Emerson Process Management ControlWave XFC - Manual (Page 30)

1-16 / Introduction CI-ControlWave XFC
1.5.2.3.1 Energy Calculation
The ControlWave XFC 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),
the user selects the downstream tap location during MVT configuration, the MVT firmware
changes the sign of the differential pressure to provide a positive DP value.
1.5.3 Archives
The ControlWave XFC stores two distinct types of archive data. The first type is Audit
Trail data, which is a recording of the various events and alarms that have an impact on
the calculated and reported rates and volumes. The second type is historical data, which
includes records of rates and volumes and other signals over time. When an archive log
becomes full, new entries replace the oldest entries in the log.
Where feasible, both forms of archive data conform to the requirements of the API Chapter
21 (the Committee on Gas Measurement's GFC document). Specifically, the averages of the
process variables stored in the data archive are for flowing periods, appropriate to their
usage in the equations, and any gas-related parameter designated an event that is changed
by an operator either remotely or locally causes an entry in the audit log.
The ControlWave XFC supports the "breaking" of a log period when an operator-entered
parameter is changed. When this occurs, the log period in process is closed out, a log is
made, and a new log is begun. This feature is disabled by default and may be enabled by
the operator. Note: To prevent several very short logs from being created due to a series of
successive configuration changes, the ControlWave XFC will not create a log which
contains less than 60 seconds (flowing or otherwise) of data. Therefore if a user enters 15
configuration changes over a 2 minute period, the log will only be broken twice.
1.5.3.1 Hourly Historical Data Log
The Hourly Data Log holds one record for every contract hour. Hourly logs hold 840 entries
or 35 days; this ensures that the previous period of hourly data is always resident in
ControlWave XFC FLASH memory. The following items are stored in the Hourly Data
Log:
• Corrected Volume
• Uncorrected Volume