Question 1

Why is the constant 0.4085?

Accepted Answer

Converting gpm × (1 ft³/7.48 gal) × (1 min/60 s) ÷ (π × (D/24)² ft²) collapses to 0.4085 × Q/D² where Q is gpm and D is inches.

Question 2

What is C900 inside diameter?

Accepted Answer

AWWA C900 PVC matches iron-pipe-size on the OUTSIDE. ID is smaller by 2 × wall thickness. A nominal 8-inch DR18 C900 pipe has an ID of about 8.43 inches.

Question 3

What flushing velocity removes biofilm?

Accepted Answer

At least 5 ft/s for unidirectional flushing; 6–8 ft/s for routine biofilm scour. Conventional flushing at 1–2 ft/s does not clean the pipe wall.

Question 4

Is this the same formula for force mains?

Accepted Answer

Yes — fluid is still water and the area calculation is the same. Sewer force mains add a solids-deposition constraint: keep velocity ≥ 2 ft/s at minimum flow.

Pipe Velocity Calculator (ft/s) from GPM and Pipe Diameter