The 'hydraulic grade line' (HGL) in a pressurised pipe:
Choose the correct answer
Total energy line including velocity head
Piezometric head (p/γ + z) at each point — represents pressure head only; below pipe = negative pressure/cavitation
Velocity profile at each section
Pipe centreline elevation profile
Correct Answer
B. Piezometric head (p/γ + z) at each point — represents pressure head only; below pipe = negative pressure/cavitation
Why is this the answer?
HGL: locus of points to which water would rise in piezometric tubes — represents p/γ + z at each section. Plotted above pipe: pressure positive (normal). Falls below pipe: negative pressure (vacuum → cavitation risk). HGL slope = −hf/L = hydraulic gradient. EGL (energy grade line) = HGL + V²/2g — always above HGL (kinetic head). HGL falls: friction losses, flow acceleration. HGL rises: pump adds energy; pressure recovery in decelerating flow.
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