13 Draught Control
Mk8 MM Manual Page | 244
13.3 Draught Control Operation
13.3.1 Operation Overview
The draught servomotor can be set on the Mk8 MM for servomotor movement only or draught control with a
pressure sensor to maintain the stack pressure.
The draught control trim functionality follows the same ideology to the EGA, where corrections are made to
air damper to compensate for the changes in exhaust gas values from their commissioned values, caused by
fluctuations in ambient conditions. For Autoflame draught control, corrections are made to the stack damper
to compensate for changes in wind and ambient conditions which cause the stack pressure to differ from the
commissioned stack pressure along the fuel curve.
Once the burner starts up and the main flame stabilised, there is a time delay where no draught control
operation occurs, set in expansion option 85. After this time delay elapses, as the air going into the burner is
increased from low fire to high fire, the boiler pressure may change, and the draught servomotor will move to
follow these changes as commissioned. If the atmospheric or stack pressures have changed from their
commissioned values along the curve, the air damper will either move in the open, or close direction to
adjust the air pressure in the stack back to its set value.
The smallest angle that the draught damper can drive to while running is defined by expansion option 83;
when the burner turns off, the draught damper will go to commissioned closed position to maintain heat
within the boiler.
If draught control is optioned off after being commissioned, then the draught servomotor will continue to
move to its commissioned angular position as the burner fires.
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