Emerson ControlWave EFM Instruction Manual page 17

1.9.6 Energy Calculation
1.9.7 Volume and Energy Integration
1.9.8 Downstream Pressure Tap
1.9.9 Historical Data Storage (Audit Records/ Archive Files)
Audit Records
(Alarms and
Events)
Revised Dec-2013
The application offers the option of using a fixed volumetric heating
value or calculating the energy content of the gas according to AGA
Report No. 5.
The application integrates and accumulates volume and energy at the
end of every calculation cycle. The application calculates the volume
for a cycle by multiplying the calculated rate by the flow time for that
cycle. The application calculates the energy for a cycle by multiplying
the volume at base conditions by the heating value.
The multivariable transducer typically measures static pressure from an
integral tap on the upstream, high-pressure leg of the differential
pressure connection. The transducer can also measure static pressure at
the downstream pressure tap, with the measurement taken from the low-
pressure 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) or an external MVT, you
select the downstream tap location during MVT configuration, the MVT
firmware changes the sign of the differential pressure to provide a
positive DP value.
The ControlWave supports two distinct types of historical data storage –
audit records and archive files.
Where feasible, both forms of archive data conform to the requirements
of the API Chapter 21. 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 audit system maintains a history of alarms and certain events that
have an impact on the calculated and reported gas flow rates and
volumes.
The application stores the most recent 500 alarms and the most recent
500 events. As new alarms/events arrive, they overwrite the oldest
entries. Internally, the ControlWave stores alarms and events separately
to prevent recurring alarms from overwriting configuration audit data
events. The application reports alarms and events in the same log.
The following circumstances generate an audit record:
Introduction
ControlWave EFM Instruction Manual
1-13