# Homework Help: Formula for water pressure, flow and power

1. Aug 7, 2007

### cens

I feel like this should be readily available, but I cannot find any information about it through google.

I am trying to find a formula that will tell me the amount of energy necessary to inject water into a system at a given pressure.

Put another way: I have a pipe of a known size and water (pipe a) is flowing through it at a know pressure (pressure x). I want to hook a pipe (pipe b) up to that original pipe a to inject additional water into it. I want to make sure that I am injecting the water into that system at a fixed flow (measured in GPM). I need to find out how much energy (say in horsepower or kPA) I would need to use to inject the water.

or graphically what is the energy necessary to inject a constant stream of water at point O, given pressure x?:

PIPE A (pressure x)
-O--------->->->->----------------->->->->----------
|
^
|P
|I
|P
|E
^
|B
|

I know that if pressure x is low, then I will not need very much energy to inject the water into that pipe, and if it is high, I will require quite a bit of energy.

I however, do not know the specific formula that is used to calculate this energy requirement. Any links, or simply the formula, if known, would be greatly appreciated.

Thanks very much.

2. Aug 9, 2007

### FredGarvin

$$P = p*\dot{V}$$

Where:
$$P$$ is the power required
$$p$$ is the pressure required
$$\dot{V}$$ is the volumetric flow rate

Make sure your units are consistent, i.e. use the standard units, m, sec, N or Lbf, ft, s.

If you are pumping water and need it quick you can use this too:

$$HP = \frac{\tex{psi}*\tex{GPM}}{1714}$$

Where:
$$HP$$ is the power in horesepower
$$psi$$ is the pumping pressure in psi(g)
$$GPM$$ is the flow rate in gallons per minute

Last edited: Aug 9, 2007
3. Apr 29, 2010

### KristoferE

I would like to calculate the power need for fulfilling a pressurized airflow stream. With water it is simple but since air is a compressible gas it gets more difficult, or not? I've seen different alternatives. Please help!

regards /k