A 12-inch diameter ductile iron transmission main carries 1,200 GPM. What is the mean water velocity inside the pipe in feet per second (fps)?

Formula Used: Velocity (fps) = Flow (cfs) ÷ Area (sq ft) = [Flow (GPM) × 0.4085] ÷ [Diameter (in)]²

Step-by-Step Mathematical Derivation:

1. Convert flow to cubic feet per second: 1,200 GPM ÷ 448.83 gpm/cfs = 2.6736 cfs.
2. Calculate 12-inch pipe area: 0.7854 × (1.0 ft)² = 0.7854 sq ft.
3. Velocity = 2.6736 cfs ÷ 0.7854 sq ft = 3.404 fps (~3.41 fps).

⚠️ Common Exam Trap: Always convert pipe diameter from inches to feet before calculating area (12 in = 1.0 ft; 8 in = 0.667 ft).