Table of Contents
Orifice Flow Basics
An orifice is a small opening through which fluid flows, driven by a pressure difference across it. Orifice plates are among the most common flow measurement devices in industrial processes. The flow rate depends on the orifice size, pressure drop, fluid density, and a discharge coefficient that accounts for real fluid effects like vena contracta contraction and friction losses.
The orifice equation is derived from Bernoulli's principle and the continuity equation. The discharge coefficient (Cd) corrects for the fact that the effective flow area is smaller than the geometric orifice area due to the vena contracta effect, where the jet continues to contract after passing through the orifice.
Orifice Equation
Where Q is volumetric flow rate, Cd is discharge coefficient, A is orifice area, ΔP is pressure difference, and ρ is fluid density.
Typical Discharge Coefficients
| Orifice Type | Cd |
|---|---|
| Sharp-edged (standard) | 0.60-0.65 |
| Well-rounded entrance | 0.95-0.98 |
| Short tube (L/D=2-3) | 0.80-0.85 |
| Borda (re-entrant) | 0.50-0.52 |
Frequently Asked Questions
What is the vena contracta?
The vena contracta is the point downstream of an orifice where the jet reaches its minimum cross-sectional area. For a sharp-edged orifice, this occurs about half a diameter downstream and the area is about 64% of the orifice area. This contraction is the main reason Cd is less than 1.
How accurate is orifice flow measurement?
Standard orifice plates per ISO 5167 can achieve ±0.5-2% accuracy when properly installed with adequate upstream straight pipe. Accuracy depends on beta ratio (orifice/pipe diameter), Reynolds number, and installation quality.