that it has a single rotor, eccentric to
the pump housing; but instead of vanes, it has radial pistons. The pistons are held against the pump
housing by centrifugal force, and the fluid is pumped by the reciprocating action of the pistons in their
bore. The fluid ports are in the center of the rotor.