# How do i work out a flow rate?

• Dominic Pew

#### Dominic Pew

Hi

I need to work out a flow rate of a fire hydrant in Liters per second.

I know the pressure is 3.5bar and the pipe diameter is 100mm

Any help would be appreciated

Thanks

Hello Dominic, welcome to PF :)

See e.g. this calculator and what it all needs to calculate a pressure drop

My advice would be to read up a bit on fluid flow , e.g. http://udel.edu/~inamdar/EGTE215/Laminar_turbulent.pdf [Broken]

Last edited by a moderator:
unfortunately this is all the information i have, i know the pipe is 100mm water main. the outlet in the hydrant is also 100mm and the pressure at this point is 3.5barpsi

Then you need to use your imagination to come up with some more constraints. If the thing is open wide, the 2.5 bar pressure difference with the outside world causes a hefty flow which in turn causes a counteracting pressure drop due to resistance. How much depends on geometry of the pipes.
The 3.5 bar will most likely also drop when it's opened.

Selected parts from this one might be nice too.

And a calculator that needs less input does a calculation for a draining tank. You get the 3.5 bar by filling in 35 m for the height. It comes back with 200 liter/s, spouting out at 26 m/s. As you see further down, it uses the Bernoulli equation and little else. At least its a rough estimate.

 forgot to insert the calculator link. done that now.