Page 145 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 123

& In parallel, the capacities of the pumps are added and & One pump running in a parallel system tends to suffer
the head of all pumps will be equal at the point where from cavitation because NPSH R of the pump in-
the discharged liquids recombine. creases drastically. Parallel pumps should have dual
. Give reasons for operation of pumps in parallel. or double seals installed to withstand cavitation when
only one pump is running. Also changes in operating
& Parallel pumps are used for a variety of reasons:
➢ Cost (two smaller pumps may cost less than a conditions may warrant increased power require-
ments for the motor when only one pump is running.
larger one).
& Pumps in series with one of the pumps running may
➢ Increase in the size of an existing plant.
warrant selection of double seals for them to avoid
➢ To compensate for a process with varying
operational problems.
capacity.
. Give pump performance curves for pumps in parallel
& It must be noted that pumps operated in parallel must
and series.
have similar head characteristics to avoid potential
& Figures 5.19 and 5.20 show centrifugal pump per-
operating problems.
formance curves for parallel and series operations.
. What are the problems involved in the operation of
. What are the general guidelines for the selection of a
pumps in parallel? How are they overcome?
centrifugal pump?
& Pumps are operated in parallel to increase pumping
& The pump should be selected based on rated
capacity and introduce flexibility in operation when
conditions.
flow capacity changes are expected.
& The BEP should be between the rated point and the
& Parallel operation is most effective with identical
normal operating point.
pumps. However, they do not have to be identical, nor
& The head/capacity characteristic curve should con-
have the same shutoff head or capacity to be
tinuously rise as flow is reduced to shut off (or zero
paralleled.
flow).
& Each pump must have a check valve to prevent
& The pump should be capable of a head increase at
reverse flow. When one of the pumps is turned off,
rated conditions by installing a larger impeller.
the flow reverses almost immediately in that line
because of the high manifold pressure supplied by the & The pump should not be operated below the minimum
operating pumps. This causes the check valve to continuous flow rate specified by the manufacturer.
close. . What are the causes of high flows in a pumping system?
& If a slow-closing check valve is installed, the flow can & When an oversized pump is used for a specific
attain a high reverse velocity before the valve closes, application.
generating high-pressure transients. Proper selection & When two or more pumps are used in parallel and one
of the check valve is necessary. of them is taken out of service due to decrease in
& If one of the pumps is relatively weaker than the demand or for maintenance.
other, the weaker pump must be started first and then . What are the effects of high flows in a pumping system?
the stronger one. & In the case of oversized pump, unless sufficient
& When draining a tank with two pumps, one should
NPSH A is provided, pump may suffer cavitation
not use a “T” with two connections. The dominant damage and power consumption will be excessive.
pump may asphyxiate the other pump. Each pump & In case of parallel pumps, flows in excess of a single-
needs its own supply pipe as illustrated in
pump capacity may result in cavitation and high-
Figure 5.18.
power consumption.
. What are the causes of low flows in pumping?
& Due to reduced demand by the process served by the
pump.
& Two pumps operating in parallel may be unsuitable
for service at reduced flows and one of the pumps on
the line may have its check valve closed by the higher
pressure developed by the stronger pump.
. What are the effects of low flows or no flows due to
closing of discharge valve in a pumping system and
FIGURE 5.18 Two pumps draining liquid from a tank. what are the remedies?