Page 133 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 111

. What is power output for a pump? Give the equation for & BEP is an important parameter in that many
its calculation. parametric calculations such as specific speed,
& Power output is the product of the total dynamic head suction specific speed, hydrodynamic size, viscos-
and the mass of liquid pumped in a given time. ity correction, head rise to shutoff, and so on are
based on capacity at BEP. Many users prefer that
& In SI units:
pumps operate within 80–110% of BEP for opti-
mum performance.
5
Power output in kW ¼ HQr=3:67 10 : ð5:10Þ
. What is specific energy for a pumping system?
where H is the total dynamic head in N-m/kg (column & Specific energy is the power consumed per unit
3
of liquid); Q is the capacity in m /h; and r is the liquid volume of fluid pumped. It is determined by mea-
3
density in kg/m . When the total dynamic head H is suring the flow delivered into the system over a
expressed in Pascals, the power output in kW is
period of time and calculating the power consumed
during the same period of time.
6
HQ=3:599 10 : ð5:11Þ & This measure takes into account all of the factors that
will influence the efficiency of an installation, not just
. What is volumetric efficiency of a pump?
pump efficiency.
& Ratio of actual volume of the fluid delivered to the
& Specific energy is a useful measure to consider when
theoretical volume that can be delivered by the pump.
evaluating combinations of pump type, model, and
. What is BEP? What is its significance?
system. Another benefit of using specific energy as a
& Best efficiency point (BEP) is the capacity at max-
measure is that it allows some approximate compar-
imum impeller diameter at which the efficiency is
isons between similar pumping installations.
highest. All points to the right or the left of BEP have
. “Generally, the larger the pump, the higher the
a lower efficiency.
efficiency.” True/False?
& BEP is a measure of optimum energy conversion.
& True.
BEP is the area on the performance curve where the
. Discuss the factors that influence pump efficiencies.
change of velocity energy into pressure energy at a
given capacity is optimum. In other words, the point & The important causes of pump inefficiencies are
where the pump is most efficient. hydraulic effects, mechanical losses, and internal
leakage.
& When sizing and selecting centrifugal pumps for a
& Hydraulic losses may be caused by boundary layer
given application, the pump efficiency at design
should be taken into consideration. The efficiency effects, disruptions of thevelocityprofileand the flow
of centrifugal pumps is stated as a percentage and separation.
represents a unit of measure describing the change of ➢ Boundary layer losses can be minimized by mak-
centrifugal force (expressed as the velocity of the ing pumps with clean, smooth, and uniform hy-
fluid) into pressure energy. draulic passages.
& The BEP is the area on the curve where the change of ➢ Mechanical grinding and polishing of hydraulic
velocity energy into pressure energy at a given flow surfaces, or modern casting techniques, can be
rate is optimum, that is, the point where the pump is used to improve the surface finish, decrease vane
most efficient. thickness and improve efficiency.
& The impeller is subject to nonsymmetrical forces ➢ Generally, the improvements in surface finishes
when operating to the right or left of the BEP. These are economically justifiable in pumps with low
forces give rise to many mechanically unstable con- specific speeds and for pumps handling pharma-
ditions such as vibration, excessive hydraulic thrust, ceutical, biological, and food products.
temperature rise, cavitation, and erosion. ➢ Shell molds, ceramic cores, and special sands
& The operation of a centrifugal pump should not be produce castings with smoother and more uniform
outside the farthest left or right efficiency curves hydraulic passages.
published by the manufacturer. ➢ Separation offlow occurs when a pump is operated
& Performance in these areas induces premature bear- well away from the BEP.
ing and mechanical seal failures due to shaft deflec- ➢ The flow separation occurs because the inci-
tion, and an increase in temperature of the process dence angle of the fluid entering the hydraulic
fluid in the pump casing causing seizure of close passage is significantly different from the angle
tolerance parts and cavitation. of the blade.