If the pipe diameter is increased while the flow remains the same, what happens to friction loss?

• A. It reduces friction loss.
• B. It remains unchanged.
• C. It doubles friction loss.
• D. It increases friction loss.

Answer: (A)

When the flow rate is kept the same, increasing the pipe diameter lowers the fluid velocity because Q = A v and A grows with D^2. Friction losses are driven by wall shear, which scales with velocity (in the common Darcy–Weisbach view, head loss goes with v^2). So as velocity drops, the friction loss drops even more strongly. In simple terms, a larger pipe gives the fluid more space to move, reducing the energy lost to friction. The result is a smaller friction loss for the same flow.