Page 115 - Fluid Power Engineering

CHAPTER 4

Hydraulic Pumps

4.1 Introduction
Hydraulic pumps are machines that act to increase the energy of the
liquid flowing through them. The three main classes of pumps are
displacement, rotodynamic, and special effect pumps. The displace-
ment pumps act to displace the liquid by contracting their oil-filled
chambers. In this way, the fluid pressure increases and the fluid is
displaced out of the pumping chamber. The rotodynamic pumps
increase mainly the kinetic energy of the liquid due to the momen-
tum exchange between the liquid and the rotor. The special effect
pumps, such as jet pumps and airlift pumps, operate using different
principles.
Rotodynamic pumps derive their name from the fact that a rotat-
ing element (rotor) is an essential part of these machines. The mutual
dynamic action between the rotor and the working fluid forms their
basic principle of operation. The blades, fixed to the rotor, form a
series of passages through which a continuous flow of fluid takes
place as the rotor rotates. The transfer of energy from the rotor to the
fluid occurs by means of rotodynamic action between the rotor and
the fluid.
Displacement pumps consist of one or several pumping cham-
bers. These chambers are closed and have nearly perfect sealing. The
volume of these chambers changes periodically with the rotation of
the pump driving shaft. The fluid is displaced from the suction line to
the delivery line by the successive expansion and contraction of the
pumping chambers. The displacement pump operation is summa-
rized in the following steps:

1. During its expansion, the pumping chamber is connected to
the suction line. The expansion develops an underpressure
inside the chamber, forcing the liquid to be sucked in.
2. When the volume of the chamber reaches its maximum value,
the chamber is separated from the suction line.
3. During the contraction period, the chamber is linked with the
pump delivery line. The fluid is then displaced to the pump

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