Knowing when a flame is present in a boiler furnace is a fundamental safety function of the burner management system. If a flame is present and the BMS does not detect it, there will be nuisance trips and the boiler cannot operate reliably. Alternatively, if the flame detection is too sensitive and does not recognize when there has been a flame failure, the fuel shutoff valves will remain open adding unburnt fuel to the boiler furnace which leads to a high risk of a boiler explosion. With this in mind, much thought has been put into the various methods of flame detection and ensuring that the BMS can reliably recognize whether a flame is present or not.

Flame Rods

There are many ways that flame detection can be done. The most basic of these is with a flame rod on smaller burners. A flame rod is a device that when energized with AC voltage (typically between 120-300 Vac) can conduct a small current through flame ionization when a flame is present in the burner. Using the burner as a ground, this completes the circuit during the negative wave of the AC cycle and causes a “rectification” effect shown below

The BMS then compares the amplitude of the positive and negative sides of the AC sine wave and from that difference, decides whether a flame is present or not. The strength of this signal is affected by the surface area of the grounding plane that the flame conducts to, and consequently when a flame rod has trouble picking up a flame, it is often caused by a bad ground connection or by carbon buildup on the flame rod or ground plane. Flame rods are a highly-common and reliable way to sense a flame in burners below 12.5mmbtu/hr. A disadvantage of flame rods is that they regularly need to be cleaned or replaced.

IR Flame Scanners

On burners that are too large for a flame rod, the most common methods for flame detection are the use of ultraviolet (UV) or Infrared (IR) flame scanners. IR scanners use sensors made of materials such as Lead-Sulfide, Lead-Selenide, or silicone phototransistors to pick up the strength of the infrared light present in a flame. A wavelength range of 400-1050 nanometers is typical, though this varies by manufacturer and type of sensing material used. IR flame scanners can also use the “flicker frequency” of a flame to distinguish between an actual flame as opposed to the radiated energy from a glowing refractory.

UV Flame Scanners

Basic UV flame scanners utilize a vacuum tube energized at a high DC voltage which emits electrons as it senses ultraviolet radiation. This UV tubes makes use of the photoelectric effect of metal and the gas multiplication effect. When the UV tube sees a sufficient amount of UV emission from the flame, electrons are released from the surface of the metal cathode and are detected as

an electric current. The current flow starts and ends abruptly and is known as an “avalanche.” A very intense source of UV radiation will produce several hundred avalanches or pulses per second. These pulses are picked up by the BMS flame amplifier and interpreted as a flame. With less radiation there will be fewer pulses per second, and upon total disappearance of the flame, the detector output ceases. Thus, the presence or absence of pulses is an indication of the presence or absence of the flame and the frequency of the pulses is a measure of flame intensity.

Problems with Basic UV Sensors

There are a couple of issues that arise from the UV tube. The first is that the tube has a limited lifespan. This can be improved by reducing the DC voltage supplied to the tube when the burner is not in operation. The second and more critical problem with UV tubes is that when they fail, they tend to fail in a “runaway” state, showing the presence of a flame at all times, even when the flame isn’t there. If the UV tube fails in the runaway state and a flame failure occurs, the BMS will not know to shut off the fuel supplied to the boiler furnace and can lead to an explosion.

UV Self-Check (UVSC)

Because of the UV tube’s tendency to fail in an unsafe way, many UV flame scanners incorporate a self-checking mechanism. This Is often done via a small actuator that moves a shutter in front of the UV tube, blocking its line of sight. This is done periodically, usually every 4-10 seconds. If the BMS receives a “Flame ON” condition from the flame sensor during the self-check period, it knows that the UV sensor has failed and it safely shuts down the burner.

Summary and Preferred Products

Flame detection is one of the most critical aspects of safe burner sequencing. The BurnerMate Universal and FlexFit burner control systems are available with IR/UV/ and UVSC options. These flame scanners include a Flame ON relay contact and a 4-20 mA flame strength output. Preferred also offers the 5002-01 UVSC flame scanner with a direct UV pulse output.