I.R.I.S. FLAME MONITORING SYSTEM P522 - Manual (Page 26)

IRIS MODEL P522 APPLICATION MANUAL
Page 26
cies than oil burners, particularly the low NOx type
burners. UV type detectors work very well on natu-
ral gas, but they are not so good on oil flames, and
work poorly on pulverized coal flames.
LOW NOx APPLICATIONS
Since the advent of low NOx burners and combustion
techniques to reduce emissions, the application of flame
monitors has been radically changed (i.e., an infrared
flicker flame monitor will not discriminate between
burners in boilers utilizing low NOx gas burners). This
is particularly true if FGR is being used. Only a UV
flame monitor with a wide linear range (S506) can be
used. In this type of application, UV radiation is spread
out all over the furnace; but the more intense radiation
is concentrated at the root of the flame envelope, so the
flame monitor must be aimed at this zone.
On the other hand, a UV flame monitor will not work
on a low NOx oil flame, because there is little UV
radiation detectable from this type of staged com-
bustion. Only an infrared flicker-type flame moni-
tor (S509, S512) will discriminate between burners.
The viewing head application tables (see Figure 11)
show most of the common applications and the view-
ing heads recommended for them. If you have enqui-
ries regarding applications that are not listed, please
contact Bob Langthorne at IRIS Systems Inc. for rec-
ommendations.
APPLICATION OF P522
Great effort has gone into designing the P522 signal
processor to make it fail safe. When properly in-
stalled and functioning as designed, it functions as a
safety device that will provide protection from boiler
or furnace explosions.
However, the output of the flame relay in most instal-
lations is not used directly to control the fuel valves.
The circuitry used between the flame relay and the safety
shutoff valve solenoid should be carefully examined.
For example: a condition where the flame
relay of the P522 feeds only to a pro-
grammable logic controller (PLC), and
the PLC controls the fuel valve, repre-
sents a dangerous installation with the po-
tential for a serious explosion.
USING A PLC WITH IRIS FLAME
MONITOR
PLCs are inherently unsafe when used by them-
selves.
1, 2, 3. Schemes have been devised to make
them safe – such as feeding outputs back to the in-
puts to check the outputs; using multiple outputs in
series for redundancy; and using “watchdog timers”
that presumably detect malfunctions. None of these
methods will make a PLC critical control loop 100
per cent safe.
PLCs cannot and should not be used by themselves
for burner control or burner management systems
without external protective circuitry to make them 100
per cent safe. The critical loops that must be pro-
tected are the main fuel trip (MFT) functions; initial
burner/igniter light off; subsequent continued burner
firing with flame monitoring; and burner shutdown.
Most burner control systems are designed for over-
all, fail safe operation. “Fail safe” means that, in the
event of a failure of any component(s) or power, the
burner or boiler will be shutdown safely. This usu-
ally is accomplished by devices such as valves, so-
lenoids and relays that use springs to return them to
a safe state when energized.
Burner control systems designed for power-to-open
and power-to-close type systems are different because
of the storage battery backup upon loss of AC power.
However, the philosophy of fail safe concepts and
safety with regard to critical loops is the same.
The external, protective circuitry required for the
boiler-common MFT function can be as simple as
using a single relay energized by a string of contacts
in series that would include the usual permissives,
proper air flow, fuel pressure, fuel temperature, etc.
More elaborate designs involving the PLC for alarm-
ing or prompts, in addition to diagnostic functions,
will complicate the MFT circuitry, but the underly-
ing philosophy must be to maintain an external, di-
rect, fail-safe function.
Also, consideration should be given to the air-purge
timing function. The dangerous condition occurs
when the purge timer times out short of the required
time delay. One way to overcome this type of mal-