The 'hydraulic gradient line' (HGL) in a pressure pipe falls below the pipe centreline when:
Choose the correct answer
Flow velocity is high
Pressure in the pipe becomes sub-atmospheric (negative) — HGL below pipe level means vacuum risk
Pump is running above capacity
Only in vertical pipes
Correct Answer
B. Pressure in the pipe becomes sub-atmospheric (negative) — HGL below pipe level means vacuum risk
Why is this the answer?
HGL = p/(ρg) + z (piezometric head). If HGL < pipe invert (centreline), then p < 0 → sub-atmospheric (negative) pressure → potential for cavitation, air entrainment, or pipe collapse (for thin-walled pipes). Designed pipes must keep HGL above pipe to maintain positive pressure throughout. Critical at summits.
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