- [John] Hi, John here. In this video we're going to look at the centrifugal pump. And I'm gonna explain to you how it works. So let's dive in. Let's do a little spin. Here is the exterior
view of the pump again. As we spin round the other side, we've got a cross-section. Now I'm not gonna go
through the terminology and the components, 'cause we covered that in another video. But let's just go through the components that we actually need to talk about. And the components are the impeller. And the volute casing. So how is it that the
centrifugal pump works? You can see from the
design and the animation that the pump's construction
is quite simple. Essentially we've got an impeller rotating within a volute casing. And as it rotates through the liquid we're creating pressure. And this pressure differential, that is the difference
between the suction side of the pump and the
discharge side of the pump, is what causes the liquid to flow. Well, in order to understand
how the centrifugal pump works we need to have a very quick look at some of the theory. So let's first pull up a open
centrifugal pump impeller. So, here we are. We're now looking at a
centrifugal semi-open type impeller. So it's like we saw
earlier was in closed type, and this one is a semi-open type, or the semi-closed, depending
on how you wanna look at it. As you can see, it is rotating in a clockwise direction. I'll give it a little spin. And we can see also that
there's a shaft in the middle of the impeller, and a shaft key which connects the shaft to the impeller itself. The suction side of the impeller is this middle area here. That's what we call the
eye of the impeller. And as we draw the liquid
in we're gonna create a negative pressure at
the eye of the impeller. And then we're gonna throw
the liquid outwards radially away from the center of the impeller towards the outer
periphery of the impeller. The reason the liquid is
thrown outwards radially, away from the center eye of the impeller, is because of the friction
between the impeller and the liquid. As the impeller moves,
some of this movement is imparted onto the liquid. And because the impeller is rotating from a center axis the
liquid has a tendency to be thrown outwards away
from the eye of the impeller. Now the force that causes this is known as centrifugal force. And because this type of
pump uses centrifugal force it's called a centrifugal pump. If you've ever driven
around the corner very fast in your car you'll know
that if you're driving around a corner and turning into the left you have a tendency to be
thrown outwards to the right. And this is centrifugal force. Now the impeller does the same job, although on a much smaller scale. It rotates very fast from
a center axis of rotation, and the liquid that is
drawn into the center is thrown outwards to the side radially away from the eye of the impeller. This centrifugal force
gives us a large increase in velocity, which we can
later turn into pressure. So let's just pause the animation, and I can show you the
flow path of the liquid. So the animation is now paused. Zoom in. Our liquid would come in. In fact let's just do it
as the liquid would flow. It would flow into the middle, like so. The liquid is then
thrown outwards radially, and it's gonna go between these two veins, or between the set of veins, I should say, between a pair of veins. And then it's gonna flow up along here. It's gonna be thrown outwards, and then it's gonna leave the impeller. So let's do this time but this time we'll just add some arrows to mark the flow. So as you can see, now
the liquid would flow out radially. And as it does so, as it flows out through the channels, the flow-path area is gonna gradually increase. And as it increases we're
gonna get a reduction in velocity and an increase in pressure. So that is essentially what
the centrifugal impeller is doing. It's converting velocity into pressure. And that's what we need in
order that we can get flow. But let's now have a look
at the theory behind this. And the theory behind it is
known as Bernoulli's Principle. Okay. So here we are. This is our Bernoulli Principle simulator. You can see we've got an underground pipe. You can assume this is gonna
be an underground water pipe. I'm gonna turn the dots off. But you can see the flow
is from left to right. And if we take our speed gauge, we can put a speed gauge on the pipe. Another speed gauge here. And we can see, in fact
I'll raise these up just ever so slightly. We can see that at these two points the flow is constant and
the speed remains the same irrespective of where
we've got our speed gauges. We can do it like this. And this makes sense because
we've got a constant flow. Now this constant flow is required in order to apply Bernoulli's Principle. Bernoulli's Principle
states that if we have a constant flow and we
change the flow-path area then we get a corresponding
change in pressure and velocity. So let's put the theory to the test. I'll take my pressure gauge and I'm gonna install it. In fact we can install it roughly... Here. So it's slightly in
front of the speed gauge in relationship to the flow. And I'll take my pressure gauge and I'll try and do roughly
the same thing again. So it's a bit further down here. And there we go. We've got the exact same pressure and the exact same speed. So Bernoulli's Principle
states that if the flow is constant and we
adjust the flow-path area then we get a change
in speed and pressure. So I'll extend the pipe and I'll make the pipe a
little bit bigger in diameter. And we can see already
the speed has dropped. It's now .7 meters per
second instead of 1.6. And the pressure has increased. We can do it also here. Again the speed has dropped. .4 meters per second. And the pressure has increased again. So we know that if we
increase the flow-path area we get a reduction in speed and in increase in pressure. And that's what Bernoulli's
Principle states. If we go the opposite way we can try and make the flow
path here slightly smaller. In fact I have to extend it a little bit because otherwise we don't
get our pressure reading. And we can see that the speed on the left is .4 meters per second. But the speed on the right has increased. So let's take this handle, reduce it. And now we can really see that if we create a massive restriction in the pipeline, then we get a huge reduction in pressure and a massive increase in speed. You can see here our speed is, or was, about 5.6, 5.7 meters per second. So, a quick recap. If the flow path becomes larger, the speed reduces and
the pressure increases. If the flow path becomes
smaller the speed increases and the pressure reduces. And you can see we got quite a lot of pressure reduction when we compare both
the left and the right. So let's now go back to our impeller and apply Bernoulli's Principle in order to figure out how it works. So, here is our impeller again. And we can see that the
flow-path area increases as the flow flows out
radially away from the center of the impeller. And we know now that this
increase in flow path is going to cause a reduction in velocity and an increase in pressure. And that is essentially how
a centrifugal pump works. Now after the impeller
usually there will be a volute casing or a diffuser. But the concept behind the
design of the volute casing and diffuser is the same
as for the impeller. We know that we need to
increase the flow path-area in order to increase the pressure. And as you can see on the diagram now, the volute casing does just that. As the liquid is discharged
from the impeller it's gonna flow around the volute casing, and we're gonna get a
reduction in velocity and an increase in pressure. And that is essentially how
a centrifugal pump works. There really is nothing more to it. So let's go back to our
main centrifugal pump model, and we'll do a very quick recap. So here we are, we're looking
at a centrifugal pump. Let's imagine we are the flow. The centrifugal pump impeller is spinning. We're gonna be drawn into the impeller eye because it's creating a negative pressure. So we've been drawn into the impeller eye. We're now gonna flow out of the veins. You can see the veins just on
the inside of the impeller. I'll see if I can actually
get through the veins. It's gonna be quite tricky because the whole thing's pretty tight. And we're flowing through
here, through here, through, further, further, further. We've been thrown out radially. And then we're gonna exit the impeller and go to the volute casing. Now we're inside the volute casing. And look, as we are
thrown out of the impeller we enter the volute casing, and notice how much more space there is as we move towards the discharge pipe. That's this pipe that we can
view at the top of the screen, compared to how much space there is on the opposite side. You can see it's getting
continually narrower if we want it to flow back the other way. So, down this side here is quite narrow. But the diameter of the volute casing is gradually increasing until we get onto this side here. And that means our flow path
is gradually getting bigger, and all of a sudden
we've got loads of space. And that means the velocity has decreased and the pressure has increased. And then we can move on our way through the rest of the system. So that's it. That's how the centrifugal pump works. If you haven't checked
out the other two videos on centrifugal pump impellers and centrifugal pump components, then I recommend you do that now. Because those videos
are also really useful. And if you like this video please do like it or
share it on social media. It really does help me out
and allows me to produce more and more content. This video lesson is taken
from the introduction Centrifugal Pumps course. So if you like the lesson, then check the video description area. And there you'll find
a link with a special discount price coupon. And if you click on
that link you'll be able to purchase the course
at a discount price. Thanks very much for you time. (mid-tempo music)
