4 Fuel Output Modes and Changeover Options
Mk8 MM Manual Page | 118
4.6 Gas Pressure Sensors and Valve Proving in DFOM with COF
There are some key differences in the operation of gas pressure sensors and valve proving between Single
Fuel Output Mode and Dual Fuel Output Mode.
Before and/or after firing the burner (according to Option 129) valve proving takes place simultaneously for
any fuel for which it is optioned. Failure of the valve proving process on either fuel will cause a lockout that
stops the burner. This does not happen in Single Fuel Output Mode.
The tests performed during valve proving are different in Dual Fuel Output Mode:
• If a fuel is about to be fired (at burner start-up or before a COF changeover to that fuel) the static line
pressure is tested against the optioned pressure limits defend in options 136, 137 and 138. This is
the same as in single fuel output (standard) mode.
• If the fuel is not about to be fired, the static line pressure is tested (at burner start-up for the 2
nd
fuel
or after a COF changeover for example) against a lower threshold, defined by Option 133 (Maximum
pressure change allowed during valve proving). This tests that the pressure is high enough for valve
proving to be valid (a drop in pressure can be detected that would cause the test to fail) but it does
not require that the fuel has enough pressure to fire.
This is to accommodate fuels that may not always be available such as waste biogas collected and
burned periodically. It means that low pressure on a fuel that isn’t about to fire will not prevent the
burner from firing on the current fuel, provided that the pressure on the 2
nd
fuel is high enough to
prove that the valve are not leaking.
If the fuel selection is changed while the burner is firing, and the newly selected fuel has a gas pressure
sensor optioned then a fuel pressure test will take place. The supply-side fuel valve for the new fuel is briefly
opened and the fuel pressure is measured. If the fuel pressure is within the optioned limits of the
commissioned pressure (Set in options 136 & 137) the fuel changeover may continue – otherwise the MM
will continue to fire on the current fuel. It should be noted that low pressure at this point will not generate a
lockout as valve proving is not being performed.
After a Fuel Change on the Fly, if valve proving is optioned for the fuel that has just been turned off then
valve proving will take place on that fuel while the burner continues to fire. The pressure must be high
enough for valve proving to be valid or a lockout will be generated, but the pressure does not need to be as
high as the commissioned limits.
When valve proving is performed on the inactive fuel while the active fuel is firing the gas pressure sensor
will be re-zeroed for 3 gas valve configurations. The gas sensor will not be re-zeroed with 2-valve
configuration (Option 13 setting 0 or 3).
Fuel pressure limits continue to be tested for the currently firing fuel as normal, including when other
operations such as valve proving are taking place on the other fuel.
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