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

Member Posts: 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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• Posts: 156Forum Coordinator

what 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?

• Posts: 7Member

Q 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.

• Posts: 156Forum Coordinator

very 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?

• Posts: 7Member

So 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.

• Posts: 7Member

Oh 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.

• Posts: 156Forum Coordinator

your 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.

• Posts: 7Member

Do 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.

• Posts: 156Forum Coordinator

actually, 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.

• Posts: 7Member

I 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.

• Posts: 156Forum Coordinator

1) 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

• Posts: 7Member

1) 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.

• UKPosts: 9,127Forum Coordinator

I 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.

• Posts: 156Forum Coordinator

one 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.