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 | ||
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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 | ||
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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. | |
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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 | 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- | ||
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ries regarding applications that are not listed, please | Burner control systems designed for | |
contact Bob Langthorne at IRIS Systems Inc. for rec- | and | |
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 | ||
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, | |
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relay of the P522 feeds only to a pro- | Also, consideration should be given to the | |
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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