Page 147 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 125

& Note: Pumps handling hydrocarbons can tolerate ➢ The motor is usually tripped or not allowed to start
lower flows than cold water, as specific volume of under these conditions. Similarly, low-flow
water is much higher than that for hydrocarbons, (cf. switches on the discharge are used to alarm and
see cavitation issues). trip the motor when a low-flow condition has
. What is the significance of the minimum flow required occurred.
by a pump? ➢ This interlock is often bypassed (manually or with
& Centrifugal pumps have a required minimum flow a timer) during pump start-up to allow the flow to
below which severe operational problems and dam- reach its normal operating point, at which time the
age can occur. Below minimum flow, pump may bypass is removed. Changing from switches to
cavitate drawing air into its suction. Its casing gets transmitters for increased accuracy and easier
filled and looses its prime. failure diagnosis is presently favored.
. What are design requirements to handle minimum flows . For a given size of already installed pump motor, how is
in a centrifugal pump system? the maximum possible size of an impeller in the pump
casing decided?
& If the primary pump discharge path can be blocked, a
recirculation line is required to accommodate the 1. Place pump discharge control valve in wide-open
minimum flow of the pump. Such lines can be position.
designed either for continuous or for controlled 2. Measure the amperage drawn by the pump?
flows.
3. Determine maximum possible impeller size that can
➢ Continuous flows are more common in smaller
be used with the existing motor, using the relation-
pumps and use a flow restriction orifice in the
ship
recirculation line to bleed off the pressure gained
from the pump. 1=3
d 2 ¼ d 1 ½A r =A 1 Š ð5:19Þ
➢ Controlled flows are normally used in higher
capacity systems where a continuous, minimum where A r is the rated capacity of the motor in amperes
flow would lead to significantly higher operating (service factor, which is 10–15% of the rated capac-
costs or the selection of a larger pump. Con- ity); A 1 is the actual amperage as measured under
trolled flows are most often accomplished with a step 2; and d 1 is the existing impeller diameter.
self-regulated pressure control valve on the
pump discharge or a flow control valve and . Which of the following two arrangements in the control
controller with the minimum flow configured of centrifugal pumps involves better energy conserva-
as the set point. tion?: Throttling the flow with a valve on pump dis-
& Excess pumping energy increases the temperature of charge or using a variable speed control on the pump
the fluid being pumped. Generally, the temperature drive?
rise is negligible, except when the pump is operating & Using a variable speed control conserves energy
near shutoff. better. It is used when flow rates are large to justify
➢ For this reason, the minimum flow recirculation additional costs in installing variable speed control.
lineshouldbeplaced insucha waythatthe & For low flow capacities, throttling is cheaper.
outlet of the line connects to a vessel instead of . What are the additional requirements for the following
the pump suction line, thus giving the fluid pump options?
time to shed its excess heat. If the fluid is added & Table 5.2 lists the additional requirements for differ-
to the suction line, the temperature increase will ent pump options.
not be dissipated. A small heat exchanger may
. “Centrifugal pumps are used as metering pumps.” True/
be placed ahead of the pump suction to achieve
False?
the same result for relatively high head pumps
& False.
( 150 m) or when handling heat-sensitive
fluids. . Under what circumstances use of inducers is desirable
on pump inlet?
➢ Low suction pressure switches interlocked into the
& Inducers are used to improve pump suction
plant control system sometimes trip the pump or
prevent its start-up when the suction pressure is conditions.
below a set value. High-pressure switches can be & Inducers improve NPSH requirements for difficult
installed on the pump discharge so as not to exceed applications.
the maximum allowable output pressure. & They assist flow of viscous fluids into pump casing.