Flushing a pipe after installation requires a flow velocity of at least __________ and preferably _________

• A. 1.0 ft/s, 2.0 ft/s
• B. 1.5 ft/s, 2.5 ft/s
• C. 2.5 ft/s, 3.5 ft/s
• D. 3.0 ft/s, 4.0 ft/s

Answer: (C)

The main idea is that flushing a newly installed pipe needs enough flow energy to lift and carry debris out of the line. You want a velocity high enough to keep sediments in suspension so they don’t settle and cling to the pipe walls, but not so high that it causes unnecessary erosion or damage.

A practical minimum to mobilize most sediments is about 2.5 ft/s. This ensures sediments don’t just settle and stay put as the water moves through. For a more thorough cleaning, aiming for around 3.5 ft/s is preferred; it improves flushing effectiveness by more reliably clearing finer particles and any residue from installation.

Remember, velocity is tied to flow rate and pipe size, since Q = velocity × cross-sectional area. So the actual flushing flow will depend on the pipe diameter, but the recommended range is 2.5 ft/s minimum and about 3.5 ft/s preferred.