Riello 20155654, Installation, Use And Maintenance Instructions

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53 20155699 GB
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8.1.7 Thermostat and control range
The thermostat function switches burner on and off on basis of
temperature and/or pressure value.
At first, the burner must be released by starting signal. Control
range is formed by en-tering controller set-point value, parame-
ters P 0054 (switch-on point), P 0055 (upper control range) and
P 0056 (burner OFF).
The shut-off hysteresis is divided into 2 ranges. The first part is
located above set-point forms the upper control range. Second
part below set-point forms the lower control range.
Control range is possibly located asymmetrically around the set-
point.
Below upper control range power control unit functions work ac-
cording to settings made in pa-rameters and default values. If the
control units’ actual value reaches shut-down range a base firing
rate request is emitted.
While set-point of control units exceeds shut-down range a con-
trol shut-off occurs.
This done by internal processing. If the actual value drops below
lower control range, a re-start can hap-pen.
NOTE:
this function may replace the control thermostat, which is re-
quired on the plant.
It does not replace a safety thermostat.
8.1.8 Control range
The control range is located around set-point. The content of
“Burner ON” parameter is sub-tracted from set-point to form
switch-on value.
“Upper control range” parameter value (P 0055) is added to set-
point to form the upper limit of control range.
The control range may therefore be located asymmetrically
around set-point.The shut-off range is limited by “Burner off” pa-
rameter (P 0056).
The parameter is added to the set-point. If this value is exceeded
a burner shut-off is initiated.
The range between “Upper control range” and “Burner OFF” is
forming the shut-off range. If the actual value reaches this range
fuel/air ratio control changes to base firing rate.
Another con-sequence is that “Burner OFF” value is in general
higher than the one of “Upper control range” value.
In all other cases there is no shut-off range and burner is imme-
diately shut-off when reaching limit.
The burner ON switching point can also be located above set-
point. In this case P0054 has to be set negative (< 0).
Key (Fig. 93)
*(1) = P 0055
*(2) = P 0056
*(3) = P 0054
*(4) = P 0048 - P 0051
8.1.9 Checking of the safety limiter
For testing purposes the safety limiter you must change the set-
point. This also causes a change in shut-off range as well.
The safety limiter can therefore be overrun in manual mode.
Fig. 93
S9617
Time
Burner OFF
Set-point *(4)
Burner ON
*(1)
*(3)
*(2)
Shut-down range
Upper range