A common rule of thumb is that you can pump through frac iron at 42 Fps. An easy way to compute how many barrels a minute that is; square the diameter and multiply by 2.0. It gets you close.
The real formula is D x D x FPSrating x .0583
If you have and internal diameter of 3.74 then:
3.74″ x 3.74″ x 42 Fps x .0583 = 34.25 Bpm RateMax
The Constant .0583 comes from dividing 60 seconds by 1029.4 = 0.058286
You hopefully remember 1029.4 from the ‘Learn section’ – you’re basically multiplying Feet per Second(FPS) to get Feet per Minute(FPM) and then divide by the 1029.4 [volume of tubulars constant].