Multiple Fire Hydrants Flowing 1

Years ago, when I did a lot of water modeling for a water district in western Los Angeles County, the County Fire Department would have required 1500+1500+500=3500 for your situation. Other jurisdictions might allow 3x1166±=3500 or some other combination, though most jurisdictions I have worked with over the years have left it to my judgement. In these cases I usually use the LACFD method that I have squirred away in my files, and which I vaguely recall having posted here at some time in the past.

I can't imagine anyone requiring 3x1500 when the total requirement is 3500 gpm. In any event, I would ask the fire department.

Normal fire fighting would not actually stress a system that hard. The 3500 gpm fire flow requirement includes a safety factor that is owned by the fire department. Same with the 20 psi minimum at the hydrant head. A friend who works for the Ventura County Fire Department told me back in the 1980s that a typical residential fire might require 500-750 gpm for 30-45 minutes, but if the fire jumps to another structure more would be needed. This is why they might require 1250 gpm for two hours.

Also keep in mind that 1500 gpm in a 6" hydrant lateral is a whopping 17 fps. At 1500 gpm you will probably lose maybe 2 psi in the lateral (L=30', C=130, d=6", Q=1500 gpm >> 4.7'=2.0 psi) and another 6 to 8 psi in the hydrant head (I have laboratory tested three American AVK hydrants at 6.3, 6.5, and 6.6 psi loss at 1500 gpm). Under these conditions, you need around 30 psi in the main to provide 20 psi at the hydrant head, not counting for elevation differences.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

thanks makes since. I actually was aware of the velocities and psi loses, which made me think 1500gpm min was wrong, but second quessed myself because of the thrust blocking that is typically installed.

In real life, you won't typically try to draw 1500 gpm from a 6" FH. That's just a conservative way to model the hydraulics of the system. For an extreme example, check what happens to the residual pressure if you put all 3500 gpm at ONE hydrant in the model.

I may have misunderstood your comment about thrust blocks, but their sizing depends on maximum system pressure plus a safety factor and not on high flow rate.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill