How do i work out a flow rate?

[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

 

[BvU]

Hello Dominic, welcome to PF :)

You might need a bit more information to get a sensible answer.
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

 

[Dominic Pew]

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

 

[BvU]

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.

[edit] forgot to insert the calculator link. done that now.

 

[PJC01]

for reaching out! Calculating flow rate involves using the equation Q = A x V, where Q is the flow rate in cubic meters per second, A is the cross-sectional area of the pipe in square meters, and V is the velocity of the fluid in meters per second. To convert cubic meters per second to liters per second, you can multiply the result by 1000.

To calculate the cross-sectional area, you can use the formula A = πr^2, where r is the radius of the pipe. In this case, the diameter is given, so you will need to divide it by 2 to get the radius. So, for a pipe with a diameter of 100mm, the radius would be 50mm or 0.05m.

To calculate the velocity, you can use the Bernoulli's equation: P1 + 1/2ρv1^2 + ρgh1 = P2 + 1/2ρv2^2 + ρgh2, where P is pressure, ρ is density, v is velocity, and h is height. In this case, you already know the pressure (3.5bar) and the height (assuming the hydrant is level with the ground). So, you can rearrange the equation to solve for v2, which is the velocity of the fluid at the end of the pipe. Once you have the velocity, you can plug it into the original equation to calculate the flow rate in cubic meters per second.

Remember to convert the result to liters per second by multiplying by 1000. I hope this helps! Let me know if you have any further questions.