after successfully completing this
lesson you'll be familiar with head to
flow curves power to flow curves net
positive suction head cavitation and how
to prevent it
the lesson will end with a test and your
score will be recorded the performance
of the pump is shown by its
characteristic curve where the flow
capacity is plotted against the delivery
pressure or developed head head is
measured in meters the study CAD is the
difference between suction head and
delivered head as the suction head
changes the static head changes when the
pump is operating the liquid will be
moving within the pipework and so a loss
due to friction will occur it's time for
a quick quiz drag the missing words to
their correct place if you're not sure
go to the previous screen to refresh
your memory
correct for a positive displacement pump
running a constant speed the flow is
constant and so the head to flow curve
should be like the red line however the
actual head to flow curve will look like
the blue line the small change is known
as slip and is due to the
compressibility of the fluid and leakage
this curve indicates that even small
restrictions in the design flow rate can
cause very high pressures to be
delivered a centrifugal pump uses the
conservation of energy principle it
changes velocity energy into pressure
energy as the differential head
increases the flow rate decreases the
performance curve looks like this
the centrifugal pump incurs head losses
due to friction the friction is caused
by the fluid changing direction when
traveling through the pump and by
clearances within the pump these losses
very with both head and flow subtracting
the losses from the ideal gives the
actual performance curve for the pop
it's time for a quick quiz drag the
missing words to their correct place if
you're not sure
go to the previous screen to refresh
your memory
correct
this graph shows the developed pressure
of a positive displacement pump is
dependent upon the power of the motor
driving the pump as supplied power
equals useful power plus losses losses
in a positive displacement pump are
mainly mechanical with the exception of
a small loss due to leakage for a
centrifugal pump as previously mentioned
losses very with pump flow and developed
head
this means that the useful power varies
a more useful curve is obtained from
plotting efficiency to flow efficiency
equals useful power / supplied power the
system should be designed to give the
normal operating flow rate at the point
of maximum efficiency the pump section
capabilities are measured by net
positive suction head or npsh short
available positive suction head or NP
sha for short is calculated from the
atmospheric pressure the static suction
head the friction loss in the pipe and
the liquid vapor pressure net positive
suction head is the atmospheric pressure
plus or minus the static suction head-
the friction loss in the pipe minus the
liquid vapor pressure the static suction
head can be positive if the suction is
above the pump the friction had varies
with the rate of liquid flow so if we
plot available net positive suction head
to flow we get the following curve for a
given rate of flow a pump will have a
net positive suction head requirement or
npsh are for short which is determined
by the manufacturer net positive suction
head requirement varies with liquid flow
these curves are an important factor
when designing a system if the pump
operates to the right of point a then
the required suction head is greater
than the available suction head
this means that vapor bubbles will occur
in the suction pipe as the vapor bubbles
move through the pump the pressure will
increase and the bubbles will collapse
this process is called cavitation and
can cause severe damage to the pump it's
time for a quick quiz on which side of
point a will cavitation occur
click on the corresponding ? if you're
not sure
go to the previous screen to refresh
your memory correct operation to the
right of point a means that vapor
bubbles will form and cavitation will be
a problem
finally when considering reciprocating
pumps a further adjustment of net
positive suction head is required due to
the inertia of the suction pulses