Which term describes flow in pipes under pressure?

• A. Open channel
• B. Closed conduit
• C. Free-flow
• D. Subsurface

Answer: (B)

When fluid is moving through a pipe system that is sealed from the atmosphere, the flow is described as closed conduit flow. The liquid is confined inside a pipe, so pressure differences—caused by pumps or head losses along the line—drive the movement, not a free surface exposed to air. Open channel flow, by contrast, has a free surface exposed to atmospheric pressure, so it isn’t pressurized inside a closed pipe. Free-flow generally refers to gravity-driven flow in an open channel, still with a free surface. Subsurface flow describes movement through soil or rock, not within a pipe system. So, flow in pipes under pressure is best described as closed conduit.