FIGURE 1. Chart depicts the spectral emissions from various fuel types. Hot refractory has been added to show where blackbody radiation can also emit significant infrared radiation. In practice, this can be ignored by the flame scanner, which looks for a flickering frequency only found in a flame.

Risk can be defined as a way to measure the possibility of an adverse event and the possible consequences should it occur. Safe operation -- a complicated subject -- can be simply defined as creating an environment where risk and the resulting consequences are avoided.

Naturally, it is important to look carefully into risk assessment of a plant or process because the consequences have the potential to be severe. The results of a major failure on a combustion system could be catastrophic. Consequences could lead to personnel injury or fatalities; damage to the system, building, environment and business (both its reputation and loss of production); and possibly other third-party liabilities.

FIGURE 2. Optimum mechanical positioning of the flame scanner allows clear sighting of the primary combustion zone of the flame. This is typically where intense mixing goes on and provides lots of flame signal as well as higher flicker frequencies than any potential background emissions.

Flame scanners are specifically designed to accurately detect the presence or absence of a target flame on a burner. By design, they will react very quickly to a change of flame condition. Depending on required codes, this is typically in a range of 1 to 4 sec. In the event of a flame failure, the scanner will signal the burner management system to immediately close the fuel safety valves, preventing unburned fuel from entering the firing chamber.

Optical flame scanners analyze the light emission from the flame at various wavelengths. Multiple detection techniques are employed depending on application, fuel type, flame condition and sensitivity required. In all cases, scanners have to be capable of distinguishing a “real” flame from any normal background emission such as hot refractory.

Flames emit light energy at spectral wavelengths from the ultraviolet range, through visible light, to the infrared range. Figure 1 shows the spectral emissions from various fuel types.

Scanners are tuned to react to light emissions in the ultraviolet (UV) or infrared spectral regions. Typically visible light is avoided to ensure the scanner is unaffected by stray light or solar energy, and the term “solar blind” is often used to describe this feature. Infrared and ultraviolet radiation are generally only found in the flame and are therefore reliable methods of detection. Other sources of infrared energy such as hot refractory do not possess the flickering quality of the flame, so it can easily be distinguished by the scanner.

FIGURE 3. Use of flame-analysis software greatly increases the user's ability to “see” what the scanner is detecting. This facilitates easier commissioning, especially in high discrimination applications. Software allows trending and logging of information in both the time and frequency domain.

Two main technologies typically are used for the sensing element within the scanner: UV tubes (Geiger Mueller principle) or solid-state devices. The UV-tube-based scanners are reliable performers for high sensitivity applications, and the solid-state units have enhanced discrimination capabilities. In application terms, a small flame being viewed from a long distance would need high sensitivity. Conversely, a firing chamber with multiple flames would need a scanner able to differentiate those flames.

High technology flame scanners have analysis capabilities and can employ many techniques to discriminate among flames in a multi-burner environment. This allows the precise tuning of the scanner to one particular target flame while ignoring the effect of other flames or background radiation in the same chamber.

Some of the complex flame-analysis methods employed in high discrimination applications are: