AGA 3 Volume Flow Rate Calculation Method II – Technical Toolboxes Knowledge Center

The accepted one-dimensional equation for mass flow through a concentric, square edged orifice meter is stated in the Equations below. The derivation is based on conservation of mass and energy, one-dimensional fluid dynamics, and empirical functions such as equations of state and thermodynamic process statements. Any derivation is accurate when all the assumptions used to develop it are valid. As a result, an empirical orifice plate coefficient of discharge is applied to the theoretical equation to adjust for multidimensional viscous fluid dynamic effects. In addition, an empirical expansion factor is applied to the theoretical equation to adjust for the reduction in fluid density that a compressible fluid experiences when it passes through an orifice plate.

The orifice meter is a mass meter from which a differential pressure signal is developed as a function of the velocity of the fluid as it passes through the orifice plate bore. Manipulation of the density variable in the equation permits calculation of flow rate in either mass or volume units. The volumetric flow rate at flowing (actual) conditions can be calculated using the following equation:

𝑞_𝑣 = 𝑞_𝑚/𝜌_𝑡,_𝑝

Orifice Plate Bore Diameter

The orifice plate bore diameter, d. is defined as the diameter at flowing conditions and can be calculated using the following equation:

Coefficient of Discharge

The concentric, square-edged, flange-tapped orifice meter coefficient of discharge, ,𝐶-𝑑.(FT), equation, developed by Reader-Harris/Gallagher (RG), is structured into distinct linkage terms and is considered to best represent the current regression data base. The equation is applicable to nominal pipe sizes of 2 inches (50 millimeters) and larger; diameter ratios (𝛽) of 0.1-0.75, provided the orifice plate bore diameter, d” is greater than 0045 inch (11.4 millimeters); and pipe Reynolds numbers (,𝑅𝑒-𝐷.) greater than or equal to 4000.

Where:
𝛽 − Diameter Ratio = 𝑑/𝐷
𝐶_𝑑(𝐹𝑇) − Coefficientof Discharge at a Specific Reynolds number for Flange−Tapped Orifice Meter
𝐶_𝑖 (𝐹𝑇) − Coefficientof Discharge at infinite Reynolds number for Flange−Tapped Orifice Meter
𝐶_𝑖(𝐶𝑇) − Coefficientof Discharge infinite Reynolds number for Corner−Tapped Orifice Meter
𝐿₁ − dimentionless correction for the tap location
𝑒 − Napoerian Constant=2.71828
𝑁₄ = 1.0 when D is in Inches=25.4 when D is in millimeters
𝑅𝑒_𝐷 − pipe Reynolds number

Reynold’s Number

The RG equation uses pipe Reynolds number as the correlating parameter to represent the change in the orifice plate coefficient of discharge, Cd, with reference to the fluid’s mass flow rate (its velocity through the orifice), the fluid density, and the fluid viscosity. The pipe Reynolds number can be calculated using the following equation:

𝐷 − Meter Tube Internal Diameter [in]
𝜋 − Universal Constant=3.14159
𝑞_𝑚 − Mass Flow Rate
𝑅𝑒_𝐷 − pipe Reynolds number
𝜇 − Absolute Viscosity of Fluid

Velocity of Approach Factor

The velocity of approach factor, ,𝐸-𝑣. , is calculated as follows:

Where:
𝛽 − Diameter Ratio
𝐸_𝑣 − Velocity Approach Factor
𝑑 − Orifice Plate Bore Diameter ,in.
𝐷 − Meter Tube Internal Diameter [in]

This assumption defines the expansion as reversible and adiabatic (no heat gain or loss). Within practical operating ranges of differential pressure, flowing pressure, and temperature, the expansion factor equation is insensitive to the value of the isentropic exponent. As a result, the assumption of a perfect or ideal isentropic exponent is reasonable for field applications.

The application of the expansion factor is valid if the following dimensionless pressure ratio criteria are followed:

Where:
𝑁₃ − Unit Conversion Factor
𝑃_𝑓1 − Absolute Static Pressure at Upstream Tap [psi]
𝑃_𝑓2 − Absolute Static Pressure at Downstream Tap [psi]
∆𝑃 − Orifice Differential Pressure [in 𝐻₂0] at 60℉

Upstream/Downstream Expansion Factor

The upstream expansion factor requires determination of the upstream static pressure, the diameter ratio, and the isentropic exponent. If the absolute static pressure is taken at the upstream differential pressure tap, then the value of the expansion factor, 𝑌₁ , shall be calculated as follows:

When the upstream static pressure is measured,

When the downstream static pressure is measured,

𝑌₁ − Upstream Expansion Factor
𝑁₃ − Unit Conversion Factor
𝑥₁ − Ratio of Differential Pressure to Asolute Static Pressure
𝑃_𝑓1 − Absolute Static Pressure at Upstream Tap [psi]
𝑃_𝑓2 − Absolute Static Pressure at Downstream Tap [psi]
∆𝑃 − Orifice Differential Pressure ,in ,𝐻-2.0.at 60℉
𝑘 − Isentropic Exponent
𝛽 − Diameter Ratio

The downstream expansion factor requires determination of the downstream static pressure, the upstream static pressure, the downstream compressibility factor, the upstream compressibility factor, the diameter ratio, and the isentropic exponent. The value of the downstream expansion factor, 𝑌₂, shall be calculated using the following equation:

When the upstream static pressure is measured,

When the downstream static pressure is measured,

𝑌₁ − Upstream Expansion Factor
𝑌₂ − Downstream Expansion Factor
𝑁₃ − Unit Conversion Factor
𝑥₁ − Ratio of Differential Pressure to Asolute Static Pressure
𝑃_𝑓1 − Absolute Static Pressure at Upstream Tap [psi]
𝑃_𝑓2 − Absolute Static Pressure at Downstream Tap [psi]
𝑍_𝑓1 − Fluid Compressibility at Upstream Tap [psi]
𝑍_𝑓2 − Fluid Compressibility at Downstream Tap [psi]
∆𝑃 − Orifice Differential Pressure ,in ,𝐻-2.0.at 60℉
𝑘 − Isentropic Exponent𝛽 − Diameter Ratio

Supercompressibility

In orifice measurement, ,𝑍-𝑏.and ,𝑍-𝑓1. appear as a ratio to the 0.5 power. This relationship is termed the supercompressibility factor and may be calculated from the following equation:

Base Pressure Factor

If contract base pressure differs from the reference base conditions (14.73 psia and 60℉), the initial calculated flow rate at reference base conditions will require conversion to the flow rate at the given base conditions.
𝐹_𝑝𝑏 = 14.73psia/𝑃_𝑏

Input Parameters

To create a new case, click the “Add Case” button

Select the AGA 3 – Volume Flow Rate Calculation – Method II application from the Regulators & Meters module list.

Enter Case Name, Location, Date and any necessary notes.

Fill out all required fields.

Make sure the values you are inputting are in the correct units.

Click the CALCULATE button.

Static Pressure Measurements are Taken from the

Linear Coefficient of Thermal Expansion

Orifice
Meter Tube

Reference Temperature Orifice Plates/Tube

Compressibility Factor at Flowing Conditions

Compressibility Factor at Base Conditions

Compressibility Factor at Standard Conditions

Mean Orifice Bore Diameter

Mean Meter Tube Internal Diameter

Real Gas Relative Density

Differential Pressure [in.H2O] at 60 deg F

Base Pressure

Static Pressure

Contract Base Temperature

Flowing Gas Temperature

Carbon Dioxide

Nitrogen

Gas Dynamic Viscosity

Isentropic Exponent

Outputs/Reports

View the results.

If an input parameter needs to be edited be sure to hit the CALCULATE button after the change.

To SAVE, fill out all required case details then click the SAVE button.

To rename an existing file, click the SAVE As button. Provide all case info then click SAVE.

To generate a REPORT, click the REPORT button.

The user may export the Case/Report by clicking the Export to Excel/PowerPoint icon.

To delete a case, click the DELETE icon near the top of the widget.

Orifice Diameter

Tube Diameter

Diameter Ratio

Velocity Approach Factor

Numeric Conversion Factor

Coefficient of Discharge

Expansion Factor

Base Pressure Factor

Base Temperature Factor

Flowing Temperature Factor

Real Gas Relative Density Factor

Super compressibility Factor

Reynolds Number

Flow Rate at Standard Conditions

Flow Rate at Base Conditions