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How centrifugal pumps work2020



hey there guys Paul here from the

engineering mindset calm in this video
we're going to be looking at centrifugal
pumps to learn the basics of how they
work the different types and where we
use them you can get more information on
centrifugal pumps shop for parts and
accessories or talk to experts on top
centrifugal pumps come in many shapes
colors and sizes but they typically look
the pumps consist of two main parts the
pump and the motor the motor is an
electrical induction motor which allows
us to convert electrical energy into
mechanical energy this mechanical energy
is used to drive the pump and move the
water the pump holds water in through
the internet and pushes it out through
the outlet as we take the unit apart we
can see that we have a fan and a
protective casing mounted at the back of
the electrical motor then inside the
motor we have the stator the stator
holds the copper coils and we're going
to look in detail at that a little later
in this video
concentric to this we have the rotor and
shaft the rotor rotates and as it
rotates so does the shaft the shaft runs
the entire length from the motor and
into the pump this thing connects into
the pumps impeller some models of
centrifugal pumps like this one will
have a separate shaft for the pump and
the motor separated shafts are joined
using a connection known as a coupling
couple pumps will usually have a bearing
house which as the name suggests houses
the bearings the shaft continues into
the pump casing
enters the casing it passes through a
gland packing and the stuffing box which
combined to form a seal the shaft then
connects onto the impeller the impeller
imparts centrifugal force onto the fluid
which enables us to move liquids such as
water through a pipe the impeller is
enclosed within the pump casing the
casing contains and directs the fire
water as the impeller pulls it in and
pushes it out therefore we have a
suction in that and a discharged outlet
at the back of the electrical motor we
see that the fan is connected to the
shaft when the motor rotates the shaft
the fan will therefore also rotate
the fan is used to cool down the
electrical motor and it will blow
ambient air over the casing to dissipate
the unwanted heat if the motor becomes
too hot the insulation on the coils
inside the motor will melt causing the
motor to short-circuit and destroy
itself
the fins on the outside perimeter of the
casing increased the surface area of the
casing which allows us to remove more
unwanted heat the electrical motor comes
in either three-phase or single phase
configuration depending on the
application we're going to look at
three-phase as this is the most common
inside the three-phase induction motor
we have three separated coils which are
wound around the stator each coil set is
connected to a different phase to
produce a rotating magnetic field when
we pass AC or alternating current
through each coil the coil will produce
an electromagnetic field which changes
in intensity as well as polarity as the
electrons passing through it change
direction between forwards and backwards
but if we connect each coil to a
different phase then the electrons will
change direction between forwards and
backwards at a different time compared
to the other phases this means that the
magnetic field of each coil will change
in intensity as well as polarity at a
different time compared to the other
phases to distribute this magnetic field
we rotate the coils 120 degrees from the
previous phase and insert them into the
stator of the motor casing this will
create the effect of a rotating magnetic
field at the center of the stator we
place the rotor and shaft the rotor will
be affected by the rotating magnetic
field and will force it to also rotate
the rotor is connected to the shaft and
the shaft runs from the fan through the
rotor all the way up to the impeller
this way when the rotor rotates so will
the impeller so now by creating the
rotating magnetic field within the motor
we spin the rotor which spins the shaft
and this spins the impeller looking at
the pump casing we find a channel for
water to flow along which is called the
volute this volute spirals around the
perimeter of the casing up to the pump
outlet this channel increases in
diameter as it makes its way to the
outlet the shaft passes through the
seals and into the pump casing or it
connects to the impeller there are many
types of impeller but most will have
these backward curved vanes which will
either be open semi open or closed with
some shrouds these backward curved vanes
do not push the water
the curves rotate with the outer edge
moving in the direction of the expanding
volute these vanes will provide the
fluid with a smooth path for the water
to flow we'll see that a little later in
this video the impeller is submerged in
water
when the impeller rotates the water
within the impeller also rotates as the
water rotates the liquid is radially
pushed out in all directions to the edge
of the impeller and into the balloon as
the water moves outwards off the
impeller it creates a region of low
pressure which pulls more water in
through the suction Inlet the water
enters into the eye of the impeller and
is trapped there between the blades as
the impeller rotates it imparts kinetic
energy or velocity onto the water
by the time the water reaches the edge
of the impeller it has reached a very
high velocity this high-speed water
flows off the impeller and into the blue
where it hits the wall of the pump
casing
this impact converts the velocity into
potential energy or pressure more water
follows behind this and so our flow
develops the volute channel has an
expanding diameter as it spirals around
the circumference of the pump casing
as it expands the velocity of the water
will decrease resulting in pressure
increasing this expanding channel
therefore allows more water to keep
joining and converting into pressure so
the discharge outlet is therefore a
higher pressure than the suction Inlet
the high pressure at the discharge
allows us to force the fluid through
pipes and into a storage tank or around
a pipe system the thickness of the
impeller and the rotational speed
affects the volume flow rate from the
pump but the diameter of the impeller
and the rotational speed will increase
the pressure it can produce centrifugal
pumps are represented in engineering
drawings with symbols like these they
can vary slightly from this so do check
the drawings information section in psh
a term you're going to hear is the NP s
H which is the acronym for net positive
suction pressure will briefly cover what
this means there are two letters at the
end of the acronym the NP s HR and the
NP SH a the R is the required MPs H each
pump is tested for this value and this
can be obtained from the pump
manufacturer by the pump operating chart
don't worry about this confusing looking
chart at this point we're going to break
it down and cover that in detail in a
dedicated video links to that in the
video description down below the r-value
is basically a warning or danger point
as the water enters the pump and flows
into the impellers eye it experiences a
lot of energy due to the friction this
will give us a pressure drop at certain
conditions the water flowing through
this section can reach boiling point
when this occurs we refer to this as
cavitation we're going to see more on
that in just a moment the other letter
was the a and this is the MPs H
available this depends on the insulation
of the pump needs to be calculated it
considers things such as insulation type
and elevation liquid temperature liquid
boiling point etc the available pressure
should always be higher than the
required value for example if we have an
insulation and we calculate the MPs H a
is 11 but the pump requires an MPs HR of
4 then the
pomp should be okay however if we
installed a pump that required an MPs HR
of 13 then the available MPs H is
insufficient and cavitation will occur
so what is cavitation as we know water
can turn from a liquid state into steam
or gas state we know that water boils at
around 100 degrees Celsius and that's
because is at sea level which has an
atmospheric pressure of 101.325 kpa but
if we went to the top of Mount Everest
then water boils here at just 71 degrees
Celsius and that's because the
atmospheric pressure has reduced to 34
kPa as the atmospheric pressure reduces
it becomes easier for the water to boil
so with the suction inlet of the pump we
know that there is going to be a
pressure drop and if this pressure is
less than the vapor pressure of the
liquid being pumped then the water can
reach boiling point when this happens
cavitation occurs during cavitation air
particles within the water will expand
as they reach boiling point these will
then collapse in on themselves very
rapidly as they collapse they will
damage the impeller as well as the pump
casing this removes small parts of metal
from the surface and if this keeps
occurring then it will eventually
destroy the pump
therefore we must insure the available
pressure is higher than the required
pressure of the pump
we use centrifugal pumps everywhere we
use them to move liquids from one tank
to another or around the system for
example we might use a small in-line
centrifugal pump in our domestic heating
circuit to move heated water around the
property we might use a large
centrifugal pump to move the condenser
water from a chillers condenser and up
to the cooling tower on the roof as part
of the centralized cooling system we're
going to look at the type of pumps and
their application in our next video in
this series okay that's it for this
video but to continual learning then
check out one of the videos on screen
now and I'll catch you there for the
next lesson

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