IRIS P522 manual Basic Burner Start Circuit

IRIS MODEL P522 APPLICATION MANUAL

function is to use an auxiliary timer, so that a time out of both the PLC and the auxiliary timer is re- quired before light off (AND relationship).

The wiring diagram in Figure 10 shows a basic ig- niter/burner circuit that can be used with a PLC to accomplish the safe operation of a burner or burners in a multiburner installation. Note that the burner shut- down (BURNER STOP SW) is included; this circuit is equally as critical as flame monitor protection.

BASIC BURNER START CIRCUIT

The sequence of operations for the basic burner start circuit is as follows (refer to Figure 12):

1.Pressing the start button (START P.B.) will start the ignition sequence, provid- ing there is no MFT present, purge is complete and all the limits for firing are satisfactory, and no flame is being de- tected. This latter function is the “safe- start-check” jargon common in the flame safeguard industry. Note that the PLC is started at the same time as time delay 1 (TD1) is pulsed ON. (The push-button switch must be actuated for at least 50 milliseconds to ensure the Delay After De-Energized(DADE) timer is started.)

2.The master relay (RM) Contact pulls in, sealing the loop around the flame relay (RF) contact, normally closed. The RM contact provides the current path to the ignition transformer and igniter valve. Note that the igniter valve contact from the PLC must be commanded closed at this time. A master relay is used so that there is adequate current-carrying capac- ity, which may not be the case for using the flame relay contacts by themselves. (The P522 flame relay contacts are rated to 2 Amps at 220 VAC.) Alternatively, additional, normally-open RM contacts can be used, either in series or parallel, to accommodate the load requirement (sometimes, two contacts in series are used to minimize contact wear caused by inductive arcing when contacts are open).

3.The DADE timer will time-out, opening the contact around the RF contact. If there is no flame detected, the RM will drop out, shutting down the sequence.

4.Once flame is detected, the PLC logic se- quence can continue by opening the burner valve, and then after a time delay, closing the igniter valve (this is called “interrupted igniter operation). The only necessary requirement is that the burner valve(s) be powered by RM contact(s), as shown. A subsequent flame failure will cause the RM to drop out, de-ener- gizing the main fuel valve(s), regardless of the state of the PLC.

This wiring diagram is not intended to illustrate all the logic that would normally be used in a multiburner system, but to show only the bare es- sentials required to make the PLC safe. Such func- tions as low-fire permissive on first burner started, flameout alarm and reset, main burner time trial for ignition, and igniter shutdown, would be part of the PLC logic.

Occasionally, it is necessary to use two viewing heads –one for the igniter and one for the burner– with one signal processor. In this case, you can switch between them (using one head at a time): the igniter viewing head when proving igniter flame, and the burner viewing head for proving main flame (re- fer to section MULTIPLE VIEWING HEADS).

A safe-start-check would have to be made on both viewing heads; you do not want to start up a burner if it is already reading other flames. You can do this by looking at both flame monitors in parallel (switch both inputs together by connecting together the SIG terminals of each viewing head, and the SIG GND terminals of each viewing head) before starting the ignition sequence. This will result in the normally- closed RF contact on the P522 having to be closed in order to start the burner. This parallel viewing head operation should only be used for the start-up sequence, because adding the flame signals together increases the unwanted background that results in poor discrimination.

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