Page 140 - Fluid mechanics, heat transfer, and mass transfer

118 PUMPS, EJECTORS, BLOWERS, AND COMPRESSORS
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undisturbed flow, P is the vapor pressure of the ➢ The most appropriate type of speed control
liquid, V is the free stream velocity of the liquid, depends on the system size and layout, fluid
and r is the liquid density. properties, and sometimes other factors.
& The ratio between the static pressure and the vapor- ➢ Bypass arrangements allow fluid to flow around a
ization pressure, an indication of the possibility of system component but at the expense of system
vaporization, is often expressed by the cavitation efficiency, since the power used to bypass any fluid
number. is wasted.
& Cavitation number depends on the nature of the ➢ Throttling valves restrict fluid flow at the expense
equipment. of pressure drops across the valves.
& As a guide, s I for blunt forms is 1.0–2.5 and for . Give a checklist of symptoms for inefficient pump
streamlined forms s I is 0.2–0.5. system operation.
. What is cavitation coefficient? & Thepumpisoversizedandhastobethrottledtodeliver
& Cavitation coefficient is defined as the right amount of flow. Energy is lost in the valve.
& Flow control valves that are highly throttled.
X
¼ NPSH=H: ð5:13Þ
& Pumps that are not running close to their best effi-
where S is the cavitation coefficient and H is the ciency points operate at lower efficiency. Throttled
pump differential head (head per stage in the case of pumps usually fall into this category.
multistage pumps) at design flow (i.e., optimum & Pumps are running with bypass, or recirculation lines
efficiency). open.
& Cavitation coefficient, a dimensionless number, is & The pump is worn and the efficiency has deteriorated.
useful in correlating specific speeds with other pump & Batch type processes in which one or more pumps
parameters. Increased values of pump cavitation operate continuously during a batch.
coefficients are associated with increased pump suc-
& Frequent on–off cycling of a pump in a continuous
tion specific speeds.
process.
& Presence of noise, due to cavitation, either at the
5.1.3 Centrifugal Pump Performance and pump or elsewhere in the system.
Operational Issues & A parallel pump system with the same number of
pumps always operating.
. What are the major components that influence efficiency
& A pump system that has undergone a change in
of a pumping system?
function, without modification.
& The major components of a typical pumping system
& A pump system with no means of measuring flow,
that have a large effect on the system efficiencies
pressure, or power.
include the selection of efficient and properly sized
& The pump/system was installed or designed incor-
electric motors, use of variable speed drives when
rectly (piping, base plate, etc.)
appropriate, proper piping inlet and outlet configura-
. What are the common centrifugal pump problems?
tions, appropriate selection and operation of valves,
Illustrate.
especially any throttling or bypass valves, pump
speed control, multiple pump arrangements, and & Figure 5.13 sums up the common centrifugal pump
bypass and throttling valves, which are the primary problems.
methods for controlling flow rates in pumping & Pump not properly primed or suction pipe not
systems. completely filled.

FIGURE 5.13 Pump suction side problems.