Small-Bore Gas Orifice Meter Flow Calculator

This Excel spreadsheet calculates the flowrate from a small-bore gas orifice meter using the ASME MFC-14M-2001 standard. This calculator is valid for pipe diameters of less than 40 mm (other restrictions are given below).

The equations are as follows

• C is the discharge coefficient. D₁ has to be supplied in m
• D₁ and D₂ are the diameter of the pipe and orifice respectively (m)
• A₁ and A₂ are the cross sectional areas of the pipe and orifice (m²)
• ΔP is the pressure drop across the orifice (Pa)
• P₁ and P_std are the upstream pressure and standard pressure
• T and Tstd are the gas temperature and standard temperature is the 
• ρ is the gas (kg m^-3)
• μ is the gas viscosity (Pa s)
• V₁ is the liquid velocity in the pipe (m s^-1)
• Re₁ is the Reynolds Number in the pipe
• β is the diameter ratio
• MW is the molecular weight of the gas (kg mol^-1)
• R is the universal gas constant (8314 J kmol^-1 K^-1)
• γ is the specific heat ratio
• e is the gas expansivity
• Q is the volumetric flowrate (m³s^-1)
• Q_std is the volumetric flowrate at standard conditions (m³s^-1)

The spreadsheet uses the ideal gas law to calculate the gas density (you just have to supply the molecular weight, pressure and temperature of the gas).

Note these restrictions to the validity of the equations

• Corner Taps: 0.1 < β < 0.8 and 12 mm < D₁ < 40 mm
• Flange Taps: 0.15 < β < 0.7 and 25 mm < D₁ < 40 mm
• D₂ > 6 mm
• Re >1000

Additionally, the discharge coefficients are only valid for the tap configurations illustrated below (as specified by the ASME MFC-14M-2001 standard).

You can choose either Corner or Flange taps with a drop-down menu in the spreadsheet, and Excel automatically uses the correct correlation for the discharge coefficient.

These equations (like nearly all orifice flow meter calculations) require an iterative solution. This is easily done with Excel's Goal Seek.  All you have to do is click a button.

Goal Seek uses an initial guess value for the Reynolds Number to calculate the discharge coefficients, and uses this to calculate the flowrate. The calculated flowrate is then used to calculate the Reynolds Number.  Goal Seek then automatically adjusts the guess and calculated values of the Reynolds number until they are the same.

Download Excel Spreadsheet to Calculate Flowrate from a Small Bore Gas Orifice Meter