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Fire hydrant flow calculation - manual

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Civilchica

Civil/Environmental
Mar 26, 2010
5
I have a project with an existing 12" water main out front, and an existing fire hydrant coming off a 150-foot long 8" dead end main onsite. We are expanding the existing building and the plan checker has asked for calculations proving we have residual pressure of 20psi for a flow of 1500gpm at the existing hydrant. We just did fire flow testing using two hydrants on the 12" main out front. Results are 70psi static, 40psi residual, 32psi pitot, 950gpm flow. I want to just do a quick hand calculation using the energy equation, but I am unsure about one thing. For my velocity at the fire hydrant, do I use the velocity based on the 2.5" hydrant orifice, or based on the 8" pipe. In the past I've used WaterCAD or the water district ran the calculations, and I don't think they factor in the fire hydrant orifice size. But I believe the fire hydrant testing does, so I don't know which to use and it makes a huge difference. Any feedback would be appreciated. Thanks.
 
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Based on what you said you don't have it. Not even close.

It appears the flow test results are reasonable. With a residual of 40 psi and using a k-factor of 150 for the hydrant it is reasonable to expect somewhere around 948 gpm through a 2 1/2" butt. That is close to what you got.

I show you have between 1 and 2 psi residual @ 1,500 gpm on the public main.


From the city main to the hydrant 150' away you will lose another 2.7 psi @ flowing 1,500 gpm assuming 150' C900 DR18 and a tee and gate valve for a total equivalent length of 209'. Loss/ft=0.013 psi. Not much loss but it will drag you down to a suction of -1.0 psi or thereabouts @ 1,500 gpm.
 
Civilchica, I am a bit late on this response however, I did a quick graph using a hydraulic graph sheet (N^1.85) and based on your hydrant flow test results, you will fall short of the 1500 gpm at 20psi. The graph show a flow of 1250 gpm at 20 psi. You'll need to increase the pressure by at leat 20 psi to get 1500 gpm.

See the JPEG attachment.

The graph sheet is a standard form used by the insurance industry for hydrant flow, sprinkler flow and fire pump flow tests.
 
 http://files.engineering.com/getfile.aspx?folder=1a5c86b2-0902-4fb8-99e4-6567ce18c5d4&file=Hydraulic_graph.jpg
About your first question, base the velocity on the 8" pipe when using Bernouilli's equation since the velocity components will cancel out. Also depending on what you are trying to calculate use the Hazen William formula.Take a look at another attachment whereby you can intermingle both equations
 
About your first question, base the velocity on the 8" pipe when using Bernouilli's equation since the velocity components will cancel out. Also depending on what you are trying to calculate use the Hazen William formula.Take a look at another attachment whereby you can use and intermingle both equations.
 
 http://files.engineering.com/getfile.aspx?folder=39031788-8a4a-4a02-9550-483823b3e4e6&file=Fluid_equations.jpg
Thanks for the feedback. I agree with the calculations showing that we have inadequate pressure. We are working with the water agency to find a solution.
 
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