Orifice Plate Sizing Calculator (ISO 5167-2) — Piping Passion

Units

Inputs

Fluid phase

Volumetric flow Q at upstream conditions

Actual volumetric flow at upstream P, T — not standard volume.

Pipe NPS

Schedule

ID will show below.

Density at upstream (ρ₁)

Dynamic viscosity (μ)

Water 20°C = 1.00 cP · Air STP = 0.018 · Light HC = 0.5 · SAE 30 = 100 cP

Differential pressure target ΔP

Range of typical DP transmitters: 10–1000 mbar full-scale.

Tap arrangement

Upstream absolute pressure P₁

Isentropic exponent κ

Air = 1.40 · Natural gas = 1.31 · Steam (sat) = 1.135 · CO₂ = 1.30

Result

Bore diameter (d)

—mm

Diameter ratio β = d / D

—

Discharge coefficient C

—

Reader-Harris/Gallagher per ISO 5167-2.

Validation

—

—

Formula & Method

qᵐ = (C / √(1 - β⁴)) · ε · (π/4) · d² · √(2 · ΔP · ρ) β = d / D (diameter ratio, valid 0.10 ≤ β ≤ 0.75) C = Reader-Harris/Gallagher equation per ISO 5167-2 §5.3.2.1 ε = expansibility factor (= 1 for liquid, < 1 for compressible) Re_D = ρ · v · D / μ ··· must be ≥ 5000 for ISO 5167-2 to apply

What this is: ISO 5167-2 is the international standard for differential-pressure flow measurement using orifice plates. The Reader-Harris/Gallagher (RHG) equation gives the discharge coefficient C as a function of β, Reynolds number, and tap geometry. Most modern flow computers and orifice-sizing programs use this equation under the hood. Tap types:

Flange taps — 1" up/1" down. Most common in process plants.
Corner taps — tap holes in the flange ring at the plate face. Used for D < 50 mm.
D and D/2 taps — pressure tappings at 1D upstream, D/2 downstream. Highest C, used in research and gas-transmission.

Range of validity (ISO 5167-2 §5.3.1.1):

50 mm ≤ D ≤ 1000 mm
0.10 ≤ β ≤ 0.75
Re_D ≥ 5000 for β ≥ 0.10

Outside these limits, accuracy is not guaranteed and you need a different primary element (Venturi, nozzle, V-cone, ultrasonic). Brownfield tip: when re-rating a metering run for higher flow, do not just swap the orifice plate. Verify the upstream/downstream straight-length requirements (ISO 5167-2 §5.3.1.4) are still met — an extra elbow installed during a tie-in upstream of the orifice can introduce 2–5% bias for years before anyone notices the meter is reading low. The fix is moving the meter, not re-sizing the plate.

⚠ For preliminary sizing only Results are based on nominal ASME dimensions and typical material densities. They do not account for manufacturing tolerances, coatings, supports, flanges, fittings, corrosion allowance, or actual site conditions. All final designs must be verified by a qualified engineer and validated against the applicable code edition. Piping Passion accepts no liability for decisions made using this tool.