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Fluid Mechanics "Centrifugal Pumps"

Centrifugal Pumps :

  • we'll just cover certain sections of it, it's on your course syllabus.
  • But we'll start off looking at centrifugal pumps. 
  • A very common type of pump. Swimming pool pumps, spa pumps, all kinds of pumps. 


  • Quite a few of them are centrifugal pumps.
  • I thought so. Okay,sometime this quarter, I don't think this early, you're going to run a test on a centrifugal pump in the fluids lab. 
  • And from that test you're going to put together some performance curves for our centrifugal pump. 
  • So, this is how we test a centrifugal pump. 

I'll go through the procedure that we use in our fluids lab. 
  • We have a pump in a circuit with water in it, from a big reservoir. 
  • The water is pumped through here, it goes around, back into the big reservoir. 
  • We have flow meters in the test setup so we know what the flow rate Q is. We have pressure gauges at points one and two.
  • A centrifugal pump is a radial type of a pump. 
  • There's an impeller blade in there. The impeller blade is rotating.
  • It spins the water outward radially, centrifugal pumps, centrifugal.
  • The purpose in life of a pump is to increase the pressure. So P2 is bigger than P1 of course.
  • That's what it's doing. 
Effects :


  • It's increasing the pressure of the fluid, water. 
  • We can write the energy equation. By the way, this is called a suction side where the water enters suction side. 
  • This is a discharge side of the pump. Suction side, discharge side. We can write the energy equation from point one to point two,right at the pump. 
  • A matter of fact, in the lab when you run the experiment,you'll see there's pressure taps in the piping. 
  • And those pressure taps are hooked up to differential pressure gauges. There's also, the pump is driven by a
  • motor. 
  • Between the motor and the pump there's a coupling. In the coupling there is a tachometer built in. So we can get the speed of the pump, the shaft that drives the pump. The speed. 
  • There's also a torque meter in that coupling. So we can get the torque on the shaft between the motor and the pump.
  • So, we can get the torque, we can get the pump speed, we can get the pressure rise across the pump. 
Results:
  • We can get the flow rate Q. We write the energy equation from point one suction side to point two, discharge side. 
  • Here's the equation. There's no straight pipe, there's straight pipe, minimal straight pipe. Maybe a few inches at the most. Forget about that.
  • So, P1 over gamma. B1 squared over 2G, Z1, HP head of the pump. This equation is in feeder meters.
  • P2 gamma. B2 squared divided by 2G, Z2. Usually this difference in elevation, Z1 to Z2, in minimal, in the matter of inches. So, we usually neglect that delta Z. And usually the
  • difference in the velocity head between one and two, is negligible so we can usually neglect that. 
  • So we're left with the pressure rise across the pump divided by gamma, equal the head developed by the head for the pump. Of course continuity tells us Q1 has to equal Q2. 
  • Okay, so that's our equation. 
  • So, what you do in the lab and the way that engineers test pumps is you run at a constant speed. 
  • Then you run different flow rates through the pump. And you get a pump performance curve. Which the pump head is plotted on the y-axis and the flow rate Q on the x-axis. And typical shape of the head curve comes off relatively constant and then drops off like that as the flow rate increases. 
  • So the pump head is constant for a while and then drops off pretty rapidly because of losses in the pump at the higher flow rates. 
  • Drops off rapidly. So that's the pump head curve. 
  • Now, besides the pump head curve, there is other curves that you can plot. Let's write these guys down. Let's take first of all, the power into the pump. W dot M . It's either in kilowatts or horsepower. Okay, that comes from where? The motor. There's a shaft from the motor to the pump.
  • So this is the shaft power. It's torque times omega. Torque times the pump speed omega. Radiance per second.
  • Pump speed. And that can give us kilowatts, or we can convert it, put pounds per second into horsepower.
  • Okay. In a laboratory environment as I told you, the shaft has a coupling on it between the pump and the motor and the coupling has a torque meter built into it and a tachometer. 
  • This is HP, this curve is for HP. Somewhat fairly linear for a while. This is the power in. And now we can also define the pump efficiency.
  • Pump efficiency, what comes out from the pump divided by what comes into the pump, the power. 
  • There is the power in right there. Okay. So, obviously it's dimensionless. W dot out is gamma QHP divided by the power in. 
  • So, with our laboratory data we take, we also in our fluids lab, we also draw the pump efficiency.
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