Page 143 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 121

FIGURE 5.16 Pump performance curves.

. Give typical performance curves for a centrifugal pump
FIGURE 5.15 Pump performance curve and system curve indicating cavitation effects.
illustrated.
& Figure 5.16 gives typical performance curves. The
solid line curves represent condition of adequate
& Conditions which will cause the pump performance NPSH A whereas the dotted lines represent condition
curve to change its position and shape are as follows: of inadequate NPSH A , that is, cavitation effect.
➢ Wear of the impeller. & As pump capacity increases head developed margin-
➢ Change in rotational speed. allydecreases inthe lower capacityranges and rapidly
➢ Change in impeller diameter. deteriorates at high capacities. Thedeteriorationleads
to breakdown conditions under cavitation conditions.
➢ Change in liquid viscosity.
& Efficiencies are low at low-flow conditions and in-
. What are the conditions that will cause a change in
crease rapidly with increased capacities and fall
system curve? What is the importance of changes in
rapidly at very high capacities.
system curve?
& Cavitation effects are noticeable as shown by the
& Static head, velocity head, and friction head.
dotted lines.
& As velocity head is generally small, changes in static
. What are the reasons of drop in head versus capacity
head and friction head will influence changes in
curve (dotted lines in Figure 5.16) originally provided
system head.
by the pump manufacturer?
➢ A change in the static head is normally a result of a
& Pumping liquids other than the one for which the
change in tank level.
manufacturer supplied the performance curves, for
➢ An increase in friction head can be caused by a
example, a higher viscosity liquid.
wide variety of conditions such as the change in a
& Wear resulting increased clearance between the im-
valve setting or buildup of solids in a strainer. This
peller and the casing, that is, worn out impeller.
will give the system curve a new slope.
& If pump driver is changed to steam turbine-driven
& Regardless of the rated capacity of the centrifugal
pump, speed could be lower than the design speed.
pump, it will only provide what the system requires.
. How is a centrifugal pump tested for cavitation?
& It is important to understand the conditions under
& When the head drops 3% or less for a given speed and
which system changes occur, the acceptability of the
capacity from design curves, that is, curves provided
new operating point on the pump curve, and the
by the manufacturer, it is an indication that cavitation
manner in which it can be moved.
is occurring.
& When the operating conditions of a system fitted with
. What are the ways by which capacity of a centrifugal
a centrifugal pump change, it is helpful to consider
can be increased?
these curves, focus on how the system is controlling
the operation of the pump and then control the system & Reduce downstream DP.
in the appropriate way. & Increase impeller size.