How a Siphon Works — and Why the Crown Pressure Matters
Discharge is set by the drop between free surfaces, not by the hump. Crown vacuum, priming, practical 7–8 m lift, and the same physics as NPSH.
Key takeaways
- Driving head is z_up − z_down, not the crown height.
- Crown pressure must stay above P_v or the siphon breaks.
- Useful lift is about 7–8 m of water in practice, less when hot or high.
- Prime the barrel and keep a single high point with air release.
A siphon looks like it is pumping over a hill. It is not. Once primed, the driving head is only the difference between the upstream and downstream free surfaces. The hill shows up as a vacuum at the crown.
Discharge
Example: Δz = 7 m, D = 0.15 m, L = 40 m, f = 0.02, K = 1.5. Then fL/D + K + 1 = 0.02×40/0.15 + 1.5 + 1 = 7.83. V = √(2×9.81×7/7.83) = 4.19 m/s, Q ≈ 74 L/s. Check the crown next — a fast siphon can still be useless if it cavitates.
Open solver: Siphon / gravity-flow calculatorCrown check
If P_c falls to P_v, the water column separates and the siphon dies. In practice you never budget the full 10.3 m of atmospheric head: dissolved gas, leaks, and imperfect priming cut the useful lift to about 7–8 m of water at sea level, less when hot or at altitude. That is the same physics as NPSH on a pump suction.
Open solver: Bernoulli energy lineOpen solver: NPSH (same vapor-pressure limit)Priming and layout
- The barrel must be filled. Vacuum pumps, a foot valve, or submerging the downstream end are the usual tricks.
- Keep the crown as low as the obstacle allows. Extra height is pure vacuum debt.
- A long downstream leg helps discharge; a long upstream leg only adds loss before the crown.
Frequently asked questions
Not with water at sea level. Atmospheric head is 10.3 m, and vapor pressure plus leaks eat the rest. Real installations budget 7–8 m to the crown.