In "How to Lose at Russian Roulette"[link at bottom of page], I taught you how to lose at Russian Roulette -- well, at least the process heating version of the game. I'd likened the act of bypassing or dialing down the combustion purge timer to playing Modified Rules Russian Roulette -- under these rules, every time you failed to blow out your brains, you put an additional round in the revolver until, finally, you succeeded (if that's the proper word for it).
In the intervening months, I've harped on this subject at various training schools, and this has prompted attendees to ask if there might be some "What if?" exceptions. Here's one that has come up frequently:
"What if the combustion air fan continues to run from the last time we shut down until we restart the oven or furnace? Doesn't that continuously purge the combustion chamber, so a prepurge on relight is unnecessary?"
Good question. Many ovens and other heating devices are cycled on and off throughout the day, and their control systems are designed to keep the combustion air blower operating during the down periods. In a variation on this theme, one plant reported running the fans between cycles to facilitate combustion chamber repairs.
I answer this question with two questions of my own.
- Does the fan run long enough and at a high enough flow rate to ensure you had at least four complete air changes in the oven? If not, you haven't met the minimum requirements of the purge.
- Are you absolutely sure that no fuel leaked into the oven during that time? If it did, and the fan was running, you may be at greater risk than if the fan was off.
Regardless, if you attempt a lightoff and it fails, you must purge. All that air put through the oven during the off period no longer matters.
NFPA 86, Ovens and Furnaces, 1999 edition, does permit relight attempts without repurging if:
- (a) The heating chamber temperature exceeds 1,400°F (760°C) and:
(b) For any fuel-fired system, if:
(1) each burner and pilot is supervised by a combustion safeguard.
(2)each burner system is equipped with safety shutoff valves.
(3) at least one burner remains operating in the common combustion chamber of the burner to be reignited.
(c) Or, for gas-fired systems only, if:
The 1,400°F minimum temperature rules out most equipment operated by Process Heating readers, but let's look closer in case you're one of the exceptions. The wording appears to be constructed around a system where you intend to shut off some of the burners in the combustion chamber while leaving on others. This requires each burner to have independent flame supervision; otherwise, the failure of one will shut down the whole zone.
If you have a multiburner system, you're allowed to skip the purge if you can meet the NFPA requirements outlined above. For most of you, however, this is only of academic interest. Your combustion system is probably equipped with the usual all-or-nothing safety system. In that case, the advice I passed on before still holds:
- 1. Do not dial down or bypass the purge timer.
2. If the burner won't light after two or three tries, quit trying and look for the cause of the problem.
3. See that all your purge times are set correctly and that there are no jumpers around the timers.