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GAA610-M
ADVANCED EMISSION GAS MONITORING SYSTEM | OI/GAA610-M-EN REV. B
… 9 Gas sampling with automatic back-purging
Cycle time
Cycle time duration
The cycle time is given as the time interval between two
automatic starts of the back-purging procedure. The higher the
dust concentration in the sample gas and the higher the sample
gas flow, the shorter this time interval must be set, to avoid a
blocking of the gas sampling probe filter.
Cycle time factory setting
The parameter ‘Cycle time’ is factory-set to 12 h.
Optimum cycle time setting
The cycle time should not be adjusted shorter than needed,
because during the back-purging procedure (approx.18 s) and
especially during the post-purge time (factory-set to 90 s) no
measurement can be made. The optimum time will have to be
found out by operational experience.
Cycle time minimum value
The cycle time should not be below a lower limit.
The back-purging procedure with cold compressed air causes a
cooling of the heated probe filter, and the filter temperature
regulation needs some time to correct this temperature
decrease.
As the filter heating regulation is a rather slow control loop, this
time will be relatively long. Therefore, the cycle time should not
fall below approx. 60 min.
Event-controlled start of the back-purging procedure by
filter plugging
Should despite the time controlled back-purging a probe filter
blocking occur caused by temporary larger amounts of dust,
with the result of a sample gas flow decrease beneath the
admissible limit, an additional back-purging procedure is started
as a result, and the probe filter is purged free in between.
Adjustment of the cycle time
To adjust the cycle time the parameters of the AO2020 program
must be changed. Please contact ABB service.
Post-purge time
Post-purge time duration
The post-purge time at the end of the back-purging procedure
must be such, that the complete pneumatic system is flushed
with the actual sample gas and the analyzer gets time to take
over the actual measuring value again.
The needed post-purge time depends on the respective layout of
the system (i.e. the length of the sample gas line) and will have
to be adjusted individually. A post-purge time of 90 s is factory-
set.
Guide for the post-purge time
A guide for the needed post-purge time is given in the table
below. Please add the times for the pneumatic system, the
analyzer and the sample gas line.
Response time (3 x T
90
, approx.)
Pneumatic system without sample gas line
27 s
plus analyzer Uras26
20 s
plus for each 10 m sample gas line I.D. = 4 mm
5 s
plus filter unit with probe tube 40, length = 1 m
45 s
Example
For an analyzer system with filter unit and 15 m sample gas line
at 60 l/h sample gas flow the post-purge time is calculated as
follows:
Post-purge time = 27 s + 20 s + 1.5 x 5 s + 45 s = 99.5 s
Adjustment of the post-purge time
To adjust the post-purge time the parameters of the AO2020
must be changed. Please contact ABB service.
