6 FC1: Legally Relevant Functionalities
6.4 Table Z AFB
The Table Z AFB calculates the compressibility factor at measurement
conditions (p, t) using a compressibility factor table. The entries of the com-
pressibility factor table (related to measurement conditions p, t) are subject
to parameterization.
In detail, the
Table Z AFB performs the following tasks:
• calculation of compressibility factor Zm at measurement conditions
(p, t) by bilinear interpolation using the compressibility factor table
• Calculation of K factor (K = Zm/Zb)
• calculation of conversion factor C
• density at base conditions, relative density, heating values, and
Wobbe indices can optionally be parameterized as constants
During operation, the Zm compressibility factor is calculated by bilinear
interpolation between the table Z values belonging to the nearest inter-
polation points for p and t. For the compressibility factor at base conditions
Zb, the functionality either uses a parameterized constant or calculates the
value via compressibility factor table.
In case that any of the inputs for pressure p or temperature t are beyond the
defined table range, no extrapolation is done. Instead, the value of the outer-
most interpolation point is used for calculating the respective compressibility
factor. In this situation, an alarm is generated and the results Zm, K and C
are marked as being in error. Same holds if pb and/or tb are lying outside
the table and Zb is parameterized to be calculated from table.
To detect a possible transition of the gas to the liquid state, the compressi-
bility factor table shall contain a zero (0) for all pairs (p, t) for which the
respective gas is supposed to be liquid. As long as liquid state is detected,
an alarm is pending. At the same time, the Zm, K and C results are marked
as being in error and the C result is set to zero (0).
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