10 Fully Metered Combustion Control
Page 146 Mk8 MM Expansion Features Installation and Set-Up Guide 04.09.2016
10.2.4 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.
10.2.5 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:
Variable Action
Ambient air pressure Average ambient pressure, set in expansion option 153
Differential air pressure Air pressure correction not used (assumed 20mbar)
Air temperature Air temperature correction not used (assumed 20
O
C)
Gas pressure Gas pressure correction not used (assumed 100mbar)
Gas temperature Gas temperature correction not used (assumed 5
O
C)
If additional Autoflame temperature and pressure sensors are used on the 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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