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6 Commissioning
6.8 Shut-off Limits
6.8.1 Run to Shut-off Limits
The VMS tolerance which was showed by every control system affects the positioning accu-
racy. These tolerances can be influenced by parameter settings.
When the combustion plant is adjusted, it has to be ensured that the flame burns safe and
stable under these tolerances. To verify that, it is necessary not only to check the pre-adjusted
setpoints of the fuel/air ratio settings but also to check the so-called shut-off limits. These are
the actuator values, which can be taken due to the permitted tolerances but not switch off.
This check is to be done for the whole firing rate range and operating range.
This may only be omitted when:
– the plant is equipped with a failsafe CO or O
2
monitoring, which will automatically off the
burner when combustion is incomplete
or
– the plant is equipped with a failsafe CO or O
2
monitoring, which will compensate the com-
bustion tolerances
or
– it is ensured that the operating mode will have enough excess air through the whole range
and the tolerance of the burner control would be of no consequence.
or
– the VMS parameters are set in such a way that the tolerances in air deficiency direction
has 0 digit, therefore only a tolerance for excess air exists.
WARNING!
If this shut-off limits check is omitted due to one of these reasons, the efficiency of the measure
is to be checked, e.g. check the stability of combustion in the preset shut-off limits in CO/O
2
monitoring.
WARNING!
Due to the evidences required by the authorities and bodies, quality management systems,
liability issues, etc, we highly recommend you to document the start-up of this shut-off limits.
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