Emerson Process Management ControlWave EFM 3808 - Manual (Page 39)

CI-ControlWave EFM Introduction / 1-23
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.
1.5.2.3.1 Energy Calculation
The ControlWave EFM offers the option of using a fixed volumetric heating value or
calculating the energy content of the gas according to AGA Report No. 5.
1.5.2.3.2 Volume and Energy Integration
Volume and energy are each integrated and accumulated at the end of every calculation
cycle. The volume for a cycle is the calculated rate multiplied by the flow time for that cycle.
The energy for a cycle is calculated by multiplying the volume at BASE conditions by the
heating value.
1.5.2.4 Downstream Pressure Tap
The multivariable transducer typically measures static pressure from an integral tap on the
upstream, high-pressure leg of the differential pressure connection. Static pressure can be
measured at the downstream pressure tap, with the measurement taken from the low-pres-
sure side to the high-pressure side. In this installation, the differential signal from the
transducer is negative. If while using the integral smart Multivariable Transmitter (MVT)