Emerson Process Management ControlWave XFC - Manual (Page 29)

CI-ControlWave XFC Introduction / 1-15
from external smart Multivariable Transmitters using either the BSAP or MODBUS
protocols. Alternatively, the inputs may be obtained via the local I/O Modules using analog
transmitters. The standard ControlWave XFC application program allows various
combinations of inputs to be selected, for a two-run metering station.
Regardless of the operating mode or the calculation interval, the ControlWave XFC
acquires samples as follows:
a. Differential pressure once per second
b. Static pressure once per second
c. Flowing temperature once per second
d. All self-test and compensation values at intervals of 4 seconds or less
1.5.2 Flow and Volume Calculations
The ControlWave XFC performs a complete flow calculation using the process variables
every second. Each calculation includes instantaneous rate according to API 14.3,
compressibility according to AGA 8 Detail or Gross method, and updates of all volumes,
totals, and archive averages. The user can select AGA3/NX-19 (1985), AGA3/AGA8,
AGA7/NX-19 or AGA7/AGA8.
1.5.2.1 Flow Rate and Flow Time Calculations (AGA3)
For orifice flow measurement, the differential pressure value is compared to a flow cutoff
value every second. If the differential pressure is less than the flow cutoff value, flow is
considered to be zero for that second. Hourly and Daily flow time is defined to be the
number of seconds for which the differential pressure exceeded the cutoff value for the
period.
The values for static and differential pressure, temperature, and flow extensions are used
as inputs to the flow equations. Users may select API 14.3 (AGA3, 1992) and AGA8
calculations, with compressibility being calculated according to AGA Report No. 8, 1992
(with 1993 errata). Both the DETAIL method and the two GROSS methods of
characterization described in AGA8 are supported. Users may also select the AGA3, 1995
and NX-19 flow equations to calculate the rate of flow.
1.5.2.2 Flow Rate Calculations and Flow Time Accumulations (AGA7)
When using PD meters, turbine meters or ultrasonic meters, the flow rate is calculated by
applying the correction factor computed by the AGA7 calculations to the frequency of the
input pulses. When the frequency drops below 1 Hz, the flow rate estimate is set to zero;
however, volume calculations are still accumulated. The flow time recorded is the time for
which the flow rate is non-zero.
1.5.2.3 Extension Calculation and Analog Averaging
For orifice meters, a flow extension is calculated every second. The extension is the square
root of the product of the absolute upstream static pressure times the differential pressure.
This extension is used in the flow rate calculation. When there is no flow, arithmetic
averages of static pressure and temperature are reported. This allows monitoring of static
pressure and temperature during shut-in periods.