Concrete
uses
Concrete
volute pumps offer a host of advantages for several industries, says
SG Joshi after examining their technical specifications
The
first concrete volute pump (CVP) in the world was installed in 1917
for raising sea-water in harbour basin in Saint Nazaire, France. This
pump is operating satisfactorily even today with flow rate of 21,600
cubic metres (cu m) per hour. These pumps are considered to be more
economical, especially in case of high-flow saline water application.
It is an ideal alternative for large Vertical Turbine (VT) pumps and
can handle large flows from 20,000 cu m per hour to 1,20,000 cu m per
hour. The performance ranges covered by VT Pump and CVPs is shown in
Figure 1A. Performance curves for CVPs are shown in Figures 1B and 1C.
| CVP
range |
 |
CVPs
came into being because of lack of availability of steel during the
First World War. It has happened many times in past that a pump originally
designed to meet the requirements of a specific service, was later found
to be suitable for a wider range of applications. The same thing happened
in case of CVPs also. These pumps are now widely used not only in coastal
areas but also in thermal / nuclear power stations, ports and shipbuilding
sites, irrigation, drainage and water supply because of the high degree
of reliability exhibited by them.
Advantages
galore
CVPs
are widely used in several countries like Germany the UK, Italy, the
Netherlands and Portugal. They are also popular in France, South Africa,
Egypt, Middle East and China. It is an interesting alternative to large
steel castings which can no longer be made in foundry and would lead
to high transportation costs and would pose difficulties in handling.
It can be easily constructed by any local civil engineering company
anywhere in the world under proper supervision. Its ability to pump
very high flow rates with very high reliability has resulted in such
type of pump being chosen for handling large volumes of water. Highest
reliability of these pumps has also eliminated the necessity for standby
pump and hence has lead to saving of much of investment and maintenance
cost. Average reliability coefficient for these pumps is as high as
99.95 per cent.
Constructing
a CVP
CVP
is civil engineered pump with very few parts / subassemblies and hence
is very simple in construction. Its construction can be grouped into
three major sections (Figure 2) as follows:
- Concrete
Volute casing, a suction elbow and a draft tube
- Rotating
assembly consisting of impeller, shaft, sleeve, coupling, thrust/
journal bearing
- Grouted
components consisting of upper and lower (suction) grouted parts,
neck piece and discharge piece
Concreting
of volute casing is divided into three phases: Concreting up to draft
level; concreting of draft tube and suction elbow and concreting of
volute and grouted components. In Figure 3, a CVP is shown in its entirety.
Hydraulic
performance
The
CVPs normally have centrifugal or mixed flow impeller and pumps are
designed in specific speed range of 70- 125. This selection is based
on operational requirement and importance of energy requirements in
operating range.
The
only objection against CVP that an orthodox designer may raise is that
a prototype is not available for testing at a laboratory, as it is directly
manufactured at site. However proven techniques of establishing performance
of suitably scaled down model is adopted in designing these pumps. Also
the advanced techniques of Computational Fluid Dynamics can be applied
to predict the performance of prototype. Pump model performance including
cavitation performance is also carried out. Full size pump testing is
not possible at factory and only site testing is carried out. Sump model
test is conducted with special care at draft tube of CVP which should
be simulated carefully.
Mechanics
The
admitted criteria for vibration level of the shaft meet the ISO 2372
standard. Although the pump rotation frequency fluctuates between 1.5
hertz and 6 hertz, and is therefore less than 10 hertz (maximum value
given by the above standard), the maximum values corresponding to 16
hertz frequency are taken into account to define three limit levels
of action.
....CONTD
