Orifice Flow Calculator

Estimate flow through a restriction orifice, or calculate bore diameter, for steam, liquids and gases.

CADBOOST THERMAL · FLOW & PRESSURE

Orifice Flow Calculator

Restriction screening · Mass flow and bore diameter

Operating conditions

Cd = 0.62 is an editable screening assumption. Use a validated coefficient for the actual restriction geometry. Upstream pressure and temperature are treated as reservoir conditions with negligible approach velocity.

Orifice Flow Visualization

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Bore diameter—
Plate thickness—
Mass discharge——
Cross section of an orifice plate and flow through its boreEnter operating conditions to update the diagram. UPSTREAMORIFICE PLATEDISCHARGE P₁ = —P₂ = — d = — t = — ṁ = —

Illustrative flow animation, not a CFD simulation or scaled pipe drawing. Thickness changes the drawing only; flow capacity still uses the selected Cd. Actual volume is referenced to upstream conditions.

Result

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Orifice flow and calculated conditions
PropertyResultUnit

Method / Guide

  1. Select the fluid, calculation mode and units. Enter upstream and downstream pressures using gauge or absolute basis; gauge pressures include the entered atmospheric pressure.
  2. For Flow from diameter, enter the bore diameter. For Diameter from required flow, enter mass flow or actual volume flow in m³/h at upstream pressure and temperature. Set Cd for the actual geometry; changing Cd directly changes predicted capacity.
  3. Saturated steam uses dry saturated inlet properties. Superheated steam and water use IAPWS-IF97; thermal oil uses the selected oil's temperature table. Air and flue gas use the entered molecular weight, isentropic exponent and Z.
  4. Select output units per result row, or apply a Units preset. Export CSV or print the calculation. Actual volume is at upstream conditions. Normal gas volume, available for air and flue gas, is defined at 0°C, 101.325 kPa absolute, Z = 1, on the same composition basis as the entered molecular weight.

Orifice visualization

The animated cross section shows calculated bore diameter and discharge flow with the entered plate thickness. Geometry is illustrative and not to scale. Thickness does not automatically adjust Cd or the flow model; select Cd for the actual bore shape and thickness. The PDF includes a static diagram with the guide.

Equations and assumptions

Area A = πd²/4. For single-phase liquids: ṁ = Cd A √(2ρΔP). For gases: the constant-exponent isentropic relation uses P₁(abs), T₁, κ and Z; the model limits mass flux once P₂/P₁ ≤ [2/(κ+1)]κ/(κ−1).

Steam uses equilibrium IAPWS-IF97 states along the inlet entropy: G(P) = ρ(P,s₁) √[2000(h₁ − h(P,s₁))]. The maximum mass flux sets the model's critical pressure; ṁ = Cd A G. Saturated steam can become wet during expansion. This homogeneous-equilibrium model assumes immediate phase equilibrium.

Required diameter is calculated by inverting the selected flow model. Aperture Reynolds number, when viscosity is available, is Re = ṁd/(Aμ). The displayed equivalent velocity is Q₁/A using upstream density; it is not local sonic or vena-contracta velocity.

Applicability

This is a preliminary restriction-capacity model. It does not apply pipe-diameter / beta-ratio corrections, pressure-tap correlations or the ISO 5167 orifice-meter method. Gas choking here is an idealized model; a real sharp-edged plate can behave differently. Liquid flashing is rejected when known downstream vapor-pressure limits are exceeded; local cavitation, two-phase slip, erosion, noise and pipe losses require separate assessment.

Steam range: inlet pressure from 0.1 to below 220.64 bar absolute, outlet pressure at least 0.00611657 bar absolute, superheated inlet temperature at most 800°C. For flue gas, enter properties for the actual gas composition, moisture basis and temperature; the provided values are illustrative.

References

NASA — compressible mass flow and choking · IAPWS-IF97 · Spirax Sarco — discharge coefficients and critical flow · Calculation library license