Page 239 - Instrumentation Reference Book 3E

Fieldbus 223

1 Stream

Mass FR
SVOL FR
Energy
Mass TOT

System alarm
Process alarm
Computer alarm

Printer
Supervisory
S-TEK supervisory
Figure 13.28 Natural gas cost metering installation. Courtesyof Spectra-Tek Ltd.

defined in AGA 8. using measured pressure and
composition data for up to 20 components. The
calculated standard density is used to derive 13.10.1 Background
standard volume. Calorific value is calculated in
accordance with IS0 6976 using the measured At present, the majority of microprocessor-
temperature, pressure, and gas composition data. based and intelligent transmitters operate in
Calorific value is used to calculate energy flow. conventional 4420 mA measurement circuits,
Mass, standard volume, and energy are each but it is inevitable that the advantages to be
available as a cumulative total; a daily total, as gained from multiplexing signals from sensors.
well as the instantaneous flow rates. Analog out- actuators, and controllers will be a feature of
puts are available for each flow rate together new process plants as well as plant upgrading.
with an incremental pulse output for mass flow However. to implement this, a commm coin-
rate. munication system, or fieldbus, needs to be
Three separate alarm outputs are provided to established.
indicate c'omputer alarm (generated by internal There are many advantages of fieldbus over
diagnostic routines), system alarm (generated by conventional analog systems. the most immediate
a fault in the internal or external measurement of which are reduced cost of installation and
circuits), and process alarms (generated by pro- maintenance, increased flexibility, and enhanced
cess conditions moving out of limits). These functionaiity. such as self-diagnostics, calibration,
hardwired alarm outputs operate in conjunction and condition monitoring. The additional bene-
with the flow computer's front panel display and fits available with these microprocessor-based
may be connected to an external annunciator and intelligent devices will also expand the use
system. of sensors and control systems.