17 Fully Metered Combustion Control
Mk8 MM Manual Page | 302
17.3.4 Mass Flow Meters
When using mass flow meters, the fuel and air rates are displayed on the MM, these are based on the 4-
20mA input signals from the mass flow meters. The control process will aim to maintain the same fuel-air
ratio as those set during the commissioning process.
If the fully metered system is set with a fuel mass flow meter, then the fuel temperature and pressure
sensors are not required for the fully metered control; however, the pressure sensor can still be used for VPS
and high/low pressure limits. If an air mass flow meter is used, then again, the air temperature and pressure
sensors are not required; however, the air pressure sensor can still be used for air pressure checking during
burner start-up e.g. purge air pressure proving.
The calorific value of the fuel is used to calculate the excess air, so this must be entered accurately.
17.3.5 Volume Flow Meters
When using volume flow meters for the fuel and mass flow rate information, corrections need to be applied to
derive the mass flow rate. The MM calculates the mass flows using the fuel density and calorific value; these
must be set accurately for the fuel being metered. If a differential pressure sensor is being used the system
can be optioned to perform square root extraction on the input, thus saving an external converter.
Based on the ideal gas flow, assuming that the humidity and the specific gas constant do not vary, the mass
flow rate is related to the volume flow rate by the following equation:
×
This formula is used internally calculated in the MM to provide a mass flow rate, this is calculated individually
for both gas and air in the MM using the following assumptions:
Average ambient pressure, set in expansion option 153
Differential air pressure
Air pressure correction not used (assumed 20mbar)
Air temperature correction not used (assumed 20
O
C)
Gas pressure correction not used (assumed 100mbar)
Gas temperature correction not used (assumed 5
O
C)
If additional Autoflame temperature and pressure sensors are used on either or both the gas and air, then
the calculated mass flow rate displayed will be more accurate. In the event of a sensor failure, the
temperature/pressure value of the failed sensor that was stored at commissioning of value is used to
calculate the displayed mass flow rate allowing the system to continue to run. If the gas and air pressure
sensors are used also used for the VPS, gas pressure limits or air sensors limits, then the burner will lockout
in the event of a sensor failure.
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