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AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
ACCOL II Reference Manual
Page AGA8Detail-1
The AGA8Detail Module uses gas component mole percent informa-
tion to compute the base compressibility (Zb), flowing compress-
ibility (Z), and supercompressibility (Fpv= Zb/Z ) for natural gas
mixtures according to the Detail Characterization method explained in
American Gas Association Report Number 8 (AGA-8), 2nd edition of
November, 1992. Up to 21 gas components can be processed by the
module. For best results, the gas component information should come
from a chromatograph.
8DETAIL
Z_FLOWING
Z_BASE
FPV
ERROR
STATUS
BASE_TEMP
BASE_PRESS
ENABLE
PRIORITY
FLOW_TEMP
STAT_PRESS
ARRAY
COLUMN
LIST
The components in the signal list or data array must be in the fixed
order defined under Component Order in this section.
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 AGA8Detail Module, the STATUS
terminal is updated to indicate the progress of the calculations. The
AGA8Detail Module’s output terminals (Z_FLOWING, Z_BASE, and
FPV) are updated only when calculations have been completed.
AGA8Detail
ACCOL II Reference Manual
Page AGA8Detail-2
AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
AGA8Detail calculations require significant amounts of memory and
computation resources, and could result in other ACCOL tasks 'slip-
ping' 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
Input/Output: Input
is the priority at which the system task which performs the
AGA8Detail 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 performance if it is so high that other tasks cannot run. The
priority should be kept as low as possible.
FLOW_TEMP Default: None, entry required
Format: Analog signal
Input/Output: Input
is the flowing gas temperature in degrees Fahrenheit.
AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
ACCOL II Reference Manual
Page AGA8Detail-3
STAT_PRESS Default: None, entry required
Format: Analog signal
Input/Output: Input
is the static pressure in psia. If this terminal has a negative value,
computations will not be performed.
BASE_TEMP Default: 600 F
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 the Compo-
nent Order portion of this section. If both the LIST and ARRAY
terminals are wired, only the LIST terminal will be used.
ACCOL II Reference Manual
Page AGA8Detail-4
AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
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 the Component Order portion of this section.
One dimensional arrays must be 1 column by N rows, where N can be
1 to 21. 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, the ARRAY termi-
nal is ignored.
COLUMN Default: None, entry required if LIST
terminal is unwired
Format: Analog signal
Input/Output: Input
is the array column containing up to 21 gas components to be used in
the calculation. This signal can be used to perform successive
AGA8Detail calculations on multiple gas streams by indexing through
the array columns. Invalid column values produce an error and no
computation is performed.
ERROR Default: None
Format: Analog signal
Input/Output: Output
displays one of the following codes. Errors are detected by the module
prior to the start of calculations.
AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
ACCOL II Reference Manual
Page AGA8Detail-5
Code Message
 0 No errors
 -1 Missing ENABLE signal
 -3 Missing input terminals: FLOW_TEMP or STAT_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 detected for STAT_PRESS or
BASE_PRESS
STATUS Default: None
Format: Analog signal
Input/Output: Output
contains one of the following status codes:
Code Message
10 Calculation complete
4 Calculating compressibility at flow conditions
3 Calculating density at flow conditions
2 Calculating gas characterization
1 Calculating compressibility at base conditions
0 Not running, check error terminal for errors
-1 No memory available
-2 No positive inputs in the component list or array
-3 Floating point error detected
-11 No convergence
-12 Root not bounded
-13 Negative square root
-14 Pressure has a negative density derivative
-15 Maximum number of iterations exceeded
ACCOL II Reference Manual
Page AGA8Detail-6
AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
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 AGA8Detail Module is executing, and if the value on
the STATUS terminal is 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 AGA8Detail Module is executing, and if the value on
the STATUS terminal is 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 AGA8Detail Module is executing, and if the value on
the STATUS terminal is 10; i.e. calculations are complete.
❏ Component Order
Gas components in lists or array columns must be in the following
order. The number of components can vary from 1 to 21, but the order
is important and must be followed. If the list or array is short; i.e., less
than 21 components, the remaining undefined components will be
given a default value of 0.
AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
ACCOL II Reference Manual
Page AGA8Detail-7
Component Name Component Name
1 Methane 11 i-Butane
2 Nitrogen 12 n-Butane
3 Carbon Dioxide 13 i-Pentane
4 Ethane 14 n-Pentane
5 Propane 15 n-Hexane
6 Water 16 n-Heptane
7 Hydrogen Sulfide 17 n-Octane
8 Hydrogen 18 n-Nonane
9 Carbon Monoxide 19 n-Decane
10 Oxygen 20 Helium
21 Argon
❏ Using the Module
Place the AGA8Detail 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 AGA8DetailModule executes, if the ENABLE signal is ON
and no previous calculation is active, the input terminals are auto-
matically 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
AGA8Detail Module ENABLE should be turned on when new compo-
nent information is available from the chromatograph. A Timer
Module might also be used to enable execution of the AGA8Detail
ACCOL II Reference Manual
Page AGA8Detail-8
AGA8Detail
American Gas Association Report No. 8 Detail Calculation Module
calculations at some frequency, e.g. once per minute. It is recom-
mended 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 AGA8Detail Module executes, the STATUS terminal is
updated. If calculations are active, the STATUS value will show the
progress of the calculations (values 1 through 4). All of these interme-
diate values may not be seen, depending on the ACCOL task rate and
other activities in the system.
When calculations are complete as indicated by the STATUS value,
the output signals Z_FLOWING, Z_BASE, and FPV are updated with
the results.
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