What is a centrifugal pump?
A centrifugal pump is a mechanical device designed to move
a fluid by means of the transfer of rotational energy from one or more driven
rotors, called impellers. Fluid enters the rapidly rotating impeller
along its axis and is cast out by centrifugal force along its circumference
through the impeller’s vane tips. The action of the impeller increases
the fluid’s velocity and pressure and also directs it towards the pump
outlet. The pump casing is specially designed to constrict the fluid from
the pump inlet, direct it into the impeller and then slow and control the fluid
before discharge.
How does a centrifugal
pump work?
The impeller is the
key component of a centrifugal pump. It consists of a series of curved
vanes. These are normally sandwiched between two discs (an enclosed
impeller). For fluids with entrained solids, an open or semi-open
impeller (backed by a single disc) is preferred (Figure 1).
Fluid enters the impeller at its axis (the ‘eye’) and exits along the circumference between the vanes. The impeller, on the opposite side to the eye, is connected through a drive shaft to a motor and rotated at high speed (typically 500-5000rpm). The rotational motion of the impeller accelerates the fluid out through the impeller vanes into the pump casing.
There are two basic designs of pump casing: volute and diffuser. The purpose in both designs is to translate the fluid flow into a controlled discharge at pressure.
In a volute casing, the impeller is offset,
effectively creating a curved funnel with an increasing cross-sectional area
towards the pump outlet. This design causes the fluid pressure to
increase towards the outlet (Figure 2).
The same basic principle applies to diffuser
designs. In this case, the fluid pressure increases as fluid is expelled
between a set of stationary vanes surrounding the impeller (Figure 3).
Diffuser designs can be tailored for specific applications and can therefore be
more efficient. Volute cases are better suited to applications involving
entrained solids or high viscosity fluids when it is advantageous to avoid the
added constrictions of diffuser vanes. The asymmetry of the volute design
can result in greater wear on the impeller and drive shaft.
What are the main features
of a centrifugal pump?
There are two main families of pumps:
centrifugal and positive displacement pumps. In comparison to the
latter, centrifugal pumps are usually specified for higher flows and for pumping
lower viscosity liquids, down to 0.1 cP. In some chemical plants, 90% of
the pumps in use will be centrifugal pumps. However, there are a number
of applications for which positive displacement pumps are preferred.
What are the limitations
of a centrifugal pump?
The efficient operation of a centrifugal pump relies on the constant, high speed rotation of its impeller. With high viscosity feeds, centrifugal pumps become increasingly inefficient: there is greater resistance and a higher pressure is needed to maintain a specific flow rate. In general, centrifugal pumps are therefore suited to low pressure, high capacity, pumping applications of liquids with viscosities between 0.1 and 200 cP.
Slurries such as mud, or high viscosity oils can cause excessive wear and overheating leading to damage and premature failures. Positive displacement pumps often operate at considerably lower speeds and are less prone to these problems.
Any pumped medium that is sensitive to
shearing (the separation of emulsions, slurries or biological liquids) can also
be damaged by the high speed of a centrifugal pump’s impeller. In such
cases, the lower speed of a positive displacement pump is preferred.
A further limitation is that, unlike a
positive displacement pump, a centrifugal pump cannot provide suction when dry:
it must initially be primed with the pumped fluid. Centrifugal pumps are
therefore not suited to any application where the supply is intermittent.
Additionally, if the feed pressure is variable, a centrifugal pump produces a
variable flow; a positive displacement pump is insensitive to changing
pressures and will provide a constant output. So, in applications where
accurate dosing is required, a positive displacement pump is preferred.