ProSoft Technology MVI56-AFC - Manual (Page 275)

MVI56-AFC ♦ ControlLogix Platform Reference
Liquid and Gas Flow Computer User Manual
ProSoft Technology, Inc. Page 275 of 316
February 25, 2011
14.2.1 Basic Metering According to Meter type
Orifice (Include V-cone): Uses AGA3 1992 / ISO 5167.
A V-cone meter is like an orifice meter, except that the V-cone is an obstruction
in the center of the pipe while an orifice is an aperture. V-cone calculation differs
from orifice calculation in the following respects:
1 The orifice Beta ratio is actually the square root of the ratio of aperture cross-
section to pipe cross-section hence for the V-cone it is calculated differently
from the two diameters.
2 The V-cone Coefficient of Discharge is entered as configuration and not
calculated. Expansion Factor (Y) is calculated differently.
Output of the calculation is mass flow rate, which is divided by density to get
volume and then integrated over time for accumulation.
Pulse: Both Gas and Liquid
Gross Volume is (pulses) / (K-factor) * (meter factor), according to API MPMS Ch
12 sec 2 1981 and 1995. Output of the standard calculation is volume flow
increment, which is then multiplied by density to get mass increment. Flow rate is
calculated in parallel to flow increment by applying to (pulse frequency) process
input the same calculation as is applied to (pulses); this technique is employed
instead of flow increment differentiation because the pulse frequency available
from the counter card in the processor is not subject to variations of timing
caused by scheduling delays in processor backplane transfer and in the firmware
of the module, thus yielding a smoother flow rate.
Correction Factors According to Product Phase
Gas
Compressibility is calculated according to the Detail Characterization Method of
AGA8 (1992). Gas density is a byproduct of this calculation. Essential input for
this calculation is molar analysis. The compressibility Z is a factor in the gas
equation PV=ZNRT, which is the rule by which gas volumes are corrected to
reference conditions.
Liquid
Temperature and pressure correction factors are calculated according to API
MPMS Ch 11 and applied according to the rules given in MPMS Ch 12. Essential
input for this calculation is Liquid Density (page 36) at either standard or flowing
conditions.
Gas Pulse Measurement
The standard applied is AGA7, which is merely a combination of the gross
volume calculation (page 275) and the gas law (PV=ZNRT) which includes
compressibility. It also specifies calculation of some intermediate factors, which
are now idiosyncratic and vestigial, having been imported from an earlier AGA3
(1985 and before) which used the "factor" method to calculate gas flow and
which has been superseded by the completely overhauled 1990/1992 AGA3.