Too many unknowns B but typically, good design for water you want to size the pipe to result in 10 to 12 ft/sec velocity for carbon steel. From what you have shared the Cameron hydraulic data book indicates 3500 to 4000 GPM gives you a range of 10.1 to 11.5 ft/sec in new 12"dia carbon steel pipe. I don't do much pvc pipe but you can probably run a little higher on it because I would think the ID would be smoother than CS. You can probably get by with flowing 6500 GPM with a velocity of 18.6 ft/sec.

Not sure this helped any.

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Jim