# What am I doing wrong? Trying to find pump power.

FluidsSkywalker
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**1**The horsepower should be around 1. I can't figure out what I'm doing wrong to get 300 HP. If anyone has any pointers, I'd appreciate it.

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## Answers

156Forum Coordinatorwhat is Q and what is V? If V is velocity then you have wrong units, it should be ft/s, not ft^3/s. Your Re is actually dimensional, ft^2, which is wrong. Can you double-check your math?

7MemberQ is volume flow rate, I wrote the units wrong, Volume would be 17.82 ft^3, Q would be 1.78ft^3/s and V which is velocity would be 145.2 ft/s. Plugging the fixed units in should make the Re dimensionless.

156Forum Coordinatorvery well. What is missing in your analysis is pump's flow rate. This is a pump spec and thus an external user input. You assume Q = Pipe volume per second. Why? Is it what the pump delivering or are you sizing up the pump for this flow rate?

7MemberSo the problem called for the tank to be emptied when the water level in the tank reached 3ft. I used that and the tank's diameter to calculate the volume. The tank is emptied in 10 seconds so I used that to find the volume flow rate. So I was trying to find a pump powerful enough to empty that volume in that amount of time.

7MemberOh and then to find the water velocity I used the volume flow rate of water emptying the tank and the cross-sectional area of the pipe.

156Forum Coordinatoryour calculations seem to be correct for the pressure head. Pump power, however, I am not so sure. Where do you get 1 HP from? But if I calculate the pump power rating, e. g. per aiche.org, for a centrifugal pump, I get 0.62 HP (eff = 1) or0.9 HP with eff = 0.75.

7MemberDo know what equation aiche.org uses for power? Your answer is a lot closer to what I'm supposed to get than the one I got.

156Forum Coordinatoractually, disregard by HP estimate, I confused per sec with per hour.

What you have is correct. So the question is why you expect 1 HP? Where is it coming from? 1HP pump delivers, what, say around 50 GPM or 0.1 ft^3/sec. Your rate is close to 800 GPM, so much more powerful pump will be needed to move such volume.

7MemberI got 1HP from my teacher when I asked them about it. They wouldn't give me any tips on what I was doing wrong or point me in any direction. So that's why I'm here. I've looked over my work multiple times and gone through the math more than once but I can't figure out why I'm getting around 300HP but I'm told I'm supposed to be getting something closer to 1 HP.

156Forum Coordinator1) can you explain me dP calculations, where do you get 11ft /32 lb ft/s^2. With this term you are not getting the units of pressure,2

2) confirm t = 10 sec, and not a larger value of 100 sec, which will give you a closer result

7Member1) Somewhere I got the equation for change in P equal headloss * density * gravity. or dP = HL * p *g. I'm not sure where the 32.2 came from, I think the example I looked at used it to convert units to be pound-force per ft^2.

2) it is in fact 10 sec. I don't know why it's that fast, but it is.

9,127Forum CoordinatorI think gravity is around 32 ft/s/s ? I've kept out as a) I can't remember far enough back to help, b) it's not simulation so others are better qualified and c) it'll take ages to covert everything into SI units.

156Forum Coordinatorone consideration missing in your calculations is hydrostatics. Water will be pushed up the pipe by the water weight in the tank, thus creating a favorable force assisting the pump. I haven't done the calculations, but it maybe enough to use a much smaller pump as opposed to 300 HP you are getting when hydrostatic upforce is neglected.