Flame scanners detect the presence or absence of a target flame on a burner, distinguishing a real flame from any normal background emission such as hot refractory -- and keep your combustion system safe.

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.

Many of the industrial production processes on which we depend make use of combustion systems to generate heat and power. These systems provide tremendous benefits but also carry inherent operating risk. Correct handling, control and supervision for any device combusting fuels is absolutely essential to providing a safe process operation.

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.

If your combustion process employs a burner management system, this provides a basic, proven method of mitigating risk and thereby increasing operational safety. These systems can range from complex multi-burner, safety instrumented systems to dedicated, individual burner, flame-safeguard controls. A flame scanner is a crucial, central component of any burner safety system and its operation and reliability should be carefully scrutinized.

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.

One common misconception about ultraviolet and infrared flame scanners is that if the operator can clearly see the visible part of the flame, so can the scanner. Ultraviolet and infrared flame scanners “see” light at a different wavelength than the human eye. So, even if the operator can see the visible part of the flame, the scanner may not be optimally sighted. Good sighting is essential for reliable operation. Figure 2 shows the classic optimum arrangement, where a clear sighting to the primary combustion zone is provided. It is recommended to always use the sighting, aim or flame signal strength facility (if available) to determine if the scanner is sighted correctly. This allows you to interpret what the scanner is actually detecting. A consistent, steady signal is a reliable signal.

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:

  • Selectivity of spectral response (for example, in the ultraviolet or infrared regions, or both).

  • Adjustable sensitivity to flame amplitude. Amplitude also is sometimes referred to as flame signal or intensity.

  • Selectable flame flicker frequency, or the frequency of the modulations within the flame.

    Some designs offer flame temperature measurement as a parallel determination of flame condition.

    In these more difficult scanning applications, use of communications and PC-based software provide an opportunity to represent the information graphically (figure 3). The user can then effectively see exactly what the scanner is detecting in a graphical format.

    Flame scanners need to be high safety integrity devices because the importance of the function they provide demands this. When selecting a flame scanner, look for agency approvals such as FM, UL and CE as this demonstrates the devices have been independently tested to meet code requirements. It is also good practice to ask your supplier or manufacturer for details about the functionality of the products. Good questions to ask relate to scanner safety integrity level, how the self-check function works, what sort of diagnostic coverage they utilize, how are they tested and what type of design verification process they go through. In fact, if in doubt about any aspect of safety, risk analysis, suitability, selection or product capabilities, consult an expert. Suppliers of safety products like these will be only too happy to offer their advice or provide detailed information to help you assess their capabilities.

    If you're looking to take your own steps to enhance safety, don't be afraid to add your own procedural layers of protection to your system. Make it a standard procedure to test and verify the safety function of your flame scanner as frequently as your particular process allows.

    In addition, next time your system shuts down due to a flame failure, take a careful look at possible reasons behind the trip. It is often easy to blame the scanner rather than the process itself. Your flame scanner is constantly keeping an eye on your safety and may be trying to let you know you have a problem. PH