For horizontal drain piping that is 4 inches or larger in diameter, what minimum computed velocity must it achieve when flowing half full?

• A. 2 FPS
• B. 1 FPS
• C. 3 FPS
• D. 4 FPS

Answer: (A)

Maintaining a self-cleansing flow in horizontal sanitary piping requires a minimum velocity so solids stay suspended and don’t settle. For pipes four inches in diameter or larger, the established minimum velocity when the flow is half full is two feet per second. This threshold keeps solids moving through the pipe under typical wastewater loads, even when the pipe isn’t full. A velocity of one foot per second would be too low for larger pipes, allowing sediment buildup, while three or four feet per second exceed the minimum and describe higher-flow conditions rather than the required baseline. So, two feet per second is the standard minimum for this case.