Flow rate through an orifice from pressure difference and discharge coefficient Cd.
Fluid Mechanics IIOrifice flow equation:
Torricelli (theoretical) velocity:
Cd accounts for both the vena contracta (contraction coefficient Cc) and friction losses (velocity coefficient Cv): Cd = Cc × Cv.
Where:
Typical discharge coefficients Cd:
| Orifice type | Cd |
|---|---|
| Sharp-edged orifice (standard) | 0.61 |
| Square-edged orifice | 0.63 |
| Well-rounded entrance / nozzle | 0.98 |
| ISA 1932 nozzle | 0.96 |
| Long-radius nozzle | 0.98 |
| Short tube orifice | 0.82 |
| Borda re-entrant tube | 0.52 |
The orifice equation is derived from Bernoulli's equation between the upstream section and the vena contracta (the narrowest point of the jet just downstream of the orifice). Cd corrects for real-fluid effects not captured by the ideal Bernoulli equation.