Page 142 - Fluid mechanics, heat transfer, and mass transfer

120 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS

& Codes require that all pump downstream equipment
E Suction ¼ D Eye N N SS ðsp:gr:Þ ð5:14Þ must be designed towithstand pump shut-in pressure.
C
. What is the effect of near shutoff (low-flow) capacity
where D Eye is the impeller eye diameter and
and head conditions on pump performance? What is the
(sp. gr.) C is the specific gravity of the fluid at
remedy?
operations conditions. If the D Eye is not available,
& Pump overheats, giving rise to serious suction pro-
it may be approximated by using 0.9 times the
blems involving vaporization and cavitation.
suction nozzle diameter for end-suction pumps and
0.75 times the suction nozzle diameter for double- & Remedy is to provide a bypass/recycle from dis-
suction pumps. charge side to suction side through cooling arrange-
ment to artificially keep a minimum safe flow through
. Name an important contributing factor for lower effi-
the pump.
ciency of a centrifugal pump.
. What is the difference between performance curve and
& Recirculation of the fluid in the casing of a centrif-
system curve for a centrifugal pump?
ugal pump is considerable, resulting unproductive
energy consumption. & The interdependency of the system and the centrif-
ugal pump can be explained with the use of the pump
. How is recirculation of the liquid inside pump casing
caused and how it is minimized? performance curve and the system curve.
& A centrifugal pump performance curve shows that
& Recirculation is caused by oversized flow channels
that allow liquid to turn around or reverse flow. This the pressure the pump can develop is reduced as the
reversal causes a vortex that attaches itself to the capacity increases. Conversely, as the capacity drops,
pressure side of the vane. the pressure it can achieveis gradually increased until
it reaches a maximum where no liquid can pass
& If there is enough energy available and the velocities
through the pump.
are high enough, damage will occur.
& The system curve represents the pressures needed at
& Suction recirculation is reduced by lowering the
different flow rates to move the product through the
peripheral velocity, whichinturn,increases
system.
NPSH. To avoid this, it is better to opt for a lower
& To simplify a comparison with the centrifugal pump
speed pump, two smaller pumps, or an increase in
performance curve, head measurement is used.
NPSH A .
& The system head consists of the following three
& Discharge recirculation is caused by flow reversal
factors:
and high velocities producing damaging vortices on
➢ Static head or the vertical elevation through which
the pressure side of the vane at the outlet.
the liquid must be lifted.
& The solution to this problem lies in the impeller
➢ Friction head or the head required to overcome the
design. The problem is the result of a mis-
friction losses in the pipe, the valves, and all the
matched casing and impeller, too little vane
fittings and equipment.
overlap in the impeller design, or trimming the
impeller below the minimum diameter for which ➢ Velocity head, which is the head required to
it was designed. accelerate the flow of liquid through the pump
. What is shut-in pressure for a centrifugal pump? (velocity head is generally quite small and often
ignored).
& Shut-in pressure is the pressure developed on the
& As the static head does not vary simply because of a
discharge side at zero flow rate, that is, when the
discharge valve is closed while the pump is in change in flow rate, the graph would show a straight
operation. line. However, both the friction head and the ve-
locity head will always vary directly with the
& Pressure developed at shutoff may be of interest
capacity.
particularly when the pump and associated equip-
ment can be isolated by valving, so that maximum & The combination of all three creates the system
design pressure of the equipment should reflect curve.
shutoff head. & When the pump curve is superimposed on the system
& Relatively high head pumps have little increase in curve, the point of intersection represents the con-
discharge pressure at shutoff. ditions (head and capacity) at which the pump will
operate, as illustrated in Figure 5.15.
& Typical shutoff head is about 25 m for mixed flow
pumps and about 350% of the head at the maximum . What are the conditions that will cause the pump
efficiency flow for axial flow pumps. performance curve change its position and shape?