AGA 3 Gas Flow Equation for Orifice Plates

Recall that we have discussed the ISO 5167 Orifice Meter Equation. While some companies use the ISO 5167 orifice gas flow equation, a vast majority of companies use the American Gas Association committee report 3 gas flow equation otherwise referred to as AGA 3 Gas Flow Equation for Orifice Plates. The flow rate computed by this equation is in standard units i.e Standard Cubic Feet per Hour or SCFH.

Any equation computing flow in standard units must predict the effective expansion of the gas as if
it were to transition from flowing conditions (the actual pressure and temperature it experiences flowing through the pipe or pipeline) to standard conditions (1 atmosphere, 60 degrees Fahrenheit). 

The compensated gas flow measurement equation published by the American Gas Association (AGA Committee Report 3) in 1992 for orifice plates with flange taps calculates this gas expansion to standard conditions with a series of factors accounting for flowing and standard (“base”) conditions. Most of these factors are represented in the AGA3  equation by different variables as shown below:

Note that SQRT in the explanations below for the AGA 3 flow equation means Square Root of the term in bracket

Where:
Q   =  Volumetric flow rate in standard cubic feet per hour - SCFH

Fn  = Numeric conversion factor (accounts for certain numeric constants, unit-         conversion coefficients, and the velocity of approach factor )

Fc  = Orifice calculation factor (a polynomial function of the orifice plate’s β ratio and Reynolds number), appropriate for flange taps.

Fsl = Slope factor (another polynomial function of the orifice plate’s β ratio and     Reynolds number), appropriate for flange taps.

Fc + Fsl = Cd = Discharge coefficient, appropriate for flange taps

Y =  Gas expansion factor (a function of β, differential pressure, static pressure, and specific heats).

Fpb = Base pressure factor = 14.73PSI/Pb

Pb   = Base pressure in PISA (Pressure in PSI absolute)

Ftb = Base temperature factor = Tb/519.67 (Temperature in degrees Rankine)

Tb  = Base temperature in degrees rankine

Ftf  = Flowing temperature factor = SQRT(519.67/Tf) (Temperature in degrees Rankine)

Tf =  Temperature at flowing conditions

Fgr = Real gas relative density factor = SQRT(1/Gr)

Gr  = Relative density of gas

Fpv = Supercompressibility factor = SQRT(Zb/Zf1)

Zb  =  Gas compressibility at base conditions (1atm, 60 degrees fahrenheit)

Zf1 =  Gas compressibility at the upstream tap

hw  = Differential pressure produced by Orifice plate ( in inches of water column)

Pf1 =  Flowing pressure of gas at the upstream tap PSIA (in PSI absolute)