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Orifice plate sizing (ISO 5167-2)

Enter fluid, maximum flow, pipe and transmitter ΔP to get the orifice bore, check the standard’s limits, the pressure loss left in the line and the straight run the installation needs.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Fluid and flow
Pipe, plate and installation

4 in schedule 40 pipe: D = 4.026 in. 6 in sch 40: 6.065 in.

—Orifice bore
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Differential pressure and permanent loss versus flow

ΔP grows with the square of flow. Vertical lines: minimum and normal flows entered.

Tap the chart to see the values.

Comparison with ISA 1932 nozzle and classical Venturi

Same ΔP and same maximum flow: permanent pressure loss of each meter. Tap a bar to see its β.

Tap the chart to see the values.

Straight lengths from ISO 5167-2 Table 3

Table row with the β just above the computed one. A: no additional uncertainty; B: +0.5% on the uncertainty of C.

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Method, assumptions and references

Equations used

qm = C ÷ √(1 − β⁴) · ε · (π/4) · d² · √(2 · ΔP · ρ1); β found by bisection until it matches the design flow
C = 0.5961 + 0.0261β² − 0.216β⁸ + 0.000521(10⁶β/ReD)^0.7 + (0.0188 + 0.0063A)β^3.5(10⁶/ReD)^0.3 + (0.043 + 0.080e^(−10L1) − 0.123e^(−7L1))(1 − 0.11A)β⁴/(1 − β⁴) − 0.031(M2′ − 0.8M2′^1.1)β^1.3
A = (19000β/ReD)^0.8; M2′ = 2L2′/(1 − β); corner: L1 = L2′ = 0; D and D/2: L1 = 1, L2′ = 0.47; flange: L1 = L2′ = 25.4/D (mm); D < 71.12 mm: + 0.011(0.75 − β)(2.8 − D/25.4)
Gas: ε = 1 − (0.351 + 0.256β⁴ + 0.93β⁸)[1 − (p2/p1)^(1/κ)]; ρ1 = p1 · M ÷ (Z · R · T1)
Permanent loss: Δϖ = [√(1 − β⁴(1 − C²)) − Cβ²] ÷ [√(1 − β⁴(1 − C²)) + Cβ²] · ΔP; power lost = qv · Δϖ
Comparison: ISA 1932 nozzle C = 0.9900 − 0.2262β^4.1 − (0.00175β² − 0.0033β^4.15)(10⁶/ReD)^1.15 (ISO 5167-3); machined Venturi C = 0.995 and loss of 10% of ΔP (ISO 5167-4)

Assumptions and limits

  • Square-edged concentric plate, single-phase steady flow, full pipe. Thermal expansion of plate and pipe neglected: enter D at operating temperature.
  • ISO 5167-2 limits: 50 ≤ D ≤ 1000 mm, d ≥ 12.5 mm, 0.10 ≤ β ≤ 0.75; ReD ≥ 5000 (and ≥ 16000β² if β > 0.56 for corner and D and D/2 taps; ≥ 170β²D for flange taps); p2/p1 ≥ 0.75.
  • Table 3 straight lengths taken from the row with the β just above the computed one (conservative side); with two fittings in series, the run between them also applies.
  • Validation: β, C and ε checked against the Python fluids library (Caleb Bell), functions C_Reader_Harris_Gallagher and orifice_expansibility, for the default examples.

Standards and technical documents

  • ISO 5167-1:2022: Measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full — Part 1: General principles and requirements
  • ISO 5167-2:2022 (e 2003): Part 2: Orifice plates
  • ISO 5167-3:2022: Part 3: Nozzles and Venturi nozzles; ISO 5167-4:2022: Part 4: Venturi tubes
  • ISO/TR 9464: Guidelines for the use of ISO 5167
  • Reader-Harris, M. Orifice Plates and Venturi Tubes. Springer, 2015
  • ASME MFC-3M: Measurement of Fluid Flow in Pipes Using Orifice, Nozzle, and Venturi

U.S. regulations

  • API MPMS Chapter 14.3 / AGA Report No. 3: orifice metering of natural gas (custody transfer)
  • ASME B31.3: Process Piping (meter runs and flanges)
  • 49 CFR Part 192: transportation of natural gas by pipeline — meter stations

Frequently asked questions

How do I size an orifice plate bore?

Fix the maximum flow and transmitter ΔP (for example, 100 inH₂O) and iterate β until the ISO 5167-2 equation with the Reader-Harris/Gallagher C gives that flow. For 200 gpm of water in a 4 in sch 40 pipe, the bore is about 2.34 in (β ≈ 0.58).

What ΔP should I choose?

Common values are 50 to 200 inH₂O (12 to 50 kPa). Higher ΔP lowers β and improves low-flow readings, but increases permanent loss and energy use.

How much straight pipe does an orifice need?

It depends on the fitting and β. Per ISO 5167-2 Table 3, a single 90° elbow needs 22 D upstream at β = 0.5 and 42 D at β = 0.6; downstream, 6 to 7 D. A fully open ball valve needs 12 D.

How much pressure does the plate lose for good?

Between 50% and 90% of the measured ΔP, depending on β. At β = 0.5 about 73%; a classical Venturi loses only 5% to 20%.