Pressure and Flow in a pipeline

[Anika]

I have a 4inch pipe and I know the pressure at the start of the pipe is 4 bar. How do you calculate the flow rate?

 

[SteamKing]

You're going to need to know something about the pressure in the pipe downstream. It also makes a difference what is flowing within the pipe.

 

[Anika]

Thank you. That is what I thought. I know it is water and that the pipe is connected to a 4bar line that supplies the water to this pipe system. The tie-in between the two lines is at a height of 4m.

 

[SteamKing]

Still not enough information.

Look, you could take an isolated pipe and pressurize it to 4 bar. There would be no flow, but the pressure inside the pipe would still be 4 bar.

If you are not familiar with the Bernoulli equation, perhaps this will help:

http://www.princeton.edu/~asmits/Bicycle_web/Bernoulli.html

Because this is water in a pipe, there will be friction, which the Bernoulli equation can be modified to include.

 

[Travis_King]

Short answer:

Unless the water is flowing into a pressurized container, it will have to discharge at atmospheric pressure. This means that between the water main (@ 4bar gage) and the outlet (atm), the liquid will have to lose 4 bar of pressure. It can lose the pressure in two ways. Losses due to static pressure loss from elevation change (outlet is higher in elevation than inlet) and dynamic pressure losses due to friction (from the water flowing through the pipe, through valves, filters, etc).

You can easily determine the static loss expected by looking at the elevation difference between inlet and outlet.

The friction loss calculation is a bit more involved, but basically: a pipe of a given material will experience a certain amount of non-reversible energy loss (realized by reduced pressure) for a given flow rate. This is true of fittings, valves, filters, elbows, etc. It's a balancing act. More flow means more pressure loss per foot of pipe. Less flow means less losses. Your flow rate is determined by the configuration of your system and how much water needs to flow through your pipes in order dissipate enough energy that the liquid leaves the system at atmospheric pressure.

Check out the Darcy Weisbach equation and all the stuff that goes along with it: http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html