Opening Characteristic: Duckbill vs Inline

The actuating force for both valve types is a differential pressure created by changing water levels: when the level outside the pipe drops below the level inside, flow occurs, opening the valve; when the outside level rises back above the inside level, the valve closes and prevents reverse flow. Because this differential acts across a different effective area on each geometry, the two valves open at different rates as pipe velocity increases.

PIPE VELOCITY

← Drag to explore

2.4 m/s

0 L/s (300mm)

G5 INLINE

0% open

The slit diaphragm sits flush inside the pipe bore, exposing a smaller effective area to the flow. Opening is delayed until velocity overcomes elastomer stiffness, then rises steeply toward full bore.

G4 DUCKBILL

0% open

Tapered rubber walls with flat sealing faces present a large projected area to flow. Even modest differential pressure peels the lips apart — opening rises quickly at low velocity, then plateaus near full bore.

VOLUME FLOW / HEADLOSS vs PIPE VELOCITY

Upstream Flow

0 L/s

Volume Through Valve

Headloss

* in unpredictable zone (<80mm)

Shaded band marks flow above the G5 inline valve's 3 m/s rated velocity limit — the G3 duckbill is rated to 6 m/s. Curves show actual volume passed, not just opening angle. The faint closing headloss curve only diverges below the 310 mmAq snap-open point.

G3 — flow

G5 — flow

G3 — headloss

G5 — headloss

G5 — closing (hysteresis)

EFFECTIVE AREA vs OPENING FORCE

The force acting to open each valve is the differential pressure multiplied by the effective area it acts on: F = P × A. For a given pressure differential, a larger effective area produces a larger opening force — this is why the G3 duckbill cracks open at a much lower pressure than the G5 inline, even though both experience the same water level difference.

0.060 m²

G5 inline — flap projected area

0.24 m²

G3 duckbill — combined wall area

4×

larger area on the G3 — roughly 4× the opening force

Comparison of Duckbill vs Inline valves