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Aga8 - Cesar Delgado

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AGA8
American Gas Association Report No. 8 Module
ACCOL II Reference Manual
Page AGA8-1
The AGA8 Module uses gas component mole percent information to
compute the base compressibility (Zb), flowing compressibility
(Z), and supercompressibility (Fpv= Zb/Z ) for natural gas mix-
tures according to the method explained in American Gas Association
Report Number 8 (AGA-8) of Dec 15, 1985.* Up to 20 gas components
can be processed by the module, and these can come from a signal list
or a data array. For best results, the gas component information
should come from a chromatograph.
This module does not perform the gas component estimation explained
in the AGA-8 Report. Such estimated gas component information in
accordance with the report can be generated by the ACCOL Charac-
terize Module described later in this manual, under 'Characterize'.
Alternately, component information can come from a signal list or
data array containing the component percentages. The components in
the list or array must be in the fixed order defined under Component
Order and Percent Requirements in this section. If the Characterize
Module generates the list or array, the components will automatically
be in the proper order.
If the ENABLE signal is ON, the module checks the entries on its
terminals. Incorrect conditions will be indicated via the ERROR
terminal signal and computations will not occur. If all conditions are
correct, then calculations are initiated.
On subsequent executions of the AGA8 Module, the STATUS terminal
BASE_TEMP
BASE_PRESS
ENABLE
PRIORITY
8
FLOW_TEMP
ARRAY
COLUMN
LIST
STAT_PRESS
Z_FLOWING
Z_BASE
FPV
ERROR
STATUS
* Users requiring the calculation method described in the
 Nov., 92 (2nd edition) of the AGA8 report should see
 'AGA8Detail' or 'AGA8Gross', later in this manual.
AGA8
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AGA8
American Gas Association Report No. 8 Module
is updated to indicate the progress of the calculations. The AGA8
Module’s output terminals (Z_FLOWING, Z_BASE, and FPV) are
updated only when calculations have been completed.
The AGA8 Module should be used only where the ACCOL FPV Module
(NX-19) will not suffice (see 'FPV', later in this manual). This is
because the AGA8 calculations require signficant amounts of memory
and computation resources, and could result in other ACCOL tasks
'slipping' from their normal execution rate (see PRIORITY terminal
below).
❏ Module Terminals
ENABLE Default: None, entry required
Format: Logical signal
Input/Output: Input
allows calculations to proceed if ON, and no calculations are already
in progress. This signal will be set OFF, automatically, when calcula-
tions begin.
PRIORITY Default: ACCOL task priority 1; (This cor-
responds to system task priority
55.)
Format: Analog signal or constant
Input/Output: Input
is the priority at which the system task which performs the AGA8
calculations will run. (System tasks are discussed in the section
'Task', later in this manual.) Values lower than 1 or larger than 64 on
this terminal will produce an error. Priority will affect system perfor-
mance if it is so high that other tasks cannot run. The priority should
be kept as low as possible.
AGA8
American Gas Association Report No. 8 Module
ACCOL II Reference Manual
Page AGA8-3
FLOW_TEMP Default: None, entry required
Format: Analog signal
Input/Output: Input
is the flowing gas temperature in degrees Fahrenheit.
STAT_PRESS Default: None, entry required
Format: Analog signal
Input/Output: Input
is the gas static pressure in psia. If this terminal has a negative value,
computations will not be performed.
BASE_TEMP Default: 600F
Format: Analog signal
Input/Output: Input
is the gas base temperature in degrees Fahrenheit.
BASE_PRESS Default: 14.73 psia
Format: Analog signal
Input/Output: Input
is the gas base pressure in psia. If the terminal has a negative value
no computations are performed.
LIST Default: None, entry required if
ARRAY terminal is not wired
Format: Analog signal
Input/Output: Input
is the number of a list containing the gas component information. The
gas components MUST be placed in the order specified in Component
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Page AGA8-4
AGA8
American Gas Association Report No. 8 Module
Order and Percent Requirements, later in this section. If both the
LIST and ARRAY terminals are wired, only the LIST terminal will be
used.
ARRAY Default: None, entry required if LIST
terminal is not wired
Format: Analog signal
Input/Output: Input
is the number of an analog array containing the gas component
information. The gas components must be placed in the order specified
in Component Order and Percent Requirements, later in this section.
One dimensional arrays must be 1 column by N rows, where N can be
1 to 20. For cases where the array contains components from multiple
gas streams, more columns can be added.
If both the LIST and ARRAY terminals are wired, only the LIST
terminal will be used.
COLUMN Default: None, entry required if LIST
terminal is unwired.
Format: Analog signal
Input/Output: Input
is the array column containing up to 20 gas components to be used in
the calculation. This signal can be used to perform successive AGA8
calculations on multiple gas streams by indexing through the array
columns. Invalid column values produce an error and no computation
is performed.
AGA8
American Gas Association Report No. 8 Module
ACCOL II Reference Manual
Page AGA8-5
ERROR Default: None
Format: Analog signal
Input/Output: Output
contains one of the following codes. Errors are detected by the module
prior to the start of calculations.
Code Message
 0 No errors
 -1 Missing ENABLE signal
 -3 Missing input terminals: FLOW_TEMP or STATIC_PRESS
or both ARRAY and LIST missing
 -4 Signal in list is not an analog signal
 -5 Not a valid list
 -6 Not a valid array
 -7 Not a valid array column
 -8 Missing FPV output terminal
 -9 Invalid PRIORITY (less than 1 or larger than 64)
-10 Negative pressure values detected for STATIC_PRESS or
BASE_PRESS
STATUS Default: None
Format: Analog signal
Input/Output: Output
contains the following status codes:
Code Message
 -3 Floating point error detected
-2 All input components are zero
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Page AGA8-6
AGA8
American Gas Association Report No. 8 Module
 Code Message (continued):
-1 No memory available
0 Not running, check error terminal for errors
 1 Combining components
 2 Mixing terms at flowing conditions
 3 Calculating density at flowing conditions
 4 Mixing at base conditions
 5 Calculating density at base conditions
10-19 Calculation complete - final root (includes iteration count)
20-23 Calculation complete - approximate root (includes approxi-
mation code)
Z_FLOWING Default: None, entry is optional
Format: Analog signal
Input/Output: Output
is the compressibility at flowing conditions. This terminal will only be
updated if the AGA8 Module is executing, and the value on the STA-
TUS terminal is at least 10, i.e., calculations are complete.
Z_BASE Default: None, entry is optional
Format: Analog signal
Input/Output: Output
is the compressibility at base conditions. This terminal will only be
updated if the AGA8 Module is executing, and the value on the STA-
TUS terminal is at least 10, i.e., calculations are complete.
FPV Default: None, entry required
Format: Analog signal
Input/Output: Output
is the supercompressibility ratio Zb/Z. This terminal will only be
updated if the AGA8 Module is executing, and the value on the STA-
TUS terminal is at least 10, i.e., calculations are complete.
AGA8
American Gas Association Report No. 8 Module
ACCOL II Reference Manual
Page AGA8-7
o Component Order and Percent Requirements
Gas components in lists or array columns must be in the following
order. The number of components can vary from 1 to 20, but the order
is important and must be followed. The Characterize Module will
produce 15 components in the proper order. You may supply up to 20
components.If the list or array is too short (i.e., not enough compo-
nents,) the remaining components are assumed to have a value of 0.
Component Name Component Name
1 Nitrogen 11 n-Pentane
2 Carbon Dioxide 12 i-Pentane
3 Hydrogen Sulphide 13 n-Hexane
4 Water 14 n-Heptane
5 Helium 15 n-Octane
6 Methane 16 n-Nonane
7 Ethane 17 n-Decane
8 Propane 18 Oxygen
9 n-Butane 19 Carbon Monoxide
10 i-Butane 20 Hydrogen
The AGA-8 Report specifies the following mole percent ranges for
results within the stated uncertainty limits:
Methane 50 to 100%
Nitrogen 0 to 50%
Carbon Dioxide 0 to 50%
Ethane 0 to 20%
Propane 0 to 5%
Butanes 0 to 3%
Pentanes 0 to 2%
Hexanes 0 to 1% (also heavier hydrocarbons)
All others 0 to 1%
In addition, the report also indicates what percentage uncertainties
can be expected for gases with compositions in the above specified
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Page AGA8-8
AGA8
American Gas Association Report No. 8 Module
ranges over various pressure and temperature regions.
Region 1:
-60 F to 180 F from 0 to 1500 psia 0.1%
Region 2:
-60 F to 180 F from 1500 to 2500 psia 0.3%
-80 F to -60 F from 0 to 2500 psia 0.3%
180 F to 240 F from 0 to 2500 psia 0.3%
Region 3:
-80 F to 240 F from 2500 to 10000 psia 0.5%
240 F to 400 F from 0 to 10000 psia 0.5%
-200 F to -80 F from 0 to 10000 psia 0.5%
Region 4:
-200 F to 400 F from 10000 to 20000 psia 1.0%
❏ Using the Module
Place the AGA8 Module in an ACCOL task which has a task priority
of 32 or less. A task rate of 1 second is recommended for efficient
monitoring of the calculations and update of the STATUS and output
signals. The value on the PRIORITY terminal of the module should
also be as low as possible.
When the AGA8 Module executes, if the ENABLE signal is ON and no
previous calculation is active, the input terminals are automatically
checked for errors. If none are detected, calculations are initiated and
the ENABLE signal is set to OFF.
Setting the ENABLE signal to ON should be controlled by logic in the
ACCOL program to limit calculation activity to a reasonable level
based on potential changes of the input information. For example, if a
chromatograph is used to provide the gas component information, the
AGA8 Module ENABLE should be turned on when new component
information is available from the chromatograph. A Timer Module
AGA8
American Gas Association Report No. 8 Module
ACCOL II Reference Manual
Page AGA8-9
might also be used to enable execution of the AGA8 calculations at
some frequency, e.g. once per minute. It is recommended that the
ENABLE signal not be set to ON while a previous calculation is
active, i.e. before the STATUS terminal value indicates completion. In
this case, a new calculation cycle will be started as soon as the current
cycle completes and the output signals are updated, therefore the
STATUS terminal completion status value will not be visible because
it will immediately change to one of the active status values.
When the AGA8 Module executes, the STATUS terminal is updated. If
calculations are active, the STATUS value will show the progress of
the calculations (values 1 through 5). All of these intermediate values
may not be seen, depending on the ACCOL task rate and other activi-
ties in the system. In general, STATUS code 5 will not be seen unless
the methane component is less than 80% or if the base pressure
exceeds 16.0 psia because the Z
b
 value requires a longer computation
in those circumstances.
When calculations are complete, the value of the STATUS terminal
signal is updated, and the output signals Z_FLOWING, Z_BASE, and
FPV are updated with the results.
The STATUS value indicates if a final root (STATUS values 10-19) or
approximate root (STATUS values 20-23) was obtained.
If the STATUS value is 10-19, subtract 9 to obtain the iteration count;
i.e. the number of passes required to determine the root.
If the STATUS value is 20-23, subtract 10 to get the approximation
code. The approximation codes are:
10 Minimum density used
11 Maximum density used
12 Root not found; last iteration used
13 No bisection convergence; last iteration used
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