Significance
of
sumps
B
Vasudeva Rao and TI Eldho talk about the importance of pump sumps for
the proper maintenance of sumps
Pump
sumps are widely used in various installations like power generation
plants and cooling water systems. Present day industrial requirements
demand pump sumps of very large capacities with good efficiency. A large
number of problems and deficiencies associated with pumping operations
are often related to pump sump design rather than mechanical imperfections.
The problems of vibrations, cavitation, rough running, lower efficiency
and reduced pump life can generally be traced back to flow conditions
in the sump and its approach area.
Designing
of a pump sump system is a complex task as it involves numerous hydraulic
factors whose variations are not exactly known. The conditions of flow
entering a pump are the net result of the approach flow and the hydraulic
design of the sump in which the pump is operated. Hydraulic conditions
in a pump sump depend on numerous local factors like the momentum and
velocity distribution of incoming flow in both horizontal and vertical
planes, velocity changes at intake piers, energy losses, screens and
other obstructions and rotational tendencies in the flow started upstream
from the area focused on. Analytical determination of these conditions
is very difficult. It is also very difficult to arrive at a mathematical
model for the pump sump design. Hence, hydraulic model studies are the
best tools for developing the pump sump designs.
Hydraulic
problems in
pump sumps
A
large number of hydraulic problems are associated with the pump sump
installations. The most important problems include, among others, the
following (Tullis, 1979):
- Surface
vortices – Aerated and non-aerated surface vortices are the generally
observed hydraulic problems. The surface vortices when severe enough
may draw air from free surface into the pump, causing unbalanced loading
of the impeller, periodic vibrations, reduction of the pump efficiency
and problems in the discharge piping.
- Subsurface
vortices – Floor and sidewalls under certain conditions emanate subsurface
vortices. The strength and location of the subsurface vortices depend
upon the circulation within the pumping sump and the relative distance
between the suction bell and the boundaries.
- Circulation
– Non-uniform approach flow that generates vortices and pre-rotation
at the throat of the pump causes circulation in the pumping sump.
The circulation affects the number, size and location of the vortices
generated within the pumping sump. It also causes pre-rotation at
the throat of the pump, changing the angle of attack of the impeller
blades from the design value, which may affect pump efficiency and
cause cavitation.
- Uneven
distribution of flow at the pump throat, which may often result in
unbalanced loading on
the impeller.
- Flow
separation and instabilities – Pump sumps having abrupt entrances
and centre piers, which cause obstructions and boundary discontinuities
resulting in flow separation and instabilities.
- Significant
time fluctuations in velocities at the pump throat causing vibrations
and cavitations.
- Inadequate
water depth in the pumping sump for suppressing cavitation.
Design
considerations of
pump sumps
While
designing the pump sump, the effects of the above mentioned hydraulic
problems should be considered. The important hydraulic effects to be
considered in the design are uneven distribution of flow in the sump
causing net flow circulation around the pump column, turbulence generated
in the approach flow to the pump and the vorticity generated at various
locations. Guidelines for hydraulic modelling provide that (Tullis,
1979): no measurable pre-rotation at the throat of the pumps be allowed;
no surface or subsurface vortices should enter into the pump; there
is no separation from the suction bell that extends to the impeller;
large-scale turbulence should be suppressed before the flow enters the
bell; the available net positive suction head should be greater than
what the pump requires.
The
pump sump designs, traditionally relied upon the....
....CONTD
