# Calculating water flow

1. Mar 21, 2007

### TSN79

I have a length of pipe 10 mm inside diameter, and a pressure of 5 bar. At the end I require about 0.2 l/s.

I know of the equation area=flow/velocity, A=q/c, but this doesn't take the pressure into consideration. How can I do this?

2. Mar 21, 2007

### Q_Goest

Hi TSN. I've attached a couple of examples of pipe flow analysis out of my college text book. The first one, 7.3, shows the use of the Darcy-Weisbach formula for a horizontal pipe. The second one, 7.4, shows a pipe with a change in elevation such that the "energy equation" (ie: Bernoulli equation) must also be used. Example 7.4 also describes the use of the "Colebrook formula". Note that the Colebrook formula is only an implicit equation which relates friction factor, Re, surface roughness and pipe diameter, so you still need to use the Darcy-Weisbach equation. Alternatively, (and I would strongly recommend) you can use the Darcy-Weisbach equation directly by using explicit equations for friction factor such as those found here:
http://www.eng-tips.com/faqs.cfm?fid=1236

Note that for various fluid restrictions such as entrance and exit losses, pipe contractions or expansions, elbows, valves and other restrictions, you should get to know how to equate all of those restrictions into a single factor known as the "resistance coefficient" (K). That factor, K, is well defined by the Crane paper #410.

K = f L / D

which can be plugged directly into the Darcy-Weisbach equation.

#### Attached Files:

• ###### Pipe Flow Examples.pdf
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3. May 12, 2007

### Mech. Engineer

the velocity is taking care of pressure here..... flow=area*velocity

Bernoulli relates the velocity with the pressure....