How does an orifice plate flow meter work?

An orifice plate creates a constriction in the pipe. The fluid speeds up through the orifice, causing a pressure drop (ΔP). Flow rate is proportional to √(ΔP), calculated using Q=C×ε×A/√(1−β⁴)×√(2ΔP/ρ) per ISO 5167.

What is the beta ratio?

Beta (β) = d/D, the ratio of orifice bore diameter to pipe internal diameter. Typical range: 0.2 to 0.75. Higher beta = less pressure loss but lower accuracy.

What is the discharge coefficient C?

The discharge coefficient accounts for real fluid effects (vena contracta, friction). For ISO 5167 corner taps, C is calculated from the Reader-Harris/Gallagher equation and is typically 0.59–0.62 for β = 0.3–0.7.

What is the expansibility factor ε?

ε (epsilon) accounts for gas compressibility. For liquids, ε = 1. For gases, ε < 1 and depends on the pressure ratio and the isentropic exponent (κ). ε ≈ 1 for small ΔP/P ratios.

What are the ISO 5167 requirements for straight pipe?

ISO 5167 requires minimum straight pipe upstream (typically 10–44 D depending on fitting type and β) and 4–8 D downstream. Flow conditioners can reduce upstream requirements.

Free Engineering Tool — #152

Orifice Plate Flow Calculator — ISO 5167

Calculate flow rate through an orifice plate per ISO 5167. Enter orifice and pipe diameters, differential pressure, and fluid density.

This calculator was developed and verified by the Vibromera engineering team, makers of the Balanset portable balancing instruments.

ISO 5167Q = C·ε·A/√(1−β⁴)·√(2ΔP/ρ)Liquids & Gases

Results

Volume Flow Rate

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Mass Flow Rate

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Beta Ratio (β = d/D)

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Discharge Coefficient C

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Orifice Velocity

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Pipe Velocity

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ISO 5167 Orifice Flow Equation

C — discharge coefficient (Reader-Harris/Gallagher equation)
ε — expansibility factor (1.0 for incompressible liquids)
β — diameter ratio d/D
ΔP — differential pressure across orifice
ρ — upstream fluid density

Simplified Discharge Coefficient

For corner taps, C ≈ 0.5959 + 0.0312β^2.1 − 0.184β⁸ (simplified Reader-Harris). For flange taps, add correction terms for pipe diameter.

⚠️ Note: This calculator uses a simplified C estimation. For custody transfer metering, use the full Reader-Harris/Gallagher equation with Reynolds number correction per ISO 5167-2.