4 Fuel Output Modes and Changeover Options
Mk8 MM Manual Page | 113
4.5.2 Checks before COF can take place
Fuel Change on the Fly can be triggered by introducing live voltage to T91 & T92 simultaneously, the
changeover can only happen if the burner is in firing mode. The MM will undergo the following checks before
going ahead with a COF fuel change:
MM Phase: Modulating – Firing
For a Fuel Change on the Fly to take place, the MM will first check whether the newly selected fuel is
commissioned, if the selected fuel is not commissioned then a warning will be displayed indicating that the
selected fuel cannot be run. This warning will clear automatically when the fuel selection is returned to the
current fuel. If the burner is firing, then it will continue to fire on the current fuel.
MM Phase: Modulating - Gas Pressure Test
If the burner is firing and the newly selected fuel has pressure sensor optioned (depending on option 125-
126 setting), a fuel pressure test will take place, during which the supply-side fuel valve (Valve 1) for the new
fuel is briefly opened and the fuel pressure is measured. If the fuel pressure is within the optioned limits (in
options 136 & 137) of the commissioned pressure then the fuel changeover process may continue, however
if the pressure for the selected fuel is not within these limits the MM will continue to fire on the current fuel.
Failure of a gas pressure test at this point causes a warning to be generated, indicating that gas pressure
prevented a fuel changeover. This warning will clear automatically when the fuel selection is returned to the
current fuel. Selecting the new fuel again will perform another gas pressure test. This test does not take
place if an oil fuel is selected.
4.5.3 COF Changeover Position
This is a position at which both fuels fire simultaneously as part of the fuel Change on the Fly process.
MM Phase: Modulating to COF
The current fuel modulates down its commissioned curve to its low fire point (or its FGR point otherwise if
optioned)
MM Phase: Fuel Changeover – Firing
From this point the MM moves to the COF Changeover Position which is generated by a combination of the
two commissioned fuel curves as follows:
• Servo 1 (Fuel 1) COF changeover position is the commissioned angle for the low fire point (or the
FGR point for fuel 1 if optioned).
• Servo 3 (Fuel 2) COF changeover position is the commissioned angle for the low fire point (or the
FGR point for fuel 2 if optioned).
• Servo 2 (Air Angle) COF changeover position is an increased air angle calculated to provide enough
air for safe combustion of the both fuels to fire simultaneously and to make sure that the combustion
is Air-Rich during the changeover process.
• Other channels, if optioned (i.e. servo 4, draught servo, and VSDs) take the larger value or more
open angle of the two fuels at the changeover point (FGR or low fire).
IMPORTANT: If a VSD is used to control the air fan in addition to the air damper servo, it is very
important to have similar VSD signal output on both fuels when commissioning the fuels in Dual Fuel
Output Mode, this is to ensure that sufficient amount of air is present for both fuels to fire at the COF
Changeover Position.
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