Combustion expert Dick Bennett explains how to use flame-monitoring systems and safety controls.

In my last column, I covered preventive maintenance items related to burners, their air and gas controls, fans and temperature control systems. This time, I'll complete the list, focusing on the safety systems.

Good preventive maintenance practices include checking valves and pressure switches as well as the burner and its flame scanner or flame rod.

Flame-Monitoring Systems

  • Check the flame-signal strength when the burner is at high and low fire. Compare with manufacturer's specs. The low fire reading can have a dual purpose: In addition to verifying the functioning of the flame-safety system, it also can give you some insight into whether low fire gas flow is set correctly. If the signal is weak or fluctuates excessively, and you've verified the scanner or flame rod is OK, the gas flow may be marginal and in need of a slight increase.

    Here's where a log of test readings really becomes useful. If you have a set of flame-signal readings taken over several weeks or months, and the readings are steadily declining, you're drifting toward a failure.

  • Remove the flame rod or scanner from the burner and examine it. Flame rods sometimes droop or warp due to heat. In dusty atmospheres, they may get coated with an insulating layer of fused or caked dust. The tubes and lenses of UV scanners often get a filmy buildup that screens out part of the radiation, causing weak signals.

  • Check the response of the flame-detection system by manually shutting off the gas flow to the burner while it's running. Required shutdown times vary with the size and vintage of the equipment, and the model of flame safeguard used, but most modern systems will drop out within 3 sec.

  • Modern flame safeguards incorporate numerous self-diagnostic features in their circuitry. One of the most important is the safe start check. By checking for a flame signal before the burner is supposed to be lit, it verifies that the flame detector and the control haven't failed in an unsafe mode. If it “sees” a flame, something's obviously wrong, because there shouldn't be any at this time, and the system shuts down.

    Normally, this circuitry is pretty reliable, but if you'd like to be extra sure, you can test it by removing the UV scanner from the burner and aiming it at the flame of a cigarette lighter while the control system is going through its preliminary startup motions. If the cycle continues through the purge, something is wrong.


Safety Controls

  • Verify the fuel safety-shutoff valves are operating properly. With the combustion system running, shut off the power to the control panel. The safety-shutoff valves should close in 1 sec or less.
  • Run a leak check on the safety-shutoff valves. This requires test petcocks in the gas line downstream of each valve (downstream body taps on the valves themselves will do) and a test switch and circuit in the control panel. In the United States, NFPA 86 is the basic safety standard for industrial ovens and furnaces, and it requires leak tests to be done at least annually. The standard also contains details of the test procedure and circuit wiring. The test essentially consists of pressurizing the closed safety-shutoff valve and watching for gas bubbles from a tube in a glass of water. By the way, a few small bubbles aren't necessarily cause for panic. A very small amount of leakage is permitted by the approval standards and by NFPA 86 -- an acknowledgment that absolute leak-tightness is difficult to achieve. Leaks exceeding those permitted by the standards should be dealt with immediately, however.

  • Verify that combustion air and gas pressure and flow switches are set properly and operating. Most switches have an indicator that shows the setting. Low air- and gas-pressure switches can be checked in place with a continuity test. Contacts should be open with no air or gas flowing, and closed when there's pressure in the line. High gas-pressure switches are tougher to check because these normally closed switches shouldn't open unless they're exposed to pressures well above normal operating levels. A functional check can be made by removing the switch from the line and checking it on a regulated compressed-air line. Be careful not to expose it to excessive pressure, though.

    The exhaust fans and, often, the recirculating fans, of most ovens are required to be monitored by flow or pressure switches. Automatic damper positions often are monitored by limit or flow switches. If these devices are interlocked into the burner safety circuit, they should be checked, too.

  • Check the purge timer setting. Some have displays or dials that show the setting, but if they don't, or if you're skeptical of the readout accuracy, run the control system through a cycle and time it with a watch. Purge settings are especially important because so many accidents have been traced to purge timers that have been dialed down or bypassed altogether (see “How to Lose at Russian Roulette,” March 1999).


Conclusion

Preventive maintenance of oven and furnace combustion systems doesn't require sophisticated instrumentation, tools or techniques, just a small amount of time and a basic knowledge of how the equipment works. The payoff, in greater reliability, energy economy, product consistency and safety, is well worth the effort.