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AGA3
American Gas Association Report No. 3 Module
Page AGA3-1
ACCOL II Reference Manual
The AGA3 Module performs the gas flow calculations specified by the
American Gas Association, Report No.3 (AGA-3) ANSI/API 2530,
1985 edition.1 The output of this module is the rate of flow of a gas
through an orifice plate in thousands of cubic feet per hour (MSCFH).
Module Terminals
DIFF_PRESS
(hw)
is the differential pressure across an orifice in inches/water at 60o F.
STAT_PRESS
is the static pressure of the flowing gas in psig.
1. In general, it is recommended that AGA3TERM be used instead of AGA3 because it
provides greater flexibility of use. Users requiring AGA3 calculations based on the Nov.,
92 (2nd edition) of the AGA3 report should use the AGA3ITER module.
Default: None, entry required
Format: Analog signal or constant
Input/Output: Input
PIPE_DIAM
ORIF_CONST
POINT
ORIF_DIAM
OUTPUT
TRACK
3
ADJ _PRESS
DIFF_ PRESS
STAT_ PRESS
FLOW_TEMP
SPEC_GRAV
FPV_IN
BASE _TEMP
BASE _PRESS
Default: None, entry required
Format: Analog signal, constant
Input/Output: Input
AGA3
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AGA3
American Gas Association Report No. 3 Module
ADJ_PRESS
is the average barometric pressure in psia. In the AGA3 Module, the
value entered as STAT_PRESS is added to ADJ_PRESS to obtain
absolute pressure.
ORIF_DIAM
is the orifice diameter in inches.
PIPE_DIAM
is the inside diameter of the pipe in inches.
ORIF_CONST
(K = Fa Fm Fl)
This constant is obtained from the multiplication of three factors. The
thermal expansion factor corrects for the change in the physical size
of the orifice plate bore due to the flowing temperature effects of the
gas. Where a mercury manometer is used, manometer and latitude
factors must also be calculated. (See Combined Orifice Constant in
this section for more details.)
Default: 14.73 psia
Format: Analog signal or constant
Input/Output: Input
Default: None; entry is mandatory
Format: Analog signal or constant
Input/Output: Input
Default: None; entry is mandatory
Format: Analog signal or constant
Input/Output: Input
Default: 1
Format: Analog signal or constant
Input/Output: Input
AGA3
American Gas Association Report No. 3 Module
Page AGA3-3
ACCOL II Reference Manual
BASE_PRESS
(Pb)
is the base or contract pressure of the gas in psia. The base pressure
specified here will be used to calculate Fpb which equals 14.73/Pb.
BASE_TEMP
(T
b
)
is the required (contract) base temperature of the flowing gas in
degrees Fahrenheit. T
b 
is used to calculate the factor F
tb
.
FLOW_TEMP
(Tf)
is the flowing temperature of the gas in degrees F. It is used to com-
pute Ftf.
FPV_IN
is the supercomcompressibility factor of the gas. This signal can be
obtained from the FPV Module.
POINT
is a construction code. This entry applies to the pipe tap and flange
Default: 14.73 psia
Format: Analog signal or constant
Input/Output: Input
Default: 600F
Format: Analog signal or constant
Input/Output: Input
Default: 600F
Format: Analog signal or constant
Input/Output: Input
Default: 1
Format: Analog signal or constant
Input/Output: Input
Default: 1
Format: Analog signal or constant
Input/Output: Input
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Page AGA3-4
AGA3
American Gas Association Report No. 3 Module
tap construction described under Base Orifice Factor in this section.
In order to change the value of this terminal while the unit is on
line, use the following procedure. Change either the PIPE_DIAM or
ORIF_DIAM value to zero. Enter the new construction code on the
POINT terminal and wait for the task to execute at least once. Set the
PIPE_DIAM or ORIF_DIAM back to the original value.
If the value of the POINT terminal is changed off line, manipulation of
the DIAM terminals as described above is not necessary.
SPEC_GRAV
(SG)
is the specific gravity of the flowing gas. This is defined as the ratio of
the density of the gas to that of dry air at standard conditions.
TRACK
is used to force the gas flow output to zero under certain measurement
conditions. It could be used to close a block valve and stop the flow.
If TRACK is an analog value, it will be compared to the value of the
differential pressure (DIFF_PRESS). As long as TRACK is below
differential pressure, the flow-computed output will be furnished to
the OUTPUT terminal. Should the track value exceed the differential
pressure value, the OUTPUT will be set to zero percent of scale. In
this way, the TRACK signal acts as a cut off limit.
If the TRACK signal is a logical variable, the ON state will allow the
flow-computed output to be furnished to the OUTPUT terminal. If the
Default: 0.6
Format: Analog signal or constant
Input/Output: Input
Default: None, entry is optional
Format: Analog or logical signal
Input/Output: Input
AGA3
American Gas Association Report No. 3 Module
Page AGA3-5
ACCOL II Reference Manual
TRACK signal is in an OFF state, the module output will be set to
zero percent of scale.
OUTPUT
(Qh)
represents the corrected gas flow rate in MSCFH (thousands of
standard cubic feet per hour).
Gas Flow Equation
The general form of the AGA-3 equation is:
Qh = C’ hw pf
where:
Qh = Quantity rate of flow at base conditions, standard cubic feet
per hour (SCFH)
C’ = Orifice flow constant
hw = Differential pressure, inches of water at 60
oF
pf = Absolute static pressure, psia
The orifice flow constant, C’, is composed of various factors, some are
fixed by the physical equipment and others vary with the state of the
flowing gas. The orifice flow constant is defined as follows:
C’ = Fb Fr Y Fpb Ftb Ftf Fg Fpv K
where:
Default: None
Format: Analog signal
Input/Output: Output
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Page AGA3-6
AGA3
American Gas Association Report No. 3 Module
Fb = Basic orifice factor for a given orifice size and pipe diameter
Fr = Reynolds number factor
Y = Expansion factor
Fpb = Pressure base factor
Ftb = Temperature base factor
Ftf = Flowing temperature factor
Fg = Specific gravity factor
Fpv = Supercompressibility factor
K = Combined orifice constant
Fb is computed using the equations contained in Appendix B of the
AGA-3 report.
The term F
r
, the Reynolds number correction factor, is calculated
from:
 b
 Fr = 1 +
 h
w
 P
f
where b is a constant for a given orifice size and pipe diameter. It is
computed using equations in Appendix B of the AGA-3 report and is
combined with the linear interpolation of Table 18.
K, the combined orifice constant, is obtained from the expression:
K = Fm Fa Fl
where:
Fm = Manometer factor for mercury-type flowmeters only
Fa = Orifice thermal expansion factor
Fl = Gravitational correction for mercury manometer factor
Y, = the expansion factor, is calculated using the equations
described in Appendix B, Section 8 of the AGA-3 report.
These equations are broken up into two factors, one of which depends
on the physical equipment, and the other which depends on the state
of the gas.
AGA3
American Gas Association Report No. 3 Module
Page AGA3-7
ACCOL II Reference Manual
The other factors are calculated as follows:
 14.73
Fpb =
 Pb
where Pb = contract base pressure
T
b
 + 459.67
Ftb =
 519.67
where T
b
 is the base temperature in degrees F
 519.67
Ftf =
 T
f
 + 459.67
Fg is defined by the following expression:
 1
Fg =
 SG
where SG is the specific gravity of the gas.
Combining the various expressions, the basic equation solved by the
gas flow block is:
 b 14.73 Tb + 459.67
 Qh = KFb * 1 + * Y * *
 hw Pf Pb 519.67
519.67 hw Pf
 * Fpv * *
 Tf + 459.67 SG
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AGA3
American Gas Association Report No. 3 Module
Combined Orifice Constant
The combined orifice constant is applied to the ORIF CONST terminal
to correct for local conditions.For example, it could be calculated from
the formula:
K = Fm Fa Fl
where:
Fl = latitude factor for mercury manometer/transducers. Specify
latitude in degrees and elevation in feet.
980.665 + ( 0.087*(latitude-45) ) - (9.4x10-5 * elevation)
 =
980.665
Fm = Manometer factor for mercury manometer/transducers
SG * Fpv
2 * Pf
 = 1 - 2.699 *
844.132 * Ta
where: Ta = ambient temperature, 
0F + 460
SG = specific gravity of the flowing gas
Fpv = supercompressibility of the flowing gas
Pf = absolute pressure of the flowing gas
When mercury manometers are not used, Fl and Fm = 1.
Fa = thermal expansion factor of orifice plate due to the temperature of
the flowing gas.
AGA3
American Gas Association Report No. 3 Module
Page AGA3-9
ACCOL II Reference Manual
For 304 or 316 stainless steel:
Fa = 1 + [ 1.85 x 10
-5 * (Tf - 68) ]
For SAE carbon steel:
Fa = 1 + [ 1.3793 x 10
-5 * (Tf - 68) ]
For 430 stainless steel:
Fa = 1 + [ 1.1364 x 10
-5 * (Tf - 68) ]
(T
f
 is the temperature of the flowing gas in degrees Fahrenheit.)
Usually these correction factors have minimal effect on the output
signal. As a result, three different approaches can be taken with
respect to this input of the module:
1. Leave this input signal unwired in which case the calculation
operates as though a default value of 1.0 were used.
2. Compute the value manually and wire a signal with the value to the
input.
3. Compute the value in a Calculator Module and apply that output to
the ORIF_CONST terminal.
Basic Orifice Factor
The AGA3 Module computes a value for the basic orifice factor inter-
nally using the orifice diameter, pipe diameter, and a code that
indicates whether pipe or flange taps are used. The pipe diameter and
orifice diameter are input signals to the AGA3 Module. Both are
specified in inches. These inputs must be wired to produce a gas flow
result. An integer code value is used at the POINT terminal to
indicate the meter construction. The following table shows the code to
be used to indicate tap type and location.
ACCOL II Reference Manual
Page AGA3-10
AGA3
American Gas Association Report No. 3 Module
Static Pressure Pipe Flange
Measured Taps Taps
Upstream 4 2
Downstream 3 1
This code affects the calculation of the expansion factor (Y). The basic
orifice factor is obtained using a linear interpolation technique based
on the AGA3 tables.
In order to save execution time, parts of the AGA-3 equation affected
by the POINT code, PIPE_DIAM or ORIF_DIAM, are not calculated
every time the task executes. PIPE_DIAM or ORIF_DIAM values can
be changed on-line and automatic recalculation of the affected param-
eters will take place. If the POINT terminal value is the only value
being changed you must also change the PIPE_DIAM or ORIF_DIAM
values as follows:
1. Disable the AGA3 Module OUTPUT via the TRACK signal to
prevent erroneous flow rates.
2. Enter the new value on the POINT terminal.
3. Set either the PIPE_DIAM or ORIF_DIAM value to zero.
4. Wait for the task to execute at least once and then set the
PIPE_DIAM or ORIF_DIAM back to the original value.
5. Re-enable the module OUTPUT via the TRACK signal.
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