Power Plant · Flow
DP Flow — ISO 5167 (Orifice, Nozzle, Venturi)
Steam, water or gas flow from a differential-pressure primary element using the full ISO 5167 equations: Reader-Harris/Gallagher discharge coefficient, Reynolds-number iteration, expansibility from the real IF97 isentropic exponent, bore and pipe thermal expansion, limits-of-use checks and a GUM uncertainty budget.
Verified against published standard
Enter the inputs and press Calculate.
About This Calculation
ISO 5167 is the international standard for measuring flow with orifice plates, nozzles and Venturi tubes. Flow follows from the differential pressure across the element, but accuracy depends on getting the discharge coefficient C (which varies with Reynolds number and diameter ratio β), the expansibility ε of the fluid, the fluid density at the upstream tap, and the true bore and pipe diameters at operating temperature right. This calculator solves the full ISO equations with exact IAPWS-IF97 steam and water properties and reports an uncertainty budget.
Frequently Asked Questions
Why does the discharge coefficient need iteration?
C depends on the pipe Reynolds number, which itself depends on the flow being calculated, so the solver repeats until flow and C agree.
Why does bore thermal expansion matter for steam?
At 540 °C an austenitic bore grows about 1–2 %, which changes the flow by a similar amount. The expansion coefficients are therefore inputs you should take from your material data.
What total uncertainty should I expect from an orifice meter?
ISO 5167-2 puts the discharge-coefficient uncertainty at about 0.5 % for diameter ratios between 0.2 and 0.6, rising outside that range. Added to the uncertainty of the Δp transmitter, the density (from pressure and temperature) and the measured diameters, a well-installed steam orifice typically lands around 1–2 %. The calculator shows each contribution so you can see which instrument dominates.
Which tap arrangement should I select?
The one the plate was manufactured and installed for. Corner, flange and D and D/2 taps give different discharge coefficients, so choosing the wrong one produces a systematic error of the order of a percent or more.
Why does the calculator warn when p2/p1 is below 0.75?
The expansibility equations in ISO 5167 were derived for pressure ratios of 0.75 or higher. At lower ratios the correlation is outside its tested range and the result should be treated with caution; use a smaller bore or a wider-range Δp measurement.