FCU 500, FCU 505 · Edition 02.17
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14 .4 High temperature operation
If firing systems are operated above 750°C (1400°F),
the system is considered to be an item of high tem-
perature equipment (see EN 746-2/NFPA 86). Flame
control must be in operation until the furnace wall
temperature has exceeded 750°C. Frequently, flame
control is dispensed with so as to achieve a particularly
high flexibility of the installation. This means that no
incorrect flame signals, e.g. signals from a UV sensor
which interprets reflected UV radiation as extraneous
signals, may lead to faults.
When a burner control unit’s HT input is activated, the
burner control unit switches to High temperature mode.
This means: the burner control unit operates without
evaluation of the flame signal. The safety function of
the device-internal flame control system is deactivated.
In High temperature mode, the gas valves are opened
by the burner control unit without flame control. The
precondition for this operating mode is that an exter-
nal flame supervision device ensures the presence of
the flame in a fail-safe manner indirectly via the tem-
perature. This is performed by the safety temperature
monitor fitted in the temperature module in conjunc-
tion with an external double thermocouple (DIN 3440).
Sensor discontinuity, sensor short-circuit, failure of a
component or mains failure must set the installation to
a safe state. Voltage may be applied to the HT input of a
burner control unit so as to activate High temperature
mode only if the temperature at the furnace wall has ex-
ceeded 750°C (1400°F). The burner control unit then
starts the burner, without monitoring the presence of
the flame.
If the temperature in the furnace chamber drops below
750°C (1400°F), the HT input must be disconnected
from the electrical power supply and the furnace must
then be operated with flame control.
Comply with the local safety regulations.
14 .4 .1 Safety temperature monitor (STM)
The FCU signals to the connected burner control units
via the fail-safe HT output that the system is in High
temperature mode (≥ 750°C pursuant to EN 746-2,
≥ 1400°F pursuant to NFPA 86). The HT output is only
enabled together with the safety interlocks.
When the HT inputs on the burner control units are ac-
tivated, the burners are started without flame control
(UV or ionization).
The double thermocouples must be positioned at the
coldest point in the combustion chamber so that they
can reliably detect whether the spontaneous ignition
temperature (≥ 750°C, ≥ 1400°F) has been exceeded.