How do i work out a flow rate?

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Discussion Overview

The discussion revolves around calculating the flow rate of a fire hydrant, specifically in liters per second, given a pressure of 3.5 bar and a pipe diameter of 100 mm. Participants explore the necessary information and constraints for an accurate calculation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant requests assistance in calculating the flow rate with the provided pressure and pipe diameter.
  • Another participant suggests that additional information is needed for a sensible answer and references a calculator for pressure drop calculations.
  • A participant notes that the only information available is the pipe diameter and pressure at the hydrant outlet, which is also 100 mm and 3.5 bar respectively.
  • One participant emphasizes the need for more constraints, mentioning that the pressure difference with the outside world will affect flow and that the pressure may drop when the hydrant is opened.
  • A rough estimate is provided using the Bernoulli equation, suggesting that a height of 35 m corresponds to the given pressure and results in a flow rate of approximately 200 liters per second at a speed of 26 m/s.

Areas of Agreement / Disagreement

Participants do not reach a consensus on how to proceed with the calculation, as there are differing views on the necessary constraints and information required.

Contextual Notes

Limitations include the lack of specific details regarding the geometry of the pipes and the potential pressure drop when the hydrant is opened, which may affect the flow rate calculation.

Dominic Pew
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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
 
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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
 
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.

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

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