IRIS manual Using a PLC with Iris Flame, Monitor, Application of P522

IRIS MODEL P522 APPLICATION MANUAL

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